Bash Reference Manual

# Bash Features ¶

This text is a brief description of the features that are present in the Bash shell (version 5.3, 24 February 2025). The Bash home page is http://www.gnu.org/software/bash/.

This is Edition 5.3, last updated 24 February 2025, of The GNU Bash Reference Manual, for `Bash`, Version 5.3.

Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (`sh`), the Korn Shell (`ksh`), and the C-shell (`csh` and its successor, `tcsh`). The following menu breaks the features up into categories, noting which features were inspired by other shells and which are specific to Bash.

This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.

## Table of Contents

## 1 Introduction ¶

### 1.1 What is Bash? ¶

Bash is the shell, or command language interpreter, for the GNU operating system. The name is an acronym for the ‘`Bourne-Again SHell`’, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell `sh`, which appeared in the Seventh Edition Bell Labs Research version of Unix.

Bash is largely compatible with `sh` and incorporates useful features from the Korn shell `ksh` and the C shell `csh`. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools portion of the IEEE POSIX specification (IEEE Standard 1003.1). It offers functional improvements over `sh` for both interactive and programming use.

While the GNU operating system provides other shells, including a version of `csh`, Bash is the default shell. Like other GNU software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems − independently-supported ports exist for Windows and other platforms.

### 1.2 What is a shell? ¶

At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions.

A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of GNU utilities. The programming language features allow these utilities to be combined. Users can create files containing commands, and these become commands themselves. These new commands have the same status as system commands in directories such as `/bin`, allowing users or groups to establish custom environments to automate their common tasks.

Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file or a string.

A shell allows execution of GNU commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands’ environments.

Shells also provide a small set of built-in commands (builtins) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, `cd`, `break`, `continue`, and `exec` cannot be implemented outside of the shell because they directly manipulate the shell itself. The `history`, `getopts`, `kill`, or `pwd` builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections.

While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.

Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. This manual describes how Bash provides all of these features.

## 2 Definitions ¶

These definitions are used throughout the remainder of this manual.

`POSIX` ¶

  A family of open system standards based on Unix. Bash is primarily concerned with the Shell and Utilities portion of the POSIX 1003.1 standard.

`blank`

  A space or tab character.

`whitespace`

  A character belonging to the `space` character class in the current locale, or for which `isspace()` returns true.

`builtin` ¶

  A command that is implemented internally by the shell itself, rather than by an executable program somewhere in the file system.

`control operator` ¶

  A `token` that performs a control function. It is a `newline` or one of the following: ‘`||`’, ‘`&&`’, ‘`&`’, ‘`;`’, ‘`;;`’, ‘`;&`’, ‘`;;&`’, ‘`|`’, ‘`|&`’, ‘`(`’, or ‘`)`’.

`exit status` ¶

  The value returned by a command to its caller. The value is restricted to eight bits, so the maximum value is 255.

`field` ¶

  A unit of text that is the result of one of the shell expansions. After expansion, when executing a command, the resulting fields are used as the command name and arguments.

`filename` ¶

  A string of characters used to identify a file.

`job` ¶

  A set of processes comprising a pipeline, and any processes descended from it, that are all in the same process group.

`job control` ¶

  A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes.

`metacharacter` ¶

  A character that, when unquoted, separates words. A metacharacter is a `space`, `tab`, `newline`, or one of the following characters: ‘`|`’, ‘`&`’, ‘`;`’, ‘`(`’, ‘`)`’, ‘`<`’, or ‘`>`’.

`name` ¶

  A `word` consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore. `Name`s are used as shell variable and function names. Also referred to as an `identifier`.

`operator` ¶

  A `control operator` or a `redirection operator`. See Redirections, for a list of redirection operators. Operators contain at least one unquoted `metacharacter`.

`process group` ¶

  A collection of related processes each having the same process group ID.

`process group ID` ¶

  A unique identifier that represents a `process group` during its lifetime.

`reserved word` ¶

  A `word` that has a special meaning to the shell. Most reserved words introduce shell flow control constructs, such as `for` and `while`.

`return status` ¶

  A synonym for `exit status`.

`signal` ¶

  A mechanism by which a process may be notified by the kernel of an event occurring in the system.

`special builtin` ¶

  A shell builtin command that has been classified as special by the POSIX standard.

`token` ¶

  A sequence of characters considered a single unit by the shell. It is either a `word` or an `operator`.

`word` ¶

  A sequence of characters treated as a unit by the shell. Words may not include unquoted `metacharacters`.

## 3 Basic Shell Features ¶

Bash is an acronym for ‘`Bourne-Again SHell`’. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, and the rules for evaluation and quoting are taken from the POSIX specification for the ‘standard’ Unix shell.

This chapter briefly summarizes the shell’s ‘building blocks’: commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.

### 3.1 Shell Syntax ¶

When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (‘`#`’), and the rest of that line.

Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command’s exit status, and makes that exit status available for further inspection or processing.

#### 3.1.1 Shell Operation ¶

The following is a brief description of the shell’s operation when it reads and executes a command. Basically, the shell does the following:

1. Reads its input from a file (see Shell Scripts), from a string supplied as an argument to the `-c` invocation option (see Invoking Bash), or from the user’s terminal.
2. Breaks the input into words and operators, obeying the quoting rules described in Quoting. These tokens are separated by `metacharacters`. This step performs alias expansion (see Aliases).
3. Parses the tokens into simple and compound commands (see Shell Commands).
4. Performs the various shell expansions (see Shell Expansions), breaking the expanded tokens into lists of filenames (see Filename Expansion) and commands and arguments.
5. Performs any necessary redirections (see Redirections) and removes the redirection operators and their operands from the argument list.
6. Executes the command (see Executing Commands).
7. Optionally waits for the command to complete and collects its exit status (see Exit Status).

#### 3.1.2 Quoting ¶

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.

Each of the shell metacharacters (see Definitions) has special meaning to the shell and must be quoted if it is to represent itself.

When the command history expansion facilities are being used (see History Expansion), the history expansion character, usually ‘`!`’, must be quoted to prevent history expansion. See Bash History Facilities, for more details concerning history expansion.

There are four quoting mechanisms: the escape character, single quotes, double quotes, and dollar-single quotes.

#### 3.1.2.1 Escape Character ¶

A non-quoted backslash ‘`\`’ is the Bash escape character. It preserves the literal value of the next character that follows, removing any special meaning it has, with the exception of `newline`. If a `\newline` pair appears, and the backslash itself is not quoted, the `\newline` is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

#### 3.1.2.2 Single Quotes ¶

Enclosing characters in single quotes (‘`'`’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

#### 3.1.2.3 Double Quotes ¶

Enclosing characters in double quotes (‘`"`’) preserves the literal value of all characters within the quotes, with the exception of ‘`$`’, ‘```’, ‘`\`’, and, when history expansion is enabled, ‘`!`’. When the shell is in POSIX mode (see Bash and POSIX), the ‘`!`’ has no special meaning within double quotes, even when history expansion is enabled. The characters ‘`$`’ and ‘```’ retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: ‘`$`’, ‘```’, ‘`"`’, ‘`\`’, or `newline`. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified.

A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ‘`!`’ appearing in double quotes is escaped using a backslash. The backslash preceding the ‘`!`’ is not removed.

The special parameters ‘`*`’ and ‘`@`’ have special meaning when in double quotes (see Shell Parameter Expansion).

#### 3.1.2.4 ANSI-C Quoting ¶

Character sequences of the form `$'string'` are treated as a special kind of single quotes. The sequence expands to string, with backslash-escaped characters in string replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

`\a`

  alert (bell)

`\b`

  backspace

`\e`
`\E`

  An escape character (not in ANSI C).

`\f`

  form feed

`\n`

  newline

`\r`

  carriage return

`\t`

  horizontal tab

`\v`

  vertical tab

`\\`

  backslash

`\'`

  single quote

`\"`

  double quote

`\?`

  question mark

`\nnn`

  The eight-bit character whose value is the octal value nnn (one to three octal digits).

`\xHH`

  The eight-bit character whose value is the hexadecimal value HH (one or two hex digits).

`\uHHHH`

  The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits).

`\UHHHHHHHH`

  The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits).

`\cx`

  A control-x character.

The expanded result is single-quoted, as if the dollar sign had not been present.

#### 3.1.2.5 Locale-Specific Translation ¶

Prefixing a double-quoted string with a dollar sign (‘`$`’), such as `$"hello, world"`, causes the string to be translated according to the current locale. The `gettext` infrastructure performs the lookup and translation, using the `LC_MESSAGES`, `TEXTDOMAINDIR`, and `TEXTDOMAIN` shell variables, as explained below. See the gettext documentation for additional details not covered here. If the current locale is `C` or `POSIX`, if there are no translations available, or if the string is not translated, the dollar sign is ignored, and the string is treated as double-quoted as described above. Since this is a form of double quoting, the string remains double-quoted by default, whether or not it is translated and replaced. If the `noexpand_translation` option is enabled using the `shopt` builtin (see The Shopt Builtin), translated strings are single-quoted instead of double-quoted.

The rest of this section is a brief overview of how you use gettext to create translations for strings in a shell script named scriptname. There are more details in the gettext documentation.

#### Creating Internationalized Scripts ¶

Once you’ve marked the strings in your script that you want to translate using $"…", you create a gettext "template" file using the command

    bash --dump-po-strings scriptname > domain.pot

The domain is your message domain. It’s just an arbitrary string that’s used to identify the files gettext needs, like a package or script name. It needs to be unique among all the message domains on systems where you install the translations, so gettext knows which translations correspond to your script. You’ll use the template file to create translations for each target language. The template file conventionally has the suffix ‘`.pot`’.

You copy this template file to a separate file for each target language you want to support (called "PO" files, which use the suffix ‘`.po`’). PO files use various naming conventions, but when you are working to translate a template file into a particular language, you first copy the template file to a file whose name is the language you want to target, with the ‘`.po`’ suffix. For instance, the Spanish translations of your strings would be in a file named ‘`es.po`’, and to get started using a message domain named "example," you would run

    cp example.pot es.po

Ultimately, PO files are often named domain.po and installed in directories that contain multiple translation files for a particular language.

Whichever naming convention you choose, you will need to translate the strings in the PO files into the appropriate languages. This has to be done manually.

When you have the translations and PO files complete, you’ll use the gettext tools to produce what are called "MO" files, which are compiled versions of the PO files the gettext tools use to look up translations efficiently. MO files are also called "message catalog" files. You use the `msgfmt` program to do this. For instance, if you had a file with Spanish translations, you could run

    msgfmt -o es.mo es.po

to produce the corresponding MO file.

Once you have the MO files, you decide where to install them and use the `TEXTDOMAINDIR` shell variable to tell the gettext tools where they are. Make sure to use the same message domain to name the MO files as you did for the PO files when you install them.

Your users will use the `LANG` or `LC_MESSAGES` shell variables to select the desired language.

You set the `TEXTDOMAIN` variable to the script’s message domain. As above, you use the message domain to name your translation files.

You, or possibly your users, set the `TEXTDOMAINDIR` variable to the name of a directory where the message catalog files are stored. If you install the message files into the system’s standard message catalog directory, you don’t need to worry about this variable.

The directory where the message catalog files are stored varies between systems. Some use the message catalog selected by the `LC_MESSAGES` shell variable. Others create the name of the message catalog from the value of the `TEXTDOMAIN` shell variable, possibly adding the ‘`.mo`’ suffix. If you use the `TEXTDOMAIN` variable, you may need to set the `TEXTDOMAINDIR` variable to the location of the message catalog files, as above. It’s common to use both variables in this fashion: `$TEXTDOMAINDIR`/`$LC_MESSAGES`/LC_MESSAGES/`$TEXTDOMAIN`.mo.

If you used that last convention, and you wanted to store the message catalog files with Spanish (es) and Esperanto (eo) translations into a local directory you use for custom translation files, you could run

    TEXTDOMAIN=example
    TEXTDOMAINDIR=/usr/local/share/locale

    cp es.mo ${TEXTDOMAINDIR}/es/LC_MESSAGES/${TEXTDOMAIN}.mo
    cp eo.mo ${TEXTDOMAINDIR}/eo/LC_MESSAGES/${TEXTDOMAIN}.mo

When all of this is done, and the message catalog files containing the compiled translations are installed in the correct location, your users will be able to see translated strings in any of the supported languages by setting the `LANG` or `LC_MESSAGES` environment variables before running your script.

#### 3.1.3 Comments ¶

In a non-interactive shell, or an interactive shell in which the `interactive_comments` option to the `shopt` builtin is enabled (see The Shopt Builtin), a word beginning with ‘`#`’ introduces a comment. A word begins at the beginning of a line, after unquoted whitespace, or after an operator. The comment causes that word and all remaining characters on that line to be ignored. An interactive shell without the `interactive_comments` option enabled does not allow comments. The `interactive_comments` option is enabled by default in interactive shells. See Interactive Shells, for a description of what makes a shell interactive.

### 3.2 Shell Commands ¶

A simple shell command such as `echo a b c` consists of the command itself followed by arguments, separated by spaces.

More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.

#### 3.2.1 Reserved Words ¶

Reserved words are words that have special meaning to the shell. They are used to begin and end the shell’s compound commands.

The following words are recognized as reserved when unquoted and the first word of a command (see below for exceptions):

`if` | `then` | `elif` | `else` | `fi` | `time`
`for` | `in` | `until` | `while` | `do` | `done`
`case` | `esac` | `coproc` | `select` | `function`
`{` | `}` | `[[` | `]]` | `!`

`in` is recognized as a reserved word if it is the third word of a `case` or `select` command. `in` and `do` are recognized as reserved words if they are the third word in a `for` command.

#### 3.2.2 Simple Commands ¶

A simple command is the kind of command that’s executed most often. It’s just a sequence of words separated by `blank`s, terminated by one of the shell’s control operators (see Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.

The return status (see Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 `waitpid` function, or 128+n if the command was terminated by signal n.

#### 3.2.3 Pipelines ¶

A `pipeline` is a sequence of one or more commands separated by one of the control operators ‘`|`’ or ‘`|&`’.

The format for a pipeline is

    [time [-p]] [!] command1 [ | or |& command2 ] ...

The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command’s output. This connection is performed before any redirections specified by command1.

If ‘`|&`’ is the pipeline operator, command1’s standard error, in addition to its standard output, is connected to command2’s standard input through the pipe; it is shorthand for `2>&1 |`. This implicit redirection of the standard error to the standard output is performed after any redirections specified by command1, consistent with that shorthand.

If the reserved word `time` precedes the pipeline, Bash prints timing statistics for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command’s execution. The `-p` option changes the output format to that specified by POSIX. When the shell is in POSIX mode (see Bash and POSIX), it does not recognize `time` as a reserved word if the next token begins with a ‘`-`’. The value of the `TIMEFORMAT` variable is a format string that specifies how the timing information should be displayed. See Bash Variables, for a description of the available formats. Providing `time` as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external `time` command cannot time these easily.

When the shell is in POSIX mode (see Bash and POSIX), you can use `time` by itself as a simple command. In this case, the shell displays the total user and system time consumed by the shell and its children. The `TIMEFORMAT` variable specifies the format of the time information.

If a pipeline is not executed asynchronously (see Lists of Commands), the shell waits for all commands in the pipeline to complete.

Each command in a multi-command pipeline, where pipes are created, is executed in its own subshell, which is a separate process (see Command Execution Environment). If the `lastpipe` option is enabled using the `shopt` builtin (see The Shopt Builtin), and job control is not active, the last element of a pipeline may be run by the shell process.

The exit status of a pipeline is the exit status of the last command in the pipeline, unless the `pipefail` option is enabled (see The Set Builtin). If `pipefail` is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ‘`!`’ precedes the pipeline, the exit status is the logical negation of the exit status as described above. If a pipeline is not executed asynchronously (see Lists of Commands), the shell waits for all commands in the pipeline to terminate before returning a value. The return status of an asynchronous pipeline is 0.

#### 3.2.4 Lists of Commands ¶

A `list` is a sequence of one or more pipelines separated by one of the operators ‘`;`’, ‘`&`’, ‘`&&`’, or ‘`||`’, and optionally terminated by one of ‘`;`’, ‘`&`’, or a `newline`.

Of these list operators, ‘`&&`’ and ‘`||`’ have equal precedence, followed by ‘`;`’ and ‘`&`’, which have equal precedence.

A sequence of one or more newlines may appear in a `list` to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator ‘`&`’, the shell executes the command asynchronously in a subshell. This is known as executing the command in the background, and these are referred to as asynchronous commands. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from `/dev/null`.

Commands separated by a ‘`;`’ are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

AND and OR lists are sequences of one or more pipelines separated by the control operators ‘`&&`’ and ‘`||`’, respectively. AND and OR lists are executed with left associativity.

An AND list has the form

    command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero (success).

An OR list has the form

    command1 || command2

command2 is executed if, and only if, command1 returns a non-zero exit status.

The return status of AND and OR lists is the exit status of the last command executed in the list.

#### 3.2.5 Compound Commands ¶

Compound commands are the shell programming language constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.

In most cases a list of commands in a compound command’s description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.

Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.

#### 3.2.5.1 Looping Constructs ¶

Bash supports the following looping constructs.

Note that wherever a ‘`;`’ appears in the description of a command’s syntax, it may be replaced with one or more newlines.

`until` ¶

  The syntax of the `until` command is:

      until test-commands; do consequent-commands; done

  Execute consequent-commands as long as test-commands has an exit status which is not zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

`while` ¶

  The syntax of the `while` command is:

      while test-commands; do consequent-commands; done

  Execute consequent-commands as long as test-commands has an exit status of zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

`for` ¶

  The syntax of the `for` command is:

      for name [ [in words ...] ; ] do commands; done

  Expand words (see Shell Expansions), and then execute commands once for each word in the resultant list, with name bound to the current word. If ‘`in words`’ is not present, the `for` command executes the commands once for each positional parameter that is set, as if ‘`in "$@"`’ had been specified (see Special Parameters).

  The return status is the exit status of the last command that executes. If there are no items in the expansion of words, no commands are executed, and the return status is zero.

  There is an alternate form of the `for` command which is similar to the C language:

      for (( expr1 ; expr2 ; expr3 )) [;] do commands ; done

  First, evaluate the arithmetic expression expr1 according to the rules described below (see Shell Arithmetic). Then, repeatedly evaluate the arithmetic expression expr2 until it evaluates to zero. Each time expr2 evaluates to a non-zero value, execute commands and evaluate the arithmetic expression expr3. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in commands that is executed, or non-zero if any of the expressions is invalid.

Use the `break` and `continue` builtins (see Bourne Shell Builtins) to control loop execution.

#### 3.2.5.2 Conditional Constructs ¶

`if` ¶

  The syntax of the `if` command is:

      if test-commands; then
        consequent-commands;
      [elif more-test-commands; then
        more-consequents;]
      [else alternate-consequents;]
      fi

  The test-commands list is executed, and if its return status is zero, the consequent-commands list is executed. If test-commands returns a non-zero status, each `elif` list is executed in turn, and if its exit status is zero, the corresponding more-consequents is executed and the command completes. If ‘`else alternate-consequents`’ is present, and the final command in the final `if` or `elif` clause has a non-zero exit status, then alternate-consequents is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

`case` ¶

  The syntax of the `case` command is:

      case word in
          [ [(] pattern [| pattern]...) command-list ;;]...
      esac

  `case` will selectively execute the command-list corresponding to the first pattern that matches word, proceeding from the first pattern to the last. The match is performed according to the rules described below in Pattern Matching. If the `nocasematch` shell option (see the description of `shopt` in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The ‘`|`’ is used to separate multiple patterns in a pattern list, and the ‘`)`’ operator terminates the pattern list. A pattern list and an associated command-list is known as a clause.

  Each clause must be terminated with ‘`;;`’, ‘`;&`’, or ‘`;;&`’. The word undergoes tilde expansion, parameter expansion, command substitution, process substitution, arithmetic expansion, and quote removal (see Shell Parameter Expansion) before the shell attempts to match the pattern. Each pattern undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, process substitution, and quote removal.

  There may be an arbitrary number of `case` clauses, each terminated by a ‘`;;`’, ‘`;&`’, or ‘`;;&`’. The first pattern that matches determines the command-list that is executed. It’s a common idiom to use ‘`*`’ as the final pattern to define the default case, since that pattern will always match.

  Here is an example using `case` in a script that could be used to describe one interesting feature of an animal:

      echo -n "Enter the name of an animal: "
      read ANIMAL
      echo -n "The $ANIMAL has "
      case $ANIMAL in
        horse | dog | cat) echo -n "four";;
        man | kangaroo ) echo -n "two";;
        *) echo -n "an unknown number of";;
      esac
      echo " legs."

  If the ‘`;;`’ operator is used, the `case` command completes after the first pattern match. Using ‘`;&`’ in place of ‘`;;`’ causes execution to continue with the command-list associated with the next clause, if any. Using ‘`;;&`’ in place of ‘`;;`’ causes the shell to test the patterns in the next clause, if any, and execute any associated command-list if the match succeeds, continuing the case statement execution as if the pattern list had not matched.

  The return status is zero if no pattern matches. Otherwise, the return status is the exit status of the last command-list executed.

`select` ¶

  The `select` construct allows the easy generation of menus. It has almost the same syntax as the `for` command:

      select name [in words ...]; do commands; done

  First, expand the list of words following `in`, generating a list of items, and print the set of expanded words on the standard error stream, each preceded by a number. If the ‘`in words`’ is omitted, print the positional parameters, as if ‘`in "$@"`’ had been specified. `select` then displays the `PS3` prompt and reads a line from the standard input. If the line consists of a number corresponding to one of the displayed words, then `select` sets the value of name to that word. If the line is empty, `select` displays the words and prompt again. If `EOF` is read, `select` completes and returns 1. Any other value read causes name to be set to null. The line read is saved in the variable `REPLY`.

  The commands are executed after each selection until a `break` command is executed, at which point the `select` command completes.

  Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.

      select fname in *;
      do
      	echo you picked $fname \($REPLY\)
      	break;
      done

`((…))`

      (( expression ))

  The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic). The expression undergoes the same expansions as if it were within double quotes, but unescaped double quote characters in expression are not treated specially and are removed. Since this can potentially result in empty strings, this command treats those as expressions that evaluate to 0. If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1.

`[[…]]` ¶

      [[ expression ]]

  Evaluate the conditional expression expression and return a status of zero (true) or non-zero (false). Expressions are composed of the primaries described below in Bash Conditional Expressions. The words between the `[[` and `]]` do not undergo word splitting and filename expansion. The shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal on those words. Conditional operators such as ‘`-f`’ must be unquoted to be recognized as primaries.

  When used with `[[`, the ‘`<`’ and ‘`>`’ operators sort lexicographically using the current locale.

  When the ‘`==`’ and ‘`!=`’ operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching, as if the `extglob` shell option were enabled. The ‘`=`’ operator is identical to ‘`==`’. If the `nocasematch` shell option (see the description of `shopt` in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (‘`==`’) or does not match (‘`!=`’) the pattern, and 1 otherwise.

  If you quote any part of the pattern, using any of the shell’s quoting mechanisms, the quoted portion is matched literally. This means every character in the quoted portion matches itself, instead of having any special pattern matching meaning.

  An additional binary operator, ‘`=~`’, is available, with the same precedence as ‘`==`’ and ‘`!=`’. When you use ‘`=~`’, the string to the right of the operator is considered a POSIX extended regular expression pattern and matched accordingly (using the POSIX `regcomp` and `regexec` interfaces usually described in regex(3)). The return value is 0 if the string matches the pattern, and 1 if it does not. If the regular expression is syntactically incorrect, the conditional expression returns 2. If the `nocasematch` shell option (see the description of `shopt` in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters.

  You can quote any part of the pattern to force the quoted portion to be matched literally instead of as a regular expression (see above). If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched literally.

  The match succeeds if the pattern matches any part of the string. If you want to force the pattern to match the entire string, anchor the pattern using the ‘`^`’ and ‘`$`’ regular expression operators.

  For example, the following will match a line (stored in the shell variable `line`) if there is a sequence of characters anywhere in the value consisting of any number, including zero, of characters in the `space` character class, immediately followed by zero or one instances of ‘`a`’, then a ‘`b`’:

      [[ $line =~ [[:space:]]*(a)?b ]]

  That means values for `line` like ‘`aab`’, ‘`aaaaaab`’, ‘`xaby`’, and ‘`ab`’ will all match, as will a line containing a ‘`b`’ anywhere in its value.

  If you want to match a character that’s special to the regular expression grammar (‘`^$|[]()\.*+?`’), it has to be quoted to remove its special meaning. This means that in the pattern ‘`xxx.txt`’, the ‘`.`’ matches any character in the string (its usual regular expression meaning), but in the pattern ‘`"xxx.txt"`’, it can only match a literal ‘`.`’.

  Likewise, if you want to include a character in your pattern that has a special meaning to the regular expression grammar, you must make sure it’s not quoted. If you want to anchor a pattern at the beginning or end of the string, for instance, you cannot quote the ‘`^`’ or ‘`$`’ characters using any form of shell quoting.

  If you want to match ‘`initial string`’ at the start of a line, the following will work:

      [[ $line =~ ^"initial string" ]]

  but this will not:

      [[ $line =~ "^initial string" ]]

  because in the second example the ‘`^`’ is quoted and doesn’t have its usual special meaning.

  It is sometimes difficult to specify a regular expression properly without using quotes, or to keep track of the quoting used by regular expressions while paying attention to shell quoting and the shell’s quote removal. Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. For example, the following is equivalent to the pattern used above:

      pattern='[[:space:]]*(a)?b'
      [[ $line =~ $pattern ]]

  Shell programmers should take special care with backslashes, since backslashes are used by both the shell and regular expressions to remove the special meaning from the following character. This means that after the shell’s word expansions complete (see Shell Expansions), any backslashes remaining in parts of the pattern that were originally not quoted can remove the special meaning of pattern characters. If any part of the pattern is quoted, the shell does its best to ensure that the regular expression treats those remaining backslashes as literal, if they appeared in a quoted portion.

  The following two sets of commands are not equivalent:

      pattern='\.'

      [[ . =~ $pattern ]]
      [[ . =~ \. ]]

      [[ . =~ "$pattern" ]]
      [[ . =~ '\.' ]]

  The first two matches will succeed, but the second two will not, because in the second two the backslash will be part of the pattern to be matched. In the first two examples, the pattern passed to the regular expression parser is ‘`\.`’. The backslash removes the special meaning from ‘`.`’, so the literal ‘`.`’ matches. In the second two examples, the pattern passed to the regular expression parser has the backslash quoted (e.g., ‘`\\\.`’), which will not match the string, since it does not contain a backslash. If the string in the first examples were anything other than ‘`.`’, say ‘`a`’, the pattern would not match, because the quoted ‘`.`’ in the pattern loses its special meaning of matching any single character.

  Bracket expressions in regular expressions can be sources of errors as well, since characters that are normally special in regular expressions lose their special meanings between brackets. However, you can use bracket expressions to match special pattern characters without quoting them, so they are sometimes useful for this purpose.

  Though it might seem like a strange way to write it, the following pattern will match a ‘`.`’ in the string:

      [[ . =~ [.] ]]

  The shell performs any word expansions before passing the pattern to the regular expression functions, so you can assume that the shell’s quoting takes precedence. As noted above, the regular expression parser will interpret any unquoted backslashes remaining in the pattern after shell expansion according to its own rules. The intention is to avoid making shell programmers quote things twice as much as possible, so shell quoting should be sufficient to quote special pattern characters where that’s necessary.

  The array variable `BASH_REMATCH` records which parts of the string matched the pattern. The element of `BASH_REMATCH` with index 0 contains the portion of the string matching the entire regular expression. Substrings matched by parenthesized subexpressions within the regular expression are saved in the remaining `BASH_REMATCH` indices. The element of `BASH_REMATCH` with index n is the portion of the string matching the nth parenthesized subexpression.

  Bash sets `BASH_REMATCH` in the global scope; declaring it as a local variable will lead to unexpected results.

  Expressions may be combined using the following operators, listed in decreasing order of precedence:

  `( expression )`

    Returns the value of expression. This may be used to override the normal precedence of operators.

  `! expression`

    True if expression is false.

  `expression1 && expression2`

    True if both expression1 and expression2 are true.

  `expression1 || expression2`

    True if either expression1 or expression2 is true.

  The `&&` and `||` operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.

#### 3.2.5.3 Grouping Commands ¶

Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.

`()`

      ( list )

  Placing a list of commands between parentheses forces the shell to create a subshell (see Command Execution Environment), and each of the commands in list is executed in that subshell environment. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.

`{}` ¶

      { list; }

  Placing a list of commands between curly braces causes the list to be executed in the current shell environment. No subshell is created. The semicolon (or newline) following list is required.

In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are reserved words, so they must be separated from the list by `blank`s or other shell metacharacters. The parentheses are operators, and are recognized as separate tokens by the shell even if they are not separated from the list by whitespace.

The exit status of both of these constructs is the exit status of list.

#### 3.2.6 Coprocesses ¶

A `coprocess` is a shell command preceded by the `coproc` reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the ‘`&`’ control operator, with a two-way pipe established between the executing shell and the coprocess.

The syntax for a coprocess is:

    coproc [NAME] command [redirections]

This creates a coprocess named NAME. command may be either a simple command (see Simple Commands) or a compound command (see Compound Commands). NAME is a shell variable name. If NAME is not supplied, the default name is `COPROC`.

The recommended form to use for a coprocess is

    coproc NAME { command; }

This form is preferred because simple commands result in the coprocess always being named `COPROC`, and it is simpler to use and more complete than the other compound commands.

There are other forms of coprocesses:

    coproc NAME compound-command
    coproc compound-command
    coproc simple-command

If command is a compound command, NAME is optional. The word following `coproc` determines whether that word is interpreted as a variable name: it is interpreted as NAME if it is not a reserved word that introduces a compound command. If command is a simple command, NAME is not allowed; this is to avoid confusion between NAME and the first word of the simple command.

When the coprocess is executed, the shell creates an array variable (see Arrays) named NAME in the context of the executing shell. The standard output of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[0]. The standard input of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[1]. This pipe is established before any redirections specified by the command (see Redirections). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions. Other than those created to execute command and process substitutions, the file descriptors are not available in subshells.

The process ID of the shell spawned to execute the coprocess is available as the value of the variable `NAME_PID`. The `wait` builtin may be used to wait for the coprocess to terminate.

Since the coprocess is created as an asynchronous command, the `coproc` command always returns success. The return status of a coprocess is the exit status of command.

#### 3.2.7 GNU Parallel ¶

There are ways to run commands in parallel that are not built into Bash. GNU Parallel is a tool to do just that.

GNU Parallel, as its name suggests, can be used to build and run commands in parallel. You may run the same command with different arguments, whether they are filenames, usernames, hostnames, or lines read from files. GNU Parallel provides shorthand references to many of the most common operations (input lines, various portions of the input line, different ways to specify the input source, and so on). Parallel can replace `xargs` or feed commands from its input sources to several different instances of Bash.

For a complete description, refer to the GNU Parallel documentation, which is available at https://www.gnu.org/software/parallel/parallel_tutorial.html.

### 3.3 Shell Functions ¶

Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" simple command. When the name of a shell function is used as a simple command name, the shell executes the list of commands associated with that function name. Shell functions are executed in the current shell context; there is no new process created to interpret them.

Functions are declared using this syntax:

    fname () compound-command [ redirections ]

or

    function fname [()] compound-command [ redirections ]

This defines a shell function named fname. The reserved word `function` is optional. If the `function` reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands). That command is usually a list enclosed between { and }, but may be any compound command listed above. If the `function` reserved word is used, but the parentheses are not supplied, the braces are recommended. When the shell is in POSIX mode (see Bash and POSIX), fname must be a valid shell name and may not be the same as one of the special builtins (see Special Builtins). When not in POSIX mode, a function name can be any unquoted shell word that does not contain ‘`$`’.

Any redirections (see Redirections) associated with the shell function are performed when the function is executed. Function definitions are deleted using the `-f` option to the `unset` builtin (see Bourne Shell Builtins).

The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.

Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by `blank`s or newlines. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. When using the braces, the list must be terminated by a semicolon, a ‘`&`’, or a newline.

compound-command is executed whenever fname is specified as the name of a simple command. Functions are executed in the context of the calling shell; there is no new process created to interpret them (contrast this with the execution of a shell script).

When a function is executed, the arguments to the function become the positional parameters during its execution (see Positional Parameters). The special parameter ‘`#`’ that expands to the number of positional parameters is updated to reflect the new set of positional parameters. Special parameter `0` is unchanged. The first element of the `FUNCNAME` variable is set to the name of the function while the function is executing.

All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the `DEBUG` and `RETURN` traps are not inherited unless the function has been given the `trace` attribute using the `declare` builtin or the `-o functrace` option has been enabled with the `set` builtin, (in which case all functions inherit the `DEBUG` and `RETURN` traps), and the `ERR` trap is not inherited unless the `-o errtrace` shell option has been enabled. See Bourne Shell Builtins, for the description of the `trap` builtin.

The `FUNCNEST` variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed the limit cause the entire command to abort.

If the builtin command `return` is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the `RETURN` trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter ‘`#`’ are restored to the values they had prior to the function’s execution. If `return` is supplied a numeric argument, that is the function’s return status; otherwise the function’s return status is the exit status of the last command executed before the `return`.

Variables local to the function are declared with the `local` builtin (local variables). Ordinarily, variables and their values are shared between a function and its caller. These variables are visible only to the function and the commands it invokes. This is particularly important when a shell function calls other functions.

In the following description, the current scope is a currently- executing function. Previous scopes consist of that function’s caller and so on, back to the "global" scope, where the shell is not executing any shell function. A local variable at the current local scope is a variable declared using the `local` or `declare` builtins in the function that is currently executing.

Local variables "shadow" variables with the same name declared at previous scopes. For instance, a local variable declared in a function hides variables with the same name declared at previous scopes, including global variables: references and assignments refer to the local variable, leaving the variables at previous scopes unmodified. When the function returns, the global variable is once again visible.

The shell uses dynamic scoping to control a variable’s visibility within functions. With dynamic scoping, visible variables and their values are a result of the sequence of function calls that caused execution to reach the current function. The value of a variable that a function sees depends on its value within its caller, if any, whether that caller is the global scope or another shell function. This is also the value that a local variable declaration shadows, and the value that is restored when the function returns.

For example, if a variable `var` is declared as local in function `func1`, and `func1` calls another function `func2`, references to `var` made from within `func2` resolve to the local variable `var` from `func1`, shadowing any global variable named `var`.

The following script demonstrates this behavior. When executed, the script displays

    In func2, var = func1 local

    func1()
    {
        local var='func1 local'
        func2
    }

    func2()
    {
        echo "In func2, var = $var"
    }

    var=global
    func1

The `unset` builtin also acts using the same dynamic scope: if a variable is local to the current scope, `unset` unsets it; otherwise the unset will refer to the variable found in any calling scope as described above. If a variable at the current local scope is unset, it remains so (appearing as unset) until it is reset in that scope or until the function returns. Once the function returns, any instance of the variable at a previous scope becomes visible. If the unset acts on a variable at a previous scope, any instance of a variable with that name that had been shadowed becomes visible (see below how the `localvar_unset` shell option changes this behavior).

The `-f` option to the `declare` (`typeset`) builtin command (see Bash Builtin Commands) lists function names and definitions. The `-F` option to `declare` or `typeset` lists the function names only (and optionally the source file and line number, if the `extdebug` shell option is enabled). Functions may be exported so that child shell processes (those created when executing a separate shell invocation) automatically have them defined with the `-f` option to the `export` builtin (see Bourne Shell Builtins). The `-f` option to the `unset` builtin (see Bourne Shell Builtins) deletes a function definition.

Functions may be recursive. The `FUNCNEST` variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, Bash places no limit on the number of recursive calls.

### 3.4 Shell Parameters ¶

A parameter is an entity that stores values. It can be a `name`, a number, or one of the special characters listed below. A variable is a parameter denoted by a `name`. A variable has a `value` and zero or more `attributes`. Attributes are assigned using the `declare` builtin command (see the description of the `declare` builtin in Bash Builtin Commands). The `export` and `readonly` builtins assign specific attributes.

A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the `unset` builtin command.

A variable is assigned to using a statement of the form

    name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (see Shell Parameter Expansion). If the variable has its `integer` attribute set, then value is evaluated as an arithmetic expression even if the `$((…))` expansion is not used (see Arithmetic Expansion). Word splitting and filename expansion are not performed. Assignment statements may also appear as arguments to the `alias`, `declare`, `typeset`, `export`, `readonly`, and `local` builtin commands (declaration commands). When in POSIX mode (see Bash and POSIX), these builtins may appear in a command after one or more instances of the `command` builtin and retain these assignment statement properties. For example,

    command export var=value

In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ‘`+=`’ operator appends to or adds to the variable’s previous value. This includes arguments to declaration commands such as `declare` that accept assignment statements. When ‘`+=`’ is applied to a variable for which the `integer` attribute has been set, the variable’s current value and value are each evaluated as arithmetic expressions, and the sum of the results is assigned as the variable’s value. The current value is usually an integer constant, but may be an expression. When ‘`+=`’ is applied to an array variable using compound assignment (see Arrays), the variable’s value is not unset (as it is when using ‘`=`’), and new values are appended to the array beginning at one greater than the array’s maximum index (for indexed arrays), or added as additional key-value pairs in an associative array. When applied to a string-valued variable, value is expanded and appended to the variable’s value.

A variable can be assigned the `nameref` attribute using the `-n` option to the `declare` or `local` builtin commands (see Bash Builtin Commands) to create a nameref, or a reference to another variable. This allows variables to be manipulated indirectly. Whenever the nameref variable is referenced, assigned to, unset, or has its attributes modified (other than using or changing the nameref attribute itself), the operation is actually performed on the variable specified by the nameref variable’s value. A nameref is commonly used within shell functions to refer to a variable whose name is passed as an argument to the function. For instance, if a variable name is passed to a shell function as its first argument, running

    declare -n ref=$1

inside the function creates a local nameref variable `ref` whose value is the variable name passed as the first argument. References and assignments to `ref`, and changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as `$1`.

If the control variable in a `for` loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference is established for each word in the list, in turn, when the loop is executed. Array variables cannot be given the nameref attribute. However, nameref variables can reference array variables and subscripted array variables. Namerefs can be unset using the `-n` option to the `unset` builtin (see Bourne Shell Builtins). Otherwise, if `unset` is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable is unset.

When the shell starts, it reads its environment and creates a shell variable from each environment variable that has a valid name, as described below (see Environment).

#### 3.4.1 Positional Parameters ¶

A positional parameter is a parameter denoted by one or more digits, other than the single digit `0`. Positional parameters are assigned from the shell’s arguments when it is invoked, and may be reassigned using the `set` builtin command. Positional parameter `N` may be referenced as `${N}`, or as `$N` when `N` consists of a single digit. Positional parameters may not be assigned to with assignment statements. The `set` and `shift` builtins are used to set and unset them (see Shell Builtin Commands). The positional parameters are temporarily replaced when a shell function is executed (see Shell Functions).

When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces. Without braces, a digit following ‘`$`’ can only refer to one of the first nine positional parameters ($1\-$9) or the special parameter $0 (see below).

#### 3.4.2 Special Parameters ¶

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. Special parameters are denoted by one of the following characters.

`*` ¶

  ($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where word expansions are performed, those words are subject to further word splitting and filename expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the `IFS` variable. That is, `"$*"` is equivalent to `"$1c$2c…"`, where c is the first character of the value of the `IFS` variable. If `IFS` is unset, the parameters are separated by spaces. If `IFS` is null, the parameters are joined without intervening separators.

`@` ¶

  ($@) Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not performed, such as the value portion of an assignment statement, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, and word splitting is performed, each parameter expands to a separate word. That is, `"$@"` is equivalent to `"$1" "$2" …`. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the expansion of the beginning part of the original word, and the expansion of the last parameter is joined with the expansion of the last part of the original word. When there are no positional parameters, `"$@"` and `$@` expand to nothing (i.e., they are removed).

`#` ¶

  ($#) Expands to the number of positional parameters in decimal.

`?` ¶

  ($?) Expands to the exit status of the most recently executed command.

`-` ¶

  ($-, a hyphen.) Expands to the current option flags as specified upon invocation, by the `set` builtin command, or those set by the shell itself (such as the `-i` option).

`$` ¶

  ($$) Expands to the process ID of the shell. In a subshell, it expands to the process ID of the invoking shell, not the subshell.

`!` ¶

  ($!) Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the `bg` builtin (see Job Control Builtins).

`0` ¶

  ($0) Expands to the name of the shell or shell script. This is set at shell initialization. If Bash is invoked with a file of commands (see Shell Scripts), `$0` is set to the name of that file. If Bash is started with the `-c` option (see Invoking Bash), then `$0` is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash, as given by argument zero.

### 3.5 Shell Expansions ¶

Expansion is performed on the command line after it has been split into `token`s. Bash performs these expansions:

- brace expansion
- tilde expansion
- parameter and variable expansion
- command substitution
- arithmetic expansion
- word splitting
- filename expansion
- quote removal

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; filename expansion; and quote removal.

On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution.

Quote removal is always performed last. It removes quote characters present in the original word, not ones resulting from one of the other expansions, unless they have been quoted themselves. See Quote Removal for more details.

Only brace expansion, word splitting, and filename expansion can increase the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of `"$@"` and `$*` (see Special Parameters), and `"${name[@]}"` and `${name[*]}` (see Arrays).

#### 3.5.1 Brace Expansion ¶

Brace expansion is a mechanism to generate arbitrary strings sharing a common prefix and suffix, either of which can be empty. This mechanism is similar to filename expansion (see Filename Expansion), but the filenames generated need not exist. Patterns to be brace expanded are formed from an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.

Brace expansions may be nested. The results of each expanded string are not sorted; brace expansion preserves left to right order. For example,

    bash$ echo a{d,c,b}e
    ade ace abe

A sequence expression takes the form `x..y[..incr]`, where x and y are either integers or letters, and incr, an optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. If either x or y begins with a zero, each generated term will contain the same number of digits, zero-padding where necessary. When letters are supplied, the expression expands to each character lexicographically between x and y, inclusive, using the C locale. Note that both x and y must be of the same type (integer or letter). When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate.

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.

A ‘`{`’ or ‘`,`’ may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ‘`${`’ is not considered eligible for brace expansion, and inhibits brace expansion until the closing ‘`}`’.

This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:

    mkdir /usr/local/src/bash/{old,new,dist,bugs}

or

    chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

Brace expansion introduces a slight incompatibility with historical versions of `sh`. `sh` does not treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a consequence of brace expansion. For example, a word entered to `sh` as ‘`file{1,2}`’ appears identically in the output. Bash outputs that word as ‘`file1 file2`’ after brace expansion. Start Bash with the `+B` option or disable brace expansion with the `+B` option to the `set` command (see Shell Builtin Commands) for strict `sh` compatibility.

#### 3.5.2 Tilde Expansion ¶

If a word begins with an unquoted tilde character (‘`~`’), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the `HOME` shell variable. If `HOME` is unset, the tilde expands to the home directory of the user executing the shell instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.

If the tilde-prefix is ‘`~+`’, the value of the shell variable `PWD` replaces the tilde-prefix. If the tilde-prefix is ‘`~-`’, the shell substitutes the value of the shell variable `OLDPWD`, if it is set.

If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a ‘`+`’ or a ‘`-`’, the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the `dirs` builtin invoked with the characters following tilde in the tilde-prefix as an argument (see The Directory Stack). If the tilde-prefix, sans the tilde, consists of a number without a leading ‘`+`’ or ‘`-`’, tilde expansion assumes ‘`+`’.

The results of tilde expansion are treated as if they were quoted, so the replacement is not subject to word splitting and filename expansion.

If the login name is invalid, or the tilde expansion fails, the tilde-prefix is left unchanged.

Bash checks each variable assignment for unquoted tilde-prefixes immediately following a ‘`:`’ or the first ‘`=`’, and performs tilde expansion in these cases. Consequently, one may use filenames with tildes in assignments to `PATH`, `MAILPATH`, and `CDPATH`, and the shell assigns the expanded value.

The following table shows how Bash treats unquoted tilde-prefixes:

`~`

  The value of `$HOME`.

`~/foo`

  `$HOME/foo`

`~fred/foo`

  The directory or file `foo` in the home directory of the user `fred`.

`~+/foo`

  `$PWD/foo`

`~-/foo`

  `${OLDPWD-'~-'}/foo`

`~N`

  The string that would be displayed by ‘`dirs +N`’.

`~+N`

  The string that would be displayed by ‘`dirs +N`’.

`~-N`

  The string that would be displayed by ‘`dirs -N`’.

Bash also performs tilde expansion on words satisfying the conditions of variable assignments (see Shell Parameters) when they appear as arguments to simple commands. Bash does not do this, except for the declaration commands listed above, when in POSIX mode.

#### 3.5.3 Shell Parameter Expansion ¶

The ‘`$`’ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name. For example, if the first positional parameter has the value ‘`a`’, then `${11}` expands to the value of the eleventh positional parameter, while `$11` expands to ‘`a1`’.

When braces are used, the matching ending brace is the first ‘`}`’ not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.

The basic form of parameter expansion is ${parameter}, which substitutes the value of parameter. The parameter is a shell parameter as described above (see Shell Parameters) or an array reference (see Arrays). The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name.

If the first character of parameter is an exclamation point (!), and parameter is not a nameref, it introduces a level of indirection. Bash uses the value formed by expanding the rest of parameter as the new parameter; this new parameter is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter. This is known as `indirect expansion`. The value is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If parameter is a nameref, this expands to the name of the variable referenced by parameter instead of performing the complete indirect expansion, for compatibility. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the forms described below (e.g., ‘`:-`’), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

`${parameter:−word}`

  If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

      $ v=123
      $ echo ${v-unset}
      123
      $ echo ${v:-unset-or-null}
      123
      $ unset v
      $ echo ${v-unset}
      unset
      $ v=
      $ echo ${v-unset}

      $ echo ${v:-unset-or-null}
      unset-or-null

`${parameter:=word}`

  If parameter is unset or null, the expansion of word is assigned to parameter, and the result of the expansion is the final value of parameter. Positional parameters and special parameters may not be assigned in this way.

      $ unset var
      $ : ${var=DEFAULT}
      $ echo $var
      DEFAULT
      $ var=
      $ : ${var=DEFAULT}
      $ echo $var

      $ var=
      $ : ${var:=DEFAULT}
      $ echo $var
      DEFAULT
      $ unset var
      $ : ${var:=DEFAULT}
      $ echo $var
      DEFAULT

`${parameter:?word}`

  If parameter is null or unset, the shell writes the expansion of word (or a message to that effect if word is not present) to the standard error and, if it is not interactive, exits with a non-zero status. An interactive shell does not exit, but does not execute the command associated with the expansion. Otherwise, the value of parameter is substituted.

      $ var=
      $ : ${var:?var is unset or null}
      bash: var: var is unset or null
      $ echo ${var?var is unset}

      $ unset var
      $ : ${var?var is unset}
      bash: var: var is unset
      $ : ${var:?var is unset or null}
      bash: var: var is unset or null
      $ var=123
      $ echo ${var:?var is unset or null}
      123

`${parameter:+word}`

  If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted. The value of parameter is not used.

      $ var=123
      $ echo ${var:+var is set and not null}
      var is set and not null
      $ echo ${var+var is set}
      var is set
      $ var=
      $ echo ${var:+var is set and not null}

      $ echo ${var+var is set}
      var is set
      $ unset var
      $ echo ${var+var is set}

      $ echo ${var:+var is set and not null}

      $

`${parameter:offset}`
`${parameter:offset:length}`

  This is referred to as Substring Expansion. It expands to up to length characters of the value of parameter starting at the character specified by offset. If parameter is ‘`@`’ or ‘`*`’, an indexed array subscripted by ‘`@`’ or ‘`*`’, or an associative array name, the results differ as described below. If :length is omitted (the first form above), this expands to the substring of the value of parameter starting at the character specified by offset and extending to the end of the value. If offset is omitted, it is treated as 0. If length is omitted, but the colon after offset is present, it is treated as 0. length and offset are arithmetic expressions (see Shell Arithmetic).

  If offset evaluates to a number less than zero, the value is used as an offset in characters from the end of the value of parameter. If length evaluates to a number less than zero, it is interpreted as an offset in characters from the end of the value of parameter rather than a number of characters, and the expansion is the characters between offset and that result.

  Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the ‘`:-`’ expansion.

  Here are some examples illustrating substring expansion on parameters and subscripted arrays:

      $ string=01234567890abcdefgh
      $ echo ${string:7}
      7890abcdefgh
      $ echo ${string:7:0}

      $ echo ${string:7:2}
      78
      $ echo ${string:7:-2}
      7890abcdef
      $ echo ${string: -7}
      bcdefgh
      $ echo ${string: -7:0}

      $ echo ${string: -7:2}
      bc
      $ echo ${string: -7:-2}
      bcdef
      $ set -- 01234567890abcdefgh
      $ echo ${1:7}
      7890abcdefgh
      $ echo ${1:7:0}

      $ echo ${1:7:2}
      78
      $ echo ${1:7:-2}
      7890abcdef
      $ echo ${1: -7}
      bcdefgh
      $ echo ${1: -7:0}

      $ echo ${1: -7:2}
      bc
      $ echo ${1: -7:-2}
      bcdef
      $ array[0]=01234567890abcdefgh
      $ echo ${array[0]:7}
      7890abcdefgh
      $ echo ${array[0]:7:0}

      $ echo ${array[0]:7:2}
      78
      $ echo ${array[0]:7:-2}
      7890abcdef
      $ echo ${array[0]: -7}
      bcdefgh
      $ echo ${array[0]: -7:0}

      $ echo ${array[0]: -7:2}
      bc
      $ echo ${array[0]: -7:-2}
      bcdef

  If parameter is ‘`@`’ or ‘`*`’, the result is length positional parameters beginning at offset. A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter (or 0 if there are no positional parameters). It is an expansion error if length evaluates to a number less than zero.

  The following examples illustrate substring expansion using positional parameters:

      $ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
      $ echo ${@:7}
      7 8 9 0 a b c d e f g h
      $ echo ${@:7:0}

      $ echo ${@:7:2}
      7 8
      $ echo ${@:7:-2}
      bash: -2: substring expression < 0
      $ echo ${@: -7:2}
      b c
      $ echo ${@:0}
      ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
      $ echo ${@:0:2}
      ./bash 1
      $ echo ${@: -7:0}

  If parameter is an indexed array name subscripted by ‘`@`’ or ‘`*`’, the result is the length members of the array beginning with `${parameter[offset]}`. A negative offset is taken relative to one greater than the maximum index of the specified array. It is an expansion error if length evaluates to a number less than zero.

  These examples show how you can use substring expansion with indexed arrays:

      $ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
      $ echo ${array[@]:7}
      7 8 9 0 a b c d e f g h
      $ echo ${array[@]:7:2}
      7 8
      $ echo ${array[@]: -7:2}
      b c
      $ echo ${array[@]: -7:-2}
      bash: -2: substring expression < 0
      $ echo ${array[@]:0}
      0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
      $ echo ${array[@]:0:2}
      0 1
      $ echo ${array[@]: -7:0}

  Substring expansion applied to an associative array produces undefined results.

  Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the positional parameters are used, `$0` is prefixed to the list.

`${!prefix*}`
`${!prefix@}`

  Expands to the names of variables whose names begin with prefix, separated by the first character of the `IFS` special variable. When ‘`@`’ is used and the expansion appears within double quotes, each variable name expands to a separate word.

`${!name[@]}`
`${!name[*]}`

  If name is an array variable, expands to the list of array indices (keys) assigned in name. If name is not an array, expands to 0 if name is set and null otherwise. When ‘`@`’ is used and the expansion appears within double quotes, each key expands to a separate word.

`${#parameter}`

  Substitutes the length in characters of the value of parameter. If parameter is ‘`*`’ or ‘`@`’, the value substituted is the number of positional parameters. If parameter is an array name subscripted by ‘`*`’ or ‘`@`’, the value substituted is the number of elements in the array. If parameter is an indexed array name subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of parameter, so negative indices count back from the end of the array, and an index of -1 references the last element.

`${parameter#word}`
`${parameter##word}`

  The word is expanded to produce a pattern and matched against the expanded value of parameter according to the rules described below (see Pattern Matching). If the pattern matches the beginning of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ‘`#`’ case) or the longest matching pattern (the ‘`##`’ case) deleted. If parameter is ‘`@`’ or ‘`*`’, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘`@`’ or ‘`*`’, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

`${parameter%word}`
`${parameter%%word}`

  The word is expanded to produce a pattern and matched against the expanded value of parameter according to the rules described below (see Pattern Matching). If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the value of parameter with the shortest matching pattern (the ‘`%`’ case) or the longest matching pattern (the ‘`%%`’ case) deleted. If parameter is ‘`@`’ or ‘`*`’, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘`@`’ or ‘`*`’, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

`${parameter/pattern/string}`
`${parameter//pattern/string}`
`${parameter/#pattern/string}`
`${parameter/%pattern/string}`

  The pattern is expanded to produce a pattern and matched against the expanded value of parameter as described below (see Pattern Matching). The longest match of pattern in the expanded value is replaced with string. string undergoes tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, and quote removal.

  In the first form above, only the first match is replaced. If there are two slashes separating parameter and pattern (the second form above), all matches of pattern are replaced with string. If pattern is preceded by ‘`#`’ (the third form above), it must match at the beginning of the expanded value of parameter. If pattern is preceded by ‘`%`’ (the fourth form above), it must match at the end of the expanded value of parameter.

  If the expansion of string is null, matches of pattern are deleted and the ‘`/`’ following pattern may be omitted.

  If the `patsub_replacement` shell option is enabled using `shopt` (see The Shopt Builtin), any unquoted instances of ‘`&`’ in string are replaced with the matching portion of pattern. This is intended to duplicate a common `sed` idiom.

  Quoting any part of string inhibits replacement in the expansion of the quoted portion, including replacement strings stored in shell variables. Backslash escapes ‘`&`’ in string; the backslash is removed in order to permit a literal ‘`&`’ in the replacement string. Users should take care if string is double-quoted to avoid unwanted interactions between the backslash and double-quoting, since backslash has special meaning within double quotes. Pattern substitution performs the check for unquoted ‘`&`’ after expanding string, so users should ensure to properly quote any occurrences of ‘`&`’ they want to be taken literally in the replacement and ensure any instances of ‘`&`’ they want to be replaced are unquoted.

  For instance,

      var=abcdef
      rep='& '
      echo ${var/abc/& }
      echo "${var/abc/& }"
      echo ${var/abc/$rep}
      echo "${var/abc/$rep}"

  will display four lines of "abc def", while

      var=abcdef
      rep='& '
      echo ${var/abc/\& }
      echo "${var/abc/\& }"
      echo ${var/abc/"& "}
      echo ${var/abc/"$rep"}

  will display four lines of "& def". Like the pattern removal operators, double quotes surrounding the replacement string quote the expanded characters, while double quotes enclosing the entire parameter substitution do not, since the expansion is performed in a context that doesn’t take any enclosing double quotes into account.

  Since backslash can escape ‘`&`’, it can also escape a backslash in the replacement string. This means that ‘`\\`’ will insert a literal backslash into the replacement, so these two `echo` commands

      var=abcdef
      rep='\\&xyz'
      echo ${var/abc/\\&xyz}
      echo ${var/abc/$rep}

  will both output ‘`\abcxyzdef`’.

  It should rarely be necessary to enclose only string in double quotes.

  If the `nocasematch` shell option (see the description of `shopt` in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters.

  If parameter is ‘`@`’ or ‘`*`’, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘`@`’ or ‘`*`’, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.

`${parameter^pattern}`
`${parameter^^pattern}`
`${parameter,pattern}`
`${parameter,,pattern}`

  This expansion modifies the case of alphabetic characters in parameter. First, the pattern is expanded to produce a pattern as described below in Pattern Matching.

  `Bash` then examines characters in the expanded value of parameter against pattern as described below. If a character matches the pattern, its case is converted. The pattern should not attempt to match more than one character.

  Using ‘`^`’ converts lowercase letters matching pattern to uppercase; ‘`,`’ converts matching uppercase letters to lowercase. The ‘`^`’ and ‘`,`’ variants examine the first character in the expanded value and convert its case if it matches pattern; the ‘`^^`’ and ‘`,,`’ variants examine all characters in the expanded value and convert each one that matches pattern. If pattern is omitted, it is treated like a ‘`?`’, which matches every character.

  If parameter is ‘`@`’ or ‘`*`’, the case modification operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘`@`’ or ‘`*`’, the case modification operation is applied to each member of the array in turn, and the expansion is the resultant list.

`${parameter@operator}`

  The expansion is either a transformation of the value of parameter or information about parameter itself, depending on the value of operator. Each operator is a single letter:

  `U`

    The expansion is a string that is the value of parameter with lowercase alphabetic characters converted to uppercase.

  `u`

    The expansion is a string that is the value of parameter with the first character converted to uppercase, if it is alphabetic.

  `L`

    The expansion is a string that is the value of parameter with uppercase alphabetic characters converted to lowercase.

  `Q`

    The expansion is a string that is the value of parameter quoted in a format that can be reused as input.

  `E`

    The expansion is a string that is the value of parameter with backslash escape sequences expanded as with the `$'…'` quoting mechanism.

  `P`

    The expansion is a string that is the result of expanding the value of parameter as if it were a prompt string (see Controlling the Prompt).

  `A`

    The expansion is a string in the form of an assignment statement or `declare` command that, if evaluated, recreates parameter with its attributes and value.

  `K`

    Produces a possibly-quoted version of the value of parameter, except that it prints the values of indexed and associative arrays as a sequence of quoted key-value pairs (see Arrays). The keys and values are quoted in a format that can be reused as input.

  `a`

    The expansion is a string consisting of flag values representing parameter’s attributes.

  `k`

    Like the ‘`K`’ transformation, but expands the keys and values of indexed and associative arrays to separate words after word splitting.

  If parameter is ‘`@`’ or ‘`*`’, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘`@`’ or ‘`*`’, the operation is applied to each member of the array in turn, and the expansion is the resultant list.

  The result of the expansion is subject to word splitting and filename expansion as described below.

#### 3.5.4 Command Substitution ¶

Command substitution allows the output of a command to replace the command itself. The standard form of command substitution occurs when a command is enclosed as follows:

    $(command)

or (deprecated)

    `command`.

Bash performs command substitution by executing command in a subshell environment and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution `$(cat file)` can be replaced by the equivalent but faster `$(< file)`.

With the old-style backquote form of substitution, backslash retains its literal meaning except when followed by ‘`$`’, ‘```’, or ‘`\`’. The first backquote not preceded by a backslash terminates the command substitution. When using the `$(command)` form, all characters between the parentheses make up the command; none are treated specially.

There is an alternate form of command substitution:

    ${c command; }

which executes command in the current execution environment and captures its output, again with trailing newlines removed.

The character c following the open brace must be a space, tab, newline, or ‘`|`’, and the close brace must be in a position where a reserved word may appear (i.e., preceded by a command terminator such as semicolon). Bash allows the close brace to be joined to the remaining characters in the word without being followed by a shell metacharacter as a reserved word would usually require.

Any side effects of command take effect immediately in the current execution environment and persist in the current environment after the command completes (e.g., the `exit` builtin exits the shell).

This type of command substitution superficially resembles executing an unnamed shell function: local variables are created as when a shell function is executing, and the `return` builtin forces command to complete; however, the rest of the execution environment, including the positional parameters, is shared with the caller.

If the first character following the open brace is a ‘`|`’, the construct expands to the value of the `REPLY` shell variable after command executes, without removing any trailing newlines, and the standard output of command remains the same as in the calling shell. Bash creates `REPLY` as an initially-unset local variable when command executes, and restores `REPLY` to the value it had before the command substitution after command completes, as with any local variable.

For example, this construct expands to ‘`12345`’, and leaves the shell variable `X` unchanged in the current execution environment:

    ${ local X=12345 ; echo $X; }

(not declaring `X` as local would modify its value in the current environment, as with normal shell function execution), while this construct does not require any output to expand to ‘`12345`’:

    ${| REPLY=12345; }

and restores `REPLY` to the value it had before the command substitution.

Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, Bash does not perform word splitting and filename expansion on the results.

#### 3.5.5 Arithmetic Expansion ¶

Arithmetic expansion evaluates an arithmetic expression and substitutes the result. The format for arithmetic expansion is:

    $(( expression ))

The expression undergoes the same expansions as if it were within double quotes, but unescaped double quote characters in expression are not treated specially and are removed. All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. Since the way Bash handles double quotes can potentially result in empty strings, arithmetic expansion treats those as expressions that evaluate to 0. Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below (see Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error, does not perform the substitution, and does not execute the command associated with the expansion.

#### 3.5.6 Process Substitution ¶

Process substitution allows a process’s input or output to be referred to using a filename. It takes the form of

    <(list)

or

    >(list)

The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion.

If the `>(list)` form is used, writing to the file provides input for list. If the `<(list)` form is used, reading the file obtains the output of list. Note that no space may appear between the `<` or `>` and the left parenthesis, otherwise the construct would be interpreted as a redirection.

Process substitution is supported on systems that support named pipes (FIFOs) or the `/dev/fd` method of naming open files.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

#### 3.5.7 Word Splitting ¶

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. Words that were not expanded are not split.

The shell treats each character of `$IFS` as a delimiter, and splits the results of the other expansions into fields using these characters as field terminators.

An IFS whitespace character is whitespace as defined above (see Definitions) that appears in the value of `IFS`. Space, tab, and newline are always considered IFS whitespace, even if they don’t appear in the locale’s `space` category.

If `IFS` is unset, word splitting behaves as if its value were `<space><tab><newline>`, and treats these characters as IFS whitespace. If the value of `IFS` is null, no word splitting occurs, but implicit null arguments (see below) are still removed.

Word splitting begins by removing sequences of IFS whitespace characters from the beginning and end of the results of the previous expansions, then splits the remaining words.

If the value of `IFS` consists solely of IFS whitespace, any sequence of IFS whitespace characters delimits a field, so a field consists of characters that are not unquoted IFS whitespace, and null fields result only from quoting.

If `IFS` contains a non-whitespace character, then any character in the value of `IFS` that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. This means that adjacent non-IFS-whitespace delimiters produce a null field. A sequence of IFS whitespace characters also delimits a field.

Explicit null arguments (`""` or `''`) are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. Expanding a parameter with no value within double quotes produces a null field, which is retained and passed to a command as an empty string.

When a quoted null argument appears as part of a word whose expansion is non-null, word splitting removes the null argument portion, leaving the non-null expansion. That is, the word `-d''` becomes `-d` after word splitting and null argument removal.

#### 3.5.8 Filename Expansion ¶

After word splitting, unless the `-f` option has been set (see The Set Builtin), Bash scans each word for the characters ‘`*`’, ‘`?`’, and ‘`[`’. If one of these characters appears, and is not quoted, then the word is regarded as a pattern, and replaced with a sorted list of filenames matching the pattern (see Pattern Matching), subject to the value of the `GLOBSORT` shell variable (see Bash Variables).

If no matching filenames are found, and the shell option `nullglob` is disabled, the word is left unchanged. If the `nullglob` option is set, and no matches are found, the word is removed. If the `failglob` shell option is set, and no matches are found, Bash prints an error message and does not execute the command. If the shell option `nocaseglob` is enabled, the match is performed without regard to the case of alphabetic characters.

When a pattern is used for filename expansion, the character ‘`.`’ at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option `dotglob` is set. In order to match the filenames `.` and `..`, the pattern must begin with ‘`.`’ (for example, ‘`.?`’), even if `dotglob` is set. If the `globskipdots` shell option is enabled, the filenames `.` and `..` never match, even if the pattern begins with a ‘`.`’. When not matching filenames, the ‘`.`’ character is not treated specially.

When matching a filename, the slash character must always be matched explicitly by a slash in the pattern, but in other matching contexts it can be matched by a special pattern character as described below (see Pattern Matching).

See the description of `shopt` in The Shopt Builtin, for a description of the `nocaseglob`, `nullglob`, `globskipdots`, `failglob`, and `dotglob` options.

The `GLOBIGNORE` shell variable may be used to restrict the set of file names matching a pattern. If `GLOBIGNORE` is set, each matching file name that also matches one of the patterns in `GLOBIGNORE` is removed from the list of matches. If the `nocaseglob` option is set, the matching against the patterns in `GLOBIGNORE` is performed without regard to case. The filenames `.` and `..` are always ignored when `GLOBIGNORE` is set and not null. However, setting `GLOBIGNORE` to a non-null value has the effect of enabling the `dotglob` shell option, so all other filenames beginning with a ‘`.`’ match. To get the old behavior of ignoring filenames beginning with a ‘`.`’, make ‘`.*`’ one of the patterns in `GLOBIGNORE`. The `dotglob` option is disabled when `GLOBIGNORE` is unset. The `GLOBIGNORE` pattern matching honors the setting of the `extglob` shell option.

The value of the `GLOBSORT` shell variable controls how the results of pathname expansion are sorted, as described below (see Bash Variables).

#### 3.5.8.1 Pattern Matching ¶

Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally.

The special pattern characters have the following meanings:

`*`

  Matches any string, including the null string. When the `globstar` shell option is enabled, and ‘`*`’ is used in a filename expansion context, two adjacent ‘`*`’s used as a single pattern match all files and zero or more directories and subdirectories. If followed by a ‘`/`’, two adjacent ‘`*`’s match only directories and subdirectories.

`?`

  Matches any single character.

`[…]`

  Matches any one of the characters enclosed between the brackets. This is known as a bracket expression and matches a single character. A pair of characters separated by a hyphen denotes a range expression; any character that falls between those two characters, inclusive, using the current locale’s collating sequence and character set, matches. If the first character following the ‘`[`’ is a ‘`!`’ or a ‘`^`’ then any character not within the range matches. To match a ‘`−`’, include it as the first or last character in the set. To match a ‘`]`’, include it as the first character in the set.

  The sorting order of characters in range expressions, and the characters included in the range, are determined by the current locale and the values of the `LC_COLLATE` and `LC_ALL` shell variables, if set.

  For example, in the default C locale, ‘`[a-dx-z]`’ is equivalent to ‘`[abcdxyz]`’. Many locales sort characters in dictionary order, and in these locales ‘`[a-dx-z]`’ is typically not equivalent to ‘`[abcdxyz]`’; it might be equivalent to ‘`[aBbCcDdxYyZz]`’, for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the `LC_COLLATE` or `LC_ALL` environment variable to the value ‘`C`’, or enable the `globasciiranges` shell option.

  Within a bracket expression, character classes can be specified using the syntax `[:`class`:]`, where class is one of the following classes defined in the POSIX standard:

      alnum   alpha   ascii   blank   cntrl   digit   graph   lower
      print   punct   space   upper   word    xdigit

  A character class matches any character belonging to that class. The `word` character class matches letters, digits, and the character ‘`_`’.

  For instance, the following pattern will match any character belonging to the `space` character class in the current locale, then any upper case letter or ‘`!`’, a dot, and finally any lower case letter or a hyphen.

      [[:space:]][[:upper:]!].[-[:lower:]]

  Within a bracket expression, an equivalence class can be specified using the syntax `[=`c`=]`, which matches all characters with the same collation weight (as defined by the current locale) as the character c.

  Within a bracket expression, the syntax `[.`symbol`.]` matches the collating symbol symbol.

If the `extglob` shell option is enabled using the `shopt` builtin, the shell recognizes several extended pattern matching operators. In the following description, a pattern-list is a list of one or more patterns separated by a ‘`|`’. When matching filenames, the `dotglob` shell option determines the set of filenames that are tested, as described above. Composite patterns may be formed using one or more of the following sub-patterns:

`?(pattern-list)`

  Matches zero or one occurrence of the given patterns.

`*(pattern-list)`

  Matches zero or more occurrences of the given patterns.

`+(pattern-list)`

  Matches one or more occurrences of the given patterns.

`@(pattern-list)`

  Matches one of the given patterns.

`!(pattern-list)`

  Matches anything except one of the given patterns.

The `extglob` option changes the behavior of the parser, since the parentheses are normally treated as operators with syntactic meaning. To ensure that extended matching patterns are parsed correctly, make sure that `extglob` is enabled before parsing constructs containing the patterns, including shell functions and command substitutions.

When matching filenames, the `dotglob` shell option determines the set of filenames that are tested: when `dotglob` is enabled, the set of filenames includes all files beginning with ‘`.`’, but the filenames `.` and `..` must be matched by a pattern or sub-pattern that begins with a dot; when it is disabled, the set does not include any filenames beginning with ‘`.`’ unless the pattern or sub-pattern begins with a ‘`.`’. If the `globskipdots` shell option is enabled, the filenames `.` and `..` never appear in the set. As above, ‘`.`’ only has a special meaning when matching filenames.

Complicated extended pattern matching against long strings is slow, especially when the patterns contain alternations and the strings contain multiple matches. Using separate matches against shorter strings, or using arrays of strings instead of a single long string, may be faster.

#### 3.5.9 Quote Removal ¶

After the preceding expansions, all unquoted occurrences of the characters ‘`\`’, ‘`'`’, and ‘`"`’ that did not result from one of the above expansions are removed.

### 3.6 Redirections ¶

Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection allows commands’ file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. When used with the `exec` builtin, redirections modify file handles in the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except `>&-` and `<&-`, the shell allocates a file descriptor greater than or equal to 10 and assigns it to {varname}. If {varname} precedes `>&-` or `<&-`, the value of varname defines the file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope of the command, which allows the shell programmer to manage the file descriptor’s lifetime manually without using the `exec` builtin. The `varredir_close` shell option manages this behavior (see The Shopt Builtin).

In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is ‘`<`’, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is ‘`>`’, the redirection refers to the standard output (file descriptor 1).

The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.

The order of redirections is significant. For example, the command

    ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

    ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described in the following table. If the operating system on which Bash is running provides these special files, Bash uses them; otherwise it emulates them internally with the behavior described below.

`/dev/fd/fd`

  If fd is a valid integer, duplicate file descriptor fd.

`/dev/stdin`

  File descriptor 0 is duplicated.

`/dev/stdout`

  File descriptor 1 is duplicated.

`/dev/stderr`

  File descriptor 2 is duplicated.

`/dev/tcp/host/port`

  If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open the corresponding TCP socket.

`/dev/udp/host/port`

  If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open the corresponding UDP socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.

#### 3.6.1 Redirecting Input ¶

Redirecting input opens the file whose name results from the expansion of word for reading on file descriptor `n`, or the standard input (file descriptor 0) if `n` is not specified.

The general format for redirecting input is:

    [n]<word

#### 3.6.2 Redirecting Output ¶

Redirecting output opens the file whose name results from the expansion of word for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.

The general format for redirecting output is:

    [n]>[|]word

If the redirection operator is ‘`>`’, and the `noclobber` option to the `set` builtin command has been enabled, the redirection fails if the file whose name results from the expansion of word exists and is a regular file. If the redirection operator is ‘`>|`’, or the redirection operator is ‘`>`’ and the `noclobber` option to the `set` builtin is not enabled, Bash attempts the redirection even if the file named by word exists.

#### 3.6.3 Appending Redirected Output ¶

Redirecting output in this fashion opens the file whose name results from the expansion of word for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created.

The general format for appending output is:

    [n]>>word

#### 3.6.4 Redirecting Standard Output and Standard Error ¶

This construct redirects both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

There are two formats for redirecting standard output and standard error:

    &>word

and

    >&word

Of the two forms, the first is preferred. This is semantically equivalent to

    >word 2>&1

When using the second form, word may not expand to a number or ‘`-`’. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.

#### 3.6.5 Appending Standard Output and Standard Error ¶

This construct appends both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

The format for appending standard output and standard error is:

    &>>word

This is semantically equivalent to

    >>word 2>&1

(see Duplicating File Descriptors below).

#### 3.6.6 Here Documents ¶

This type of redirection instructs the shell to read input from the current source until it reads a line containing only delimiter (with no trailing blanks). All of the lines read up to that point then become the standard input (or file descriptor n if n is specified) for a command.

The format of here-documents is:

    [n]<<[−]word
            here-document
    delimiter

The shell does not perform parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion on word.

If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, delimiter is word itself, and the here-document text is treated similarly to a double-quoted string: all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence `\newline` is treated literally, and ‘`\`’ must be used to quote the characters ‘`\`’, ‘`$`’, and ‘```’; however, double quote characters have no special meaning.

If the redirection operator is ‘`<<-`’, the shell strips leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.

If the delimiter is not quoted, the `\<newline>` sequence is treated as a line continuation: the two lines are joined and the backslash-newline is removed. This happens while reading the here-document, before the check for the ending delimiter, so joined lines can form the end delimiter.

#### 3.6.7 Here Strings ¶

A variant of here documents, the format is:

    [n]<<< word

The word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Filename expansion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified).

#### 3.6.8 Duplicating File Descriptors ¶

The redirection operator

    [n]<&word

is used to duplicate input file descriptors. If word expands to one or more digits, file descriptor n is made to be a copy of that file descriptor. It is a redirection error if the digits in word do not specify a file descriptor open for input. If word evaluates to ‘`-`’, file descriptor n is closed. If n is not specified, this uses the standard input (file descriptor 0).

The operator

    [n]>&word

is used similarly to duplicate output file descriptors. If n is not specified, this uses the standard output (file descriptor 1). It is a redirection error if the digits in word do not specify a file descriptor open for output. If word evaluates to ‘`-`’, file descriptor n is closed. As a special case, if n is omitted, and word does not expand to one or more digits or ‘`-`’, this redirects the standard output and standard error as described previously.

#### 3.6.9 Moving File Descriptors ¶

The redirection operator

    [n]<&digit-

moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.

Similarly, the redirection operator

    [n]>&digit-

moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.

#### 3.6.10 Opening File Descriptors for Reading and Writing ¶

The redirection operator

    [n]<>word

opens the file whose name is the expansion of word for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.

### 3.7 Executing Commands ¶

#### 3.7.1 Simple Command Expansion ¶

When the shell executes a simple command, it performs the following expansions, assignments, and redirections, from left to right, in the following order.

1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.
2. The words that are not variable assignments or redirections are expanded (see Shell Expansions). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.
3. Redirections are performed as described above (see Redirections).
4. The text after the ‘`=`’ in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment. In the case of such a command (one that consists only of assignment statements and redirections), assignment statements are performed before redirections. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.

If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a zero status.

#### 3.7.2 Command Search and Execution ¶

After a command has been split into words, if it results in a simple command and an optional list of arguments, the shell performs the following actions.

1. If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described in Shell Functions.
2. If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked.
3. If the name is neither a shell function nor a builtin, and contains no slashes, Bash searches each element of `$PATH` for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple `PATH` searches (see the description of `hash` in Bourne Shell Builtins). Bash performs a full search of the directories in `$PATH` only if the command is not found in the hash table. If the search is unsuccessful, the shell searches for a defined shell function named `command_not_found_handle`. If that function exists, it is invoked in a separate execution environment with the original command and the original command’s arguments as its arguments, and the function’s exit status becomes the exit status of that subshell. If that function is not defined, the shell prints an error message and returns an exit status of 127.
4. If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments supplied, if any.
5. If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a shell script, a file containing shell commands, and the shell executes it as described in Shell Scripts.
6. If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status.

#### 3.7.3 Command Execution Environment ¶

The shell has an execution environment, which consists of the following:

- Open files inherited by the shell at invocation, as modified by redirections supplied to the `exec` builtin.
- The current working directory as set by `cd`, `pushd`, or `popd`, or inherited by the shell at invocation.
- The file creation mode mask as set by `umask` or inherited from the shell’s parent.
- Current traps set by `trap`.
- Shell parameters that are set by variable assignment or with `set` or inherited from the shell’s parent in the environment.
- Shell functions defined during execution or inherited from the shell’s parent in the environment.
- Options enabled at invocation (either by default or with command-line arguments) or by `set`.
- Options enabled by `shopt` (see The Shopt Builtin).
- Shell aliases defined with `alias` (see Aliases).
- Various process IDs, including those of background jobs (see Lists of Commands), the value of `$$`, and the value of `$PPID`.

When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.

- The shell’s open files, plus any modifications and additions specified by redirections to the command.
- The current working directory.
- The file creation mode mask.
- Shell variables and functions marked for export, along with variables exported for the command, passed in the environment (see Environment).
- Traps caught by the shell are reset to the values inherited from the shell’s parent, and traps ignored by the shell are ignored.

A command invoked in this separate environment cannot affect the shell’s execution environment.

A subshell is a copy of the shell process.

Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline, except possibly in the last element depending on the value of the `lastpipe` shell option (see The Shopt Builtin), are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell’s execution environment.

When the shell is in POSIX mode, subshells spawned to execute command substitutions inherit the value of the `-e` option from the parent shell. When not in POSIX mode, Bash clears the `-e` option in such subshells See the description of the `inherit_errexit` shell option (see Bash Builtin Commands) for how to control this behavior when not in POSIX mode.

If a command is followed by a ‘`&`’ and job control is not active, the default standard input for the command is the empty file `/dev/null`. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.

#### 3.7.4 Environment ¶

When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form `name=value`.

Bash provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for `export` to child processes. Executed commands inherit the environment. The `export`, ‘`declare -x`’, and `unset` commands modify the environment by adding and deleting parameters and functions. If the value of a parameter in the environment is modified, the new value automatically becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell’s initial environment, whose values may be modified in the shell, less any pairs removed by the `unset` and ‘`export -n`’ commands, plus any additions via the `export` and ‘`declare -x`’ commands.

If any parameter assignment statements, as described in Shell Parameters, appear before a simple command, the variable assignments are part of that command’s environment for as long as it executes. These assignment statements affect only the environment seen by that command. If these assignments precede a call to a shell function, the variables are local to the function and exported to that function’s children.

If the `-k` option is set (see The Set Builtin), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.

When Bash invokes an external command, the variable ‘`$_`’ is set to the full pathname of the command and passed to that command in its environment.

#### 3.7.5 Exit Status ¶

The exit status of an executed command is the value returned by the `waitpid` system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes.

For the shell’s purposes, a command which exits with a zero exit status has succeeded. So while an exit status of zero indicates success, a non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes.

When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.

If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.

If a command fails because of an error during expansion or redirection, the exit status is greater than zero.

The exit status is used by the Bash conditional commands (see Conditional Constructs) and some of the list constructs (see Lists of Commands).

All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage, generally invalid options or missing arguments.

The exit status of the last command is available in the special parameter $? (see Special Parameters).

Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in which case it exits with a non-zero value. See also the `exit` builtin command (see Bourne Shell Builtins.

#### 3.7.6 Signals ¶

When Bash is interactive, in the absence of any traps, it ignores `SIGTERM` (so that ‘`kill 0`’ does not kill an interactive shell), and catches and handles `SIGINT` (so that the `wait` builtin is interruptible). When Bash receives a `SIGINT`, it breaks out of any executing loops. In all cases, Bash ignores `SIGQUIT`. If job control is in effect (see Job Control), Bash ignores `SIGTTIN`, `SIGTTOU`, and `SIGTSTP`.

The `trap` builtin modifies the shell’s signal handling, as described below (see Bourne Shell Builtins.

Non-builtin commands Bash executes have signal handlers set to the values inherited by the shell from its parent, unless `trap` sets them to be ignored, in which case the child process will ignore them as well. When job control is not in effect, asynchronous commands ignore `SIGINT` and `SIGQUIT` in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals `SIGTTIN`, `SIGTTOU`, and `SIGTSTP`.

The shell exits by default upon receipt of a `SIGHUP`. Before exiting, an interactive shell resends the `SIGHUP` to all jobs, running or stopped. The shell sends `SIGCONT` to stopped jobs to ensure that they receive the `SIGHUP` (See Job Control, for more information about running and stopped jobs). To prevent the shell from sending the `SIGHUP` signal to a particular job, remove it from the jobs table with the `disown` builtin (see Job Control Builtins) or mark it not to receive `SIGHUP` using `disown -h`.

If the `huponexit` shell option has been set using `shopt` (see The Shopt Builtin), Bash sends a `SIGHUP` to all jobs when an interactive login shell exits.

If Bash is waiting for a command to complete and receives a signal for which a trap has been set, it will not execute the trap until the command completes. If Bash is waiting for an asynchronous command via the `wait` builtin, and it receives a signal for which a trap has been set, the `wait` builtin will return immediately with an exit status greater than 128, immediately after which the shell executes the trap.

When job control is not enabled, and Bash is waiting for a foreground command to complete, the shell receives keyboard-generated signals such as `SIGINT` (usually generated by ‘`^C`’) that users commonly intend to send to that command. This happens because the shell and the command are in the same process group as the terminal, and ‘`^C`’ sends `SIGINT` to all processes in that process group. Since Bash does not enable job control by default when the shell is not interactive, this scenario is most common in non-interactive shells.

When job control is enabled, and Bash is waiting for a foreground command to complete, the shell does not receive keyboard-generated signals, because it is not in the same process group as the terminal. This scenario is most common in interactive shells, where Bash attempts to enable job control by default. See Job Control, for a more in-depth discussion of process groups.

When job control is not enabled, and Bash receives `SIGINT` while waiting for a foreground command, it waits until that foreground command terminates and then decides what to do about the `SIGINT`:

1. If the command terminates due to the `SIGINT`, Bash concludes that the user meant to send the `SIGINT` to the shell as well, and acts on the `SIGINT` (e.g., by running a `SIGINT` trap, exiting a non-interactive shell, or returning to the top level to read a new command).
2. If the command does not terminate due to `SIGINT`, the program handled the `SIGINT` itself and did not treat it as a fatal signal. In that case, Bash does not treat `SIGINT` as a fatal signal, either, instead assuming that the `SIGINT` was used as part of the program’s normal operation (e.g., `emacs` uses it to abort editing commands) or deliberately discarded. However, Bash will run any trap set on `SIGINT`, as it does with any other trapped signal it receives while it is waiting for the foreground command to complete, for compatibility.

When job control is enabled, Bash does not receive keyboard-generated signals such as `SIGINT` while it is waiting for a foreground command. An interactive shell does not pay attention to the `SIGINT`, even if the foreground command terminates as a result, other than noting its exit status. If the shell is not interactive, and the foreground command terminates due to the `SIGINT`, Bash pretends it received the `SIGINT` itself (scenario 1 above), for compatibility.

### 3.8 Shell Scripts ¶

A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the `-c` nor `-s` option is supplied (see Invoking Bash), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. If the filename does not contain any slashes, the shell first searches for the file in the current directory, and looks in the directories in `$PATH` if not found there.

Bash tries to determine whether the file is a text file or a binary, and will not execute files it determines to be binaries.

When Bash runs a shell script, it sets the special parameter `0` to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset.

A shell script may be made executable by using the `chmod` command to turn on the execute bit. When Bash finds such a file while searching the `$PATH` for a command, it creates a new instance of itself to execute it. In other words, executing

    filename arguments

is equivalent to executing

    bash filename arguments

if `filename` is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of `hash` in Bourne Shell Builtins) are retained by the child.

The GNU operating system, and most versions of Unix, make this a part of the operating system’s command execution mechanism. If the first line of a script begins with the two characters ‘`#!`’, the remainder of the line specifies an interpreter for the program and, depending on the operating system, one or more optional arguments for that interpreter. Thus, you can specify Bash, `awk`, Perl, or some other interpreter and write the rest of the script file in that language.

The arguments to the interpreter consist of one or more optional arguments following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments supplied to the script. The details of how the interpreter line is split into an interpreter name and a set of arguments vary across systems. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and a single argument to a maximum of 32 characters, so it’s not portable to assume that using more than one argument will work.

Bash scripts often begin with `#! /bin/bash` (assuming that Bash has been installed in `/bin`), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell. It’s a common idiom to use `env` to find `bash` even if it’s been installed in another directory: `#!/usr/bin/env bash` will find the first occurrence of `bash` in `$PATH`.

## 4 Shell Builtin Commands ¶

Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command (see Simple Commands), the shell executes the command directly, without invoking another program. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities.

This section briefly describes the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash.

Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see Job Control Builtins), the directory stack (see Directory Stack Builtins), the command history (see Bash History Builtins), and the programmable completion facilities (see Programmable Completion Builtins).

Many of the builtins have been extended by POSIX or Bash.

Unless otherwise noted, each builtin command documented as accepting options preceded by ‘`-`’ accepts ‘`--`’ to signify the end of the options. The `:`, `true`, `false`, and `test`/`[` builtins do not accept options and do not treat ‘`--`’ specially. The `exit`, `logout`, `return`, `break`, `continue`, `let`, and `shift` builtins accept and process arguments beginning with ‘`-`’ without requiring ‘`--`’. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with ‘`-`’ as invalid options and require ‘`--`’ to prevent this interpretation.

### 4.1 Bourne Shell Builtins ¶

The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX standard.

`: (a colon)` ¶

      : [arguments]

  Do nothing beyond expanding arguments and performing redirections. The return status is zero.

`. (a period)` ¶

      . [-p path] filename [arguments]

  The `.` command reads and execute commands from the filename argument in the current shell context.

  If filename does not contain a slash, `.` searches for it. If `-p` is supplied, `.` treats path as a colon-separated list of directories in which to find filename; otherwise, `.` uses the directories in `PATH` to find filename. filename does not need to be executable. When Bash is not in POSIX mode, it searches the current directory if filename is not found in `$PATH`, but does not search the current directory if `-p` is supplied. If the `sourcepath` option (see The Shopt Builtin) is turned off, `.` does not search `PATH`.

  If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged.

  If the `-T` option is enabled, `.` inherits any trap on `DEBUG`; if it is not, any `DEBUG` trap string is saved and restored around the call to `.`, and `.` unsets the `DEBUG` trap while it executes. If `-T` is not set, and the sourced file changes the `DEBUG` trap, the new value persists after `.` completes. The return status is the exit status of the last command executed from filename, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to `source`.

`break` ¶

      break [n]

  Exit from a `for`, `while`, `until`, or `select` loop. If n is supplied, `break` exits the nth enclosing loop. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

`cd` ¶

      cd [-L] [-@] [directory]
      cd -P [-e] [-@] [directory]

  Change the current working directory to directory. If directory is not supplied, the value of the `HOME` shell variable is used as directory. If the shell variable `CDPATH` exists, and directory does not begin with a slash, `cd` uses it as a search path: `cd` searches each directory name in `CDPATH` for directory, with alternative directory names in `CDPATH` separated by a colon (‘`:`’). A null directory name in `CDPATH` means the same thing as the current directory.

  The `-P` option means not to follow symbolic links: symbolic links are resolved while `cd` is traversing directory and before processing an instance of `..` in directory.

  By default, or when the `-L` option is supplied, symbolic links in directory are resolved after `cd` processes an instance of `..` in directory.

  If `..` appears in directory, `cd` processes it by removing the immediately preceding pathname component, back to a slash or the beginning of directory, and verifying that the portion of directory it has processed to that point is still a valid directory name after removing the pathname component. If it is not a valid directory name, `cd` returns a non-zero status.

  If the `-e` option is supplied with `-P` and `cd` cannot successfully determine the current working directory after a successful directory change, it returns a non-zero status.

  On systems that support it, the `-@` option presents the extended attributes associated with a file as a directory.

  If directory is ‘`-`’, it is converted to `$OLDPWD` before attempting the directory change.

  If `cd` uses a non-empty directory name from `CDPATH`, or if ‘`-`’ is the first argument, and the directory change is successful, `cd` writes the absolute pathname of the new working directory to the standard output.

  If the directory change is successful, `cd` sets the value of the `PWD` environment variable to the new directory name, and sets the `OLDPWD` environment variable to the value of the current working directory before the change.

  The return status is zero if the directory is successfully changed, non-zero otherwise.

`continue` ¶

      continue [n]

  `continue` resumes the next iteration of an enclosing `for`, `while`, `until`, or `select` loop. If n is supplied, Bash resumes the execution of the nth enclosing loop. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

`eval` ¶

      eval [arguments]

  The arguments are concatenated together into a single command, separated by spaces. Bash then reads and executes this command and returns its exit status as the exit status of `eval`. If there are no arguments or only empty arguments, the return status is zero.

`exec` ¶

      exec [-cl] [-a name] [command [arguments]]

  If command is supplied, it replaces the shell without creating a new process. command cannot be a shell builtin or function. The arguments become the arguments to command If the `-l` option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what the `login` program does. The `-c` option causes command to be executed with an empty environment. If `-a` is supplied, the shell passes name as the zeroth argument to command.

  If command cannot be executed for some reason, a non-interactive shell exits, unless the `execfail` shell option is enabled. In that case, it returns a non-zero status. An interactive shell returns a non-zero status if the file cannot be executed. A subshell exits unconditionally if `exec` fails.

  If command is not specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero.

`exit` ¶

      exit [n]

  Exit the shell, returning a status of n to the shell’s parent. If n is omitted, the exit status is that of the last command executed. Any trap on `EXIT` is executed before the shell terminates.

`export` ¶

      export [-fn] [-p] [name[=value]]

  Mark each name to be passed to subsequently executed commands in the environment. If the `-f` option is supplied, the names refer to shell functions; otherwise the names refer to shell variables.

  The `-n` option means to unexport each name: no longer mark it for export. If no names are supplied, or if the `-p` option is given, `export` displays a list of names of all exported variables on the standard output. The `-p` option displays output in a form that may be reused as input.

  `export` allows the value of a variable to be set at the same time it is exported or unexported by following the variable name with =value. This sets the value of the variable is to value while modifying the export attribute.

  The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or `-f` is supplied with a name that is not a shell function.

`false` ¶

      false

  Does nothing; returns a non-zero status.

`getopts` ¶

      getopts optstring name [arg ...]

  `getopts` is used by shell scripts or functions to parse positional parameters and obtain options and their arguments. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by whitespace. The colon (‘`:`’) and question mark (‘`?`’) may not be used as option characters.

  Each time it is invoked, `getopts` places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable `OPTIND`. `OPTIND` is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, `getopts` places that argument into the variable `OPTARG`.

  The shell does not reset `OPTIND` automatically; it must be manually reset between multiple calls to `getopts` within the same shell invocation to use a new set of parameters.

  When it reaches the end of options, `getopts` exits with a return value greater than zero. `OPTIND` is set to the index of the first non-option argument, and name is set to ‘`?`’.

  `getopts` normally parses the positional parameters, but if more arguments are supplied as arg values, `getopts` parses those instead.

  `getopts` can report errors in two ways. If the first character of optstring is a colon, `getopts` uses silent error reporting. In normal operation, `getopts` prints diagnostic messages when it encounters invalid options or missing option arguments. If the variable `OPTERR` is set to 0, `getopts` does not display any error messages, even if the first character of `optstring` is not a colon.

  If `getopts` detects an invalid option, it places ‘`?`’ into name and, if not silent, prints an error message and unsets `OPTARG`. If `getopts` is silent, it assigns the option character found to `OPTARG` and does not print a diagnostic message.

  If a required argument is not found, and `getopts` is not silent, it sets the value of name to a question mark (‘`?`’), unsets `OPTARG`, and prints a diagnostic message. If `getopts` is silent, it sets the value of name to a colon (‘`:`’), and sets `OPTARG` to the option character found.

  `getopts` returns true if an option, specified or unspecified, is found. It returns false when it encounters the end of options or if an error occurs.

`hash` ¶

      hash [-r] [-p filename] [-dt] [name]

  Each time `hash` is invoked, it remembers the full filenames of the commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in `$PATH`. Any previously-remembered filename associated with name is discarded. The `-p` option inhibits the path search, and `hash` uses filename as the location of name.

  The `-r` option causes the shell to forget all remembered locations. Assigning to the `PATH` variable also clears all hashed filenames. The `-d` option causes the shell to forget the remembered location of each name.

  If the `-t` option is supplied, `hash` prints the full pathname corresponding to each name. If multiple name arguments are supplied with `-t`, `hash` prints each name before the corresponding hashed full path. The `-l` option displays output in a format that may be reused as input.

  If no arguments are given, or if only `-l` is supplied, `hash` prints information about remembered commands. The `-t`, `-d`, and `-p` options (the options that act on the name arguments) are mutually exclusive. Only one will be active. If more than one is supplied, `-t` has higher priority than `-p`, and both have higher priority than `-d`.

  The return status is zero unless a name is not found or an invalid option is supplied.

`pwd` ¶

      pwd [-LP]

  Print the absolute pathname of the current working directory. If the `-P` option is supplied, or the `-o physical` option to the `set` builtin (see The Set Builtin) is enabled, the pathname printed will not contain symbolic links. If the `-L` option is supplied, the pathname printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.

`readonly` ¶

      readonly [-aAf] [-p] [name[=value]] ...

  Mark each name as readonly. The values of these names may not be changed by subsequent assignment or unset. If the `-f` option is supplied, each name refers to a shell function. The `-a` option means each name refers to an indexed array variable; the `-A` option means each name refers to an associative array variable. If both options are supplied, `-A` takes precedence. If no name arguments are supplied, or if the `-p` option is supplied, print a list of all readonly names. The other options may be used to restrict the output to a subset of the set of readonly names. The `-p` option displays output in a format that may be reused as input.

  `readonly` allows the value of a variable to be set at the same time the readonly attribute is changed by following the variable name with =value. This sets the value of the variable is to value while modifying the readonly attribute.

  The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the `-f` option is supplied with a name that is not a shell function.

`return` ¶

      return [n]

  Stop executing a shell function or sourced file and return the value n to its caller. If n is not supplied, the return value is the exit status of the last command executed. If `return` is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If `return` is executed during a `DEBUG` trap, the last command used to determine the status is the last command executed by the trap handler before `return` was invoked.

  When `return` is used to terminate execution of a script being executed with the `.` (`source`) builtin, it returns either n or the exit status of the last command executed within the script as the exit status of the script. If n is supplied, the return value is its least significant 8 bits.

  Any command associated with the `RETURN` trap is executed before execution resumes after the function or script.

  The return status is non-zero if `return` is supplied a non-numeric argument or is used outside a function and not during the execution of a script by `.` or `source`.

`shift` ¶

      shift [n]

  Shift the positional parameters to the left by n: the positional parameters from n+1 … `$#` are renamed to `$1` … `$#`-n. Parameters represented by the numbers `$#` down to `$#`-n+1 are unset. n must be a non-negative number less than or equal to `$#`. If n is not supplied, it is assumed to be 1. If n is zero or greater than `$#`, the positional parameters are not changed. The return status is zero unless n is greater than `$#` or less than zero, non-zero otherwise.

`test` ¶
`[`

      test expr

  Evaluate a conditional expression expr and return a status of 0 (true) or 1 (false). Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in Bash Conditional Expressions. `test` does not accept any options, nor does it accept and ignore an argument of `--` as signifying the end of options. When using the `[` form, the last argument to the command must be a `]`.

  Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. `test` uses operator precedence when there are five or more arguments.

  `! expr`

    True if expr is false.

  `( expr )`

    Returns the value of expr. This may be used to override normal operator precedence.

  `expr1 -a expr2`

    True if both expr1 and expr2 are true.

  `expr1 -o expr2`

    True if either expr1 or expr2 is true.

  The `test` and `[` builtins evaluate conditional expressions using a set of rules based on the number of arguments.

  0 arguments

    The expression is false.

  1 argument

    The expression is true if, and only if, the argument is not null.

  2 arguments

    If the first argument is ‘`!`’, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (see Bash Conditional Expressions), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false.

  3 arguments

    The following conditions are applied in the order listed.

    1. If the second argument is one of the binary conditional operators (see Bash Conditional Expressions), the result of the expression is the result of the binary test using the first and third arguments as operands. The ‘`-a`’ and ‘`-o`’ operators are considered binary operators when there are three arguments.
    2. If the first argument is ‘`!`’, the value is the negation of the two-argument test using the second and third arguments.
    3. If the first argument is exactly ‘`(`’ and the third argument is exactly ‘`)`’, the result is the one-argument test of the second argument.
    4. Otherwise, the expression is false.
  4 arguments

    The following conditions are applied in the order listed.

    1. If the first argument is ‘`!`’, the result is the negation of the three-argument expression composed of the remaining arguments.
    2. If the first argument is exactly ‘`(`’ and the fourth argument is exactly ‘`)`’, the result is the two-argument test of the second and third arguments.
    3. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.
  5 or more arguments

    The expression is parsed and evaluated according to precedence using the rules listed above.

  If the shell is in POSIX mode, or if the expression is part of the `[[` command, the ‘`<`’ and ‘`>`’ operators sort using the current locale. If the shell is not in POSIX mode, the `test` and ‘`[`’ commands sort lexicographically using ASCII ordering.

  The historical operator-precedence parsing with 4 or more arguments can lead to ambiguities when it encounters strings that look like primaries. The POSIX standard has deprecated the `-a` and `-o` primaries and enclosing expressions within parentheses. Scripts should no longer use them. It’s much more reliable to restrict test invocations to a single primary, and to replace uses of `-a` and `-o` with the shell’s `&&` and `||` list operators. For example, use

      test -n string1 && test -n string2

  instead of

      test -n string1 -a -n string2

`times` ¶

      times

  Print out the user and system times used by the shell and its children. The return status is zero.

`trap` ¶

      trap [-lpP] [action] [sigspec ...]

  The action is a command that is read and executed when the shell receives any of the signals sigspec. If action is absent (and there is a single sigspec) or equal to ‘`-`’, each specified sigspec’s disposition is reset to the value it had when the shell was started. If action is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes.

  If no arguments are supplied, `trap` prints the actions associated with each trapped signal as a set of `trap` commands that can be reused as shell input to restore the current signal dispositions.

  If action is not present and `-p` has been supplied, `trap` displays the trap commands associated with each sigspec, or, if no sigspecs are supplied, for all trapped signals, as a set of `trap` commands that can be reused as shell input to restore the current signal dispositions. The `-P` option behaves similarly, but displays only the actions associated with each sigspec argument. `-P` requires at least one sigspec argument. The `-P` or `-p` options may be used in a subshell environment (e.g., command substitution) and, as long as they are used before `trap` is used to change a signal’s handling, will display the state of its parent’s traps.

  The `-l` option prints a list of signal names and their corresponding numbers. Each sigspec is either a signal name or a signal number. Signal names are case insensitive and the `SIG` prefix is optional. If `-l` is supplied with no sigspec arguments, it prints a list of valid signal names.

  If a sigspec is `0` or `EXIT`, action is executed when the shell exits. If a sigspec is `DEBUG`, action is executed before every simple command, `for` command, `case` command, `select` command, (( arithmetic command, [[ conditional command, arithmetic `for` command, and before the first command executes in a shell function. Refer to the description of the `extdebug` shell option (see The Shopt Builtin) for details of its effect on the `DEBUG` trap. If a sigspec is `RETURN`, action is executed each time a shell function or a script executed with the `.` or `source` builtins finishes executing.

  If a sigspec is `ERR`, action is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions. The `ERR` trap is not executed if the failed command is part of the command list immediately following an `until` or `while` reserved word, part of the test following the `if` or `elif` reserved words, part of a command executed in a `&&` or `||` list except the command following the final `&&` or `||`, any command in a pipeline but the last, (subject to the state of the `pipefail` shell option), or if the command’s return status is being inverted using `!`. These are the same conditions obeyed by the `errexit` (`-e`) option.

  When the shell is not interactive, signals ignored upon entry to a non-interactive shell cannot be trapped or reset. Interactive shells permit trapping signals ignored on entry. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created.

  The return status is zero unless a sigspec does not specify a valid signal; non-zero otherwise.

`true` ¶

      true

  Does nothing, returns a 0 status.

`umask` ¶

      umask [-p] [-S] [mode]

  Set the shell process’s file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the `chmod` command. If mode is omitted, `umask` prints the current value of the mask. If the `-S` option is supplied without a mode argument, `umask` prints the mask in a symbolic format; the default output is an octal number. If the `-p` option is supplied, and mode is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no mode argument is supplied, and non-zero otherwise.

  Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from `7`. Thus, a umask of `022` results in permissions of `755`.

`unset` ¶

      unset [-fnv] [name]

  Remove each variable or function name. If the `-v` option is given, each name refers to a shell variable and that variable is removed. If the `-f` option is given, the names refer to shell functions, and the function definition is removed. If the `-n` option is supplied, and name is a variable with the `nameref` attribute, name will be unset rather than the variable it references. `-n` has no effect if the `-f` option is supplied. If no options are supplied, each name refers to a variable; if there is no variable by that name, a function with that name, if any, is unset. Readonly variables and functions may not be unset. When variables or functions are removed, they are also removed from the environment passed to subsequent commands. Some shell variables may not be unset. Some shell variables lose their special behavior if they are unset; such behavior is noted in the description of the individual variables. The return status is zero unless a name is readonly or may not be unset.

### 4.2 Bash Builtin Commands ¶

This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX standard.

`alias` ¶

      alias [-p] [name[=value] ...]

  Without arguments or with the `-p` option, `alias` prints the list of aliases on the standard output in a form that allows them to be reused as input. If arguments are supplied, define an alias for each name whose value is given. If no value is given, print the name and value of the alias name. A trailing space in value causes the next word to be checked for alias substitution when the alias is expanded during command parsing. `alias` returns true unless a name is given (without a corresponding =value) for which no alias has been defined. Aliases are described in Aliases.

`bind` ¶

      bind [-m keymap] [-lsvSVX]
      bind [-m keymap] [-q function] [-u function] [-r keyseq]
      bind [-m keymap] -f filename
      bind [-m keymap] -x keyseq[: ]shell-command
      bind [-m keymap] keyseq:function-name
      bind [-m keymap] keyseq:readline-command
      bind [-m keymap] -p|-P [readline-command]
      bind readline-command-line

  Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro or to a shell command, or set a Readline variable. Each non-option argument is a key binding or command as it would appear in a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument; e.g., ‘`"\C-x\C-r":re-read-init-file`’.

  In the following descriptions, options that display output in a form available to be re-read format their output as commands that would appear in a Readline initialization file or that would be supplied as individual arguments to a `bind` command.

  Options, if supplied, have the following meanings:

  `-m keymap`

    Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are `emacs`, `emacs-standard`, `emacs-meta`, `emacs-ctlx`, `vi`, `vi-move`, `vi-command`, and `vi-insert`. `vi` is equivalent to `vi-command` (`vi-move` is also a synonym); `emacs` is equivalent to `emacs-standard`.

  `-l`

    List the names of all Readline functions.

  `-p`

    Display Readline function names and bindings in such a way that they can be used as an argument to a subsequent `bind` command or in a Readline initialization file. If arguments remain after option processing, `bind` treats them as readline command names and restricts output to those names.

  `-P`

    List current Readline function names and bindings. If arguments remain after option processing, `bind` treats them as readline command names and restricts output to those names.

  `-s`

    Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as an argument to a subsequent `bind` command or in a Readline initialization file.

  `-S`

    Display Readline key sequences bound to macros and the strings they output.

  `-v`

    Display Readline variable names and values in such a way that they can be used as an argument to a subsequent `bind` command or in a Readline initialization file.

  `-V`

    List current Readline variable names and values.

  `-f filename`

    Read key bindings from filename.

  `-q function`

    Display key sequences that invoke the named Readline function.

  `-u function`

    Unbind all key sequences bound to the named Readline function.

  `-r keyseq`

    Remove any current binding for keyseq.

  `-x keyseq:shell-command`

    Cause shell-command to be executed whenever keyseq is entered. The separator between keyseq and shell-command is either whitespace or a colon optionally followed by whitespace. If the separator is whitespace, shell-command must be enclosed in double quotes and Readline expands any of its special backslash-escapes in shell-command before saving it. If the separator is a colon, any enclosing double quotes are optional, and Readline does not expand the command string before saving it. Since the entire key binding expression must be a single argument, it should be enclosed in single quotes. When shell-command is executed, the shell sets the `READLINE_LINE` variable to the contents of the Readline line buffer and the `READLINE_POINT` and `READLINE_MARK` variables to the current location of the insertion point and the saved insertion point (the mark), respectively. The shell assigns any numeric argument the user supplied to the `READLINE_ARGUMENT` variable. If there was no argument, that variable is not set. If the executed command changes the value of any of `READLINE_LINE`, `READLINE_POINT`, or `READLINE_MARK`, those new values will be reflected in the editing state.

  `-X`

    List all key sequences bound to shell commands and the associated commands in a format that can be reused as an argument to a subsequent `bind` command.

  The return status is zero unless an invalid option is supplied or an error occurs.

`builtin` ¶

      builtin [shell-builtin [args]]

  Execute the specified shell builtin shell-builtin, passing it args, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if shell-builtin is not a shell builtin command.

`caller` ¶

      caller [expr]

  Returns the context of any active subroutine call (a shell function or a script executed with the `.` or `source` builtins).

  Without expr, `caller` displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as expr, `caller` displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0.

  The return value is 0 unless the shell is not executing a subroutine call or expr does not correspond to a valid position in the call stack.

`command` ¶

      command [-pVv] command [arguments ...]

  The `command` builtin runs command with arguments ignoring any shell function named command. Only shell builtin commands or commands found by searching the `PATH` are executed. If there is a shell function named `ls`, running ‘`command ls`’ within the function will execute the external command `ls` instead of calling the function recursively. The `-p` option means to use a default value for `PATH` that is guaranteed to find all of the standard utilities. The return status in this case is 127 if command cannot be found or an error occurred, and the exit status of command otherwise.

  If either the `-V` or `-v` option is supplied, `command` prints a description of command. The `-v` option displays a single word indicating the command or file name used to invoke command; the `-V` option produces a more verbose description. In this case, the return status is zero if command is found, and non-zero if not.

`declare` ¶

      declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]

  Declare variables and give them attributes. If no names are given, then display the values of variables or shell functions instead.

  The `-p` option will display the attributes and values of each name. When `-p` is used with name arguments, additional options, other than `-f` and `-F`, are ignored.

  When `-p` is supplied without name arguments, `declare` will display the attributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with `-p`, `declare` will display the attributes and values of all shell variables. The `-f` option restricts the display to shell functions.

  The `-F` option inhibits the display of function definitions; only the function name and attributes are printed. If the `extdebug` shell option is enabled using `shopt` (see The Shopt Builtin), the source file name and line number where each name is defined are displayed as well. `-F` implies `-f`.

  The `-g` option forces variables to be created or modified at the global scope, even when `declare` is executed in a shell function. It is ignored in when `declare` is not executed in a shell function.

  The `-I` option causes local variables to inherit the attributes (except the `nameref` attribute) and value of any existing variable with the same name at a surrounding scope. If there is no existing variable, the local variable is initially unset.

  The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:

  `-a`

    Each name is an indexed array variable (see Arrays).

  `-A`

    Each name is an associative array variable (see Arrays).

  `-f`

    Each name refers to a shell function.

  `-i`

    The variable is to be treated as an integer; arithmetic evaluation (see Shell Arithmetic) is performed when the variable is assigned a value.

  `-l`

    When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.

  `-n`

    Give each name the `nameref` attribute, making it a name reference to another variable. That other variable is defined by the value of name. All references, assignments, and attribute modifications to name, except for those using or changing the `-n` attribute itself, are performed on the variable referenced by name’s value. The nameref attribute cannot be applied to array variables.

  `-r`

    Make names readonly. These names cannot then be assigned values by subsequent assignment statements or unset.

  `-t`

    Give each name the `trace` attribute. Traced functions inherit the `DEBUG` and `RETURN` traps from the calling shell. The trace attribute has no special meaning for variables.

  `-u`

    When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled.

  `-x`

    Mark each name for export to subsequent commands via the environment.

  Using ‘`+`’ instead of ‘`-`’ turns off the specified attribute instead, with the exceptions that ‘`+a`’ and ‘`+A`’ may not be used to destroy array variables and ‘`+r`’ will not remove the readonly attribute.

  When used in a function, `declare` makes each name local, as with the `local` command, unless the `-g` option is supplied. If a variable name is followed by =value, the value of the variable is set to value.

  When using `-a` or `-A` and the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments.

  The return status is zero unless an invalid option is encountered, an attempt is made to define a function using ‘`-f foo=bar`’, an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see Arrays), one of the names is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with `-f`.

`echo` ¶

      echo [-neE] [arg ...]

  Output the args, separated by spaces, terminated with a newline. The return status is 0 unless a write error occurs. If `-n` is specified, the trailing newline is not printed.

  If the `-e` option is given, `echo` interprets the following backslash-escaped characters. The `-E` option disables interpretation of these escape characters, even on systems where they are interpreted by default. The `xpg_echo` shell option determines whether or not `echo` interprets any options and expands these escape characters. `echo` does not interpret `--` to mean the end of options.

  `echo` interprets the following escape sequences:

  `\a`

    alert (bell)

  `\b`

    backspace

  `\c`

    suppress further output

  `\e`
  `\E`

    escape

  `\f`

    form feed

  `\n`

    new line

  `\r`

    carriage return

  `\t`

    horizontal tab

  `\v`

    vertical tab

  `\\`

    backslash

  `\0nnn`

    The eight-bit character whose value is the octal value nnn (zero to three octal digits).

  `\xHH`

    The eight-bit character whose value is the hexadecimal value HH (one or two hex digits).

  `\uHHHH`

    The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits).

  `\UHHHHHHHH`

    The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits).

  `echo` writes any unrecognized backslash-escaped characters unchanged.

`enable` ¶

      enable [-a] [-dnps] [-f filename] [name ...]

  Enable and disable builtin shell commands. Disabling a builtin allows an executable file which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before files.

  If `-n` is supplied, the names are disabled. Otherwise names are enabled. For example, to use the `test` binary found using `$PATH` instead of the shell builtin version, type ‘`enable -n test`’.

  If the `-p` option is supplied, or no name arguments are supplied, print a list of shell builtins. With no other arguments, the list consists of all enabled shell builtins. The `-n` option means to print only disabled builtins. The `-a` option means to list each builtin with an indication of whether or not it is enabled. The `-s` option means to restrict `enable` to the POSIX special builtins.

  The `-f` option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. If filename does not contain a slash. Bash will use the value of the `BASH_LOADABLES_PATH` variable as a colon-separated list of directories in which to search for filename. The default for `BASH_LOADABLES_PATH` is system-dependent, and may include "." to force a search of the current directory. The `-d` option will delete a builtin loaded with `-f`. If `-s` is used with `-f`, the new builtin becomes a POSIX special builtin (see Special Builtins).

  If no options are supplied and a name is not a shell builtin, `enable` will attempt to load name from a shared object named name, as if the command were ‘`enable -f name name`’.

  The return status is zero unless a name is not a shell builtin or there is an error loading a new builtin from a shared object.

`help` ¶

      help [-dms] [pattern]

  Display helpful information about builtin commands. If pattern is specified, `help` gives detailed help on all commands matching pattern as described below; otherwise it displays a list of all builtins and shell compound commands.

  Options, if supplied, have the following meanings:

  `-d`

    Display a short description of each pattern

  `-m`

    Display the description of each pattern in a manpage-like format

  `-s`

    Display only a short usage synopsis for each pattern

  If pattern contains pattern matching characters (see Pattern Matching) it’s treated as a shell pattern and `help` prints the description of each help topic matching pattern.

  If not, and pattern exactly matches the name of a help topic, `help` prints the description associated with that topic. Otherwise, `help` performs prefix matching and prints the descriptions of all matching help topics.

  The return status is zero unless no command matches pattern.

`let` ¶

      let expression [expression ...]

  The `let` builtin allows arithmetic to be performed on shell variables. Each expression is evaluated as an arithmetic expression according to the rules given below in Shell Arithmetic. If the last expression evaluates to 0, `let` returns 1; otherwise `let` returns 0.

`local` ¶

      local [option] name[=value] ...

  For each argument, create a local variable named name, and assign it value. The option can be any of the options accepted by `declare`. `local` can only be used within a function; it makes the variable name have a visible scope restricted to that function and its children. It is an error to use `local` when not within a function.

  If name is ‘`-`’, it makes the set of shell options local to the function in which `local` is invoked: any shell options changed using the `set` builtin inside the function after the call to `local` are restored to their original values when the function returns. The restore is performed as if a series of `set` commands were executed to restore the values that were in place before the function.

  With no operands, `local` writes a list of local variables to the standard output.

  The return status is zero unless `local` is used outside a function, an invalid name is supplied, or name is a readonly variable.

`logout` ¶

      logout [n]

  Exit a login shell, returning a status of n to the shell’s parent.

`mapfile` ¶

      mapfile [-d delim] [-n count] [-O origin] [-s count]
          [-t] [-u fd] [-C callback] [-c quantum] [array]

  Read lines from the standard input, or from file descriptor fd if the `-u` option is supplied, into the indexed array variable array. The variable `MAPFILE` is the default array. Options, if supplied, have the following meanings:

  `-d`

    Use the first character of delim to terminate each input line, rather than newline. If delim is the empty string, `mapfile` will terminate a line when it reads a NUL character.

  `-n`

    Copy at most count lines. If count is 0, copy all lines.

  `-O`

    Begin assigning to array at index origin. The default index is 0.

  `-s`

    Discard the first count lines read.

  `-t`

    Remove a trailing delim (default newline) from each line read.

  `-u`

    Read lines from file descriptor fd instead of the standard input.

  `-C`

    Evaluate callback each time quantum lines are read. The `-c` option specifies quantum.

  `-c`

    Specify the number of lines read between each call to callback.

  If `-C` is specified without `-c`, the default quantum is 5000. When callback is evaluated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments. callback is evaluated after the line is read but before the array element is assigned.

  If not supplied with an explicit origin, `mapfile` will clear array before assigning to it.

  `mapfile` returns zero unless an invalid option or option argument is supplied, array is invalid or unassignable, or if array is not an indexed array.

`printf` ¶

      printf [-v var] format [arguments]

  Write the formatted arguments to the standard output under the control of the format. The `-v` option assigns the output to the variable var rather than printing it to the standard output.

  The format is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument. In addition to the standard `printf(3)` format characters `cCsSndiouxXeEfFgGaA`, `printf` interprets the following additional format specifiers:

  `%b`

    Causes `printf` to expand backslash escape sequences in the corresponding argument in the same way as `echo -e` (see Bash Builtin Commands).

  `%q`

    Causes `printf` to output the corresponding argument in a format that can be reused as shell input. `%q` and `%Q`P use the ANSI-C quoting style (see ANSI-C Quoting) if any characters in the argument string require it, and backslash quoting otherwise. If the format string uses the `printf` alternate form, these two formats quote the argument string using single quotes.

  `%Q`

    like `%q`, but applies any supplied precision to the argument before quoting it.

  `%(datefmt)T`

    Causes `printf` to output the date-time string resulting from using datefmt as a format string for `strftime`(3). The corresponding argument is an integer representing the number of seconds since the epoch. This format specifier recognizes Two special argument values: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been supplied. This is an exception to the usual `printf` behavior.

  The %b, %q, and %T format specifiers all use the field width and precision arguments from the format specification and write that many bytes from (or use that wide a field for) the expanded argument, which usually contains more characters than the original.

  The %n format specifier accepts a corresponding argument that is treated as a shell variable name.

  The %s and %c format specifiers accept an l (long) modifier, which forces them to convert the argument string to a wide-character string and apply any supplied field width and precision in terms of characters, not bytes. The %S and %C format specifiers are equivalent to %ls and %lc, respectively.

  Arguments to non-string format specifiers are treated as C language constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the numeric value of the following character, using the current locale.

  The format is reused as necessary to consume all of the arguments. If the format requires more arguments than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero if an invalid option is supplied or a write or assignment error occurs.

`read` ¶

      read [-Eers] [-a aname] [-d delim] [-i text] [-n nchars]
          [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

  Read one line from the standard input, or from the file descriptor fd supplied as an argument to the `-u` option, split it into words as described above in Word Splitting, and assign the first word to the first name, the second word to the second name, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of the `IFS` variable are used to split the line into words using the same rules the shell uses for expansion (described above in Word Splitting). The backslash character ‘`\`’ removes any special meaning for the next character read and is used for line continuation.

  Options, if supplied, have the following meanings:

  `-a aname`

    The words are assigned to sequential indices of the array variable aname, starting at 0. All elements are removed from aname before the assignment. Other name arguments are ignored.

  `-d delim`

    The first character of delim terminates the input line, rather than newline. If delim is the empty string, `read` will terminate a line when it reads a NUL character.

  `-e`

    If the standard input is coming from a terminal, `read` uses Readline (see Command Line Editing) to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Readline’s default filename completion.

  `-E`

    If the standard input is coming from a terminal, `read` uses Readline (see Command Line Editing) to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Bash’s default completion, including programmable completion.

  `-i text`

    If Readline is being used to read the line, `read` places text into the editing buffer before editing begins.

  `-n nchars`

    `read` returns after reading nchars characters rather than waiting for a complete line of input, unless it encounters EOF or `read` times out, but honors a delimiter if it reads fewer than nchars characters before the delimiter.

  `-N nchars`

    `read` returns after reading exactly nchars characters rather than waiting for a complete line of input, unless it encounters EOF or `read` times out. Delimiter characters in the input are not treated specially and do not cause `read` to return until it has read nchars characters. The result is not split on the characters in `IFS`; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the `-r` option below).

  `-p prompt`

    Display prompt, without a trailing newline, before attempting to read any input, but only if input is coming from a terminal.

  `-r`

    If this option is given, backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not then be used as a line continuation.

  `-s`

    Silent mode. If input is coming from a terminal, characters are not echoed.

  `-t timeout`

    Cause `read` to time out and return failure if it does not read a complete line of input (or a specified number of characters) within timeout seconds. timeout may be a decimal number with a fractional portion following the decimal point. This option is only effective if `read` is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If `read` times out, it saves any partial input read into the specified variable name, and returns a status greater than 128. If timeout is 0, `read` returns immediately, without trying to read any data. In this case, the exit status is 0 if input is available on the specified file descriptor, or the read will return EOF, non-zero otherwise.

  `-u fd`

    Read input from file descriptor fd instead of the standard input.

  Other than the case where delim is the empty string, `read` ignores any NUL characters in the input.

  If no names are supplied, `read` assigns the line read, without the ending delimiter but otherwise unmodified, to the variable `REPLY`.

  The exit status is zero, unless end-of-file is encountered, `read` times out (in which case the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as the argument to `-u`.

`readarray` ¶

      readarray [-d delim] [-n count] [-O origin] [-s count]
          [-t] [-u fd] [-C callback] [-c quantum] [array]

  Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the `-u` option is supplied.

  A synonym for `mapfile`.

`source` ¶

      source [-p path] filename [arguments]

  A synonym for `.` (see Bourne Shell Builtins).

`type` ¶

      type [-afptP] [name ...]

  Indicate how each name would be interpreted if used as a command name.

  If the `-t` option is used, `type` prints a single word which is one of ‘`alias`’, ‘`keyword`’, ‘`function`’, ‘`builtin`’, or ‘`file`’, if name is an alias, shell reserved word, shell function, shell builtin, or executable file, respectively. If the name is not found, `type` prints nothing and returns a failure status.

  If the `-p` option is used, `type` either returns the name of the executable file that would be found by searching `$PATH` for `name`, or nothing if `-t` would not return ‘`file`’.

  The `-P` option forces a path search for each name, even if `-t` would not return ‘`file`’.

  If a name is present in the table of hashed commands, options `-p` and `-P` print the hashed value, which is not necessarily the file that appears first in `$PATH`.

  If the `-a` option is used, `type` returns all of the places that contain a command named name. This includes aliases, reserved words, functions, and builtins, but the path search options (`-p` and `-P`) can be supplied to restrict the output to executable files. If `-a` is supplied with `-p`, `type` does not look in the table of hashed commands, and only performs a `PATH` search for name.

  If the `-f` option is used, `type` does not attempt to find shell functions, as with the `command` builtin.

  The return status is zero if all of the names are found, non-zero if any are not found.

`typeset` ¶

      typeset [-afFgrxilnrtux] [-p] [name[=value] ...]

  The `typeset` command is supplied for compatibility with the Korn shell. It is a synonym for the `declare` builtin command.

`ulimit` ¶

      ulimit [-HS] -a
      ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [limit]

  `ulimit` provides control over the resources available to the shell and to processes it starts, on systems that allow such control. If an option is given, it is interpreted as follows:

  `-S`

    Change and report the soft limit associated with a resource.

  `-H`

    Change and report the hard limit associated with a resource.

  `-a`

    Report all current limits; no limits are set.

  `-b`

    The maximum socket buffer size.

  `-c`

    The maximum size of core files created.

  `-d`

    The maximum size of a process’s data segment.

  `-e`

    The maximum scheduling priority ("nice").

  `-f`

    The maximum size of files written by the shell and its children.

  `-i`

    The maximum number of pending signals.

  `-k`

    The maximum number of kqueues that may be allocated.

  `-l`

    The maximum size that may be locked into memory.

  `-m`

    The maximum resident set size (many systems do not honor this limit).

  `-n`

    The maximum number of open file descriptors (most systems do not allow this value to be set).

  `-p`

    The pipe buffer size.

  `-q`

    The maximum number of bytes in POSIX message queues.

  `-r`

    The maximum real-time scheduling priority.

  `-s`

    The maximum stack size.

  `-t`

    The maximum amount of cpu time in seconds.

  `-u`

    The maximum number of processes available to a single user.

  `-v`

    The maximum amount of virtual memory available to the shell, and, on some systems, to its children.

  `-x`

    The maximum number of file locks.

  `-P`

    The maximum number of pseudoterminals.

  `-R`

    The maximum time a real-time process can run before blocking, in microseconds.

  `-T`

    The maximum number of threads.

  If limit is supplied, and the `-a` option is not used, limit is the new value of the specified resource. The special limit values `hard`, `soft`, and `unlimited` stand for the current hard limit, the current soft limit, and no limit, respectively. A hard limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the hard limit. Otherwise, `ulimit` prints the current value of the soft limit for the specified resource, unless the `-H` option is supplied. When more than one resource is specified, the limit name and unit, if appropriate, are printed before the value. When setting new limits, if neither `-H` nor `-S` is supplied, `ulimit` sets both the hard and soft limits. If no option is supplied, then `-f` is assumed.

  Values are in 1024-byte increments, except for `-t`, which is in seconds; `-R`, which is in microseconds; `-p`, which is in units of 512-byte blocks; `-P`, `-T`, `-b`, `-k`, `-n` and `-u`, which are unscaled values; and, when in POSIX mode (see Bash and POSIX), `-c` and `-f`, which are in 512-byte increments.

  The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit.

`unalias` ¶

      unalias [-a] [name ... ]

  Remove each name from the list of aliases. If `-a` is supplied, remove all aliases. The return value is true unless a supplied name is not a defined alias. Aliases are described in Aliases.

### 4.3 Modifying Shell Behavior ¶

#### 4.3.1 The Set Builtin ¶

This builtin is so complicated that it deserves its own section. `set` allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables.

`set` ¶

      set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [argument ...]
      set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [argument ...]
      set -o
      set +o

  If no options or arguments are supplied, `set` displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In POSIX mode, only shell variables are listed.

  When options are supplied, they set or unset shell attributes. Any arguments remaining after option processing replace the positional parameters.

  Options, if specified, have the following meanings:

  `-a`

    Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.

  `-b`

    Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt or, under some circumstances, when a foreground command exits. This is effective only when job control is enabled.

  `-e`

    Exit immediately if a pipeline (see Pipelines), which may consist of a single simple command (see Simple Commands), a list (see Lists of Commands), or a compound command (see Compound Commands) returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a `while` or `until` reserved word, part of the test in an `if` statement, part of any command executed in a `&&` or `||` list except the command following the final `&&` or `||`, any command in a pipeline but the last (subject to the state of the `pipefail` shell option), or if the command’s return status is being inverted with `!`. If a compound command other than a subshell returns a non-zero status because a command failed while `-e` was being ignored, the shell does not exit. A trap on `ERR`, if set, is executed before the shell exits.

    This option applies to the shell environment and each subshell environment separately (see Command Execution Environment), and may cause subshells to exit before executing all the commands in the subshell.

    If a compound command or shell function executes in a context where `-e` is being ignored, none of the commands executed within the compound command or function body will be affected by the `-e` setting, even if `-e` is set and a command returns a failure status. If a compound command or shell function sets `-e` while executing in a context where `-e` is ignored, that setting will not have any effect until the compound command or the command containing the function call completes.

  `-f`

    Disable filename expansion (globbing).

  `-h`

    Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default.

  `-k`

    All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.

  `-m`

    Job control is enabled (see Job Control). All processes run in a separate process group. When a background job completes, the shell prints a line containing its exit status.

  `-n`

    Read commands but do not execute them. This may be used to check a script for syntax errors. This option is ignored by interactive shells.

  `-o option-name`

    Set the option corresponding to option-name. If `-o` is supplied with no option-name, `set` prints the current shell options settings. If `+o` is supplied with no option-name, `set` prints a series of `set` commands to recreate the current option settings on the standard output. Valid option names are:

    `allexport`

      Same as `-a`.

    `braceexpand`

      Same as `-B`.

    `emacs`

      Use an `emacs`-style line editing interface (see Command Line Editing). This also affects the editing interface used for `read -e`.

    `errexit`

      Same as `-e`.

    `errtrace`

      Same as `-E`.

    `functrace`

      Same as `-T`.

    `hashall`

      Same as `-h`.

    `histexpand`

      Same as `-H`.

    `history`

      Enable command history, as described in Bash History Facilities. This option is on by default in interactive shells.

    `ignoreeof`

      An interactive shell will not exit upon reading EOF.

    `keyword`

      Same as `-k`.

    `monitor`

      Same as `-m`.

    `noclobber`

      Same as `-C`.

    `noexec`

      Same as `-n`.

    `noglob`

      Same as `-f`.

    `nolog`

      Currently ignored.

    `notify`

      Same as `-b`.

    `nounset`

      Same as `-u`.

    `onecmd`

      Same as `-t`.

    `physical`

      Same as `-P`.

    `pipefail`

      If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.

    `posix`

      Enable POSIX mode; change the behavior of Bash where the default operation differs from the POSIX standard to match the standard (see Bash and POSIX). This is intended to make Bash behave as a strict superset of that standard.

    `privileged`

      Same as `-p`.

    `verbose`

      Same as `-v`.

    `vi`

      Use a `vi`-style line editing interface. This also affects the editing interface used for `read -e`.

    `xtrace`

      Same as `-x`.

  `-p`

    Turn on privileged mode. In this mode, the `$BASH_ENV` and `$ENV` files are not processed, shell functions are not inherited from the environment, and the `SHELLOPTS`, `BASHOPTS`, `CDPATH` and `GLOBIGNORE` variables, if they appear in the environment, are ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the `-p` option is not supplied, these actions are taken and the effective user id is set to the real user id. If the `-p` option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.

  `-r`

    Enable restricted shell mode (see The Restricted Shell). This option cannot be unset once it has been set.

  `-t`

    Exit after reading and executing one command.

  `-u`

    Treat unset variables and parameters other than the special parameters ‘`@`’ or ‘`*`’, or array variables subscripted with ‘`@`’ or ‘`*`’, as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.

  `-v`

    Print shell input lines to standard error as they are read.

  `-x`

    Print a trace of simple commands, `for` commands, `case` commands, `select` commands, and arithmetic `for` commands and their arguments or associated word lists to the standard error after they are expanded and before they are executed. The shell prints the expanded value of the `PS4` variable before the command and its expanded arguments.

  `-B`

    The shell will perform brace expansion (see Brace Expansion). This option is on by default.

  `-C`

    Prevent output redirection using ‘`>`’, ‘`>&`’, and ‘`<>`’ from overwriting existing files. Using the redirection operator ‘`>|`’ instead of ‘`>`’ will override this and force the creation of an output file.

  `-E`

    If set, any trap on `ERR` is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The `ERR` trap is normally not inherited in such cases.

  `-H`

    Enable ‘`!`’ style history substitution (see History Expansion). This option is on by default for interactive shells.

  `-P`

    If set, Bash does not resolve symbolic links when executing commands such as `cd` which change the current directory. It uses the physical directory structure instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory.

    For example, if `/usr/sys` is a symbolic link to `/usr/local/sys` then:

        $ cd /usr/sys; echo $PWD
        /usr/sys
        $ cd ..; pwd
        /usr

    If `set -P` is on, then:

        $ cd /usr/sys; echo $PWD
        /usr/local/sys
        $ cd ..; pwd
        /usr/local

  `-T`

    If set, any traps on `DEBUG` and `RETURN` are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The `DEBUG` and `RETURN` traps are normally not inherited in such cases.

  `--`

    If no arguments follow this option, unset the positional parameters. Otherwise, the positional parameters are set to the arguments, even if some of them begin with a ‘`-`’.

  `-`

    Signal the end of options, and assign all remaining arguments to the positional parameters. The `-x` and `-v` options are turned off. If there are no arguments, the positional parameters remain unchanged.

  Using ‘`+`’ rather than ‘`-`’ causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in `$-`.

  The remaining N arguments are positional parameters and are assigned, in order, to `$1`, `$2`, … `$N`. The special parameter `#` is set to N.

  The return status is always zero unless an invalid option is supplied.

#### 4.3.2 The Shopt Builtin ¶

This builtin allows you to change additional optional shell behavior.

`shopt` ¶

      shopt [-pqsu] [-o] [optname ...]

  Toggle the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the `-o` option is used, those available with the `-o` option to the `set` builtin command (see The Set Builtin).

  With no options, or with the `-p` option, display a list of all settable options, with an indication of whether or not each is set; if any optnames are supplied, the output is restricted to those options. The `-p` option displays output in a form that may be reused as input.

  Other options have the following meanings:

  `-s`

    Enable (set) each optname.

  `-u`

    Disable (unset) each optname.

  `-q`

    Suppresses normal output; the return status indicates whether the optname is set or unset. If multiple optname arguments are supplied with `-q`, the return status is zero if all optnames are enabled; non-zero otherwise.

  `-o`

    Restricts the values of optname to be those defined for the `-o` option to the `set` builtin (see The Set Builtin).

  If either `-s` or `-u` is used with no optname arguments, `shopt` shows only those options which are set or unset, respectively.

  Unless otherwise noted, the `shopt` options are disabled (off) by default.

  The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.

  The list of `shopt` options is:

  `array_expand_once`

    If set, the shell suppresses multiple evaluation of associative and indexed array subscripts during arithmetic expression evaluation, while executing builtins that can perform variable assignments, and while executing builtins that perform array dereferencing.

  `assoc_expand_once`

    Deprecated; a synonym for `array_expand_once`.

  `autocd`

    If set, a command name that is the name of a directory is executed as if it were the argument to the `cd` command. This option is only used by interactive shells.

  `bash_source_fullpath`

    If set, filenames added to the `BASH_SOURCE` array variable are converted to full pathnames (see Bash Variables).

  `cdable_vars`

    If this is set, an argument to the `cd` builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to.

  `cdspell`

    If set, the `cd` command attempts to correct minor errors in the spelling of a directory component. Minor errors include transposed characters, a missing character, and one extra character. If `cd` corrects the directory name, it prints the corrected filename, and the command proceeds. This option is only used by interactive shells.

  `checkhash`

    If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, Bash performs a normal path search.

  `checkjobs`

    If set, Bash lists the status of any stopped and running jobs before exiting an interactive shell. If any jobs are running, Bash defers the exit until a second exit is attempted without an intervening command (see Job Control). The shell always postpones exiting if any jobs are stopped.

  `checkwinsize`

    If set, Bash checks the window size after each external (non-builtin) command and, if necessary, updates the values of `LINES` and `COLUMNS`, using the file descriptor associated with stderr if it is a terminal. This option is enabled by default.

  `cmdhist`

    If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. This option is enabled by default, but only has an effect if command history is enabled (see Bash History Facilities).

  `compat31`
  `compat32`
  `compat40`
  `compat41`
  `compat42`
  `compat43`
  `compat44`

    These control aspects of the shell’s compatibility mode (see Shell Compatibility Mode).

  `complete_fullquote`

    If set, Bash quotes all shell metacharacters in filenames and directory names when performing completion. If not set, Bash removes metacharacters such as the dollar sign from the set of characters that will be quoted in completed filenames when these metacharacters appear in shell variable references in words to be completed. This means that dollar signs in variable names that expand to directories will not be quoted; however, any dollar signs appearing in filenames will not be quoted, either. This is active only when Bash is using backslashes to quote completed filenames. This variable is set by default, which is the default Bash behavior in versions through 4.2.

  `direxpand`

    If set, Bash replaces directory names with the results of word expansion when performing filename completion. This changes the contents of the Readline editing buffer. If not set, Bash attempts to preserve what the user typed.

  `dirspell`

    If set, Bash attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist.

  `dotglob`

    If set, Bash includes filenames beginning with a ‘`.`’ in the results of filename expansion. The filenames `.` and `..` must always be matched explicitly, even if `dotglob` is set.

  `execfail`

    If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the `exec` builtin. An interactive shell does not exit if `exec` fails.

  `expand_aliases`

    If set, aliases are expanded as described below under Aliases, Aliases. This option is enabled by default for interactive shells.

  `extdebug`

    If set at shell invocation, or in a shell startup file, arrange to execute the debugger profile before the shell starts, identical to the `--debugger` option. If set after invocation, behavior intended for use by debuggers is enabled:

    1. The `-F` option to the `declare` builtin (see Bash Builtin Commands) displays the source file name and line number corresponding to each function name supplied as an argument.
    2. If the command run by the `DEBUG` trap returns a non-zero value, the next command is skipped and not executed.
    3. If the command run by the `DEBUG` trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the `.` or `source` builtins), the shell simulates a call to `return`.
    4. `BASH_ARGC` and `BASH_ARGV` are updated as described in their descriptions (see Bash Variables).
    5. Function tracing is enabled: command substitution, shell functions, and subshells invoked with `( command )` inherit the `DEBUG` and `RETURN` traps.
    6. Error tracing is enabled: command substitution, shell functions, and subshells invoked with `( command )` inherit the `ERR` trap.
  `extglob`

    If set, enable the extended pattern matching features described above (see Pattern Matching).

  `extquote`

    If set, `$'string'` and `$"string"` quoting is performed within `${parameter}` expansions enclosed in double quotes. This option is enabled by default.

  `failglob`

    If set, patterns which fail to match filenames during filename expansion result in an expansion error.

  `force_fignore`

    If set, the suffixes specified by the `FIGNORE` shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See Bash Variables, for a description of `FIGNORE`. This option is enabled by default.

  `globasciiranges`

    If set, range expressions used in pattern matching bracket expressions (see Pattern Matching) behave as if in the traditional C locale when performing comparisons. That is, pattern matching does not take the current locale’s collating sequence into account, so ‘`b`’ will not collate between ‘`A`’ and ‘`B`’, and upper-case and lower-case ASCII characters will collate together.

  `globskipdots`

    If set, filename expansion will never match the filenames `.` and `..`, even if the pattern begins with a ‘`.`’. This option is enabled by default.

  `globstar`

    If set, the pattern ‘`**`’ used in a filename expansion context will match all files and zero or more directories and subdirectories. If the pattern is followed by a ‘`/`’, only directories and subdirectories match.

  `gnu_errfmt`

    If set, shell error messages are written in the standard GNU error message format.

  `histappend`

    If set, the history list is appended to the file named by the value of the `HISTFILE` variable when the shell exits, rather than overwriting the file.

  `histreedit`

    If set, and Readline is being used, the user is given the opportunity to re-edit a failed history substitution.

  `histverify`

    If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification.

  `hostcomplete`

    If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a ‘`@`’ is being completed (see Letting Readline Type For You). This option is enabled by default.

  `huponexit`

    If set, Bash will send `SIGHUP` to all jobs when an interactive login shell exits (see Signals).

  `inherit_errexit`

    If set, command substitution inherits the value of the `errexit` option, instead of unsetting it in the subshell environment. This option is enabled when POSIX mode is enabled.

  `interactive_comments`

    In an interactive shell, a word beginning with ‘`#`’ causes that word and all remaining characters on that line to be ignored, as in a non-interactive shell. This option is enabled by default.

  `lastpipe`

    If set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment.

  `lithist`

    If enabled, and the `cmdhist` option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible.

  `localvar_inherit`

    If set, local variables inherit the value and attributes of a variable of the same name that exists at a previous scope before any new value is assigned. The `nameref` attribute is not inherited.

  `localvar_unset`

    If set, calling `unset` on local variables in previous function scopes marks them so subsequent lookups find them unset until that function returns. This is identical to the behavior of unsetting local variables at the current function scope.

  `login_shell`

    The shell sets this option if it is started as a login shell (see Invoking Bash). The value may not be changed.

  `mailwarn`

    If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, Bash displays the message `"The mail in mailfile has been read"`.

  `no_empty_cmd_completion`

    If set, and Readline is being used, Bash does not search the `PATH` for possible completions when completion is attempted on an empty line.

  `nocaseglob`

    If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion.

  `nocasematch`

    If set, Bash matches patterns in a case-insensitive fashion when performing matching while executing `case` or `[[` conditional commands (see Conditional Constructs, when performing pattern substitution word expansions, or when filtering possible completions as part of programmable completion.

  `noexpand_translation`

    If set, Bash encloses the translated results of $"…" quoting in single quotes instead of double quotes. If the string is not translated, this has no effect.

  `nullglob`

    If set, filename expansion patterns which match no files (see Filename Expansion) expand to nothing and are removed, rather than expanding to themselves.

  `patsub_replacement`

    If set, Bash expands occurrences of ‘`&`’ in the replacement string of pattern substitution to the text matched by the pattern, as described above (see Shell Parameter Expansion). This option is enabled by default.

  `progcomp`

    If set, enable the programmable completion facilities (see Programmable Completion). This option is enabled by default.

  `progcomp_alias`

    If set, and programmable completion is enabled, Bash treats a command name that doesn’t have any completions as a possible alias and attempts alias expansion. If it has an alias, Bash attempts programmable completion using the command word resulting from the expanded alias.

  `promptvars`

    If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described below (see Controlling the Prompt). This option is enabled by default.

  `restricted_shell`

    The shell sets this option if it is started in restricted mode (see The Restricted Shell). The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted.

  `shift_verbose`

    If this is set, the `shift` builtin prints an error message when the shift count exceeds the number of positional parameters.

  `sourcepath`

    If set, the `.` (`source`) builtin uses the value of `PATH` to find the directory containing the file supplied as an argument when the `-p` option is not supplied. This option is enabled by default.

  `varredir_close`

    If set, the shell automatically closes file descriptors assigned using the `{varname}` redirection syntax (see Redirections) instead of leaving them open when the command completes.

  `xpg_echo`

    If set, the `echo` builtin expands backslash-escape sequences by default. If the `posix` shell option (see The Set Builtin) is also enabled, `echo` does not interpret any options.

### 4.4 Special Builtins ¶

For historical reasons, the POSIX standard has classified several builtin commands as special. When Bash is executing in POSIX mode, the special builtins differ from other builtin commands in three respects:

1. Special builtins are found before shell functions during command lookup.
2. If a special builtin returns an error status, a non-interactive shell exits.
3. Assignment statements preceding the command stay in effect in the shell environment after the command completes.

When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash POSIX mode is described in Bash and POSIX.

These are the POSIX special builtins:

    break : . source continue eval exec exit export readonly return set
    shift times trap unset

## 5 Shell Variables ¶

This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables.

### 5.1 Bourne Shell Variables ¶

Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.

`CDPATH` ¶

  A colon-separated list of directories used as a search path for the `cd` builtin command.

`HOME` ¶

  The current user’s home directory; the default for the `cd` builtin command. The value of this variable is also used by tilde expansion (see Tilde Expansion).

`IFS` ¶

  A list of characters that separate fields; used when the shell splits words as part of expansion and by the `read` builtin to split lines into words. See Word Splitting, for a description of word splitting.

`MAIL` ¶

  If the value is set to a filename or directory name and the `MAILPATH` variable is not set, Bash informs the user of the arrival of mail in the specified file or Maildir-format directory.

`MAILPATH` ¶

  A colon-separated list of filenames which the shell periodically checks for new mail. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the filename from the message with a ‘`?`’. When used in the text of the message, `$_` expands to the name of the current mail file.

`OPTARG` ¶

  The value of the last option argument processed by the `getopts` builtin.

`OPTIND` ¶

  The index of the next argument to be processed by the `getopts` builtin.

`PATH` ¶

  A colon-separated list of directories in which the shell looks for commands. A zero-length (null) directory name in the value of `PATH` indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. The default path is system-dependent, and is set by the administrator who installs `bash`. A common value is "/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin".

`PS1` ¶

  The primary prompt string. The default value is ‘`\s-\v\$` ’. See Controlling the Prompt, for the complete list of escape sequences that are expanded before `PS1` is displayed.

`PS2` ¶

  The secondary prompt string. The default value is ‘`>` ’. `PS2` is expanded in the same way as `PS1` before being displayed.

### 5.2 Bash Variables ¶

These variables are set or used by Bash, but other shells do not normally treat them specially.

A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see Job Control Variables).

`_` ¶

  ($_, an underscore.) This has a number of meanings depending on context. At shell startup, $_ set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, it expands to the last argument to the previous simple command executed in the foreground, after expansion. It is also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, $_ expands to the name of the mail file.

`BASH` ¶

  The full pathname used to execute the current instance of Bash.

`BASHOPTS` ¶

  A colon-separated list of enabled shell options. Each word in the list is a valid argument for the `-s` option to the `shopt` builtin command (see The Shopt Builtin). The options appearing in `BASHOPTS` are those reported as ‘`on`’ by ‘`shopt`’. If this variable is in the environment when Bash starts up, the shell enables each option in the list before reading any startup files. If this variable is exported, child shells will enable each option in the list. This variable is readonly.

`BASHPID` ¶

  Expands to the process ID of the current Bash process. This differs from `$$` under certain circumstances, such as subshells that do not require Bash to be re-initialized. Assignments to `BASHPID` have no effect. If `BASHPID` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_ALIASES` ¶

  An associative array variable whose members correspond to the internal list of aliases as maintained by the `alias` builtin. (see Bourne Shell Builtins). Elements added to this array appear in the alias list; however, unsetting array elements currently does not cause aliases to be removed from the alias list. If `BASH_ALIASES` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_ARGC` ¶

  An array variable whose values are the number of parameters in each frame of the current Bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with `.` or `source`) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto `BASH_ARGC`. The shell sets `BASH_ARGC` only when in extended debugging mode (see The Shopt Builtin for a description of the `extdebug` option to the `shopt` builtin). Setting `extdebug` after the shell has started to execute a subroutine, or referencing this variable when `extdebug` is not set, may result in inconsistent values. Assignments to `BASH_ARGC` have no effect, and it may not be unset.

`BASH_ARGV` ¶

  An array variable containing all of the parameters in the current Bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the shell pushes the supplied parameters onto `BASH_ARGV`. The shell sets `BASH_ARGV` only when in extended debugging mode (see The Shopt Builtin for a description of the `extdebug` option to the `shopt` builtin). Setting `extdebug` after the shell has started to execute a script, or referencing this variable when `extdebug` is not set, may result in inconsistent values. Assignments to `BASH_ARGV` have no effect, and it may not be unset.

`BASH_ARGV0` ¶

  When referenced, this variable expands to the name of the shell or shell script (identical to `$0`; See Special Parameters, for the description of special parameter 0). Assigning a value to `BASH_ARGV0` sets `$0` to the same value. If `BASH_ARGV0` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_CMDS` ¶

  An associative array variable whose members correspond to the internal hash table of commands as maintained by the `hash` builtin (see Bourne Shell Builtins). Adding elements to this array makes them appear in the hash table; however, unsetting array elements currently does not remove command names from the hash table. If `BASH_CMDS` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_COMMAND` ¶

  Expands to the command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If `BASH_COMMAND` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_COMPAT` ¶

  The value is used to set the shell’s compatibility level. See Shell Compatibility Mode, for a description of the various compatibility levels and their effects. The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility level. If `BASH_COMPAT` is unset or set to the empty string, the compatibility level is set to the default for the current version. If `BASH_COMPAT` is set to a value that is not one of the valid compatibility levels, the shell prints an error message and sets the compatibility level to the default for the current version. A subset of the valid values correspond to the compatibility levels described below (see Shell Compatibility Mode). For example, 4.2 and 42 are valid values that correspond to the `compat42` `shopt` option and set the compatibility level to 42. The current version is also a valid value.

`BASH_ENV` ¶

  If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. Bash does not use `PATH` to search for the resultant filename. See Bash Startup Files.

`BASH_EXECUTION_STRING` ¶

  The command argument to the `-c` invocation option.

`BASH_LINENO` ¶

  An array variable whose members are the line numbers in source files where each corresponding member of `FUNCNAME` was invoked. `${BASH_LINENO[$i]}` is the line number in the source file (`${BASH_SOURCE[$i+1]}`) where `${FUNCNAME[$i]}` was called (or `${BASH_LINENO[$i-1]}` if referenced within another shell function). Use `LINENO` to obtain the current line number. Assignments to `BASH_LINENO` have no effect, and it may not be unset.

`BASH_LOADABLES_PATH` ¶

  A colon-separated list of directories in which the `enable` command looks for dynamically loadable builtins.

`BASH_MONOSECONDS` ¶

  Each time this variable is referenced, it expands to the value returned by the system’s monotonic clock, if one is available. If there is no monotonic clock, this is equivalent to `EPOCHSECONDS`. If `BASH_MONOSECONDS` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_REMATCH` ¶

  An array variable whose members are assigned by the ‘`=~`’ binary operator to the `[[` conditional command (see Conditional Constructs). The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression.

`BASH_SOURCE` ¶

  An array variable whose members are the source filenames where the corresponding shell function names in the `FUNCNAME` array variable are defined. The shell function `${FUNCNAME[$i]}` is defined in the file `${BASH_SOURCE[$i]}` and called from `${BASH_SOURCE[$i+1]}` Assignments to `BASH_SOURCE` have no effect, and it may not be unset.

`BASH_SUBSHELL` ¶

  Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If `BASH_SUBSHELL` is unset, it loses its special properties, even if it is subsequently reset.

`BASH_TRAPSIG` ¶

  Set to the signal number corresponding to the trap action being executed during its execution. See the description of `trap` (see Bourne Shell Builtins) for information about signal numbers and trap execution.

`BASH_VERSINFO` ¶

  A readonly array variable (see Arrays) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows:

  `BASH_VERSINFO[0]`

    The major version number (the release).

  `BASH_VERSINFO[1]`

    The minor version number (the version).

  `BASH_VERSINFO[2]`

    The patch level.

  `BASH_VERSINFO[3]`

    The build version.

  `BASH_VERSINFO[4]`

    The release status (e.g., `beta`).

  `BASH_VERSINFO[5]`

    The value of `MACHTYPE`.

`BASH_VERSION` ¶

  Expands to a string describing the version of this instance of Bash (e.g., 5.2.37(3)-release).

`BASH_XTRACEFD` ¶

  If set to an integer corresponding to a valid file descriptor, Bash writes the trace output generated when ‘`set -x`’ is enabled to that file descriptor, instead of the standard error. This allows tracing output to be separated from diagnostic and error messages. The file descriptor is closed when `BASH_XTRACEFD` is unset or assigned a new value. Unsetting `BASH_XTRACEFD` or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting `BASH_XTRACEFD` to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed.

`CHILD_MAX` ¶

  Set the number of exited child status values for the shell to remember. Bash will not allow this value to be decreased below a POSIX-mandated minimum, and there is a maximum value (currently 8192) that this may not exceed. The minimum value is system-dependent.

`COLUMNS` ¶

  Used by the `select` command to determine the terminal width when printing selection lists. Automatically set if the `checkwinsize` option is enabled (see The Shopt Builtin), or in an interactive shell upon receipt of a `SIGWINCH`.

`COMP_CWORD` ¶

  An index into `${COMP_WORDS}` of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion).

`COMP_KEY` ¶

  The key (or final key of a key sequence) used to invoke the current completion function. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

`COMP_LINE` ¶

  The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

`COMP_POINT` ¶

  The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to `${#COMP_LINE}`. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

`COMP_TYPE` ¶

  Set to an integer value corresponding to the type of attempted completion that caused a completion function to be called: `TAB`, for normal completion, ‘`?`’, for listing completions after successive tabs, ‘`!`’, for listing alternatives on partial word completion, ‘`@`’, to list completions if the word is not unmodified, or ‘`%`’, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

`COMP_WORDBREAKS` ¶

  The set of characters that the Readline library treats as word separators when performing word completion. If `COMP_WORDBREAKS` is unset, it loses its special properties, even if it is subsequently reset.

`COMP_WORDS` ¶

  An array variable consisting of the individual words in the current command line. The line is split into words as Readline would split it, using `COMP_WORDBREAKS` as described above. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion).

`COMPREPLY` ¶

  An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Programmable Completion). Each array element contains one possible completion.

`COPROC` ¶

  An array variable created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses).

`DIRSTACK` ¶

  An array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the `dirs` builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the `pushd` and `popd` builtins must be used to add and remove directories. Assigning to this variable does not change the current directory. If `DIRSTACK` is unset, it loses its special properties, even if it is subsequently reset.

`EMACS` ¶

  If Bash finds this variable in the environment when the shell starts, and its value is ‘`t`’, Bash assumes that the shell is running in an Emacs shell buffer and disables line editing.

`ENV` ¶

  Expanded and executed similarly to `BASH_ENV` (see Bash Startup Files) when an interactive shell is invoked in POSIX mode (see Bash and POSIX).

`EPOCHREALTIME` ¶

  Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch as a floating-point value with micro-second granularity (see the documentation for the C library function `time` for the definition of Epoch). Assignments to `EPOCHREALTIME` are ignored. If `EPOCHREALTIME` is unset, it loses its special properties, even if it is subsequently reset.

`EPOCHSECONDS` ¶

  Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see the documentation for the C library function `time` for the definition of Epoch). Assignments to `EPOCHSECONDS` are ignored. If `EPOCHSECONDS` is unset, it loses its special properties, even if it is subsequently reset.

`EUID` ¶

  The numeric effective user id of the current user. This variable is readonly.

`EXECIGNORE` ¶

  A colon-separated list of shell patterns (see Pattern Matching) defining the set of filenames to be ignored by command search using `PATH`. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via `PATH` lookup. This does not affect the behavior of the `[`, `test`, and `[[` commands. Full pathnames in the command hash table are not subject to `EXECIGNORE`. Use this variable to ignore shared library files that have the executable bit set, but are not executable files. The pattern matching honors the setting of the `extglob` shell option.

`FCEDIT` ¶

  The editor used as a default by the `fc` builtin command.

`FIGNORE` ¶

  A colon-separated list of suffixes to ignore when performing filename completion. A filename whose suffix matches one of the entries in `FIGNORE` is excluded from the list of matched filenames. A sample value is ‘`.o:~`’

`FUNCNAME` ¶

  An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is `"main"`. This variable exists only when a shell function is executing. Assignments to `FUNCNAME` have no effect. If `FUNCNAME` is unset, it loses its special properties, even if it is subsequently reset.

  This variable can be used with `BASH_LINENO` and `BASH_SOURCE`. Each element of `FUNCNAME` has corresponding elements in `BASH_LINENO` and `BASH_SOURCE` to describe the call stack. For instance, `${FUNCNAME[$i]}` was called from the file `${BASH_SOURCE[$i+1]}` at line number `${BASH_LINENO[$i]}`. The `caller` builtin displays the current call stack using this information.

`FUNCNEST` ¶

  A numeric value greater than 0 defines a maximum function nesting level. Function invocations that exceed this nesting level cause the current command to abort.

`GLOBIGNORE` ¶

  A colon-separated list of patterns defining the set of file names to be ignored by filename expansion. If a file name matched by a filename expansion pattern also matches one of the patterns in `GLOBIGNORE`, it is removed from the list of matches. The pattern matching honors the setting of the `extglob` shell option.

`GLOBSORT` ¶

  Controls how the results of filename expansion are sorted. The value of this variable specifies the sort criteria and sort order for the results of filename expansion. If this variable is unset or set to the null string, filename expansion uses the historical behavior of sorting by name, in ascending lexicographic order as determined by the `LC_COLLATE` shell variable.

  If set, a valid value begins with an optional ‘`+`’, which is ignored, or ‘`-`’, which reverses the sort order from ascending to descending, followed by a sort specifier. The valid sort specifiers are ‘`name`’, ‘`numeric`’, ‘`size`’, ‘`mtime`’, ‘`atime`’, ‘`ctime`’, and ‘`blocks`’, which sort the files on name, names in numeric rather than lexicographic order, file size, modification time, access time, inode change time, and number of blocks, respectively. If any of the non-name keys compare as equal (e.g., if two files are the same size), sorting uses the name as a secondary sort key.

  For example, a value of `-mtime` sorts the results in descending order by modification time (newest first).

  The ‘`numeric`’ specifier treats names consisting solely of digits as numbers and sorts them using their numeric value (so “2” sorts before “10”, for example). When using ‘`numeric`’, names containing non-digits sort after all the all-digit names and are sorted by name using the traditional behavior.

  A sort specifier of ‘`nosort`’ disables sorting completely; Bash returns the results in the order they are read from the file system, ignoring any leading ‘`-`’.

  If the sort specifier is missing, it defaults to name, so a value of ‘`+`’ is equivalent to the null string, and a value of ‘`-`’ sorts by name in descending order.

  Any invalid value restores the historical sorting behavior.

`GROUPS` ¶

  An array variable containing the list of groups of which the current user is a member. Assignments to `GROUPS` have no effect. If `GROUPS` is unset, it loses its special properties, even if it is subsequently reset.

`histchars` ¶

  The two or three characters which control history expansion, quick substitution, and tokenization (see History Expansion). The first character is the history expansion character, the character which begins a history expansion, normally ‘`!`’. The second character is the quick substitution character, normally ‘`^`’. When it appears as the first character on the line, history substitution repeats the previous command, replacing one string with another. The optional third character is the history comment character, normally ‘`#`’, which indicates that the remainder of the line is a comment when it appears as the first character of a word. The history comment character disables history substitution for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.

`HISTCMD` ¶

  The history number, or index in the history list, of the current command. Assignments to `HISTCMD` have no effect. If `HISTCMD` is unset, it loses its special properties, even if it is subsequently reset.

`HISTCONTROL` ¶

  A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ‘`ignorespace`’, lines which begin with a space character are not saved in the history list. A value of ‘`ignoredups`’ causes lines which match the previous history entry not to be saved. A value of ‘`ignoreboth`’ is shorthand for ‘`ignorespace`’ and ‘`ignoredups`’. A value of ‘`erasedups`’ causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If `HISTCONTROL` is unset, or does not include a valid value, Bash saves all lines read by the shell parser on the history list, subject to the value of `HISTIGNORE`. If the first line of a multi-line compound command was saved, the second and subsequent lines are not tested, and are added to the history regardless of the value of `HISTCONTROL`. If the first line was not saved, the second and subsequent lines of the command are not saved either.

`HISTFILE` ¶

  The name of the file to which the command history is saved. Bash assigns a default value of `~/.bash_history`. If `HISTFILE` is unset or null, the shell does not save the command history when it exits.

`HISTFILESIZE` ¶

  The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines by removing the oldest entries. The history file is also truncated to this size after writing it when a shell exits or by the `history` builtin. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of `HISTSIZE` after reading any startup files.

`HISTIGNORE` ¶

  A colon-separated list of patterns used to decide which command lines should be saved on the history list. If a command line matches one of the patterns in the value of `HISTIGNORE`, it is not saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (Bash does not implicitly append a ‘`*`’). Each pattern is tested against the line after the checks specified by `HISTCONTROL` are applied. In addition to the normal shell pattern matching characters, ‘`&`’ matches the previous history line. A backslash escapes the ‘`&`’; the backslash is removed before attempting a match. If the first line of a multi-line compound command was saved, the second and subsequent lines are not tested, and are added to the history regardless of the value of `HISTIGNORE`. If the first line was not saved, the second and subsequent lines of the command are not saved either. The pattern matching honors the setting of the `extglob` shell option.

  `HISTIGNORE` subsumes some of the function of `HISTCONTROL`. A pattern of ‘`&`’ is identical to `ignoredups`, and a pattern of ‘`[ ]*`’ is identical to `ignorespace`. Combining these two patterns, separating them with a colon, provides the functionality of `ignoreboth`.

`HISTSIZE` ¶

  The maximum number of commands to remember on the history list. If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit). The shell sets the default value to 500 after reading any startup files.

`HISTTIMEFORMAT` ¶

  If this variable is set and not null, its value is used as a format string for `strftime`(3) to print the time stamp associated with each history entry displayed by the `history` builtin. If this variable is set, the shell writes time stamps to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines.

`HOSTFILE` ¶

  Contains the name of a file in the same format as `/etc/hosts` that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, Bash adds the contents of the new file to the existing list. If `HOSTFILE` is set, but has no value, or does not name a readable file, Bash attempts to read `/etc/hosts` to obtain the list of possible hostname completions. When `HOSTFILE` is unset, Bash clears the hostname list.

`HOSTNAME` ¶

  The name of the current host.

`HOSTTYPE` ¶

  A string describing the machine Bash is running on.

`IGNOREEOF` ¶

  Controls the action of the shell on receipt of an `EOF` character as the sole input. If set, the value is the number of consecutive `EOF` characters that can be read as the first character on an input line before Bash exits. If the variable is set but does not have a numeric value, or the value is null, then the default is 10. If the variable is unset, then `EOF` signifies the end of input to the shell. This is only in effect for interactive shells.

`INPUTRC` ¶

  The name of the Readline initialization file, overriding the default of `~/.inputrc`.

`INSIDE_EMACS` ¶

  If Bash finds this variable in the environment when the shell starts, it assumes that the shell is running in an Emacs shell buffer and may disable line editing depending on the value of `TERM`.

`LANG` ¶

  Used to determine the locale category for any category not specifically selected with a variable starting with `LC_`.

`LC_ALL` ¶

  This variable overrides the value of `LANG` and any other `LC_` variable specifying a locale category.

`LC_COLLATE` ¶

  This variable determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within filename expansion and pattern matching (see Filename Expansion).

`LC_CTYPE` ¶

  This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (see Filename Expansion).

`LC_MESSAGES` ¶

  This variable determines the locale used to translate double-quoted strings preceded by a ‘`$`’ (see Locale-Specific Translation).

`LC_NUMERIC` ¶

  This variable determines the locale category used for number formatting.

`LC_TIME` ¶

  This variable determines the locale category used for data and time formatting.

`LINENO` ¶

  The line number in the script or shell function currently executing. Line numbers start with 1. When not in a script or function, the value is not guaranteed to be meaningful. If `LINENO` is unset, it loses its special properties, even if it is subsequently reset.

`LINES` ¶

  Used by the `select` command to determine the column length for printing selection lists. Automatically set if the `checkwinsize` option is enabled (see The Shopt Builtin), or in an interactive shell upon receipt of a `SIGWINCH`.

`MACHTYPE` ¶

  A string that fully describes the system type on which Bash is executing, in the standard GNU cpu-company-system format.

`MAILCHECK` ¶

  How often (in seconds) that the shell should check for mail in the files specified in the `MAILPATH` or `MAIL` variables. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.

`MAPFILE` ¶

  An array variable created to hold the text read by the `mapfile` builtin when no variable name is supplied.

`OLDPWD` ¶

  The previous working directory as set by the `cd` builtin.

`OPTERR` ¶

  If set to the value 1, Bash displays error messages generated by the `getopts` builtin command. `OPTERR` is initialized to 1 each time the shell is invoked.

`OSTYPE` ¶

  A string describing the operating system Bash is running on.

`PIPESTATUS` ¶

  An array variable (see Arrays) containing a list of exit status values from the commands in the most-recently-executed foreground pipeline, which may consist of only a simple command (see Shell Commands). Bash sets `PIPESTATUS` after executing multi-element pipelines, timed and negated pipelines, simple commands, subshells created with the ‘`(`’ operator, the `[[` and `((` compound commands, and after error conditions that result in the shell aborting command execution.

`POSIXLY_CORRECT` ¶

  If this variable is in the environment when Bash starts, the shell enters POSIX mode (see Bash and POSIX) before reading the startup files, as if the `--posix` invocation option had been supplied. If it is set while the shell is running, Bash enables POSIX mode, as if the command

      set -o posix

  had been executed. When the shell enters POSIX mode, it sets this variable if it was not already set.

`PPID` ¶

  The process ID of the shell’s parent process. This variable is readonly.

`PROMPT_COMMAND` ¶

  If this variable is set, and is an array, the value of each set element is interpreted as a command to execute before printing the primary prompt (`$PS1`). If this is set but not an array variable, its value is used as a command to execute instead.

`PROMPT_DIRTRIM` ¶

  If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the `\w` and `\W` prompt string escapes (see Controlling the Prompt). Characters removed are replaced with an ellipsis.

`PS0` ¶

  The value of this parameter is expanded like `PS1` and displayed by interactive shells after reading a command and before the command is executed.

`PS3` ¶

  The value of this variable is used as the prompt for the `select` command. If this variable is not set, the `select` command prompts with ‘`#?` ’

`PS4` ¶

  The value of this parameter is expanded like `PS1` and the expanded value is the prompt printed before the command line is echoed when the `-x` option is set (see The Set Builtin). The first character of the expanded value is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is ‘`+` ’.

`PWD` ¶

  The current working directory as set by the `cd` builtin.

`RANDOM` ¶

  Each time this parameter is referenced, it expands to a random integer between 0 and 32767. Assigning a value to `RANDOM` initializes (seeds) the sequence of random numbers. Seeding the random number generator with the same constant value produces the same sequence of values. If `RANDOM` is unset, it loses its special properties, even if it is subsequently reset.

`READLINE_ARGUMENT` ¶

  Any numeric argument given to a Readline command that was defined using ‘`bind -x`’ (see Bash Builtin Commands when it was invoked.

`READLINE_LINE` ¶

  The contents of the Readline line buffer, for use with ‘`bind -x`’ (see Bash Builtin Commands).

`READLINE_MARK` ¶

  The position of the mark (saved insertion point) in the Readline line buffer, for use with ‘`bind -x`’ (see Bash Builtin Commands). The characters between the insertion point and the mark are often called the region.

`READLINE_POINT` ¶

  The position of the insertion point in the Readline line buffer, for use with ‘`bind -x`’ (see Bash Builtin Commands).

`REPLY` ¶

  The default variable for the `read` builtin; set to the line read when `read` is not supplied a variable name argument.

`SECONDS` ¶

  This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment. The number of seconds at shell invocation and the current time are always determined by querying the system clock at one-second resolution. If `SECONDS` is unset, it loses its special properties, even if it is subsequently reset.

`SHELL` ¶

  This environment variable expands to the full pathname to the shell. If it is not set when the shell starts, Bash assigns to it the full pathname of the current user’s login shell.

`SHELLOPTS` ¶

  A colon-separated list of enabled shell options. Each word in the list is a valid argument for the `-o` option to the `set` builtin command (see The Set Builtin). The options appearing in `SHELLOPTS` are those reported as ‘`on`’ by ‘`set -o`’. If this variable is in the environment when Bash starts up, the shell enables each option in the list before reading any startup files. If this variable is exported, child shells will enable each option in the list. This variable is readonly.

`SHLVL` ¶

  Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.

`SRANDOM` ¶

  This variable expands to a 32-bit pseudo-random number each time it is referenced. The random number generator is not linear on systems that support `/dev/urandom` or `arc4random`, so each returned number has no relationship to the numbers preceding it. The random number generator cannot be seeded, so assignments to this variable have no effect. If `SRANDOM` is unset, it loses its special properties, even if it is subsequently reset.

`TIMEFORMAT` ¶

  The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the `time` reserved word should be displayed. The ‘`%`’ character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the brackets denote optional portions.

  `%%`

    A literal ‘`%`’.

  `%[p][l]R`

    The elapsed time in seconds.

  `%[p][l]U`

    The number of CPU seconds spent in user mode.

  `%[p][l]S`

    The number of CPU seconds spent in system mode.

  `%P`

    The CPU percentage, computed as (%U + %S) / %R.

  The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. `time` prints at most six digits after the decimal point; values of p greater than 6 are changed to 6. If p is not specified, `time` prints three digits after the decimal point.

  The optional `l` specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.

  If this variable is not set, Bash acts as if it had the value

      $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'

  If the value is null, Bash does not display any timing information. A trailing newline is added when the format string is displayed.

`TMOUT` ¶

  If set to a value greater than zero, the `read` builtin uses the value as its default timeout (see Bash Builtin Commands). The `select` command (see Conditional Constructs) terminates if input does not arrive after `TMOUT` seconds when input is coming from a terminal.

  In an interactive shell, the value is interpreted as the number of seconds to wait for a line of input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if a complete line of input does not arrive.

`TMPDIR` ¶

  If set, Bash uses its value as the name of a directory in which Bash creates temporary files for the shell’s use.

`UID` ¶

  The numeric real user id of the current user. This variable is readonly.

## 6 Bash Features ¶

This chapter describes features unique to Bash.

### 6.1 Invoking Bash ¶

    bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option]
        [-O shopt_option] [argument ...]
    bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option]
        [-O shopt_option] -c string [argument ...]
    bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option]
        [-O shopt_option] [argument ...]

All of the single-character options used with the `set` builtin (see The Set Builtin) can be used as options when the shell is invoked. In addition, there are several multi-character options that you can use. These options must appear on the command line before the single-character options to be recognized.

`--debugger`

  Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see The Shopt Builtin for a description of the `extdebug` option to the `shopt` builtin).

`--dump-po-strings`

  Print a list of all double-quoted strings preceded by ‘`$`’ on the standard output in the GNU `gettext` PO (portable object) file format. Equivalent to `-D` except for the output format.

`--dump-strings`

  Equivalent to `-D`.

`--help`

  Display a usage message on standard output and exit successfully.

`--init-file filename`
`--rcfile filename`

  Execute commands from filename (instead of `~/.bashrc`) in an interactive shell.

`--login`

  Equivalent to `-l`.

`--noediting`

  Do not use the GNU Readline library (see Command Line Editing) to read command lines when the shell is interactive.

`--noprofile`

  Don’t load the system-wide startup file `/etc/profile` or any of the personal initialization files `~/.bash_profile`, `~/.bash_login`, or `~/.profile` when Bash is invoked as a login shell.

`--norc`

  Don’t read the `~/.bashrc` initialization file in an interactive shell. This is on by default if the shell is invoked as `sh`.

`--posix`

  Enable POSIX mode; change the behavior of Bash where the default operation differs from the POSIX standard to match the standard. This is intended to make Bash behave as a strict superset of that standard. See Bash and POSIX, for a description of the Bash POSIX mode.

`--restricted`

  Equivalent to `-r`. Make the shell a restricted shell (see The Restricted Shell).

`--verbose`

  Equivalent to `-v`. Print shell input lines as they’re read.

`--version`

  Show version information for this instance of Bash on the standard output and exit successfully.

There are several single-character options that may be supplied at invocation which are not available with the `set` builtin.

`-c`

  Read and execute commands from the first non-option argument command_string, then exit. If there are arguments after the command_string, the first argument is assigned to `$0` and any remaining arguments are assigned to the positional parameters. The assignment to `$0` sets the name of the shell, which is used in warning and error messages.

`-i`

  Force the shell to run interactively. Interactive shells are described in Interactive Shells.

`-l`

  Make this shell act as if it had been directly invoked by login. When the shell is interactive, this is equivalent to starting a login shell with ‘`exec -l bash`’. When the shell is not interactive, it will read and execute the login shell startup files. ‘`exec bash -l`’ or ‘`exec bash --login`’ will replace the current shell with a Bash login shell. See Bash Startup Files, for a description of the special behavior of a login shell.

`-r`

  Make the shell a restricted shell (see The Restricted Shell).

`-s`

  If this option is present, or if no arguments remain after option processing, then Bash reads commands from the standard input. This option allows the positional parameters to be set when invoking an interactive shell or when reading input through a pipe.

`-D`

  Print a list of all double-quoted strings preceded by ‘`$`’ on the standard output. These are the strings that are subject to language translation when the current locale is not `C` or `POSIX` (see Locale-Specific Translation). This implies the `-n` option; no commands will be executed.

`[-+]O [shopt_option]`

  shopt_option is one of the shell options accepted by the `shopt` builtin (see The Shopt Builtin). If shopt_option is present, `-O` sets the value of that option; `+O` unsets it. If shopt_option is not supplied, Bash prints the names and values of the shell options accepted by `shopt` on the standard output. If the invocation option is `+O`, the output is displayed in a format that may be reused as input.

`--`

  A `--` signals the end of options and disables further option processing. Any arguments after the `--` are treated as a shell script filename (see Shell Scripts) and arguments passed to that script.

`-`

  Equivalent to `--`.

A login shell is one whose first character of argument zero is ‘`-`’, or one invoked with the `--login` option.

An interactive shell is one started without non-option arguments, unless `-s` is specified, without specifying the `-c` option, and whose standard input and standard error are both connected to terminals (as determined by isatty(3)), or one started with the `-i` option. See Interactive Shells, for more information.

If arguments remain after option processing, and neither the `-c` nor the `-s` option has been supplied, the first argument is treated as the name of a file containing shell commands (see Shell Scripts). When Bash is invoked in this fashion, `$0` is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash’s exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0. Bash first attempts to open the file in the current directory, and, if no file is found, searches the directories in `PATH` for the script.

### 6.2 Bash Startup Files ¶

This section describes how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in filenames as described above under Tilde Expansion (see Tilde Expansion).

Interactive shells are described in Interactive Shells.

#### Invoked as an interactive login shell, or with `--login` ¶

When Bash is invoked as an interactive login shell, or as a non-interactive shell with the `--login` option, it first reads and executes commands from the file `/etc/profile`, if that file exists. After reading that file, it looks for `~/.bash_profile`, `~/.bash_login`, and `~/.profile`, in that order, and reads and executes commands from the first one that exists and is readable. The `--noprofile` option inhibits this behavior.

When an interactive login shell exits, or a non-interactive login shell executes the `exit` builtin command, Bash reads and executes commands from the file `~/.bash_logout`, if it exists.

#### Invoked as an interactive non-login shell ¶

When Bash runs as an interactive shell that is not a login shell, it reads and executes commands from `~/.bashrc`, if that file exists. The `--norc` option inhibits this behavior. The `--rcfile file` option causes Bash to use file instead of `~/.bashrc`.

So, typically, your `~/.bash_profile` contains the line

    if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

after (or before) any login-specific initializations.

#### Invoked non-interactively ¶

When Bash is started non-interactively, to run a shell script, for example, it looks for the variable `BASH_ENV` in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:

    if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but does not the value of the `PATH` variable to search for the filename.

As noted above, if a non-interactive shell is invoked with the `--login` option, Bash attempts to read and execute commands from the login shell startup files.

#### Invoked with name `sh` ¶

If Bash is invoked with the name `sh`, it tries to mimic the startup behavior of historical versions of `sh` as closely as possible, while conforming to the POSIX standard as well.

When invoked as an interactive login shell, or as a non-interactive shell with the `--login` option, it first attempts to read and execute commands from `/etc/profile` and `~/.profile`, in that order. The `--noprofile` option inhibits this behavior.

When invoked as an interactive shell with the name `sh`, Bash looks for the variable `ENV`, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as `sh` does not attempt to read and execute commands from any other startup files, the `--rcfile` option has no effect.

A non-interactive shell invoked with the name `sh` does not attempt to read any other startup files.

When invoked as `sh`, Bash enters POSIX mode after reading the startup files.

#### Invoked in POSIX mode ¶

When Bash is started in POSIX mode, as with the `--posix` command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the `ENV` variable and read and execute commands from the file whose name is the expanded value. No other startup files are read.

#### Invoked by remote shell daemon ¶

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the historical and rarely-seen remote shell daemon, usually `rshd`, or the secure shell daemon `sshd`. If Bash determines it is being run non-interactively in this fashion, it reads and executes commands from `~/.bashrc`, if that file exists and is readable. Bash does not read this file if invoked as `sh`. The `--norc` option inhibits this behavior, and the `--rcfile` option makes Bash use a different file instead of `~/.bashrc`, but neither `rshd` nor `sshd` generally invoke the shell with those options or allow them to be specified.

#### Invoked with unequal effective and real UID/GIDs ¶

If Bash is started with the effective user (group) id not equal to the real user (group) id, and the `-p` option is not supplied, no startup files are read, shell functions are not inherited from the environment, the `SHELLOPTS`, `BASHOPTS`, `CDPATH`, and `GLOBIGNORE` variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the `-p` option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.

### 6.3 Interactive Shells ¶

#### 6.3.1 What is an Interactive Shell? ¶

An interactive shell is one started without non-option arguments (unless `-s` is specified) and without specifying the `-c` option, whose input and error output are both connected to terminals (as determined by `isatty(3)`), or one started with the `-i` option.

An interactive shell generally reads from and writes to a user’s terminal.

The `-s` invocation option may be used to set the positional parameters when an interactive shell starts.

#### 6.3.2 Is this Shell Interactive? ¶

To determine within a startup script whether or not Bash is running interactively, test the value of the ‘`-`’ special parameter. It contains `i` when the shell is interactive. For example:

    case "$-" in
    *i*)	echo This shell is interactive ;;
    *)	echo This shell is not interactive ;;
    esac

Alternatively, startup scripts may examine the variable `PS1`; it is unset in non-interactive shells, and set in interactive shells. Thus:

    if [ -z "$PS1" ]; then
            echo This shell is not interactive
    else
            echo This shell is interactive
    fi

#### 6.3.3 Interactive Shell Behavior ¶

When the shell is running interactively, it changes its behavior in several ways.

1. Bash reads and executes startup files as described in Bash Startup Files.
2. Job Control (see Job Control) is enabled by default. When job control is in effect, Bash ignores the keyboard-generated job control signals `SIGTTIN`, `SIGTTOU`, and `SIGTSTP`.
3. Bash executes the values of the set elements of the `PROMPT_COMMAND` array variable as commands before printing the primary prompt, `$PS1` (see Bash Variables).
4. Bash expands and displays `PS1` before reading the first line of a command, and expands and displays `PS2` before reading the second and subsequent lines of a multi-line command. Bash expands and displays `PS0` after it reads a command but before executing it. See Controlling the Prompt, for a complete list of prompt string escape sequences.
5. Bash uses Readline (see Command Line Editing) to read commands from the user’s terminal.
6. Bash inspects the value of the `ignoreeof` option to `set -o` instead of exiting immediately when it receives an `EOF` on its standard input when reading a command (see The Set Builtin).
7. Bash enables Command history (see Bash History Facilities) and history expansion (see History Expansion) by default. When a shell with history enabled exits, Bash saves the command history to the file named by `$HISTFILE`.
8. Alias expansion (see Aliases) is performed by default.
9. In the absence of any traps, Bash ignores `SIGTERM` (see Signals).
10. In the absence of any traps, `SIGINT` is caught and handled (see Signals). `SIGINT` will interrupt some shell builtins.
11. An interactive login shell sends a `SIGHUP` to all jobs on exit if the `huponexit` shell option has been enabled (see Signals).
12. The `-n` option has no effect, whether at invocation or when using ‘`set -n`’ (see The Set Builtin).
13. Bash will check for mail periodically, depending on the values of the `MAIL`, `MAILPATH`, and `MAILCHECK` shell variables (see Bash Variables).
14. The shell will not exit on expansion errors due to references to unbound shell variables after ‘`set -u`’ has been enabled (see The Set Builtin).
15. The shell will not exit on expansion errors caused by var being unset or null in `${var:?word}` expansions (see Shell Parameter Expansion).
16. Redirection errors encountered by shell builtins will not cause the shell to exit.
17. When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see Bash and POSIX).
18. A failed `exec` will not cause the shell to exit (see Bourne Shell Builtins).
19. Parser syntax errors will not cause the shell to exit.
20. If the `cdspell` shell option is enabled, the shell will attempt simple spelling correction for directory arguments to the `cd` builtin (see the description of the `cdspell` option to the `shopt` builtin in The Shopt Builtin). The `cdspell` option is only effective in interactive shells.
21. The shell will check the value of the `TMOUT` variable and exit if a command is not read within the specified number of seconds after printing `$PS1` (see Bash Variables).

### 6.4 Bash Conditional Expressions ¶

Conditional expressions are used by the `[[` compound command (see Conditional Constructs) and the `test` and `[` builtin commands (see Bourne Shell Builtins). The `test` and `[` commands determine their behavior based on the number of arguments; see the descriptions of those commands for any other command-specific actions.

Expressions may be unary or binary, and are formed from the primaries listed below. Unary expressions are often used to examine the status of a file or shell variable. Binary operators are used for string, numeric, and file attribute comparisons.

Bash handles several filenames specially when they are used in expressions. If the operating system on which Bash is running provides these special files, Bash uses them; otherwise it emulates them internally with this behavior: If the file argument to one of the primaries is of the form `/dev/fd/N`, then Bash checks file descriptor N. If the file argument to one of the primaries is one of `/dev/stdin`, `/dev/stdout`, or `/dev/stderr`, Bash checks file descriptor 0, 1, or 2, respectively.

When used with `[[`, the ‘`<`’ and ‘`>`’ operators sort lexicographically using the current locale. The `test` command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.

`-a file`

  True if file exists.

`-b file`

  True if file exists and is a block special file.

`-c file`

  True if file exists and is a character special file.

`-d file`

  True if file exists and is a directory.

`-e file`

  True if file exists.

`-f file`

  True if file exists and is a regular file.

`-g file`

  True if file exists and its set-group-id bit is set.

`-h file`

  True if file exists and is a symbolic link.

`-k file`

  True if file exists and its "sticky" bit is set.

`-p file`

  True if file exists and is a named pipe (FIFO).

`-r file`

  True if file exists and is readable.

`-s file`

  True if file exists and has a size greater than zero.

`-t fd`

  True if file descriptor fd is open and refers to a terminal.

`-u file`

  True if file exists and its set-user-id bit is set.

`-w file`

  True if file exists and is writable.

`-x file`

  True if file exists and is executable.

`-G file`

  True if file exists and is owned by the effective group id.

`-L file`

  True if file exists and is a symbolic link.

`-N file`

  True if file exists and has been modified since it was last accessed.

`-O file`

  True if file exists and is owned by the effective user id.

`-S file`

  True if file exists and is a socket.

`file1 -ef file2`

  True if file1 and file2 refer to the same device and inode numbers.

`file1 -nt file2`

  True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

`file1 -ot file2`

  True if file1 is older than file2, or if file2 exists and file1 does not.

`-o optname`

  True if the shell option optname is enabled. The list of options appears in the description of the `-o` option to the `set` builtin (see The Set Builtin).

`-v varname`

  True if the shell variable varname is set (has been assigned a value). If varname is an indexed array variable name subscripted by ‘`@`’ or ‘`*`’, this returns true if the array has any set elements. If varname is an associative array variable name subscripted by ‘`@`’ or ‘`*`’, this returns true if an element with that key is set.

`-R varname`

  True if the shell variable varname is set and is a name reference.

`-z string`

  True if the length of string is zero.

`-n string`
`string`

  True if the length of string is non-zero.

`string1 == string2`
`string1 = string2`

  True if the strings are equal. When used with the `[[` command, this performs pattern matching as described above (see Conditional Constructs).

  ‘`=`’ should be used with the `test` command for POSIX conformance.

`string1 != string2`

  True if the strings are not equal.

`string1 < string2`

  True if string1 sorts before string2 lexicographically.

`string1 > string2`

  True if string1 sorts after string2 lexicographically.

`arg1 OP arg2`

  `OP` is one of ‘`-eq`’, ‘`-ne`’, ‘`-lt`’, ‘`-le`’, ‘`-gt`’, or ‘`-ge`’. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers. When used with the `[[` command, arg1 and arg2 are evaluated as arithmetic expressions (see Shell Arithmetic). Since the expansions the `[[` command performs on arg1 and arg2 can potentially result in empty strings, arithmetic expression evaluation treats those as expressions that evaluate to 0.

### 6.5 Shell Arithmetic ¶

The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by using the `((` compound command, the `let` and `declare` builtins, the arithmetic `for` command, the `[[` conditional command, or the `-i` option to the `declare` builtin.

Evaluation is done in the largest fixed-width integers available, with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.

`id++ id--`

  variable post-increment and post-decrement

`++id --id`

  variable pre-increment and pre-decrement

`- +`

  unary minus and plus

`! ~`

  logical and bitwise negation

`**`

  exponentiation

`* / %`

  multiplication, division, remainder

`+ -`

  addition, subtraction

`<< >>`

  left and right bitwise shifts

`<= >= < >`

  comparison

`== !=`

  equality and inequality

`&`

  bitwise AND

`^`

  bitwise exclusive OR

`|`

  bitwise OR

`&&`

  logical AND

`||`

  logical OR

`expr ? if-true-expr : if-false-expr`

  conditional operator

`= *= /= %= += -= <<= >>= &= ^= |=`

  assignment

`expr1 , expr2`

  comma

Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. This means you can use x, where x is a shell variable name, in an arithmetic expression, and the shell will evaluate its value as an expression and use the result. A shell variable that is null or unset evaluates to 0 when referenced by name in an expression.

The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the `integer` attribute using ‘`declare -i`’ is assigned a value. A null value evaluates to 0. A shell variable need not have its `integer` attribute turned on to be used in an expression.

Integer constants follow the C language definition, without suffixes or character constants. Constants with a leading 0 are interpreted as octal numbers. A leading ‘`0x`’ or ‘`0X`’ denotes hexadecimal. Otherwise, numbers take the form [base`#`]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base`#` is omitted, then base 10 is used. When specifying n, if a non-digit is required, the digits greater than 9 are represented by the lowercase letters, the uppercase letters, ‘`@`’, and ‘`_`’, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.

Operators are evaluated in precedence order. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.

### 6.6 Aliases ¶

Aliases allow a string to be substituted for a word that is in a position in the input where it can be the first word of a simple command. Aliases have names and corresponding values that are set and unset using the `alias` and `unalias` builtin commands (see Shell Builtin Commands).

If the shell reads an unquoted word in the right position, it checks the word to see if it matches an alias name. If it matches, the shell replaces the word with the alias value, and reads that value as if it had been read instead of the word. The shell doesn’t look at any characters following the word before attempting alias substitution.

The characters ‘`/`’, ‘`$`’, ‘```’, ‘`=`’ and any of the shell metacharacters or quoting characters listed above may not appear in an alias name. The replacement text may contain any valid shell input, including shell metacharacters. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias `ls` to `"ls -F"`, for instance, and Bash does not try to recursively expand the replacement text.

If the last character of the alias value is a `blank`, then the shell checks the next command word following the alias for alias expansion.

Aliases are created and listed with the `alias` command, and removed with the `unalias` command.

There is no mechanism for using arguments in the replacement text, as in `csh`. If arguments are needed, use a shell function (see Shell Functions) instead.

Aliases are not expanded when the shell is not interactive, unless the `expand_aliases` shell option is set using `shopt` (see The Shopt Builtin).

The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one complete line of input, and all lines that make up a compound command, before executing any of the commands on that line or the compound command. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the shell reads the next line of input, and an alias definition in a compound command does not take effect until the shell parses and executes the entire compound command. The commands following the alias definition on that line, or in the rest of a compound command, are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use `alias` in compound commands.

For almost every purpose, shell functions are preferable to aliases.

### 6.7 Arrays ¶

Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the `declare` builtin explicitly declares an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Indexed arrays are referenced using arithmetic expressions that must expand to an integer (see Shell Arithmetic)) and are zero-based; associative arrays use arbitrary strings. Unless otherwise noted, indexed array indices must be non-negative integers.

The shell performs parameter and variable expansion, arithmetic expansion, command substitution, and quote removal on indexed array subscripts. Since this can potentially result in empty strings, subscript indexing treats those as expressions that evaluate to 0.

The shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, and quote removal on associative array subscripts. Empty strings cannot be used as associative array keys.

Bash automatically creates an indexed array if any variable is assigned to using the syntax

    name[subscript]=value

The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero. To explicitly declare an indexed array, use

    declare -a name

(see Bash Builtin Commands). The syntax

    declare -a name[subscript]

is also accepted; the subscript is ignored.

Associative arrays are created using

    declare -A name

Attributes may be specified for an array variable using the `declare` and `readonly` builtins. Each attribute applies to all members of an array.

Arrays are assigned using compound assignments of the form

    name=(value1 value2 ... )

where each value may be of the form `[subscript]=`string. Indexed array assignments do not require anything but string.

Each value in the list undergoes the shell expansions described above (see Shell Expansions), but values that are valid variable assignments including the brackets and subscript do not undergo brace expansion and word splitting, as with individual variable assignments.

When assigning to indexed arrays, if the optional subscript is supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero.

When assigning to an associative array, the words in a compound assignment may be either assignment statements, for which the subscript is required, or a list of words that is interpreted as a sequence of alternating keys and values: name=(key1 value1 key2 value2 … ). These are treated identically to name=( [key1]=value1 [key2]=value2 … ). The first word in the list determines how the remaining words are interpreted; all assignments in a list must be of the same type. When using key/value pairs, the keys may not be missing or empty; a final missing value is treated like the empty string.

This syntax is also accepted by the `declare` builtin. Individual array elements may be assigned to using the `name[subscript]=value` syntax introduced above.

When assigning to an indexed array, if name is subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of name, so negative indices count back from the end of the array, and an index of -1 references the last element.

The ‘`+=`’ operator appends to an array variable when assigning using the compound assignment syntax; see Shell Parameters above.

An array element is referenced using `${name[subscript]}`. The braces are required to avoid conflicts with the shell’s filename expansion operators. If the subscript is ‘`@`’ or ‘`*`’, the word expands to all members of the array name, unless otherwise noted in the description of a builtin or word expansion. These subscripts differ only when the word appears within double quotes. If the word is double-quoted, `${name[*]}` expands to a single word with the value of each array member separated by the first character of the `IFS` variable, and `${name[@]}` expands each element of name to a separate word. When there are no array members, `${name[@]}` expands to nothing. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the expansion of the original word, and the expansion of the last parameter is joined with the last part of the expansion of the original word. This is analogous to the expansion of the special parameters ‘`@`’ and ‘`*`’.

`${#name[subscript]}` expands to the length of `${name[subscript]}`. If subscript is ‘`@`’ or ‘`*`’, the expansion is the number of elements in the array.

If the subscript used to reference an element of an indexed array evaluates to a number less than zero, it is interpreted as relative to one greater than the maximum index of the array, so negative indices count back from the end of the array, and an index of -1 refers to the last element.

Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0. Any reference to a variable using a valid subscript is valid; Bash creates an array if necessary.

An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.

It is possible to obtain the keys (indices) of an array as well as the values. ${!name[@]} and ${!name[*]} expand to the indices assigned in array variable name. The treatment when in double quotes is similar to the expansion of the special parameters ‘`@`’ and ‘`*`’ within double quotes.

The `unset` builtin is used to destroy arrays. `unset name[subscript]` unsets the array element at index subscript. Negative subscripts to indexed arrays are interpreted as described above. Unsetting the last element of an array variable does not unset the variable. `unset name`, where name is an array, removes the entire array. `unset name[subscript]` behaves differently depending on the array type when subscript is ‘`*`’ or ‘`@`’. When name is an associative array, it removes the element with key ‘`*`’ or ‘`@`’. If name is an indexed array, `unset` removes all of the elements, but does not remove the array itself.

When using a variable name with a subscript as an argument to a command, such as with `unset`, without using the word expansion syntax described above (e.g., unset a[4]), the argument is subject to the shell’s filename expansion. Quote the argument if pathname expansion is not desired (e.g., unset ’a[4]’).

The `declare`, `local`, and `readonly` builtins each accept a `-a` option to specify an indexed array and a `-A` option to specify an associative array. If both options are supplied, `-A` takes precedence. The `read` builtin accepts a `-a` option to assign a list of words read from the standard input to an array, and can read values from the standard input into individual array elements. The `set` and `declare` builtins display array values in a way that allows them to be reused as input. Other builtins accept array name arguments as well (e.g., `mapfile`); see the descriptions of individual builtins for details. The shell provides a number of builtin array variables.

### 6.8 The Directory Stack ¶

The directory stack is a list of recently-visited directories. The `pushd` builtin adds directories to the stack as it changes the current directory, and the `popd` builtin removes specified directories from the stack and changes the current directory to the directory removed. The `dirs` builtin displays the contents of the directory stack. The current directory is always the "top" of the directory stack.

The contents of the directory stack are also visible as the value of the `DIRSTACK` shell variable.

#### 6.8.1 Directory Stack Builtins ¶

`dirs` ¶

      dirs [-clpv] [+N | -N]

  Without options, display the list of currently remembered directories. Directories are added to the list with the `pushd` command; the `popd` command removes directories from the list. The current directory is always the first directory in the stack.

  Options, if supplied, have the following meanings:

  `-c`

    Clears the directory stack by deleting all of the elements.

  `-l`

    Produces a listing using full pathnames; the default listing format uses a tilde to denote the home directory.

  `-p`

    Causes `dirs` to print the directory stack with one entry per line.

  `-v`

    Causes `dirs` to print the directory stack with one entry per line, prefixing each entry with its index in the stack.

  `+N`

    Displays the Nth directory (counting from the left of the list printed by `dirs` when invoked without options), starting with zero.

  `-N`

    Displays the Nth directory (counting from the right of the list printed by `dirs` when invoked without options), starting with zero.

`popd` ¶

      popd [-n] [+N | -N]

  Remove elements from the directory stack. The elements are numbered from 0 starting at the first directory listed by `dirs`; that is, `popd` is equivalent to `popd +0`.

  When no arguments are given, `popd` removes the top directory from the stack and changes to the new top directory.

  Arguments, if supplied, have the following meanings:

  `-n`

    Suppress the normal change of directory when removing directories from the stack, only manipulate the stack.

  `+N`

    Remove the Nth directory (counting from the left of the list printed by `dirs`), starting with zero, from the stack.

  `-N`

    Remove the Nth directory (counting from the right of the list printed by `dirs`), starting with zero, from the stack.

  If the top element of the directory stack is modified, and the `-n` option was not supplied, `popd` uses the `cd` builtin to change to the directory at the top of the stack. If the `cd` fails, `popd` returns a non-zero value.

  Otherwise, `popd` returns an unsuccessful status if an invalid option is specified, the directory stack is empty, or N specifies a non-existent directory stack entry.

  If the `popd` command is successful, Bash runs `dirs` to show the final contents of the directory stack, and the return status is 0.

`pushd` ¶

      pushd [-n] [+N | -N | dir]

  Add a directory to the top of the directory stack, or rotate the stack, making the new top of the stack the current working directory. With no arguments, `pushd` exchanges the top two elements of the directory stack.

  Arguments, if supplied, have the following meanings:

  `-n`

    Suppress the normal change of directory when rotating or adding directories to the stack, only manipulate the stack.

  `+N`

    Rotate the stack so that the Nth directory (counting from the left of the list printed by `dirs`, starting with zero) is at the top.

  `-N`

    Rotate the stack so that the Nth directory (counting from the right of the list printed by `dirs`, starting with zero) is at the top.

  `dir`

    Make dir be the top of the stack.

  After the stack has been modified, if the `-n` option was not supplied, `pushd` uses the `cd` builtin to change to the directory at the top of the stack. If the `cd` fails, `pushd` returns a non-zero value.

  Otherwise, if no arguments are supplied, `pushd` returns zero unless the directory stack is empty. When rotating the directory stack, `pushd` returns zero unless the directory stack is empty or N specifies a non-existent directory stack element.

  If the `pushd` command is successful, Bash runs `dirs` to show the final contents of the directory stack.

### 6.9 Controlling the Prompt ¶

In addition, the following table describes the special characters which can appear in the prompt variables `PS0`, `PS1`, `PS2`, and `PS4`:

`\a`

  A bell character.

`\d`

  The date, in "Weekday Month Date" format (e.g., "Tue May 26").

`\D{format}`

  The format is passed to `strftime`(3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation. The braces are required.

`\e`

  An escape character.

`\h`

  The hostname, up to the first ‘`.`’.

`\H`

  The hostname.

`\j`

  The number of jobs currently managed by the shell.

`\l`

  The basename of the shell’s terminal device name (e.g., "ttys0").

`\n`

  A newline.

`\r`

  A carriage return.

`\s`

  The name of the shell: the basename of `$0` (the portion following the final slash).

`\t`

  The time, in 24-hour HH:MM:SS format.

`\T`

  The time, in 12-hour HH:MM:SS format.

`\@`

  The time, in 12-hour am/pm format.

`\A`

  The time, in 24-hour HH:MM format.

`\u`

  The username of the current user.

`\v`

  The Bash version (e.g., 2.00).

`\V`

  The Bash release, version + patchlevel (e.g., 2.00.0).

`\w`

  The value of the `PWD` shell variable (`$PWD`), with `$HOME` abbreviated with a tilde (uses the `$PROMPT_DIRTRIM` variable).

`\W`

  The basename of `$PWD`, with `$HOME` abbreviated with a tilde.

`\!`

  The history number of this command.

`\#`

  The command number of this command.

`\$`

  If the effective uid is 0, `#`, otherwise `$`.

`\nnn`

  The character whose ASCII code is the octal value nnn.

`\\`

  A backslash.

`\[`

  Begin a sequence of non-printing characters. Thiss could be used to embed a terminal control sequence into the prompt.

`\]`

  End a sequence of non-printing characters.

The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see Bash History Facilities), while the command number is the position in the sequence of commands executed during the current shell session.

After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the `promptvars` shell option (see The Shopt Builtin). This can have unwanted side effects if escaped portions of the string appear within command substitution or contain characters special to word expansion.

### 6.10 The Restricted Shell ¶

If Bash is started with the name `rbash`, or the `--restricted` or `-r` option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. A restricted shell behaves identically to `bash` with the exception that the following are disallowed or not performed:

- Changing directories with the `cd` builtin.
- Setting or unsetting the values of the `SHELL`, `PATH`, `HISTFILE`, `ENV`, or `BASH_ENV` variables.
- Specifying command names containing slashes.
- Specifying a filename containing a slash as an argument to the `.` builtin command.
- Using the `-p` option to the `.` builtin command to specify a search path.
- Specifying a filename containing a slash as an argument to the `history` builtin command.
- Specifying a filename containing a slash as an argument to the `-p` option to the `hash` builtin command.
- Importing function definitions from the shell environment at startup.
- Parsing the value of `SHELLOPTS` from the shell environment at startup.
- Redirecting output using the ‘`>`’, ‘`>|`’, ‘`<>`’, ‘`>&`’, ‘`&>`’, and ‘`>>`’ redirection operators.
- Using the `exec` builtin to replace the shell with another command.
- Adding or deleting builtin commands with the `-f` and `-d` options to the `enable` builtin.
- Using the `enable` builtin command to enable disabled shell builtins.
- Specifying the `-p` option to the `command` builtin.
- Turning off restricted mode with ‘`set +r`’ or ‘`shopt -u restricted_shell`’.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (see Shell Scripts), `rbash` turns off any restrictions in the shell spawned to execute the script.

The restricted shell mode is only one component of a useful restricted environment. It should be accompanied by setting `PATH` to a value that allows execution of only a few verified commands (commands that allow shell escapes are particularly vulnerable), changing the current directory to a non-writable directory other than `$HOME` after login, not allowing the restricted shell to execute shell scripts, and cleaning the environment of variables that cause some commands to modify their behavior (e.g., `VISUAL` or `PAGER`).

Modern systems provide more secure ways to implement a restricted environment, such as `jails`, `zones`, or `containers`.

### 6.11 Bash and POSIX ¶

#### 6.11.1 What is POSIX? ¶

POSIX is the name for a family of standards based on Unix. A number of Unix services, tools, and functions are part of the standard, ranging from the basic system calls and C library functions to common applications and tools to system administration and management.

The POSIX Shell and Utilities standard was originally developed by IEEE Working Group 1003.2 (POSIX.2). The first edition of the 1003.2 standard was published in 1992. It was merged with the original IEEE 1003.1 Working Group and is currently maintained by the Austin Group (a joint working group of the IEEE, The Open Group and ISO/IEC SC22/WG15). Today the Shell and Utilities are a volume within the set of documents that make up IEEE Std 1003.1-2024, and thus the former POSIX.2 (from 1992) is now part of the current unified POSIX standard.

The Shell and Utilities volume concentrates on the command interpreter interface and utility programs commonly executed from the command line or by other programs. The standard is freely available on the web at https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html.

Bash is concerned with the aspects of the shell’s behavior defined by the POSIX Shell and Utilities volume. The shell command language has of course been standardized, including the basic flow control and program execution constructs, I/O redirection and pipelines, argument handling, variable expansion, and quoting.

The special builtins, which must be implemented as part of the shell to provide the desired functionality, are specified as being part of the shell; examples of these are `eval` and `export`. Other utilities appear in the sections of POSIX not devoted to the shell which are commonly (and in some cases must be) implemented as builtin commands, such as `read` and `test`. POSIX also specifies aspects of the shell’s interactive behavior, including job control and command line editing. Only vi-style line editing commands have been standardized; emacs editing commands were left out due to objections.

#### 6.11.2 Bash POSIX Mode ¶

Although Bash is an implementation of the POSIX shell specification, there are areas where the Bash default behavior differs from the specification. The Bash posix mode changes the Bash behavior in these areas so that it conforms more strictly to the standard.

Starting Bash with the `--posix` command-line option or executing ‘`set -o posix`’ while Bash is running will cause Bash to conform more closely to the POSIX standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.

When invoked as `sh`, Bash enters POSIX mode after reading the startup files.

The following list is what’s changed when POSIX mode is in effect:

1. Bash ensures that the `POSIXLY_CORRECT` variable is set.
2. Bash reads and executes the POSIX startup files (`$ENV`) rather than the normal Bash files (see Bash Startup Files.
3. Alias expansion is always enabled, even in non-interactive shells.
4. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.
5. Alias expansion is performed when initially parsing a command substitution. The default (non-posix) mode generally defers it, when enabled, until the command substitution is executed. This means that command substitution will not expand aliases that are defined after the command substitution is initially parsed (e.g., as part of a function definition).
6. The `time` reserved word may be used by itself as a simple command. When used in this way, it displays timing statistics for the shell and its completed children. The `TIMEFORMAT` variable controls the format of the timing information.
7. The parser does not recognize `time` as a reserved word if the next token begins with a ‘`-`’.
8. When parsing and expanding a ${…} expansion that appears within double quotes, single quotes are no longer special and cannot be used to quote a closing brace or other special character, unless the operator is one of those defined to perform pattern removal. In this case, they do not have to appear as matched pairs.
9. Redirection operators do not perform filename expansion on the word in a redirection unless the shell is interactive.
10. Redirection operators do not perform word splitting on the word in a redirection.
11. Function names may not be the same as one of the POSIX special builtins.
12. Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.
13. While variable indirection is available, it may not be applied to the ‘`#`’ and ‘`?`’ special parameters.
14. Expanding the ‘`*`’ special parameter in a pattern context where the expansion is double-quoted does not treat the `$*` as if it were double-quoted.
15. A double quote character (‘`"`’) is treated specially when it appears in a backquoted command substitution in the body of a here-document that undergoes expansion. That means, for example, that a backslash preceding a double quote character will escape it and the backslash will be removed.
16. Command substitutions don’t set the ‘`?`’ special parameter. The exit status of a simple command without a command word is still the exit status of the last command substitution that occurred while evaluating the variable assignments and redirections in that command, but that does not happen until after all of the assignments and redirections.
17. Literal tildes that appear as the first character in elements of the `PATH` variable are not expanded as described above under Tilde Expansion.
18. Command lookup finds POSIX special builtins before shell functions, including output printed by the `type` and `command` builtins.
19. Even if a shell function whose name contains a slash was defined before entering POSIX mode, the shell will not execute a function whose name contains one or more slashes.
20. When a command in the hash table no longer exists, Bash will re-search `$PATH` to find the new location. This is also available with ‘`shopt -s checkhash`’.
21. Bash will not insert a command without the execute bit set into the command hash table, even if it returns it as a (last-ditch) result from a `$PATH` search.
22. The message printed by the job control code and builtins when a job exits with a non-zero status is ‘Done(status)’.
23. The message printed by the job control code and builtins when a job is stopped is ‘Stopped(signame)’, where signame is, for example, `SIGTSTP`.
24. If the shell is interactive, Bash does not perform job notifications between executing commands in lists separated by ‘`;`’ or newline. Non-interactive shells print status messages after a foreground job in a list completes.
25. If the shell is interactive, Bash waits until the next prompt before printing the status of a background job that changes status or a foreground job that terminates due to a signal. Non-interactive shells print status messages after a foreground job completes.
26. Bash permanently removes jobs from the jobs table after notifying the user of their termination via the `wait` or `jobs` builtins. It removes the job from the jobs list after notifying the user of its termination, but the status is still available via `wait`, as long as `wait` is supplied a PID argument.
27. The `vi` editing mode will invoke the `vi` editor directly when the ‘`v`’ command is run, instead of checking `$VISUAL` and `$EDITOR`.
28. Prompt expansion enables the POSIX `PS1` and `PS2` expansions of ‘`!`’ to the history number and ‘`!!`’ to ‘`!`’, and Bash performs parameter expansion on the values of `PS1` and `PS2` regardless of the setting of the `promptvars` option.
29. The default history file is `~/.sh_history` (this is the default value the shell assigns to `$HISTFILE`).
30. The ‘`!`’ character does not introduce history expansion within a double-quoted string, even if the `histexpand` option is enabled.
31. When printing shell function definitions (e.g., by `type`), Bash does not print the `function` reserved word unless necessary.
32. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.
33. Non-interactive shells exit if a parameter expansion error occurs.
34. If a POSIX special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the POSIX standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on.
35. A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable.
36. A non-interactive shell exits with an error status if a variable assignment error occurs in an assignment statement preceding a special builtin, but not with any other simple command. For any other simple command, the shell aborts execution of that command, and execution continues at the top level ("the shell shall not perform any further processing of the command in which the error occurred").
37. A non-interactive shell exits with an error status if the iteration variable in a `for` statement or the selection variable in a `select` statement is a readonly variable or has an invalid name.
38. Non-interactive shells exit if filename in `.` filename is not found.
39. Non-interactive shells exit if there is a syntax error in a script read with the `.` or `source` builtins, or in a string processed by the `eval` builtin.
40. Non-interactive shells exit if the `export`, `readonly` or `unset` builtin commands get an argument that is not a valid identifier, and they are not operating on shell functions. These errors force an exit because these are special builtins.
41. Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.
42. The `command` builtin does not prevent builtins that take assignment statements as arguments from expanding them as assignment statements; when not in POSIX mode, declaration commands lose their assignment statement expansion properties when preceded by `command`.
43. Enabling POSIX mode has the effect of setting the `inherit_errexit` option, so subshells spawned to execute command substitutions inherit the value of the `-e` option from the parent shell. When the `inherit_errexit` option is not enabled, Bash clears the `-e` option in such subshells.
44. Enabling POSIX mode has the effect of setting the `shift_verbose` option, so numeric arguments to `shift` that exceed the number of positional parameters will result in an error message.
45. Enabling POSIX mode has the effect of setting the `interactive_comments` option (see Comments).
46. The `.` and `source` builtins do not search the current directory for the filename argument if it is not found by searching `PATH`.
47. When the `alias` builtin displays alias definitions, it does not display them with a leading ‘`alias` ’ unless the `-p` option is supplied.
48. The `bg` builtin uses the required format to describe each job placed in the background, which does not include an indication of whether the job is the current or previous job.
49. When the `cd` builtin is invoked in logical mode, and the pathname constructed from `$PWD` and the directory name supplied as an argument does not refer to an existing directory, `cd` will fail instead of falling back to physical mode.
50. When the `cd` builtin cannot change a directory because the length of the pathname constructed from `$PWD` and the directory name supplied as an argument exceeds `PATH_MAX` when canonicalized, `cd` will attempt to use the supplied directory name.
51. When the `xpg_echo` option is enabled, Bash does not attempt to interpret any arguments to `echo` as options. `echo` displays each argument after converting escape sequences.
52. The `export` and `readonly` builtin commands display their output in the format required by POSIX.
53. When listing the history, the `fc` builtin does not include an indication of whether or not a history entry has been modified.
54. The default editor used by `fc` is `ed`.
55. `fc` treats extra arguments as an error instead of ignoring them.
56. If there are too many arguments supplied to `fc -s`, `fc` prints an error message and returns failure.
57. The output of ‘`kill -l`’ prints all the signal names on a single line, separated by spaces, without the ‘`SIG`’ prefix.
58. The `kill` builtin does not accept signal names with a ‘`SIG`’ prefix.
59. The `kill` builtin returns a failure status if any of the pid or job arguments are invalid or if sending the specified signal to any of them fails. In default mode, `kill` returns success if the signal was successfully sent to any of the specified processes.
60. The `printf` builtin uses `double` (via `strtod`) to convert arguments corresponding to floating point conversion specifiers, instead of `long double` if it’s available. The ‘`L`’ length modifier forces `printf` to use `long double` if it’s available.
61. The `pwd` builtin verifies that the value it prints is the same as the current directory, even if it is not asked to check the file system with the `-P` option.
62. The `read` builtin may be interrupted by a signal for which a trap has been set. If Bash receives a trapped signal while executing `read`, the trap handler executes and `read` returns an exit status greater than 128.
63. When the `set` builtin is invoked without options, it does not display shell function names and definitions.
64. When the `set` builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.
65. The `test` builtin compares strings using the current locale when evaluating the ‘`<`’ and ‘`>`’ binary operators.
66. The `test` builtin’s `-t` unary primary requires an argument. Historical versions of `test` made the argument optional in certain cases, and Bash attempts to accommodate those for backwards compatibility.
67. The `trap` builtin displays signal names without the leading `SIG`.
68. The `trap` builtin doesn’t check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is, unless that argument consists solely of digits and is a valid signal number. If users want to reset the handler for a given signal to the original disposition, they should use ‘`-`’ as the first argument.
69. `trap -p` without arguments displays signals whose dispositions are set to SIG_DFL and those that were ignored when the shell started, not just trapped signals.
70. The `type` and `command` builtins will not report a non-executable file as having been found, though the shell will attempt to execute such a file if it is the only so-named file found in `$PATH`.
71. The `ulimit` builtin uses a block size of 512 bytes for the `-c` and `-f` options.
72. The `unset` builtin with the `-v` option specified returns a fatal error if it attempts to unset a `readonly` or `non-unsettable` variable, which causes a non-interactive shell to exit.
73. When asked to unset a variable that appears in an assignment statement preceding the command, the `unset` builtin attempts to unset a variable of the same name in the current or previous scope as well. This implements the required "if an assigned variable is further modified by the utility, the modifications made by the utility shall persist" behavior.
74. The arrival of `SIGCHLD` when a trap is set on `SIGCHLD` does not interrupt the `wait` builtin and cause it to return immediately. The trap command is run once for each child that exits.
75. Bash removes an exited background process’s status from the list of such statuses after the `wait` builtin returns it.

There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:

1. The `fc` builtin checks `$EDITOR` as a program to edit history entries if `FCEDIT` is unset, rather than defaulting directly to `ed`. `fc` uses `ed` if `EDITOR` is unset.
2. As noted above, Bash requires the `xpg_echo` option to be enabled for the `echo` builtin to be fully conformant.

Bash can be configured to be POSIX-conformant by default, by specifying the `--enable-strict-posix-default` to `configure` when building (see Optional Features).

### 6.12 Shell Compatibility Mode ¶

Bash-4.0 introduced the concept of a shell compatibility level, specified as a set of options to the shopt builtin (`compat31`, `compat32`, `compat40`, `compat41`, and so on). There is only one current compatibility level – each option is mutually exclusive. The compatibility level is intended to allow users to select behavior from previous versions that is incompatible with newer versions while they migrate scripts to use current features and behavior. It’s intended to be a temporary solution.

This section does not mention behavior that is standard for a particular version (e.g., setting `compat32` means that quoting the right hand side of the regexp matching operator quotes special regexp characters in the word, which is default behavior in bash-3.2 and subsequent versions).

If a user enables, say, `compat32`, it may affect the behavior of other compatibility levels up to and including the current compatibility level. The idea is that each compatibility level controls behavior that changed in that version of Bash, but that behavior may have been present in earlier versions. For instance, the change to use locale-based comparisons with the `[[` command came in bash-4.1, and earlier versions used ASCII-based comparisons, so enabling `compat32` will enable ASCII-based comparisons as well. That granularity may not be sufficient for all uses, and as a result users should employ compatibility levels carefully. Read the documentation for a particular feature to find out the current behavior.

Bash-4.3 introduced a new shell variable: `BASH_COMPAT`. The value assigned to this variable (a decimal version number like 4.2, or an integer corresponding to the `compat`NN option, like 42) determines the compatibility level.

Starting with bash-4.4, Bash began deprecating older compatibility levels. Eventually, the options will be removed in favor of `BASH_COMPAT`.

Bash-5.0 was the final version for which there was an individual shopt option for the previous version. `BASH_COMPAT` is the only mechanism to control the compatibility level in versions newer than bash-5.0.

The following table describes the behavior changes controlled by each compatibility level setting. The `compat`NN tag is used as shorthand for setting the compatibility level to NN using one of the following mechanisms. For versions prior to bash-5.0, the compatibility level may be set using the corresponding `compat`NN shopt option. For bash-4.3 and later versions, the `BASH_COMPAT` variable is preferred, and it is required for bash-5.1 and later versions.

`compat31`
  - Quoting the rhs of the `[[` command’s regexp matching operator (=~) has no special effect
`compat40`
  - The ‘`<`’ and ‘`>`’ operators to the `[[` command do not consider the current locale when comparing strings; they use ASCII ordering. Bash versions prior to bash-4.1 use ASCII collation and strcmp(3); bash-4.1 and later use the current locale’s collation sequence and strcoll(3).
`compat41`
  - In POSIX mode, `time` may be followed by options and still be recognized as a reserved word (this is POSIX interpretation 267).
  - In POSIX mode, the parser requires that an even number of single quotes occur in the word portion of a double-quoted ${…} parameter expansion and treats them specially, so that characters within the single quotes are considered quoted (this is POSIX interpretation 221).
`compat42`
  - The replacement string in double-quoted pattern substitution does not undergo quote removal, as it does in versions after bash-4.2.
  - In POSIX mode, single quotes are considered special when expanding the word portion of a double-quoted ${…} parameter expansion and can be used to quote a closing brace or other special character (this is part of POSIX interpretation 221); in later versions, single quotes are not special within double-quoted word expansions.
`compat43`
  - Word expansion errors are considered non-fatal errors that cause the current command to fail, even in POSIX mode (the default behavior is to make them fatal errors that cause the shell to exit).
  - When executing a shell function, the loop state (while/until/etc.) is not reset, so `break` or `continue` in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this.
`compat44`
  - The shell sets up the values used by `BASH_ARGV` and `BASH_ARGC` so they can expand to the shell’s positional parameters even if extended debugging mode is not enabled.
  - A subshell inherits loops from its parent context, so `break` or `continue` will cause the subshell to exit. Bash-5.0 and later reset the loop state to prevent the exit.
  - Variable assignments preceding builtins like `export` and `readonly` that set attributes continue to affect variables with the same name in the calling environment even if the shell is not in POSIX mode.
`compat50 (set using BASH_COMPAT)`
  - Bash-5.1 changed the way `$RANDOM` is generated to introduce slightly more randomness. If the shell compatibility level is set to 50 or lower, it reverts to the method from bash-5.0 and previous versions, so seeding the random number generator by assigning a value to `RANDOM` will produce the same sequence as in bash-5.0.
  - If the command hash table is empty, Bash versions prior to bash-5.1 printed an informational message to that effect, even when producing output that can be reused as input. Bash-5.1 suppresses that message when the `-l` option is supplied.
`compat51 (set using BASH_COMPAT)`
  - The `unset` builtin will unset the array `a` given an argument like ‘`a[@]`’. Bash-5.2 will unset an element with key ‘`@`’ (associative arrays) or remove all the elements without unsetting the array (indexed arrays).
  - Arithmetic commands ( ((…)) ) and the expressions in an arithmetic for statement can be expanded more than once.
  - Expressions used as arguments to arithmetic operators in the `[[` conditional command can be expanded more than once.
  - The expressions in substring parameter brace expansion can be expanded more than once.
  - The expressions in the $(( … )) word expansion can be expanded more than once.
  - Arithmetic expressions used as indexed array subscripts can be expanded more than once.
  - `test -v`, when given an argument of ‘`A[@]`’, where A is an existing associative array, will return true if the array has any set elements. Bash-5.2 will look for and report on a key named ‘`@`’.
  - the ${parameter[:]=value} word expansion will return value, before any variable-specific transformations have been performed (e.g., converting to lowercase). Bash-5.2 will return the final value assigned to the variable.
  - Parsing command substitutions will behave as if extended globbing (see The Shopt Builtin) is enabled, so that parsing a command substitution containing an extglob pattern (say, as part of a shell function) will not fail. This assumes the intent is to enable extglob before the command is executed and word expansions are performed. It will fail at word expansion time if extglob hasn’t been enabled by the time the command is executed.
`compat52 (set using BASH_COMPAT)`
  - The `test` builtin uses its historical algorithm to parse parenthesized subexpressions when given five or more arguments.
  - If the `-p` or `-P` option is supplied to the `bind` builtin, `bind` treats any arguments remaining after option processing as bindable command names, and displays any key sequences bound to those commands, instead of treating the arguments as key sequences to bind.
  - Interactive shells will notify the user of completed jobs while sourcing a script. Newer versions defer notification until script execution completes.

## 7 Job Control ¶

This chapter discusses what job control is, how it works, and how Bash allows you to access its facilities.

### 7.1 Job Control Basics ¶

Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the operating system kernel’s terminal driver and Bash.

The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which the `jobs` command will display. Each job has a job number, which `jobs` displays between brackets. Job numbers start at 1. When Bash starts a job asynchronously, it prints a line that looks like:

    [1] 25647

indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, each process has a process group ID, and the operating system maintains the notion of a current terminal process group ID. This terminal process group ID is associated with the controlling terminal.

Processes that have the same process group ID are said to be part of the same process group. Members of the foreground process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as `SIGINT`. Processes in the foreground process group are said to be foreground processes. Background processes are those whose process group ID differs from the controlling terminal’s; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or, if the user so specifies with `stty tostop`, write to the controlling terminal. The system sends a `SIGTTIN` (`SIGTTOU`) signal to background processes which attempt to read from (write to when `tostop` is in effect) the terminal, which, unless caught, suspends the process.

If the operating system on which Bash is running supports job control, Bash contains facilities to use it. Typing the suspend character (typically ‘`^Z`’, Control-Z) while a process is running stops that process and returns control to Bash. Typing the delayed suspend character (typically ‘`^Y`’, Control-Y) causes the process to stop when it attempts to read input from the terminal, and returns control to Bash. The user then manipulates the state of this job, using the `bg` command to continue it in the background, the `fg` command to continue it in the foreground, or the `kill` command to kill it. The suspend character takes effect immediately, and has the additional side effect of discarding any pending output and typeahead. If you want to force a background process to stop, or stop a process that’s not associated with your terminal session, send it the `SIGSTOP` signal using `kill`.

There are a number of ways to refer to a job in the shell. The ‘`%`’ character introduces a job specification (jobspec).

Job number `n` may be referred to as ‘`%n`’. A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, ‘`%ce`’ refers to a job whose command name begins with ‘`ce`’. Using ‘`%?ce`’, on the other hand, refers to any job containing the string ‘`ce`’ in its command line. If the prefix or substring matches more than one job, Bash reports an error.

The symbols ‘`%%`’ and ‘`%+`’ refer to the shell’s notion of the current job. A single ‘`%`’ (with no accompanying job specification) also refers to the current job. ‘`%-`’ refers to the previous job. When a job starts in the background, a job stops while in the foreground, or a job is resumed in the background, it becomes the current job. The job that was the current job becomes the previous job. When the current job terminates, the previous job becomes the current job. If there is only a single job, ‘`%+`’ and ‘`%-`’ can both be used to refer to that job. In output pertaining to jobs (e.g., the output of the `jobs` command), the current job is always marked with a ‘`+`’, and the previous job with a ‘`-`’.

Simply naming a job can be used to bring it into the foreground: ‘`%1`’ is a synonym for ‘`fg %1`’, bringing job 1 from the background into the foreground. Similarly, ‘`%1 &`’ resumes job 1 in the background, equivalent to ‘`bg %1`’.

The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before notifying the user about changes in a job’s status so as to not interrupt any other output, though it will notify of changes in a job’s status after a foreground command in a list completes, before executing the next command in the list. If the `-b` option to the `set` builtin is enabled, Bash reports status changes immediately (see The Set Builtin). Bash executes any trap on `SIGCHLD` for each child process that terminates.

When a job terminates and Bash notifies the user about it, Bash removes the job from the jobs table. It will not appear in `jobs` output, but `wait` will report its exit status, as long as it’s supplied the process ID associated with the job as an argument. When the table is empty, job numbers start over at 1.

If a user attempts to exit Bash while jobs are stopped, (or running, if the `checkjobs` option is enabled – see The Shopt Builtin), the shell prints a warning message, and if the `checkjobs` option is enabled, lists the jobs and their statuses. The `jobs` command may then be used to inspect their status. If the user immediately attempts to exit again, without an intervening command, Bash does not print another warning, and terminates any stopped jobs.

When the shell is waiting for a job or process using the `wait` builtin, and job control is enabled, `wait` will return when the job changes state. The `-f` option causes `wait` to wait until the job or process terminates before returning.

### 7.2 Job Control Builtins ¶

`bg` ¶

      bg [jobspec ...]

  Resume each suspended job jobspec in the background, as if it had been started with ‘`&`’. If jobspec is not supplied, the shell uses its notion of the current job. `bg` returns zero unless it is run when job control is not enabled, or, when run with job control enabled, any jobspec was not found or specifies a job that was started without job control.

`fg` ¶

      fg [jobspec]

  Resume the job jobspec in the foreground and make it the current job. If jobspec is not supplied, `fg` resumes the current job. The return status is that of the command placed into the foreground, or non-zero if run when job control is disabled or, when run with job control enabled, jobspec does not specify a valid job or jobspec specifies a job that was started without job control.

`jobs` ¶

      jobs [-lnprs] [jobspec]
      jobs -x command [arguments]

  The first form lists the active jobs. The options have the following meanings:

  `-l`

    List process IDs in addition to the normal information.

  `-n`

    Display information only about jobs that have changed status since the user was last notified of their status.

  `-p`

    List only the process ID of the job’s process group leader.

  `-r`

    Display only running jobs.

  `-s`

    Display only stopped jobs.

  If jobspec is supplied, `jobs` restricts output to information about that job. If jobspec is not supplied, `jobs` lists the status of all jobs. The return status is zero unless an invalid option is encountered or an invalid jobspec is supplied.

  If the `-x` option is supplied, `jobs` replaces any jobspec found in command or arguments with the corresponding process group ID, and executes command, passing it arguments, returning its exit status.

`kill` ¶

      kill [-s sigspec] [-n signum] [-sigspec] id [...]
      kill -l|-L [exit_status]

  Send a signal specified by sigspec or signum to the processes named by each id. Each id may be a job specification jobspec or process ID pid. sigspec is either a case-insensitive signal name such as `SIGINT` (with or without the `SIG` prefix) or a signal number; signum is a signal number. If sigspec and signum are not present, `kill` sends `SIGTERM`.

  The `-l` option lists the signal names. If any arguments are supplied when `-l` is supplied, `kill` lists the names of the signals corresponding to the arguments, and the return status is zero. exit_status is a number specifying a signal number or the exit status of a process terminated by a signal; if it is supplied, `kill` prints the name of the signal that caused the process to terminate. `kill` assumes that process exit statuses are greater than 128; anything less than that is a signal number. The `-L` option is equivalent to `-l`.

  The return status is zero if at least one signal was successfully sent, or non-zero if an error occurs or an invalid option is encountered.

`wait` ¶

      wait [-fn] [-p varname] [id ...]

  Wait until the child process specified by each id exits and return the exit status of the last id. Each id may be a process ID pid or a job specification jobspec; if a jobspec is supplied, `wait` waits for all processes in the job.

  If no options or ids are supplied, `wait` waits for all running background jobs and the last-executed process substitution, if its process id is the same as $!, and the return status is zero.

  If the `-n` option is supplied, `wait` waits for any one of the ids or, if no ids are supplied, any job or process substitution, to complete and returns its exit status. If none of the supplied ids is a child of the shell, or if no arguments are supplied and the shell has no unwaited-for children, the exit status is 127.

  If the `-p` option is supplied, `wait` assigns the process or job identifier of the job for which the exit status is returned to the variable varname named by the option argument. The variable, which cannot be readonly, will be unset initially, before any assignment. This is useful only when used with the `-n` option.

  Supplying the `-f` option, when job control is enabled, forces `wait` to wait for each id to terminate before returning its status, instead of returning when it changes status.

  If none of the ids specify one of the shell’s an active child processes, the return status is 127. If `wait` is interrupted by a signal, any varname will remain unset, and the return status will be greater than 128, as described above (see Signals). Otherwise, the return status is the exit status of the last id.

`disown` ¶

      disown [-ar] [-h] [id ...]

  Without options, remove each id from the table of active jobs. Each id may be a job specification jobspec or a process ID pid; if id is a pid, `disown` uses the job containing pid as jobspec.

  If the `-h` option is supplied, `disown` does not remove the jobs corresponding to each `id` from the jobs table, but rather marks them so the shell does not send `SIGHUP` to the job if the shell receives a `SIGHUP`.

  If no id is supplied, the `-a` option means to remove or mark all jobs; the `-r` option without an id argument removes or marks running jobs. If no id is supplied, and neither the `-a` nor the `-r` option is supplied, `disown` removes or marks the current job.

  The return value is 0 unless an id does not specify a valid job.

`suspend` ¶

      suspend [-f]

  Suspend the execution of this shell until it receives a `SIGCONT` signal. A login shell, or a shell without job control enabled, cannot be suspended; the `-f` option will override this and force the suspension. The return status is 0 unless the shell is a login shell or job control is not enabled and `-f` is not supplied.

When job control is not active, the `kill` and `wait` builtins do not accept jobspec arguments. They must be supplied process IDs.

### 7.3 Job Control Variables ¶

`auto_resume` ¶

  This variable controls how the shell interacts with the user and job control. If this variable exists then simple commands consisting of only a single word, without redirections, are treated as candidates for resumption of an existing job. There is no ambiguity allowed; if there is more than one job beginning with or containing the word, then this selects the most recently accessed job. The name of a stopped job, in this context, is the command line used to start it, as displayed by `jobs`. If this variable is set to the value ‘`exact`’, the word must match the name of a stopped job exactly; if set to ‘`substring`’, the word needs to match a substring of the name of a stopped job. The ‘`substring`’ value provides functionality analogous to the ‘`%?string`’ job ID (see Job Control Basics). If set to any other value (e.g., ‘`prefix`’), the word must be a prefix of a stopped job’s name; this provides functionality analogous to the ‘`%string`’ job ID.

## 8 Command Line Editing ¶

This chapter describes the basic features of the GNU command line editing interface. Command line editing is provided by the Readline library, which is used by several different programs, including Bash. Command line editing is enabled by default when using an interactive shell, unless the `--noediting` option is supplied at shell invocation. Line editing is also used when using the `-e` option to the `read` builtin command (see Bash Builtin Commands). By default, the line editing commands are similar to those of Emacs; a vi-style line editing interface is also available. Line editing can be enabled at any time using the `-o emacs` or `-o vi` options to the `set` builtin command (see The Set Builtin), or disabled using the `+o emacs` or `+o vi` options to `set`.

### 8.1 Introduction to Line Editing ¶

The following paragraphs use Emacs style to describe the notation used to represent keystrokes.

The text `C-k` is read as ‘Control-K’ and describes the character produced when the `k` key is pressed while the Control key is depressed.

The text `M-k` is read as ‘Meta-K’ and describes the character produced when the Meta key (if you have one) is depressed, and the `k` key is pressed (a meta character), then both are released. The Meta key is labeled `ALT` or `Option` on many keyboards. On keyboards with two keys labeled `ALT` (usually to either side of the space bar), the `ALT` on the left side is generally set to work as a Meta key. One of the `ALT` keys may also be configured as some other modifier, such as a Compose key for typing accented characters.

On some keyboards, the Meta key modifier produces characters with the eighth bit (0200) set. You can use the `enable-meta-key` variable to control whether or not it does this, if the keyboard allows it. On many others, the terminal or terminal emulator converts the metafied key to a key sequence beginning with `ESC` as described in the next paragraph.

If you do not have a Meta or `ALT` key, or another key working as a Meta key, you can generally achieve the latter effect by typing `ESC` first, and then typing `k`. The `ESC` character is known as the meta prefix).

Either process is known as metafying the `k` key.

If your Meta key produces a key sequence with the `ESC` meta prefix, you can make `M-key` key bindings you specify (see `Key Bindings` in Readline Init File Syntax) do the same thing by setting the `force-meta-prefix` variable.

The text `M-C-k` is read as ‘Meta-Control-k’ and describes the character produced by metafying `C-k`.

In addition, several keys have their own names. Specifically, `DEL`, `ESC`, `LFD`, `SPC`, `RET`, and `TAB` all stand for themselves when seen in this text, or in an init file (see Readline Init File). If your keyboard lacks a `LFD` key, typing `C-j` will output the appropriate character. The `RET` key may be labeled `Return` or `Enter` on some keyboards.

### 8.2 Readline Interaction ¶

Often during an interactive session you type in a long line of text, only to notice that the first word on the line is misspelled. The Readline library gives you a set of commands for manipulating the text as you type it in, allowing you to just fix your typo, and not forcing you to retype the majority of the line. Using these editing commands, you move the cursor to the place that needs correction, and delete or insert the text of the corrections. Then, when you are satisfied with the line, you simply press `RET`. You do not have to be at the end of the line to press `RET`; the entire line is accepted regardless of the location of the cursor within the line.

#### 8.2.1 Readline Bare Essentials ¶

In order to enter characters into the line, simply type them. The typed character appears where the cursor was, and then the cursor moves one space to the right. If you mistype a character, you can use your erase character to back up and delete the mistyped character.

Sometimes you may mistype a character, and not notice the error until you have typed several other characters. In that case, you can type `C-b` to move the cursor to the left, and then correct your mistake. Afterwards, you can move the cursor to the right with `C-f`.

When you add text in the middle of a line, you will notice that characters to the right of the cursor are ‘pushed over’ to make room for the text that you have inserted. Likewise, when you delete text behind the cursor, characters to the right of the cursor are ‘pulled back’ to fill in the blank space created by the removal of the text. These are the bare essentials for editing the text of an input line:

`C-b`

  Move back one character.

`C-f`

  Move forward one character.

`DEL` or `Backspace`

  Delete the character to the left of the cursor.

`C-d`

  Delete the character underneath the cursor.

Printing characters

  Insert the character into the line at the cursor.

`C-_` or `C-x C-u`

  Undo the last editing command. You can undo all the way back to an empty line.

Depending on your configuration, the `Backspace` key might be set to delete the character to the left of the cursor and the `DEL` key set to delete the character underneath the cursor, like `C-d`, rather than the character to the left of the cursor.

#### 8.2.2 Readline Movement Commands ¶

The above table describes the most basic keystrokes that you need in order to do editing of the input line. For your convenience, many other commands are available in addition to `C-b`, `C-f`, `C-d`, and `DEL`. Here are some commands for moving more rapidly within the line.

`C-a`

  Move to the start of the line.

`C-e`

  Move to the end of the line.

`M-f`

  Move forward a word, where a word is composed of letters and digits.

`M-b`

  Move backward a word.

`C-l`

  Clear the screen, reprinting the current line at the top.

Notice how `C-f` moves forward a character, while `M-f` moves forward a word. It is a loose convention that control keystrokes operate on characters while meta keystrokes operate on words.

#### 8.2.3 Readline Killing Commands ¶

Killing text means to delete the text from the line, but to save it away for later use, usually by yanking (re-inserting) it back into the line. (‘Cut’ and ‘paste’ are more recent jargon for ‘kill’ and ‘yank’.)

If the description for a command says that it ‘kills’ text, then you can be sure that you can get the text back in a different (or the same) place later.

When you use a kill command, the text is saved in a kill-ring. Any number of consecutive kills save all of the killed text together, so that when you yank it back, you get it all. The kill ring is not line specific; the text that you killed on a previously typed line is available to be yanked back later, when you are typing another line.

Here is the list of commands for killing text.

`C-k`

  Kill the text from the current cursor position to the end of the line.

`M-d`

  Kill from the cursor to the end of the current word, or, if between words, to the end of the next word. Word boundaries are the same as those used by `M-f`.

`M-`DEL``

  Kill from the cursor to the start of the current word, or, if between words, to the start of the previous word. Word boundaries are the same as those used by `M-b`.

`C-w`

  Kill from the cursor to the previous whitespace. This is different than `M-`DEL`` because the word boundaries differ.

Here is how to yank the text back into the line. Yanking means to copy the most-recently-killed text from the kill buffer into the line at the current cursor position.

`C-y`

  Yank the most recently killed text back into the buffer at the cursor.

`M-y`

  Rotate the kill-ring, and yank the new top. You can only do this if the prior command is `C-y` or `M-y`.

#### 8.2.4 Readline Arguments ¶

You can pass numeric arguments to Readline commands. Sometimes the argument acts as a repeat count, other times it is the sign of the argument that is significant. If you pass a negative argument to a command which normally acts in a forward direction, that command will act in a backward direction. For example, to kill text back to the start of the line, you might type ‘`M-- C-k`’.

The general way to pass numeric arguments to a command is to type meta digits before the command. If the first ‘digit’ typed is a minus sign (‘`-`’), then the sign of the argument will be negative. Once you have typed one meta digit to get the argument started, you can type the remainder of the digits, and then the command. For example, to give the `C-d` command an argument of 10, you could type ‘`M-1 0 C-d`’, which will delete the next ten characters on the input line.

#### 8.2.5 Searching for Commands in the History ¶

Readline provides commands for searching through the command history (see Bash History Facilities) for lines containing a specified string. There are two search modes: incremental and non-incremental.

Incremental searches begin before the user has finished typing the search string. As each character of the search string is typed, Readline displays the next entry from the history matching the string typed so far. An incremental search requires only as many characters as needed to find the desired history entry. When using emacs editing mode, type `C-r` to search backward in the history for a particular string. Typing `C-s` searches forward through the history. The characters present in the value of the `isearch-terminators` variable are used to terminate an incremental search. If that variable has not been assigned a value, the `ESC` and `C-j` characters terminate an incremental search. `C-g` aborts an incremental search and restores the original line. When the search is terminated, the history entry containing the search string becomes the current line.

To find other matching entries in the history list, type `C-r` or `C-s` as appropriate. This searches backward or forward in the history for the next entry matching the search string typed so far. Any other key sequence bound to a Readline command terminates the search and executes that command. For instance, a `RET` terminates the search and accepts the line, thereby executing the command from the history list. A movement command will terminate the search, make the last line found the current line, and begin editing.

Readline remembers the last incremental search string. If two `C-r`s are typed without any intervening characters defining a new search string, Readline uses any remembered search string.

Non-incremental searches read the entire search string before starting to search for matching history entries. The search string may be typed by the user or be part of the contents of the current line.

### 8.3 Readline Init File ¶

Although the Readline library comes with a set of Emacs-like keybindings installed by default, it is possible to use a different set of keybindings. Any user can customize programs that use Readline by putting commands in an inputrc file, conventionally in their home directory. The name of this file is taken from the value of the shell variable `INPUTRC`. If that variable is unset, the default is `~/.inputrc`. If that file does not exist or cannot be read, Readline looks for `/etc/inputrc`. The `bind` builtin command can also be used to set Readline keybindings and variables. See Bash Builtin Commands.

When a program that uses the Readline library starts up, Readline reads the init file and sets any variables and key bindings it contains.

In addition, the `C-x C-r` command re-reads this init file, thus incorporating any changes that you might have made to it.

#### 8.3.1 Readline Init File Syntax ¶

There are only a few basic constructs allowed in the Readline init file. Blank lines are ignored. Lines beginning with a ‘`#`’ are comments. Lines beginning with a ‘`$`’ indicate conditional constructs (see Conditional Init Constructs). Other lines denote variable settings and key bindings.

Variable Settings

  You can modify the run-time behavior of Readline by altering the values of variables in Readline using the `set` command within the init file. The syntax is simple:

      set variable value

  Here, for example, is how to change from the default Emacs-like key binding to use `vi` line editing commands:

      set editing-mode vi

  Variable names and values, where appropriate, are recognized without regard to case. Unrecognized variable names are ignored.

  Boolean variables (those that can be set to on or off) are set to on if the value is null or empty, on (case-insensitive), or 1. Any other value results in the variable being set to off.

  The `bind -V` command lists the current Readline variable names and values. See Bash Builtin Commands.

  A great deal of run-time behavior is changeable with the following variables.

  `active-region-start-color` ¶

    A string variable that controls the text color and background when displaying the text in the active region (see the description of `enable-active-region` below). This string must not take up any physical character positions on the display, so it should consist only of terminal escape sequences. It is output to the terminal before displaying the text in the active region. This variable is reset to the default value whenever the terminal type changes. The default value is the string that puts the terminal in standout mode, as obtained from the terminal’s terminfo description. A sample value might be ‘`\e[01;33m`’.

  `active-region-end-color` ¶

    A string variable that “undoes” the effects of `active-region-start-color` and restores “normal” terminal display appearance after displaying text in the active region. This string must not take up any physical character positions on the display, so it should consist only of terminal escape sequences. It is output to the terminal after displaying the text in the active region. This variable is reset to the default value whenever the terminal type changes. The default value is the string that restores the terminal from standout mode, as obtained from the terminal’s terminfo description. A sample value might be ‘`\e[0m`’.

  `bell-style` ¶

    Controls what happens when Readline wants to ring the terminal bell. If set to ‘`none`’, Readline never rings the bell. If set to ‘`visible`’, Readline uses a visible bell if one is available. If set to ‘`audible`’ (the default), Readline attempts to ring the terminal’s bell.

  `bind-tty-special-chars` ¶

    If set to ‘`on`’ (the default), Readline attempts to bind the control characters that are treated specially by the kernel’s terminal driver to their Readline equivalents. These override the default Readline bindings described here. Type ‘`stty -a`’ at a Bash prompt to see your current terminal settings, including the special control characters (usually `cchars`).

  `blink-matching-paren` ¶

    If set to ‘`on`’, Readline attempts to briefly move the cursor to an opening parenthesis when a closing parenthesis is inserted. The default is ‘`off`’.

  `colored-completion-prefix` ¶

    If set to ‘`on`’, when listing completions, Readline displays the common prefix of the set of possible completions using a different color. The color definitions are taken from the value of the `LS_COLORS` environment variable. If there is a color definition in `LS_COLORS` for the custom suffix ‘`readline-colored-completion-prefix`’, Readline uses this color for the common prefix instead of its default. The default is ‘`off`’.

  `colored-stats` ¶

    If set to ‘`on`’, Readline displays possible completions using different colors to indicate their file type. The color definitions are taken from the value of the `LS_COLORS` environment variable. The default is ‘`off`’.

  `comment-begin` ¶

    The string to insert at the beginning of the line by the `insert-comment` command. The default value is `"#"`.

  `completion-display-width` ¶

    The number of screen columns used to display possible matches when performing completion. The value is ignored if it is less than 0 or greater than the terminal screen width. A value of 0 causes matches to be displayed one per line. The default value is -1.

  `completion-ignore-case` ¶

    If set to ‘`on`’, Readline performs filename matching and completion in a case-insensitive fashion. The default value is ‘`off`’.

  `completion-map-case` ¶

    If set to ‘`on`’, and completion-ignore-case is enabled, Readline treats hyphens (‘`-`’) and underscores (‘`_`’) as equivalent when performing case-insensitive filename matching and completion. The default value is ‘`off`’.

  `completion-prefix-display-length` ¶

    The maximum length in characters of the common prefix of a list of possible completions that is displayed without modification. When set to a value greater than zero, Readline replaces common prefixes longer than this value with an ellipsis when displaying possible completions. If a completion begins with a period, and Readline is completing filenames, it uses three underscores instead of an ellipsis.

  `completion-query-items` ¶

    The number of possible completions that determines when the user is asked whether the list of possibilities should be displayed. If the number of possible completions is greater than or equal to this value, Readline asks whether or not the user wishes to view them; otherwise, Readline simply lists the completions. This variable must be set to an integer value greater than or equal to zero. A zero value means Readline should never ask; negative values are treated as zero. The default limit is `100`.

  `convert-meta` ¶

    If set to ‘`on`’, Readline converts characters it reads that have the eighth bit set to an ASCII key sequence by clearing the eighth bit and prefixing an `ESC` character, converting them to a meta-prefixed key sequence. The default value is ‘`on`’, but Readline sets it to ‘`off`’ if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the `LC_CTYPE` locale category, and may change if the locale changes. This variable also affects key bindings; see the description of `force-meta-prefix` below.

  `disable-completion` ¶

    If set to ‘`On`’, Readline inhibits word completion. Completion characters are inserted into the line as if they had been mapped to `self-insert`. The default is ‘`off`’.

  `echo-control-characters` ¶

    When set to ‘`on`’, on operating systems that indicate they support it, Readline echoes a character corresponding to a signal generated from the keyboard. The default is ‘`on`’.

  `editing-mode` ¶

    The `editing-mode` variable controls the default set of key bindings. By default, Readline starts up in emacs editing mode, where the keystrokes are most similar to Emacs. This variable can be set to either ‘`emacs`’ or ‘`vi`’.

  `emacs-mode-string` ¶

    If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when emacs editing mode is active. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The ‘`\1`’ and ‘`\2`’ escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. The default is ‘`@`’.

  `enable-active-region` ¶

    point is the current cursor position, and mark refers to a saved cursor position (see Commands For Moving). The text between the point and mark is referred to as the region. When this variable is set to ‘`On`’, Readline allows certain commands to designate the region as active. When the region is active, Readline highlights the text in the region using the value of the `active-region-start-color`, which defaults to the string that enables the terminal’s standout mode. The active region shows the text inserted by bracketed-paste and any matching text found by incremental and non-incremental history searches. The default is ‘`On`’.

  `enable-bracketed-paste` ¶

    When set to ‘`On`’, Readline configures the terminal to insert each paste into the editing buffer as a single string of characters, instead of treating each character as if it had been read from the keyboard. This is called putting the terminal into bracketed paste mode; it prevents Readline from executing any editing commands bound to key sequences appearing in the pasted text. The default is ‘`On`’.

  `enable-keypad` ¶

    When set to ‘`on`’, Readline tries to enable the application keypad when it is called. Some systems need this to enable the arrow keys. The default is ‘`off`’.

  `enable-meta-key` ¶

    When set to ‘`on`’, Readline tries to enable any meta modifier key the terminal claims to support when it is called. On many terminals, the Meta key is used to send eight-bit characters; this variable checks for the terminal capability that indicates the terminal can enable and disable a mode that sets the eighth bit of a character (0200) if the Meta key is held down when the character is typed (a meta character). The default is ‘`on`’.

  `expand-tilde` ¶

    If set to ‘`on`’, Readline attempts tilde expansion when it attempts word completion. The default is ‘`off`’.

  `force-meta-prefix` ¶

    If set to ‘`on`’, Readline modifies its behavior when binding key sequences containing `\M-` or `Meta-` (see `Key Bindings` in Readline Init File Syntax) by converting a key sequence of the form `\M-`C or `Meta-`C to the two-character sequence `ESC` C (adding the meta prefix). If `force-meta-prefix` is set to ‘`off`’ (the default), Readline uses the value of the `convert-meta` variable to determine whether to perform this conversion: if `convert-meta` is ‘`on`’, Readline performs the conversion described above; if it is ‘`off`’, Readline converts C to a meta character by setting the eighth bit (0200). The default is ‘`off`’.

  `history-preserve-point` ¶

    If set to ‘`on`’, the history code attempts to place the point (the current cursor position) at the same location on each history line retrieved with `previous-history` or `next-history`. The default is ‘`off`’.

  `history-size` ¶

    Set the maximum number of history entries saved in the history list. If set to zero, any existing history entries are deleted and no new entries are saved. If set to a value less than zero, the number of history entries is not limited. By default, Bash sets the maximum number of history entries to the value of the `HISTSIZE` shell variable. If you try to set history-size to a non-numeric value, the maximum number of history entries will be set to 500.

  `horizontal-scroll-mode` ¶

    Setting this variable to ‘`on`’ means that the text of the lines being edited will scroll horizontally on a single screen line when the lines are longer than the width of the screen, instead of wrapping onto a new screen line. This variable is automatically set to ‘`on`’ for terminals of height 1. By default, this variable is set to ‘`off`’.

  `input-meta` ¶

    If set to ‘`on`’, Readline enables eight-bit input (that is, it does not clear the eighth bit in the characters it reads), regardless of what the terminal claims it can support. The default value is ‘`off`’, but Readline sets it to ‘`on`’ if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the `LC_CTYPE` locale category, and its value may change if the locale changes. The name `meta-flag` is a synonym for `input-meta`.

  `isearch-terminators` ¶

    The string of characters that should terminate an incremental search without subsequently executing the character as a command (see Searching for Commands in the History). If this variable has not been given a value, the characters `ESC` and `C-j` terminate an incremental search.

  `keymap` ¶

    Sets Readline’s idea of the current keymap for key binding commands. Built-in `keymap` names are `emacs`, `emacs-standard`, `emacs-meta`, `emacs-ctlx`, `vi`, `vi-move`, `vi-command`, and `vi-insert`. `vi` is equivalent to `vi-command` (`vi-move` is also a synonym); `emacs` is equivalent to `emacs-standard`. Applications may add additional names. The default value is `emacs`; the value of the `editing-mode` variable also affects the default keymap.

  `keyseq-timeout`

    Specifies the duration Readline will wait for a character when reading an ambiguous key sequence (one that can form a complete key sequence using the input read so far, or can take additional input to complete a longer key sequence). If Readline doesn’t receive any input within the timeout, it uses the shorter but complete key sequence. Readline uses this value to determine whether or not input is available on the current input source (`rl_instream` by default). The value is specified in milliseconds, so a value of 1000 means that Readline will wait one second for additional input. If this variable is set to a value less than or equal to zero, or to a non-numeric value, Readline waits until another key is pressed to decide which key sequence to complete. The default value is `500`.

  `mark-directories`

    If set to ‘`on`’, completed directory names have a slash appended. The default is ‘`on`’.

  `mark-modified-lines` ¶

    When this variable is set to ‘`on`’, Readline displays an asterisk (‘`*`’) at the start of history lines which have been modified. This variable is ‘`off`’ by default.

  `mark-symlinked-directories` ¶

    If set to ‘`on`’, completed names which are symbolic links to directories have a slash appended, subject to the value of `mark-directories`. The default is ‘`off`’.

  `match-hidden-files` ¶

    This variable, when set to ‘`on`’, forces Readline to match files whose names begin with a ‘`.`’ (hidden files) when performing filename completion. If set to ‘`off`’, the user must include the leading ‘`.`’ in the filename to be completed. This variable is ‘`on`’ by default.

  `menu-complete-display-prefix` ¶

    If set to ‘`on`’, menu completion displays the common prefix of the list of possible completions (which may be empty) before cycling through the list. The default is ‘`off`’.

  `output-meta` ¶

    If set to ‘`on`’, Readline displays characters with the eighth bit set directly rather than as a meta-prefixed escape sequence. The default is ‘`off`’, but Readline sets it to ‘`on`’ if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the `LC_CTYPE` locale category, and its value may change if the locale changes.

  `page-completions` ¶

    If set to ‘`on`’, Readline uses an internal pager resembling more(1) to display a screenful of possible completions at a time. This variable is ‘`on`’ by default.

  `prefer-visible-bell`

    See `bell-style`.

  `print-completions-horizontally`

    If set to ‘`on`’, Readline displays completions with matches sorted horizontally in alphabetical order, rather than down the screen. The default is ‘`off`’.

  `revert-all-at-newline` ¶

    If set to ‘`on`’, Readline will undo all changes to history lines before returning when executing `accept-line`. By default, history lines may be modified and retain individual undo lists across calls to `readline()`. The default is ‘`off`’.

  `search-ignore-case` ¶

    If set to ‘`on`’, Readline performs incremental and non-incremental history list searches in a case-insensitive fashion. The default value is ‘`off`’.

  `show-all-if-ambiguous` ¶

    This alters the default behavior of the completion functions. If set to ‘`on`’, words which have more than one possible completion cause the matches to be listed immediately instead of ringing the bell. The default value is ‘`off`’.

  `show-all-if-unmodified` ¶

    This alters the default behavior of the completion functions in a fashion similar to show-all-if-ambiguous. If set to ‘`on`’, words which have more than one possible completion without any possible partial completion (the possible completions don’t share a common prefix) cause the matches to be listed immediately instead of ringing the bell. The default value is ‘`off`’.

  `show-mode-in-prompt` ¶

    If set to ‘`on`’, add a string to the beginning of the prompt indicating the editing mode: emacs, vi command, or vi insertion. The mode strings are user-settable (e.g., emacs-mode-string). The default value is ‘`off`’.

  `skip-completed-text` ¶

    If set to ‘`on`’, this alters the default completion behavior when inserting a single match into the line. It’s only active when performing completion in the middle of a word. If enabled, Readline does not insert characters from the completion that match characters after point in the word being completed, so portions of the word following the cursor are not duplicated. For instance, if this is enabled, attempting completion when the cursor is after the first ‘`e`’ in ‘`Makefile`’ will result in ‘`Makefile`’ rather than ‘`Makefilefile`’, assuming there is a single possible completion. The default value is ‘`off`’.

  `vi-cmd-mode-string` ¶

    If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in command mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The ‘`\1`’ and ‘`\2`’ escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. The default is ‘`(cmd)`’.

  `vi-ins-mode-string` ¶

    If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in insertion mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The ‘`\1`’ and ‘`\2`’ escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. The default is ‘`(ins)`’.

  `visible-stats` ¶

    If set to ‘`on`’, a character denoting a file’s type is appended to the filename when listing possible completions. The default is ‘`off`’.

Key Bindings

  The syntax for controlling key bindings in the init file is simple. First you need to find the name of the command that you want to change. The following sections contain tables of the command name, the default keybinding, if any, and a short description of what the command does.

  Once you know the name of the command, simply place on a line in the init file the name of the key you wish to bind the command to, a colon, and then the name of the command. There can be no space between the key name and the colon – that will be interpreted as part of the key name. The name of the key can be expressed in different ways, depending on what you find most comfortable.

  In addition to command names, Readline allows keys to be bound to a string that is inserted when the key is pressed (a macro). The difference between a macro and a command is that a macro is enclosed in single or double quotes.

  The `bind -p` command displays Readline function names and bindings in a format that can be put directly into an initialization file. See Bash Builtin Commands.

  keyname: function-name or macro

    keyname is the name of a key spelled out in English. For example:

        Control-u: universal-argument
        Meta-Rubout: backward-kill-word
        Control-o: "> output"

    In the example above, `C-u` is bound to the function `universal-argument`, `M-DEL` is bound to the function `backward-kill-word`, and `C-o` is bound to run the macro expressed on the right hand side (that is, to insert the text ‘`> output`’ into the line).

    This key binding syntax recognizes a number of symbolic character names: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT (a destructive backspace), SPACE, SPC, and TAB.

  "keyseq": function-name or macro

    keyseq differs from keyname above in that strings denoting an entire key sequence can be specified, by placing the key sequence in double quotes. Some GNU Emacs style key escapes can be used, as in the following example, but none of the special character names are recognized.

        "\C-u": universal-argument
        "\C-x\C-r": re-read-init-file
        "\e[11~": "Function Key 1"

    In the above example, `C-u` is again bound to the function `universal-argument` (just as it was in the first example), ‘``C-x` `C-r``’ is bound to the function `re-read-init-file`, and ‘``ESC` `[` `1` `1` `~``’ is bound to insert the text ‘`Function Key 1`’.

  The following GNU Emacs style escape sequences are available when specifying key sequences:

  ``\C-``

    A control prefix.

  ``\M-``

    Adding the meta prefix or converting the following character to a meta character, as described above under `force-meta-prefix` (see `Variable Settings` in Readline Init File Syntax).

  ``\e``

    An escape character.

  ``\\``

    Backslash.

  ``\"``

    `"`, a double quotation mark.

  ``\'``

    `'`, a single quote or apostrophe.

  In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is available:

  `\a`

    alert (bell)

  `\b`

    backspace

  `\d`

    delete

  `\f`

    form feed

  `\n`

    newline

  `\r`

    carriage return

  `\t`

    horizontal tab

  `\v`

    vertical tab

  `\nnn`

    The eight-bit character whose value is the octal value nnn (one to three digits).

  `\xHH`

    The eight-bit character whose value is the hexadecimal value HH (one or two hex digits).

  When entering the text of a macro, single or double quotes must be used to indicate a macro definition. Unquoted text is assumed to be a function name. The backslash escapes described above are expanded in the macro body. Backslash will quote any other character in the macro text, including ‘`"`’ and ‘`'`’. For example, the following binding will make ‘``C-x` \`’ insert a single ‘`\`’ into the line:

      "\C-x\\": "\\"

#### 8.3.2 Conditional Init Constructs ¶

Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. There are four parser directives available.

`$if`

  The `$if` construct allows bindings to be made based on the editing mode, the terminal being used, or the application using Readline. The text of the test, after any comparison operator, extends to the end of the line; unless otherwise noted, no characters are required to isolate it.

  `mode`

    The `mode=` form of the `$if` directive is used to test whether Readline is in `emacs` or `vi` mode. This may be used in conjunction with the ‘`set keymap`’ command, for instance, to set bindings in the `emacs-standard` and `emacs-ctlx` keymaps only if Readline is starting out in `emacs` mode.

  `term`

    The `term=` form may be used to include terminal-specific key bindings, perhaps to bind the key sequences output by the terminal’s function keys. The word on the right side of the ‘`=`’ is tested against both the full name of the terminal and the portion of the terminal name before the first ‘`-`’. This allows `xterm` to match both `xterm` and `xterm-256color`, for instance.

  `version`

    The `version` test may be used to perform comparisons against specific Readline versions. The `version` expands to the current Readline version. The set of comparison operators includes ‘`=`’ (and ‘`==`’), ‘`!=`’, ‘`<=`’, ‘`>=`’, ‘`<`’, and ‘`>`’. The version number supplied on the right side of the operator consists of a major version number, an optional decimal point, and an optional minor version (e.g., ‘`7.1`’). If the minor version is omitted, it defaults to ‘`0`’. The operator may be separated from the string `version` and from the version number argument by whitespace. The following example sets a variable if the Readline version being used is 7.0 or newer:

        $if version >= 7.0
        set show-mode-in-prompt on
        $endif

  `application`

    The application construct is used to include application-specific settings. Each program using the Readline library sets the application name, and you can test for a particular value. This could be used to bind key sequences to functions useful for a specific program. For instance, the following command adds a key sequence that quotes the current or previous word in Bash:

        $if Bash
        # Quote the current or previous word
        "\C-xq": "\eb\"\ef\""
        $endif

  `variable`

    The variable construct provides simple equality tests for Readline variables and values. The permitted comparison operators are ‘`=`’, ‘`==`’, and ‘`!=`’. The variable name must be separated from the comparison operator by whitespace; the operator may be separated from the value on the right hand side by whitespace. String and boolean variables may be tested. Boolean variables must be tested against the values on and off. The following example is equivalent to the `mode=emacs` test described above:

        $if editing-mode == emacs
        set show-mode-in-prompt on
        $endif

`$else`

  Commands in this branch of the `$if` directive are executed if the test fails.

`$endif`

  This command, as seen in the previous example, terminates an `$if` command.

`$include`

  This directive takes a single filename as an argument and reads commands and key bindings from that file. For example, the following directive reads from `/etc/inputrc`:

      $include /etc/inputrc

#### 8.3.3 Sample Init File ¶

Here is an example of an inputrc file. This illustrates key binding, variable assignment, and conditional syntax.

    # This file controls the behavior of line input editing for
    # programs that use the GNU Readline library.  Existing
    # programs include FTP, Bash, and GDB.
    #
    # You can re-read the inputrc file with C-x C-r.
    # Lines beginning with '#' are comments.
    #
    # First, include any system-wide bindings and variable
    # assignments from /etc/Inputrc
    $include /etc/Inputrc

    #
    # Set various bindings for emacs mode.

    set editing-mode emacs

    $if mode=emacs

    Meta-Control-h:	backward-kill-word	Text after the function name is ignored

    #
    # Arrow keys in keypad mode
    #
    #"\M-OD":        backward-char
    #"\M-OC":        forward-char
    #"\M-OA":        previous-history
    #"\M-OB":        next-history
    #
    # Arrow keys in ANSI mode
    #
    "\M-[D":        backward-char
    "\M-[C":        forward-char
    "\M-[A":        previous-history
    "\M-[B":        next-history
    #
    # Arrow keys in 8 bit keypad mode
    #
    #"\M-\C-OD":       backward-char
    #"\M-\C-OC":       forward-char
    #"\M-\C-OA":       previous-history
    #"\M-\C-OB":       next-history
    #
    # Arrow keys in 8 bit ANSI mode
    #
    #"\M-\C-[D":       backward-char
    #"\M-\C-[C":       forward-char
    #"\M-\C-[A":       previous-history
    #"\M-\C-[B":       next-history

    C-q: quoted-insert

    $endif

    # An old-style binding.  This happens to be the default.
    TAB: complete

    # Macros that are convenient for shell interaction
    $if Bash
    # edit the path
    "\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
    # prepare to type a quoted word --
    # insert open and close double quotes
    # and move to just after the open quote
    "\C-x\"": "\"\"\C-b"
    # insert a backslash (testing backslash escapes
    # in sequences and macros)
    "\C-x\\": "\\"
    # Quote the current or previous word
    "\C-xq": "\eb\"\ef\""
    # Add a binding to refresh the line, which is unbound
    "\C-xr": redraw-current-line
    # Edit variable on current line.
    "\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
    $endif

    # use a visible bell if one is available
    set bell-style visible

    # don't strip characters to 7 bits when reading
    set input-meta on

    # allow iso-latin1 characters to be inserted rather
    # than converted to prefix-meta sequences
    set convert-meta off

    # display characters with the eighth bit set directly
    # rather than as meta-prefixed characters
    set output-meta on

    # if there are 150 or more possible completions for a word,
    # ask whether or not the user wants to see all of them
    set completion-query-items 150

    # For FTP
    $if Ftp
    "\C-xg": "get \M-?"
    "\C-xt": "put \M-?"
    "\M-.": yank-last-arg
    $endif

### 8.4 Bindable Readline Commands ¶

This section describes Readline commands that may be bound to key sequences. You can list your key bindings by executing `bind -P` or, for a more terse format, suitable for an inputrc file, `bind -p`. (See Bash Builtin Commands.) Command names without an accompanying key sequence are unbound by default.

In the following descriptions, point refers to the current cursor position, and mark refers to a cursor position saved by the `set-mark` command. The text between the point and mark is referred to as the region. Readline has the concept of an active region: when the region is active, Readline redisplay highlights the region using the value of the `active-region-start-color` variable. The `enable-active-region` variable turns this on and off. Several commands set the region to active; those are noted below.

#### 8.4.1 Commands For Moving ¶

`beginning-of-line (C-a)` ¶

  Move to the start of the current line. This may also be bound to the Home key on some keyboards.

`end-of-line (C-e)` ¶

  Move to the end of the line. This may also be bound to the End key on some keyboards.

`forward-char (C-f)` ¶

  Move forward a character. This may also be bound to the right arrow key on some keyboards.

`backward-char (C-b)` ¶

  Move back a character. This may also be bound to the left arrow key on some keyboards.

`forward-word (M-f)` ¶

  Move forward to the end of the next word. Words are composed of letters and digits.

`backward-word (M-b)` ¶

  Move back to the start of the current or previous word. Words are composed of letters and digits.

`shell-forward-word (M-C-f)` ¶

  Move forward to the end of the next word. Words are delimited by non-quoted shell metacharacters.

`shell-backward-word (M-C-b)` ¶

  Move back to the start of the current or previous word. Words are delimited by non-quoted shell metacharacters.

`previous-screen-line ()` ¶

  Attempt to move point to the same physical screen column on the previous physical screen line. This will not have the desired effect if the current Readline line does not take up more than one physical line or if point is not greater than the length of the prompt plus the screen width.

`next-screen-line ()` ¶

  Attempt to move point to the same physical screen column on the next physical screen line. This will not have the desired effect if the current Readline line does not take up more than one physical line or if the length of the current Readline line is not greater than the length of the prompt plus the screen width.

`clear-display (M-C-l)` ¶

  Clear the screen and, if possible, the terminal’s scrollback buffer, then redraw the current line, leaving the current line at the top of the screen.

`clear-screen (C-l)` ¶

  Clear the screen, then redraw the current line, leaving the current line at the top of the screen. If given a numeric argument, this refreshes the current line without clearing the screen.

`redraw-current-line ()` ¶

  Refresh the current line. By default, this is unbound.

#### 8.4.2 Commands For Manipulating The History ¶

`accept-line (Newline or Return)` ¶

  Accept the line regardless of where the cursor is. If this line is non-empty, add it to the history list according to the setting of the `HISTCONTROL` and `HISTIGNORE` variables. If this line is a modified history line, then restore the history line to its original state.

`previous-history (C-p)` ¶

  Move ‘back’ through the history list, fetching the previous command. This may also be bound to the up arrow key on some keyboards.

`next-history (C-n)` ¶

  Move ‘forward’ through the history list, fetching the next command. This may also be bound to the down arrow key on some keyboards.

`beginning-of-history (M-<)` ¶

  Move to the first line in the history.

`end-of-history (M->)` ¶

  Move to the end of the input history, i.e., the line currently being entered.

`reverse-search-history (C-r)` ¶

  Search backward starting at the current line and moving ‘up’ through the history as necessary. This is an incremental search. This command sets the region to the matched text and activates the region.

`forward-search-history (C-s)` ¶

  Search forward starting at the current line and moving ‘down’ through the history as necessary. This is an incremental search. This command sets the region to the matched text and activates the region.

`non-incremental-reverse-search-history (M-p)` ¶

  Search backward starting at the current line and moving ‘up’ through the history as necessary using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.

`non-incremental-forward-search-history (M-n)` ¶

  Search forward starting at the current line and moving ‘down’ through the history as necessary using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.

`history-search-backward ()` ¶

  Search backward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. By default, this command is unbound, but may be bound to the Page Down key on some keyboards.

`history-search-forward ()` ¶

  Search forward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. By default, this command is unbound, but may be bound to the Page Up key on some keyboards.

`history-substring-search-backward ()` ¶

  Search backward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search. By default, this command is unbound.

`history-substring-search-forward ()` ¶

  Search forward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search. By default, this command is unbound.

`yank-nth-arg (M-C-y)` ¶

  Insert the first argument to the previous command (usually the second word on the previous line) at point. With an argument n, insert the nth word from the previous command (the words in the previous command begin with word 0). A negative argument inserts the nth word from the end of the previous command. Once the argument n is computed, this uses the history expansion facilities to extract the nth word, as if the ‘`!n`’ history expansion had been specified.

`yank-last-arg (M-. or M-_)` ¶

  Insert last argument to the previous command (the last word of the previous history entry). With a numeric argument, behave exactly like `yank-nth-arg`. Successive calls to `yank-last-arg` move back through the history list, inserting the last word (or the word specified by the argument to the first call) of each line in turn. Any numeric argument supplied to these successive calls determines the direction to move through the history. A negative argument switches the direction through the history (back or forward). This uses the history expansion facilities to extract the last word, as if the ‘`!$`’ history expansion had been specified.

`operate-and-get-next (C-o)` ¶

  Accept the current line for return to the calling application as if a newline had been entered, and fetch the next line relative to the current line from the history for editing. A numeric argument, if supplied, specifies the history entry to use instead of the current line.

`fetch-history ()` ¶

  With a numeric argument, fetch that entry from the history list and make it the current line. Without an argument, move back to the first entry in the history list.

#### 8.4.3 Commands For Changing Text ¶

`end-of-file (usually C-d)` ¶

  The character indicating end-of-file as set, for example, by `stty`. If this character is read when there are no characters on the line, and point is at the beginning of the line, Readline interprets it as the end of input and returns EOF.

`delete-char (C-d)` ¶

  Delete the character at point. If this function is bound to the same character as the tty EOF character, as `C-d` commonly is, see above for the effects. This may also be bound to the Delete key on some keyboards.

`backward-delete-char (Rubout)` ¶

  Delete the character behind the cursor. A numeric argument means to kill the characters, saving them on the kill ring, instead of deleting them.

`forward-backward-delete-char ()` ¶

  Delete the character under the cursor, unless the cursor is at the end of the line, in which case the character behind the cursor is deleted. By default, this is not bound to a key.

`quoted-insert (C-q or C-v)` ¶

  Add the next character typed to the line verbatim. This is how to insert key sequences like `C-q`, for example.

`self-insert (a, b, A, 1, !, …)` ¶

  Insert the character typed.

`bracketed-paste-begin ()` ¶

  This function is intended to be bound to the "bracketed paste" escape sequence sent by some terminals, and such a binding is assigned by default. It allows Readline to insert the pasted text as a single unit without treating each character as if it had been read from the keyboard. The characters are inserted as if each one was bound to `self-insert` instead of executing any editing commands.

  Bracketed paste sets the region (the characters between point and the mark) to the inserted text. It sets the active region.

`transpose-chars (C-t)` ¶

  Drag the character before the cursor forward over the character at the cursor, moving the cursor forward as well. If the insertion point is at the end of the line, then this transposes the last two characters of the line. Negative arguments have no effect.

`transpose-words (M-t)` ¶

  Drag the word before point past the word after point, moving point past that word as well. If the insertion point is at the end of the line, this transposes the last two words on the line.

`shell-transpose-words (M-C-t)` ¶

  Drag the word before point past the word after point, moving point past that word as well. If the insertion point is at the end of the line, this transposes the last two words on the line. Word boundaries are the same as `shell-forward-word` and `shell-backward-word`.

`upcase-word (M-u)` ¶

  Uppercase the current (or following) word. With a negative argument, uppercase the previous word, but do not move the cursor.

`downcase-word (M-l)` ¶

  Lowercase the current (or following) word. With a negative argument, lowercase the previous word, but do not move the cursor.

`capitalize-word (M-c)` ¶

  Capitalize the current (or following) word. With a negative argument, capitalize the previous word, but do not move the cursor.

`overwrite-mode ()` ¶

  Toggle overwrite mode. With an explicit positive numeric argument, switches to overwrite mode. With an explicit non-positive numeric argument, switches to insert mode. This command affects only `emacs` mode; `vi` mode does overwrite differently. Each call to `readline()` starts in insert mode.

  In overwrite mode, characters bound to `self-insert` replace the text at point rather than pushing the text to the right. Characters bound to `backward-delete-char` replace the character before point with a space.

  By default, this command is unbound, but may be bound to the Insert key on some keyboards.

#### 8.4.4 Killing And Yanking ¶

`kill-line (C-k)` ¶

  Kill the text from point to the end of the current line. With a negative numeric argument, kill backward from the cursor to the beginning of the line.

`backward-kill-line (C-x Rubout)` ¶

  Kill backward from the cursor to the beginning of the current line. With a negative numeric argument, kill forward from the cursor to the end of the line.

`unix-line-discard (C-u)` ¶

  Kill backward from the cursor to the beginning of the current line.

`kill-whole-line ()` ¶

  Kill all characters on the current line, no matter where point is. By default, this is unbound.

`kill-word (M-d)` ¶

  Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as `forward-word`.

`backward-kill-word (M-`DEL`)` ¶

  Kill the word behind point. Word boundaries are the same as `backward-word`.

`shell-kill-word (M-C-d)` ¶

  Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as `shell-forward-word`.

`shell-backward-kill-word ()` ¶

  Kill the word behind point. Word boundaries are the same as `shell-backward-word`.

`unix-word-rubout (C-w)` ¶

  Kill the word behind point, using white space as a word boundary, saving the killed text on the kill-ring.

`unix-filename-rubout ()` ¶

  Kill the word behind point, using white space and the slash character as the word boundaries, saving the killed text on the kill-ring.

`delete-horizontal-space ()` ¶

  Delete all spaces and tabs around point. By default, this is unbound.

`kill-region ()` ¶

  Kill the text in the current region. By default, this command is unbound.

`copy-region-as-kill ()` ¶

  Copy the text in the region to the kill buffer, so it can be yanked right away. By default, this command is unbound.

`copy-backward-word ()` ¶

  Copy the word before point to the kill buffer. The word boundaries are the same as `backward-word`. By default, this command is unbound.

`copy-forward-word ()` ¶

  Copy the word following point to the kill buffer. The word boundaries are the same as `forward-word`. By default, this command is unbound.

`yank (C-y)` ¶

  Yank the top of the kill ring into the buffer at point.

`yank-pop (M-y)` ¶

  Rotate the kill-ring, and yank the new top. You can only do this if the prior command is `yank` or `yank-pop`.

#### 8.4.5 Specifying Numeric Arguments ¶

`digit-argument (`M-0`, `M-1`, … `M--`)` ¶

  Add this digit to the argument already accumulating, or start a new argument. `M--` starts a negative argument.

`universal-argument ()` ¶

  This is another way to specify an argument. If this command is followed by one or more digits, optionally with a leading minus sign, those digits define the argument. If the command is followed by digits, executing `universal-argument` again ends the numeric argument, but is otherwise ignored. As a special case, if this command is immediately followed by a character that is neither a digit nor minus sign, the argument count for the next command is multiplied by four. The argument count is initially one, so executing this function the first time makes the argument count four, a second time makes the argument count sixteen, and so on. By default, this is not bound to a key.

#### 8.4.6 Letting Readline Type For You ¶

`complete (`TAB`)` ¶

  Attempt to perform completion on the text before point. The actual completion performed is application-specific. Bash attempts completion by first checking for any programmable completions for the command word (see Programmable Completion), otherwise treating the text as a variable (if the text begins with ‘`$`’), username (if the text begins with ‘`~`’), hostname (if the text begins with ‘`@`’), or command (including aliases, functions, and builtins) in turn. If none of these produces a match, it falls back to filename completion.

`possible-completions (M-?)` ¶

  List the possible completions of the text before point. When displaying completions, Readline sets the number of columns used for display to the value of `completion-display-width`, the value of the environment variable `COLUMNS`, or the screen width, in that order.

`insert-completions (M-*)` ¶

  Insert all completions of the text before point that would have been generated by `possible-completions`, separated by a space.

`menu-complete ()` ¶

  Similar to `complete`, but replaces the word to be completed with a single match from the list of possible completions. Repeatedly executing `menu-complete` steps through the list of possible completions, inserting each match in turn. At the end of the list of completions, `menu-complete` rings the bell (subject to the setting of `bell-style`) and restores the original text. An argument of n moves n positions forward in the list of matches; a negative argument moves backward through the list. This command is intended to be bound to `TAB`, but is unbound by default.

`menu-complete-backward ()` ¶

  Identical to `menu-complete`, but moves backward through the list of possible completions, as if `menu-complete` had been given a negative argument. This command is unbound by default.

`export-completions ()` ¶

  Perform completion on the word before point as described above and write the list of possible completions to Readline’s output stream using the following format, writing information on separate lines:

  - the number of matches N;
  - the word being completed;
  - S:E, where S and E are the start and end offsets of the word in the Readline line buffer; then
  - each match, one per line

  If there are no matches, the first line will be “0”, and this command does not print any output after the S:E. If there is only a single match, this prints a single line containing it. If there is more than one match, this prints the common prefix of the matches, which may be empty, on the first line after the S:E, then the matches on subsequent lines. In this case, N will include the first line with the common prefix.

  The user or application should be able to accommodate the possibility of a blank line. The intent is that the user or application reads N lines after the line containing S:E to obtain the match list. This command is unbound by default.

`delete-char-or-list ()` ¶

  Deletes the character under the cursor if not at the beginning or end of the line (like `delete-char`). At the end of the line, it behaves identically to `possible-completions`. This command is unbound by default.

`complete-filename (M-/)` ¶

  Attempt filename completion on the text before point.

`possible-filename-completions (C-x /)` ¶

  List the possible completions of the text before point, treating it as a filename.

`complete-username (M-~)` ¶

  Attempt completion on the text before point, treating it as a username.

`possible-username-completions (C-x ~)` ¶

  List the possible completions of the text before point, treating it as a username.

`complete-variable (M-$)` ¶

  Attempt completion on the text before point, treating it as a shell variable.

`possible-variable-completions (C-x $)` ¶

  List the possible completions of the text before point, treating it as a shell variable.

`complete-hostname (M-@)` ¶

  Attempt completion on the text before point, treating it as a hostname.

`possible-hostname-completions (C-x @)` ¶

  List the possible completions of the text before point, treating it as a hostname.

`complete-command (M-!)` ¶

  Attempt completion on the text before point, treating it as a command name. Command completion attempts to match the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames, in that order.

`possible-command-completions (C-x !)` ¶

  List the possible completions of the text before point, treating it as a command name.

`dynamic-complete-history (M-`TAB`)` ¶

  Attempt completion on the text before point, comparing the text against history list entries for possible completion matches.

`dabbrev-expand ()` ¶

  Attempt menu completion on the text before point, comparing the text against lines from the history list for possible completion matches.

`complete-into-braces (M-{)` ¶

  Perform filename completion and insert the list of possible completions enclosed within braces so the list is available to the shell (see Brace Expansion).

#### 8.4.7 Keyboard Macros ¶

`start-kbd-macro (C-x ()` ¶

  Begin saving the characters typed into the current keyboard macro.

`end-kbd-macro (C-x ))` ¶

  Stop saving the characters typed into the current keyboard macro and save the definition.

`call-last-kbd-macro (C-x e)` ¶

  Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the keyboard.

`print-last-kbd-macro ()` ¶

  Print the last keyboard macro defined in a format suitable for the inputrc file.

#### 8.4.8 Some Miscellaneous Commands ¶

`re-read-init-file (C-x C-r)` ¶

  Read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there.

`abort (C-g)` ¶

  Abort the current editing command and ring the terminal’s bell (subject to the setting of `bell-style`).

`do-lowercase-version (M-A, M-B, M-x, …)` ¶

  If the metafied character x is upper case, run the command that is bound to the corresponding metafied lower case character. The behavior is undefined if x is already lower case.

`prefix-meta (`ESC`)` ¶

  Metafy the next character typed. Typing ‘``ESC` f`’ is equivalent to typing `M-f`.

`undo (C-_ or C-x C-u)` ¶

  Incremental undo, separately remembered for each line.

`revert-line (M-r)` ¶

  Undo all changes made to this line. This is like executing the `undo` command enough times to get back to the initial state.

`tilde-expand (M-&)` ¶

  Perform tilde expansion on the current word.

`set-mark (C-@)` ¶

  Set the mark to the point. If a numeric argument is supplied, set the mark to that position.

`exchange-point-and-mark (C-x C-x)` ¶

  Swap the point with the mark. Set the current cursor position to the saved position, then set the mark to the old cursor position.

`character-search (C-])` ¶

  Read a character and move point to the next occurrence of that character. A negative argument searches for previous occurrences.

`character-search-backward (M-C-])` ¶

  Read a character and move point to the previous occurrence of that character. A negative argument searches for subsequent occurrences.

`skip-csi-sequence ()` ¶

  Read enough characters to consume a multi-key sequence such as those defined for keys like Home and End. CSI sequences begin with a Control Sequence Indicator (CSI), usually `ESC [`. If this sequence is bound to "\e[", keys producing CSI sequences have no effect unless explicitly bound to a Readline command, instead of inserting stray characters into the editing buffer. This is unbound by default, but usually bound to `ESC [`.

`insert-comment (M-#)` ¶

  Without a numeric argument, insert the value of the `comment-begin` variable at the beginning of the current line. If a numeric argument is supplied, this command acts as a toggle: if the characters at the beginning of the line do not match the value of `comment-begin`, insert the value; otherwise delete the characters in `comment-begin` from the beginning of the line. In either case, the line is accepted as if a newline had been typed. The default value of `comment-begin` causes this command to make the current line a shell comment. If a numeric argument causes the comment character to be removed, the line will be executed by the shell.

`dump-functions ()` ¶

  Print all of the functions and their key bindings to the Readline output stream. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file. This command is unbound by default.

`dump-variables ()` ¶

  Print all of the settable variables and their values to the Readline output stream. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file. This command is unbound by default.

`dump-macros ()` ¶

  Print all of the Readline key sequences bound to macros and the strings they output to the Readline output stream. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file. This command is unbound by default.

`execute-named-command (M-x)` ¶

  Read a bindable Readline command name from the input and execute the function to which it’s bound, as if the key sequence to which it was bound appeared in the input. If this function is supplied with a numeric argument, it passes that argument to the function it executes.

`spell-correct-word (C-x s)` ¶

  Perform spelling correction on the current word, treating it as a directory or filename, in the same way as the `cdspell` shell option. Word boundaries are the same as those used by `shell-forward-word`.

`glob-complete-word (M-g)` ¶

  Treat the word before point as a pattern for pathname expansion, with an asterisk implicitly appended, then use the pattern to generate a list of matching file names for possible completions.

`glob-expand-word (C-x *)` ¶

  Treat the word before point as a pattern for pathname expansion, and insert the list of matching file names, replacing the word. If a numeric argument is supplied, append a ‘`*`’ before pathname expansion.

`glob-list-expansions (C-x g)` ¶

  Display the list of expansions that would have been generated by `glob-expand-word`, and redisplay the line. If a numeric argument is supplied, append a ‘`*`’ before pathname expansion.

`shell-expand-line (M-C-e)` ¶

  Expand the line by performing shell word expansions. This performs alias and history expansion, $’string’ and $"string" quoting, tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, word splitting, and quote removal. An explicit argument suppresses command and process substitution.

`history-expand-line (M-^)` ¶

  Perform history expansion on the current line.

`magic-space ()` ¶

  Perform history expansion on the current line and insert a space (see History Expansion).

`alias-expand-line ()` ¶

  Perform alias expansion on the current line (see Aliases).

`history-and-alias-expand-line ()` ¶

  Perform history and alias expansion on the current line.

`insert-last-argument (M-. or M-_)` ¶

  A synonym for `yank-last-arg`.

`edit-and-execute-command (C-x C-e)` ¶

  Invoke an editor on the current command line, and execute the result as shell commands. Bash attempts to invoke `$VISUAL`, `$EDITOR`, and `emacs` as the editor, in that order.

`display-shell-version (C-x C-v)` ¶

  Display version information about the current instance of Bash.

### 8.5 Readline vi Mode ¶

While the Readline library does not have a full set of `vi` editing functions, it does contain enough to allow simple editing of the line. The Readline `vi` mode behaves as specified in the `sh` description in the POSIX standard.

You can use the ‘`set -o emacs`’ and ‘`set -o vi`’ commands (see The Set Builtin) to switch interactively between `emacs` and `vi` editing modes, The Readline default is `emacs` mode.

When you enter a line in `vi` mode, you are already placed in ‘insertion’ mode, as if you had typed an ‘`i`’. Pressing `ESC` switches you into ‘command’ mode, where you can edit the text of the line with the standard `vi` movement keys, move to previous history lines with ‘`k`’ and subsequent lines with ‘`j`’, and so forth.

### 8.6 Programmable Completion ¶

When the user attempts word completion for a command or an argument to a command for which a completion specification (a compspec) has been defined using the `complete` builtin (see Programmable Completion Builtins), Readline invokes the programmable completion facilities.

First, Bash identifies the command name. If a compspec has been defined for that command, the compspec is used to generate the list of possible completions for the word. If the command word is the empty string (completion attempted at the beginning of an empty line), Bash uses any compspec defined with the `-E` option to `complete`. The `-I` option to `complete` indicates that the command word is the first non-assignment word on the line, or after a command delimiter such as ‘`;`’ or ‘`|`’. This usually indicates command name completion.

If the command word is a full pathname, Bash searches for a compspec for the full pathname first. If there is no compspec for the full pathname, Bash attempts to find a compspec for the portion following the final slash. If those searches do not result in a compspec, or if there is no compspec for the command word, Bash uses any compspec defined with the `-D` option to `complete` as the default. If there is no default compspec, Bash performs alias expansion on the command word as a final resort, and attempts to find a compspec for the command word resulting from any successful expansion.

If a compspec is not found, Bash performs its default completion described above (see Letting Readline Type For You). Otherwise, once a compspec has been found, Bash uses it to generate the list of matching words.

First, Bash performs the actions specified by the compspec. This only returns matches which are prefixes of the word being completed. When the `-f` or `-d` option is used for filename or directory name completion, Bash uses shell the variable `FIGNORE` to filter the matches. See Bash Variables, for a description of `FIGNORE`.

Next, programmable completion generates matches specified by a pathname expansion pattern supplied as an argument to the `-G` option. The words generated by the pattern need not match the word being completed. Bash uses the `FIGNORE` variable to filter the matches, but does not use the `GLOBIGNORE` shell variable.

Next, completion considers the string specified as the argument to the `-W` option. The string is first split using the characters in the `IFS` special variable as delimiters. This honors shell quoting within the string, in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of `IFS`. Each word is then expanded using brace expansion, tilde expansion, parameter and variable expansion, command substitution, and arithmetic expansion, as described above (see Shell Expansions). The results are split using the rules described above (see Word Splitting). The results of the expansion are prefix-matched against the word being completed, and the matching words become possible completions.

After these matches have been generated, Bash executes any shell function or command specified with the `-F` and `-C` options. When the command or function is invoked, Bash assigns values to the `COMP_LINE`, `COMP_POINT`, `COMP_KEY`, and `COMP_TYPE` variables as described above (see Bash Variables). If a shell function is being invoked, Bash also sets the `COMP_WORDS` and `COMP_CWORD` variables. When the function or command is invoked, the first argument ($1) is the name of the command whose arguments are being completed, the second argument ($2) is the word being completed, and the third argument ($3) is the word preceding the word being completed on the current command line. There is no filtering of the generated completions against the word being completed; the function or command has complete freedom in generating the matches and they do not need to match a prefix of the word.

Any function specified with `-F` is invoked first. The function may use any of the shell facilities, including the `compgen` and `compopt` builtins described below (see Programmable Completion Builtins), to generate the matches. It must put the possible completions in the `COMPREPLY` array variable, one per array element.

Next, any command specified with the `-C` option is invoked in an environment equivalent to command substitution. It should print a list of completions, one per line, to the standard output. Backslash will escape a newline, if necessary. These are added to the set of possible completions.

After generating all of the possible completions, Bash applies any filter specified with the `-X` option to the completions in the list. The filter is a pattern as used for pathname expansion; a ‘`&`’ in the pattern is replaced with the text of the word being completed. A literal ‘`&`’ may be escaped with a backslash; the backslash is removed before attempting a match. Any completion that matches the pattern is removed from the list. A leading ‘`!`’ negates the pattern; in this case Bash removes any completion that does not match the pattern. If the `nocasematch` shell option is enabled (see the description of `shopt` in The Shopt Builtin), Bash performs the match without regard to the case of alphabetic characters.

Finally, programmable completion adds any prefix and suffix specified with the `-P` and `-S` options, respectively, to each completion, and returns the result to Readline as the list of possible completions.

If the previously-applied actions do not generate any matches, and the `-o dirnames` option was supplied to `complete` when the compspec was defined, Bash attempts directory name completion.

If the `-o plusdirs` option was supplied to `complete` when the compspec was defined, Bash attempts directory name completion and adds any matches to the set of possible completions.

By default, if a compspec is found, whatever it generates is returned to the completion code as the full set of possible completions. The default Bash completions and the Readline default of filename completion are disabled. If the `-o bashdefault` option was supplied to `complete` when the compspec was defined, and the compspec generates no matches, Bash attempts its default completions. If the compspec and, if attempted, the default Bash completions generate no matches, and the `-o default` option was supplied to `complete` when the compspec was defined, programmable completion performs Readline’s default completion.

The options supplied to `complete` and `compopt` can control how Readline treats the completions. For instance, the `-o fullquote` option tells Readline to quote the matches as if they were filenames. See the description of `complete` (see Programmable Completion Builtins) for details.

When a compspec indicates that it wants directory name completion, the programmable completion functions force Readline to append a slash to completed names which are symbolic links to directories, subject to the value of the mark-directories Readline variable, regardless of the setting of the mark-symlinked-directories Readline variable.

There is some support for dynamically modifying completions. This is most useful when used in combination with a default completion specified with `-D`. It’s possible for shell functions executed as completion functions to indicate that completion should be retried by returning an exit status of 124. If a shell function returns 124, and changes the compspec associated with the command on which completion is being attempted (supplied as the first argument when the function is executed), programmable completion restarts from the beginning, with an attempt to find a new compspec for that command. This can be used to build a set of completions dynamically as completion is attempted, rather than loading them all at once.

For instance, assuming that there is a library of compspecs, each kept in a file corresponding to the name of the command, the following default completion function would load completions dynamically:

    _completion_loader()
    {
        . "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
    }
    complete -D -F _completion_loader -o bashdefault -o default

### 8.7 Programmable Completion Builtins ¶

Three builtin commands are available to manipulate the programmable completion facilities: one to specify how the arguments to a particular command are to be completed, and two to modify the completion as it is happening.

`compgen` ¶

      compgen [-V varname] [option] [word]

  Generate possible completion matches for word according to the options, which may be any option accepted by the `complete` builtin with the exceptions of `-p`, `-r`, `-D`, `-E`, and `-I`, and write the matches to the standard output.

  If the `-V` option is supplied, `compgen` stores the generated completions into the indexed array variable varname instead of writing them to the standard output.

  When using the `-F` or `-C` options, the various shell variables set by the programmable completion facilities, while available, will not have useful values.

  The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. If word is specified, only those completions matching word will be displayed or stored.

  The return value is true unless an invalid option is supplied, or no matches were generated.

`complete` ¶

      complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action]
      [-G globpat] [-W wordlist] [-F function] [-C command]
      [-X filterpat] [-P prefix] [-S suffix] name [name ...]
      complete -pr [-DEI] [name ...]

  Specify how arguments to each name should be completed.

  If the `-p` option is supplied, or if no options or names are supplied, print existing completion specifications in a way that allows them to be reused as input. The `-r` option removes a completion specification for each name, or, if no names are supplied, all completion specifications.

  The `-D` option indicates that other supplied options and actions should apply to the “default” command completion; that is, completion attempted on a command for which no completion has previously been defined. The `-E` option indicates that other supplied options and actions should apply to “empty” command completion; that is, completion attempted on a blank line. The `-I` option indicates that other supplied options and actions should apply to completion on the initial non-assignment word on the line, or after a command delimiter such as ‘`;`’ or ‘`|`’, which is usually command name completion. If multiple options are supplied, the `-D` option takes precedence over `-E`, and both take precedence over `-I`. If any of `-D`, `-E`, or `-I` are supplied, any other name arguments are ignored; these completions only apply to the case specified by the option.

  The process of applying these completion specifications when word completion is attempted is described above (see Programmable Completion).

  Other options, if specified, have the following meanings. The arguments to the `-G`, `-W`, and `-X` options (and, if necessary, the `-P` and `-S` options) should be quoted to protect them from expansion before the `complete` builtin is invoked.

  `-o comp-option`

    The comp-option controls several aspects of the compspec’s behavior beyond the simple generation of completions. comp-option may be one of:

    `bashdefault`

      Perform the rest of the default Bash completions if the compspec generates no matches.

    `default`

      Use Readline’s default filename completion if the compspec generates no matches.

    `dirnames`

      Perform directory name completion if the compspec generates no matches.

    `filenames`

      Tell Readline that the compspec generates filenames, so it can perform any filename-specific processing (such as adding a slash to directory names, quoting special characters, or suppressing trailing spaces). This option is intended to be used with shell functions specified with `-F`.

    `fullquote`

      Tell Readline to quote all the completed words even if they are not filenames.

    `noquote`

      Tell Readline not to quote the completed words if they are filenames (quoting filenames is the default).

    `nosort`

      Tell Readline not to sort the list of possible completions alphabetically.

    `nospace`

      Tell Readline not to append a space (the default) to words completed at the end of the line.

    `plusdirs`

      After generating any matches defined by the compspec, attempt directory name completion and add any matches to the results of the other actions.

  `-A action`

    The action may be one of the following to generate a list of possible completions:

    `alias`

      Alias names. May also be specified as `-a`.

    `arrayvar`

      Array variable names.

    `binding`

      Readline key binding names (see Bindable Readline Commands).

    `builtin`

      Names of shell builtin commands. May also be specified as `-b`.

    `command`

      Command names. May also be specified as `-c`.

    `directory`

      Directory names. May also be specified as `-d`.

    `disabled`

      Names of disabled shell builtins.

    `enabled`

      Names of enabled shell builtins.

    `export`

      Names of exported shell variables. May also be specified as `-e`.

    `file`

      File and directory names, similar to Readline’s filename completion. May also be specified as `-f`.

    `function`

      Names of shell functions.

    `group`

      Group names. May also be specified as `-g`.

    `helptopic`

      Help topics as accepted by the `help` builtin (see Bash Builtin Commands).

    `hostname`

      Hostnames, as taken from the file specified by the `HOSTFILE` shell variable (see Bash Variables).

    `job`

      Job names, if job control is active. May also be specified as `-j`.

    `keyword`

      Shell reserved words. May also be specified as `-k`.

    `running`

      Names of running jobs, if job control is active.

    `service`

      Service names. May also be specified as `-s`.

    `setopt`

      Valid arguments for the `-o` option to the `set` builtin (see The Set Builtin).

    `shopt`

      Shell option names as accepted by the `shopt` builtin (see Bash Builtin Commands).

    `signal`

      Signal names.

    `stopped`

      Names of stopped jobs, if job control is active.

    `user`

      User names. May also be specified as `-u`.

    `variable`

      Names of all shell variables. May also be specified as `-v`.

  `-C command`

    command is executed in a subshell environment, and its output is used as the possible completions. Arguments are passed as with the `-F` option.

  `-F function`

    The shell function function is executed in the current shell environment. When it is executed, the first argument ($1) is the name of the command whose arguments are being completed, the second argument ($2) is the word being completed, and the third argument ($3) is the word preceding the word being completed, as described above (see Programmable Completion). When `function` finishes, programmable completion retrieves the possible completions from the value of the `COMPREPLY` array variable.

  `-G globpat`

    Expand the filename expansion pattern globpat to generate the possible completions.

  `-P prefix`

    Add prefix to the beginning of each possible completion after all other options have been applied.

  `-S suffix`

    Append suffix to each possible completion after all other options have been applied.

  `-W wordlist`

    Split the wordlist using the characters in the `IFS` special variable as delimiters, and expand each resulting word. Shell quoting is honored within wordlist in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of `IFS`. The possible completions are the members of the resultant list which match a prefix of the word being completed.

  `-X filterpat`

    filterpat is a pattern as used for filename expansion. It is applied to the list of possible completions generated by the preceding options and arguments, and each completion matching filterpat is removed from the list. A leading ‘`!`’ in filterpat negates the pattern; in this case, any completion not matching filterpat is removed.

  The return value is true unless an invalid option is supplied, an option other than `-p`, `-r`, `-D`, `-E`, or `-I` is supplied without a name argument, an attempt is made to remove a completion specification for a name for which no specification exists, or an error occurs adding a completion specification.

`compopt` ¶

      compopt [-o option] [-DEI] [+o option] [name]

  Modify completion options for each name according to the options, or for the currently-executing completion if no names are supplied. If no options are given, display the completion options for each name or the current completion. The possible values of option are those valid for the `complete` builtin described above.

  The `-D` option indicates that other supplied options should apply to the “default” command completion; the `-E` option indicates that other supplied options should apply to “empty” command completion; and the `-I` option indicates that other supplied options should apply to completion on the initial word on the line. These are determined in the same way as the `complete` builtin.

  If multiple options are supplied, the `-D` option takes precedence over `-E`, and both take precedence over `-I`

  The return value is true unless an invalid option is supplied, an attempt is made to modify the options for a name for which no completion specification exists, or an output error occurs.

### 8.8 A Programmable Completion Example ¶

The most common way to obtain additional completion functionality beyond the default actions `complete` and `compgen` provide is to use a shell function and bind it to a particular command using `complete -F`.

The following function provides completions for the `cd` builtin. It is a reasonably good example of what shell functions must do when used for completion. This function uses the word passed as `$2` to determine the directory name to complete. You can also use the `COMP_WORDS` array variable; the current word is indexed by the `COMP_CWORD` variable.

The function relies on the `complete` and `compgen` builtins to do much of the work, adding only the things that the Bash `cd` does beyond accepting basic directory names: tilde expansion (see Tilde Expansion), searching directories in $CDPATH, which is described above (see Bourne Shell Builtins), and basic support for the `cdable_vars` shell option (see The Shopt Builtin). `_comp_cd` modifies the value of IFS so that it contains only a newline to accommodate file names containing spaces and tabs – `compgen` prints the possible completions it generates one per line.

Possible completions go into the COMPREPLY array variable, one completion per array element. The programmable completion system retrieves the completions from there when the function returns.

    # A completion function for the cd builtin
    # based on the cd completion function from the bash_completion package
    _comp_cd()
    {
        local IFS=$' \t\n'    # normalize IFS
        local cur _skipdot _cdpath
        local i j k

        # Tilde expansion, which also expands tilde to full pathname
        case "$2" in
        \~*)    eval cur="$2" ;;
        *)      cur=$2 ;;
        esac

        # no cdpath or absolute pathname -- straight directory completion
        if [[ -z "${CDPATH:-}" ]] || [[ "$cur" == @(./*|../*|/*) ]]; then
            # compgen prints paths one per line; could also use while loop
            IFS=$'\n'
            COMPREPLY=( $(compgen -d -- "$cur") )
            IFS=$' \t\n'
        # CDPATH+directories in the current directory if not in CDPATH
        else
            IFS=$'\n'
            _skipdot=false
            # preprocess CDPATH to convert null directory names to .
            _cdpath=${CDPATH/#:/.:}
            _cdpath=${_cdpath//::/:.:}
            _cdpath=${_cdpath/%:/:.}
            for i in ${_cdpath//:/$'\n'}; do
                if [[ $i -ef . ]]; then _skipdot=true; fi
                k="${#COMPREPLY[@]}"
                for j in $( compgen -d -- "$i/$cur" ); do
                    COMPREPLY[k++]=${j#$i/}        # cut off directory
                done
            done
            $_skipdot || COMPREPLY+=( $(compgen -d -- "$cur") )
            IFS=$' \t\n'
        fi

        # variable names if appropriate shell option set and no completions
        if shopt -q cdable_vars && [[ ${#COMPREPLY[@]} -eq 0 ]]; then
            COMPREPLY=( $(compgen -v -- "$cur") )
        fi

        return 0
    }

We install the completion function using the `-F` option to `complete`:

    # Tell readline to quote appropriate and append slashes to directories;
    # use the bash default completion for other arguments
    complete -o filenames -o nospace -o bashdefault -F _comp_cd cd

Since we’d like Bash and Readline to take care of some of the other details for us, we use several other options to tell Bash and Readline what to do. The `-o filenames` option tells Readline that the possible completions should be treated as filenames, and quoted appropriately. That option will also cause Readline to append a slash to filenames it can determine are directories (which is why we might want to extend `_comp_cd` to append a slash if we’re using directories found via CDPATH: Readline can’t tell those completions are directories). The `-o nospace` option tells Readline to not append a space character to the directory name, in case we want to append to it. The `-o bashdefault` option brings in the rest of the “Bash default” completions – possible completions that Bash adds to the default Readline set. These include things like command name completion, variable completion for words beginning with ‘`$`’ or ‘`${`’, completions containing pathname expansion patterns (see Filename Expansion), and so on.

Once installed using `complete`, `_comp_cd` will be called every time we attempt word completion for a `cd` command.

Many more examples – an extensive collection of completions for most of the common GNU, Unix, and Linux commands – are available as part of the bash_completion project. This is installed by default on many GNU/Linux distributions. Originally written by Ian Macdonald, the project now lives at https://github.com/scop/bash-completion/. There are ports for other systems such as Solaris and Mac OS X.

An older version of the bash_completion package is distributed with bash in the `examples/complete` subdirectory.

## 9 Using History Interactively ¶

This chapter describes how to use the GNU History Library interactively, from a user’s standpoint. It should be considered a user’s guide. For information on using the GNU History Library in other programs, see the GNU Readline Library Manual.

### 9.1 Bash History Facilities ¶

When the `-o history` option to the `set` builtin is enabled (see The Set Builtin), the shell provides access to the command history, the list of commands previously typed. The value of the `HISTSIZE` shell variable is used as the number of commands to save in a history list: the shell saves the text of the last `$HISTSIZE` commands (default 500). The shell stores each command in the history list prior to parameter and variable expansion but after history expansion is performed, subject to the values of the shell variables `HISTIGNORE` and `HISTCONTROL`.

When the shell starts up, Bash initializes the history list by reading history entries from the file named by the `HISTFILE` variable (default `~/.bash_history`). This is referred to as the history file. The history file is truncated, if necessary, to contain no more than the number of history entries specified by the value of the `HISTFILESIZE` variable. If `HISTFILESIZE` is unset, or set to null, a non-numeric value, or a numeric value less than zero, the history file is not truncated.

When the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history entry. These timestamps are optionally displayed depending on the value of the `HISTTIMEFORMAT` variable (see Bash Variables). When present, history timestamps delimit history entries, making multi-line entries possible.

When a shell with history enabled exits, Bash copies the last `$HISTSIZE` entries from the history list to the file named by `$HISTFILE`. If the `histappend` shell option is set (see Bash Builtin Commands), Bash appends the entries to the history file, otherwise it overwrites the history file. If `HISTFILE` is unset or null, or if the history file is unwritable, the history is not saved. After saving the history, Bash truncates the history file to contain no more than `$HISTFILESIZE` lines as described above.

If the `HISTTIMEFORMAT` variable is set, the shell writes the timestamp information associated with each history entry to the history file, marked with the history comment character, so timestamps are preserved across shell sessions. When the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history entry. As above, when using `HISTTIMEFORMAT`, the timestamps delimit multi-line history entries.

The `fc` builtin command will list or edit and re-execute a portion of the history list. The `history` builtin can display or modify the history list and manipulate the history file. When using command-line editing, search commands are available in each editing mode that provide access to the history list (see Commands For Manipulating The History).

The shell allows control over which commands are saved on the history list. The `HISTCONTROL` and `HISTIGNORE` variables are used to save only a subset of the commands entered. If the `cmdhist` shell option is enabled, the shell attempts to save each line of a multi-line command in the same history entry, adding semicolons where necessary to preserve syntactic correctness. The `lithist` shell option modifies `cmdhist` by saving the command with embedded newlines instead of semicolons. The `shopt` builtin is used to set these options. See The Shopt Builtin, for a description of `shopt`.

### 9.2 Bash History Builtins ¶

Bash provides two builtin commands which manipulate the history list and history file.

`fc` ¶

      fc [-e ename] [-lnr] [first] [last]
      fc -s [pat=rep] [command]

  The first form selects a range of commands from first to last from the history list and displays or edits and re-executes them. Both first and last may be specified as a string (to locate the most recent command beginning with that string) or as a number (an index into the history list, where a negative number is used as an offset from the current command number).

  When listing, a first or last of 0 is equivalent to -1 and -0 is equivalent to the current command (usually the `fc` command); otherwise 0 is equivalent to -1 and -0 is invalid.

  If last is not specified, it is set to the current command for listing and to first otherwise. If first is not specified, it is set to the previous command for editing and −16 for listing.

  If the `-l` flag is supplied, the commands are listed on standard output. The `-n` flag suppresses the command numbers when listing. The `-r` flag reverses the order of the listing.

  Otherwise, `fc` invokes the editor named by ename on a file containing those commands. If ename is not supplied, `fc` uses the value of the following variable expansion: `${FCEDIT:-${EDITOR:-vi}}`. This says to use the value of the `FCEDIT` variable if set, or the value of the `EDITOR` variable if that is set, or `vi` if neither is set. When editing is complete, `fc` reads the file of edited commands and echoes and executes them.

  In the second form, `fc` re-executes command after replacing each instance of pat in the selected command with rep. command is interpreted the same as first above.

  A useful alias to use with the `fc` command is `r='fc -s'`, so that typing ‘`r cc`’ runs the last command beginning with `cc` and typing ‘`r`’ re-executes the last command (see Aliases).

  If the first form is used, the return value is zero unless an invalid option is encountered or first or last specify history lines out of range. When editing and re-executing a file of commands, the return value is the value of the last command executed or failure if an error occurs with the temporary file. If the second form is used, the return status is that of the re-executed command, unless command does not specify a valid history entry, in which case `fc` returns a non-zero status.

`history` ¶

      history [n]
      history -c
      history -d offset
      history -d start-end
      history [-anrw] [filename]
      history -ps arg

  With no options, display the history list with numbers. Entries prefixed with a ‘`*`’ have been modified. An argument of n lists only the last n entries. If the shell variable `HISTTIMEFORMAT` is set and not null, it is used as a format string for `strftime`(3) to display the time stamp associated with each displayed history entry. If `history` uses `HISTTIMEFORMAT`, it does not print an intervening space between the formatted time stamp and the history entry.

  Options, if supplied, have the following meanings:

  `-c`

    Clear the history list. This may be combined with the other options to replace the history list.

  `-d offset`

    Delete the history entry at position offset. If offset is positive, it should be specified as it appears when the history is displayed. If offset is negative, it is interpreted as relative to one greater than the last history position, so negative indices count back from the end of the history, and an index of ‘`-1`’ refers to the current `history -d` command.

  `-d start-end`

    Delete the range of history entries between positions start and end, inclusive. Positive and negative values for start and end are interpreted as described above.

  `-a`

    Append the "new" history lines to the history file. These are history lines entered since the beginning of the current Bash session, but not already appended to the history file.

  `-n`

    Read the history lines not already read from the history file and add them to the current history list. These are lines appended to the history file since the beginning of the current Bash session.

  `-r`

    Read the history file and append its contents to the history list.

  `-w`

    Write the current history list to the history file, overwriting the history file.

  `-p`

    Perform history substitution on the args and display the result on the standard output, without storing the results in the history list.

  `-s`

    Add the args to the end of the history list as a single entry. The last command in the history list is removed before adding the args.

  If a filename argument is supplied with any of the `-w`, `-r`, `-a`, or `-n` options, Bash uses filename as the history file. If not, it uses the value of the `HISTFILE` variable. If `HISTFILE` is unset or null, these options have no effect.

  If the `HISTTIMEFORMAT` variable is set, `history` writes the time stamp information associated with each history entry to the history file, marked with the history comment character as described above. When the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history entry.

  The return value is 0 unless an invalid option is encountered, an error occurs while reading or writing the history file, an invalid offset or range is supplied as an argument to `-d`, or the history expansion supplied as an argument to `-p` fails.

### 9.3 History Expansion ¶

The shell provides a history expansion feature that is similar to the history expansion provided by `csh` (also referred to as history substitution where appropriate). This section describes the syntax used to manipulate the history information.

History expansion is enabled by default for interactive shells, and can be disabled using the `+H` option to the `set` builtin command (see The Set Builtin). Non-interactive shells do not perform history expansion by default, but it can be enabled with `set -H`.

History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.

History expansion is performed immediately after a complete line is read, before the shell breaks it into words, and is performed on each line individually. Bash attempts to inform the history expansion functions about quoting still in effect from previous lines.

History expansion takes place in two parts. The first is to determine which entry from the history list should be used during substitution. The second is to select portions of that entry to include into the current one.

The entry selected from the history is called the event, and the portions of that entry that are acted upon are words. Various modifiers are available to manipulate the selected words. The entry is split into words in the same fashion that Bash does when reading input, so that several words surrounded by quotes are considered one word. The event designator selects the event, the optional word designator selects words from the event, and various optional modifiers are available to manipulate the selected words.

History expansions are introduced by the appearance of the history expansion character, which is ‘`!`’ by default. History expansions may appear anywhere in the input, but do not nest.

History expansion implements shell-like quoting conventions: a backslash can be used to remove the special handling for the next character; single quotes enclose verbatim sequences of characters, and can be used to inhibit history expansion; and characters enclosed within double quotes may be subject to history expansion, since backslash can escape the history expansion character, but single quotes may not, since they are not treated specially within double quotes.

When using the shell, only ‘`\`’ and ‘`'`’ may be used to escape the history expansion character, but the history expansion character is also treated as quoted if it immediately precedes the closing double quote in a double-quoted string.

Several characters inhibit history expansion if found immediately following the history expansion character, even if it is unquoted: space, tab, newline, carriage return, ‘`=`’, and the other shell metacharacters.

There is a special abbreviation for substitution, active when the quick substitution character (described above under `histchars`) is the first character on the line. It selects the previous history list entry, using an event designator equivalent to `!!`, and substitutes one string for another in that entry. It is described below (see Event Designators). This is the only history expansion that does not begin with the history expansion character.

Several shell options settable with the `shopt` builtin (see The Shopt Builtin) modify history expansion behavior If the `histverify` shell option is enabled, and Readline is being used, history substitutions are not immediately passed to the shell parser. Instead, the expanded line is reloaded into the Readline editing buffer for further modification. If Readline is being used, and the `histreedit` shell option is enabled, a failed history expansion is reloaded into the Readline editing buffer for correction.

The `-p` option to the `history` builtin command shows what a history expansion will do before using it. The `-s` option to the `history` builtin may be used to add commands to the end of the history list without actually executing them, so that they are available for subsequent recall. This is most useful in conjunction with Readline.

The shell allows control of the various characters used by the history expansion mechanism with the `histchars` variable, as explained above (see Bash Variables). The shell uses the history comment character to mark history timestamps when writing the history file.

#### 9.3.1 Event Designators ¶

An event designator is a reference to an entry in the history list. The event designator consists of the portion of the word beginning with the history expansion character, and ending with the word designator if one is present, or the end of the word. Unless the reference is absolute, events are relative to the current position in the history list.

`!`

  Start a history substitution, except when followed by a space, tab, the end of the line, ‘`=`’, or the rest of the shell metacharacters defined above (see Definitions).

`!n`

  Refer to history list entry n.

`!-n`

  Refer to the history entry minus n.

`!!`

  Refer to the previous entry. This is a synonym for ‘`!-1`’.

`!string`

  Refer to the most recent command preceding the current position in the history list starting with string.

`!?string[?]`

  Refer to the most recent command preceding the current position in the history list containing string. The trailing ‘`?`’ may be omitted if the string is followed immediately by a newline. If string is missing, this uses the string from the most recent search; it is an error if there is no previous search string.

`^string1^string2^`

  Quick Substitution. Repeat the last command, replacing string1 with string2. Equivalent to `!!:s^string1^string2^`.

`!#`

  The entire command line typed so far.

#### 9.3.2 Word Designators ¶

Word designators are used to select desired words from the event. They are optional; if the word designator isn’t supplied, the history expansion uses the entire event. A ‘`:`’ separates the event specification from the word designator. It may be omitted if the word designator begins with a ‘`^`’, ‘`$`’, ‘`*`’, ‘`-`’, or ‘`%`’. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). That first word is usually the command word, and the arguments begin with the second word. Words are inserted into the current line separated by single spaces.

For example,

`!!`

  designates the preceding command. When you type this, the preceding command is repeated in toto.

`!!:$`

  designates the last word of the preceding command. This may be shortened to `!$`.

`!fi:2`

  designates the second argument of the most recent command starting with the letters `fi`.

Here are the word designators:

`0 (zero)`

  The `0`th word. For the shell, and many other, applications, this is the command word.

`n`

  The nth word.

`^`

  The first argument: word 1.

`$`

  The last word. This is usually the last argument, but expands to the zeroth word if there is only one word in the line.

`%`

  The first word matched by the most recent ‘`?string?`’ search, if the search string begins with a character that is part of a word. By default, searches begin at the end of each line and proceed to the beginning, so the first word matched is the one closest to the end of the line.

`x-y`

  A range of words; ‘`-y`’ abbreviates ‘`0-y`’.

`*`

  All of the words, except the `0`th. This is a synonym for ‘`1-$`’. It is not an error to use ‘`*`’ if there is just one word in the event; it expands to the empty string in that case.

`x*`

  Abbreviates ‘`x-$`’.

`x-`

  Abbreviates ‘`x-$`’ like ‘`x*`’, but omits the last word. If ‘`x`’ is missing, it defaults to 0.

If a word designator is supplied without an event specification, the previous command is used as the event, equivalent to `!!`.

#### 9.3.3 Modifiers ¶

After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a ‘`:`’. These modify, or edit, the word or words selected from the history event.

`h`

  Remove a trailing filename component, leaving only the head.

`t`

  Remove all leading filename components, leaving the tail.

`r`

  Remove a trailing suffix of the form ‘`.suffix`’, leaving the basename.

`e`

  Remove all but the trailing suffix.

`p`

  Print the new command but do not execute it.

`q`

  Quote the substituted words, escaping further substitutions.

`x`

  Quote the substituted words as with ‘`q`’, but break into words at spaces, tabs, and newlines. The ‘`q`’ and ‘`x`’ modifiers are mutually exclusive; expansion uses the last one supplied.

`s/old/new/`

  Substitute new for the first occurrence of old in the event line. Any character may be used as the delimiter in place of ‘`/`’. The delimiter may be quoted in old and new with a single backslash. If ‘`&`’ appears in new, it is replaced with old. A single backslash quotes the ‘`&`’ in old and new. If old is null, it is set to the last old substituted, or, if no previous history substitutions took place, the last string in a !?string`[?]` search. If new is null, each matching old is deleted. The final delimiter is optional if it is the last character on the input line.

`&`

  Repeat the previous substitution.

`g`
`a`

  Cause changes to be applied over the entire event line. This is used in conjunction with ‘`s`’, as in `gs/old/new/`, or with ‘`&`’.

`G`

  Apply the following ‘`s`’ or ‘`&`’ modifier once to each word in the event.

## 10 Installing Bash ¶

This chapter provides basic instructions for installing Bash on the various supported platforms. The distribution supports the GNU operating systems, nearly every version of Unix, and several non-Unix systems such as BeOS and Interix. Other independent ports exist for Windows platforms.

### 10.1 Basic Installation ¶

These are installation instructions for Bash.

The simplest way to compile Bash is:

1. `cd` to the directory containing the source code and type ‘`./configure`’ to configure Bash for your system. If you’re using `csh` on an old version of System V, you might need to type ‘`sh ./configure`’ instead to prevent `csh` from trying to execute `configure` itself.

   Running `configure` takes some time. While running, it prints messages telling which features it is checking for.

2. Type ‘`make`’ to compile Bash and build the `bashbug` bug reporting script.
3. Optionally, type ‘`make tests`’ to run the Bash test suite.
4. Type ‘`make install`’ to install `bash` and `bashbug`. This will also install the manual pages and Info file, message translation files, some supplemental documentation, a number of example loadable builtin commands, and a set of header files for developing loadable builtins. You may need additional privileges to install `bash` to your desired destination, which may require ‘`sudo make install`’. More information about controlling the locations where `bash` and other files are installed is below (see Installation Names).

The `configure` shell script attempts to guess correct values for various system-dependent variables used during compilation. It uses those values to create a `Makefile` in each directory of the package (the top directory, the `builtins`, `doc`, `po`, and `support` directories, each directory under `lib`, and several others). It also creates a `config.h` file containing system-dependent definitions. Finally, it creates a shell script named `config.status` that you can run in the future to recreate the current configuration, a file `config.cache` that saves the results of its tests to speed up reconfiguring, and a file `config.log` containing compiler output (useful mainly for debugging `configure`). If at some point `config.cache` contains results you don’t want to keep, you may remove or edit it.

To find out more about the options and arguments that the `configure` script understands, type

    bash-4.2$ ./configure --help

at the Bash prompt in your Bash source directory.

If you want to build Bash in a directory separate from the source directory – to build for multiple architectures, for example – just use the full path to the configure script. The following commands will build Bash in a directory under `/usr/local/build` from the source code in `/usr/local/src/bash-4.4`:

    mkdir /usr/local/build/bash-4.4
    cd /usr/local/build/bash-4.4
    bash /usr/local/src/bash-4.4/configure
    make

See Compiling For Multiple Architectures for more information about building in a directory separate from the source.

If you need to do unusual things to compile Bash, please try to figure out how `configure` could check whether or not to do them, and mail diffs or instructions to bash-maintainers@gnu.org so they can be considered for the next release.

The file `configure.ac` is used to create `configure` by a program called Autoconf. You only need `configure.ac` if you want to change it or regenerate `configure` using a newer version of Autoconf. If you do this, make sure you are using Autoconf version 2.69 or newer.

You can remove the program binaries and object files from the source code directory by typing ‘`make clean`’. To also remove the files that `configure` created (so you can compile Bash for a different kind of computer), type ‘`make distclean`’.

### 10.2 Compilers and Options ¶

Some systems require unusual options for compilation or linking that the `configure` script does not know about. You can give `configure` initial values for variables by setting them in the environment. Using a Bourne-compatible shell, you can do that on the command line like this:

    CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure

On systems that have the `env` program, you can do it like this:

    env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure

The configuration process uses GCC to build Bash if it is available.

### 10.3 Compiling For Multiple Architectures ¶

You can compile Bash for more than one kind of computer at the same time, by placing the object files for each architecture in their own directory. To do this, you must use a version of `make` that supports the `VPATH` variable, such as GNU `make`. `cd` to the directory where you want the object files and executables to go and run the `configure` script from the source directory (see Basic Installation). You may need to supply the `--srcdir=PATH` argument to tell `configure` where the source files are. `configure` automatically checks for the source code in the directory that `configure` is in and in `..`.

If you have to use a `make` that does not support the `VPATH` variable, you can compile Bash for one architecture at a time in the source code directory. After you have installed Bash for one architecture, use ‘`make distclean`’ before reconfiguring for another architecture.

Alternatively, if your system supports symbolic links, you can use the `support/mkclone` script to create a build tree which has symbolic links back to each file in the source directory. Here’s an example that creates a build directory in the current directory from a source directory `/usr/gnu/src/bash-2.0`:

    bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .

The `mkclone` script requires Bash, so you must have already built Bash for at least one architecture before you can create build directories for other architectures.

### 10.4 Installation Names ¶

By default, ‘`make install`’ will install into `/usr/local/bin`, `/usr/local/man`, etc.; that is, the installation prefix defaults to `/usr/local`. You can specify an installation prefix other than `/usr/local` by giving `configure` the option `--prefix=PATH`, or by specifying a value for the `prefix` ‘`make`’ variable when running ‘`make install`’ (e.g., ‘`make install prefix=PATH`’). The `prefix` variable provides a default for `exec_prefix` and other variables used when installing Bash.

You can specify separate installation prefixes for architecture-specific files and architecture-independent files. If you give `configure` the option `--exec-prefix=PATH`, ‘`make install`’ will use PATH as the prefix for installing programs and libraries. Documentation and other data files will still use the regular prefix.

If you would like to change the installation locations for a single run, you can specify these variables as arguments to `make`: ‘`make install exec_prefix=/`’ will install `bash` and `bashbug` into `/bin` instead of the default `/usr/local/bin`.

If you want to see the files Bash will install and where it will install them without changing anything on your system, specify the variable `DESTDIR` as an argument to `make`. Its value should be the absolute directory path you’d like to use as the root of your sample installation tree. For example,

    mkdir /fs1/bash-install
    make install DESTDIR=/fs1/bash-install

will install `bash` into `/fs1/bash-install/usr/local/bin/bash`, the documentation into directories within `/fs1/bash-install/usr/local/share`, the example loadable builtins into `/fs1/bash-install/usr/local/lib/bash`, and so on. You can use the usual `exec_prefix` and `prefix` variables to alter the directory paths beneath the value of `DESTDIR`.

The GNU Makefile standards provide a more complete description of these variables and their effects.

### 10.5 Specifying the System Type ¶

There may be some features `configure` can not figure out automatically, but needs to determine by the type of host Bash will run on. Usually `configure` can figure that out, but if it prints a message saying it can not guess the host type, give it the `--host=TYPE` option. ‘`TYPE`’ can either be a short name for the system type, such as ‘`sun4`’, or a canonical name with three fields: ‘`CPU-COMPANY-SYSTEM`’ (e.g., ‘`i386-unknown-freebsd4.2`’).

See the file `support/config.sub` for the possible values of each field.

### 10.6 Sharing Defaults ¶

If you want to set default values for `configure` scripts to share, you can create a site shell script called `config.site` that gives default values for variables like `CC`, `cache_file`, and `prefix`. `configure` looks for `PREFIX/share/config.site` if it exists, then `PREFIX/etc/config.site` if it exists. Or, you can set the `CONFIG_SITE` environment variable to the location of the site script. A warning: the Bash `configure` looks for a site script, but not all `configure` scripts do.

### 10.7 Operation Controls ¶

`configure` recognizes the following options to control how it operates.

`--cache-file=file`

  Use and save the results of the tests in file instead of `./config.cache`. Set file to `/dev/null` to disable caching, for debugging `configure`.

`--help`

  Print a summary of the options to `configure`, and exit.

`--quiet`
`--silent`
`-q`

  Do not print messages saying which checks are being made.

`--srcdir=dir`

  Look for the Bash source code in directory dir. Usually `configure` can determine that directory automatically.

`--version`

  Print the version of Autoconf used to generate the `configure` script, and exit.

`configure` also accepts some other, not widely used, boilerplate options. ‘`configure --help`’ prints the complete list.

### 10.8 Optional Features ¶

The Bash `configure` has a number of `--enable-feature` options, where feature indicates an optional part of Bash. There are also several `--with-package` options, where package is something like ‘`bash-malloc`’ or ‘`afs`’. To turn off the default use of a package, use `--without-package`. To configure Bash without a feature that is enabled by default, use `--disable-feature`.

Here is a complete list of the `--enable-` and `--with-` options that the Bash `configure` recognizes.

`--with-afs`

  Define if you are using the Andrew File System from Transarc.

`--with-bash-malloc`

  Use the Bash version of `malloc` in the directory `lib/malloc`. This is not the same `malloc` that appears in GNU libc, but a custom version originally derived from the 4.2 BSD `malloc`. This `malloc` is very fast, but wastes some space on each allocation, though it uses several techniques to minimize the waste. This option is enabled by default. The `NOTES` file contains a list of systems for which this should be turned off, and `configure` disables this option automatically for a number of systems.

`--with-curses`

  Use the curses library instead of the termcap library. `configure` usually chooses this automatically, since most systems include the termcap functions in the curses library.

`--with-gnu-malloc`

  A synonym for `--with-bash-malloc`.

`--with-installed-readline[=PREFIX]`

  Define this to make Bash link with a locally-installed version of Readline rather than the version in `lib/readline`. This works only with Readline 5.0 and later versions. If PREFIX is `yes` or not supplied, `configure` uses the values of the make variables `includedir` and `libdir`, which are subdirectories of `prefix` by default, to find the installed version of Readline if it is not in the standard system include and library directories. If PREFIX is `no`, Bash links with the version in `lib/readline`. If PREFIX is set to any other value, `configure` treats it as a directory pathname and looks for the installed version of Readline in subdirectories of that directory (include files in PREFIX/`include` and the library in PREFIX/`lib`). The Bash default is to link with a static library built in the `lib/readline` subdirectory of the build directory.

`--with-libintl-prefix[=PREFIX]`

  Define this to make Bash link with a locally-installed version of the libintl library instead of the version in `lib/intl`.

`--with-libiconv-prefix[=PREFIX]`

  Define this to make Bash look for libiconv in PREFIX instead of the standard system locations. The Bash distribution does not include this library.

`--enable-minimal-config`

  This produces a shell with minimal features, closer to the historical Bourne shell.

There are several `--enable-` options that alter how Bash is compiled, linked, and installed, rather than changing run-time features.

`--enable-largefile`

  Enable support for large files if the operating system requires special compiler options to build programs which can access large files. This is enabled by default, if the operating system provides large file support.

`--enable-profiling`

  This builds a Bash binary that produces profiling information to be processed by `gprof` each time it is executed.

`--enable-separate-helpfiles`

  Use external files for the documentation displayed by the `help` builtin instead of storing the text internally.

`--enable-static-link`

  This causes Bash to be linked statically, if `gcc` is being used. This could be used to build a version to use as root’s shell.

The ‘`minimal-config`’ option can be used to disable all of the following options, but it is processed first, so individual options may be enabled using ‘`enable-feature`’.

All of the following options except for ‘`alt-array-implementation`’, ‘`disabled-builtins`’, ‘`direxpand-default`’, ‘`strict-posix-default`’, and ‘`xpg-echo-default`’ are enabled by default, unless the operating system does not provide the necessary support.

`--enable-alias`

  Allow alias expansion and include the `alias` and `unalias` builtins (see Aliases).

`--enable-alt-array-implementation`

  This builds Bash using an alternate implementation of arrays (see Arrays) that provides faster access at the expense of using more memory (sometimes many times more, depending on how sparse an array is).

`--enable-arith-for-command`

  Include support for the alternate form of the `for` command that behaves like the C language `for` statement (see Looping Constructs).

`--enable-array-variables`

  Include support for one-dimensional array shell variables (see Arrays).

`--enable-bang-history`

  Include support for `csh`-like history substitution (see History Expansion).

`--enable-bash-source-fullpath-default`

  Set the default value of the `bash_source_fullpath` shell option described above under The Shopt Builtin to be enabled. This controls how filenames are assigned to the `BASH_SOURCE` array variable.

`--enable-brace-expansion`

  Include `csh`-like brace expansion ( `b{a,b}c` → `bac bbc` ). See Brace Expansion, for a complete description.

`--enable-casemod-attributes`

  Include support for case-modifying attributes in the `declare` builtin and assignment statements. Variables with the `uppercase` attribute, for example, will have their values converted to uppercase upon assignment.

`--enable-casemod-expansion`

  Include support for case-modifying word expansions.

`--enable-command-timing`

  Include support for recognizing `time` as a reserved word and for displaying timing statistics for the pipeline following `time` (see Pipelines). This allows timing pipelines, shell compound commands, shell builtins, and shell functions, which an external command cannot do easily.

`--enable-cond-command`

  Include support for the `[[` conditional command. (see Conditional Constructs).

`--enable-cond-regexp`

  Include support for matching POSIX regular expressions using the ‘`=~`’ binary operator in the `[[` conditional command. (see Conditional Constructs).

`--enable-coprocesses`

  Include support for coprocesses and the `coproc` reserved word (see Pipelines).

`--enable-debugger`

  Include support for the Bash debugger (distributed separately).

`--enable-dev-fd-stat-broken`

  If calling `stat` on /dev/fd/N returns different results than calling `fstat` on file descriptor N, supply this option to enable a workaround. This has implications for conditional commands that test file attributes.

`--enable-direxpand-default`

  Cause the `direxpand` shell option (see The Shopt Builtin) to be enabled by default when the shell starts. It is normally disabled by default.

`--enable-directory-stack`

  Include support for a `csh`-like directory stack and the `pushd`, `popd`, and `dirs` builtins (see The Directory Stack).

`--enable-disabled-builtins`

  Allow builtin commands to be invoked via ‘`builtin xxx`’ even after `xxx` has been disabled using ‘`enable -n xxx`’. See Bash Builtin Commands, for details of the `builtin` and `enable` builtin commands.

`--enable-dparen-arithmetic`

  Include support for the `((…))` command (see Conditional Constructs).

`--enable-extended-glob`

  Include support for the extended pattern matching features described above under Pattern Matching.

`--enable-extended-glob-default`

  Set the default value of the `extglob` shell option described above under The Shopt Builtin to be enabled.

`--enable-function-import`

  Include support for importing function definitions exported by another instance of the shell from the environment. This option is enabled by default.

`--enable-glob-asciiranges-default`

  Set the default value of the `globasciiranges` shell option described above under The Shopt Builtin to be enabled. This controls the behavior of character ranges when used in pattern matching bracket expressions.

`--enable-help-builtin`

  Include the `help` builtin, which displays help on shell builtins and variables (see Bash Builtin Commands).

`--enable-history`

  Include command history and the `fc` and `history` builtin commands (see Bash History Facilities).

`--enable-job-control`

  This enables the job control features (see Job Control), if the operating system supports them.

`--enable-multibyte`

  This enables support for multibyte characters if the operating system provides the necessary support.

`--enable-net-redirections`

  This enables the special handling of filenames of the form `/dev/tcp/host/port` and `/dev/udp/host/port` when used in redirections (see Redirections).

`--enable-process-substitution`

  This enables process substitution (see Process Substitution) if the operating system provides the necessary support.

`--enable-progcomp`

  Enable the programmable completion facilities (see Programmable Completion). If Readline is not enabled, this option has no effect.

`--enable-prompt-string-decoding`

  Turn on the interpretation of a number of backslash-escaped characters in the `$PS0`, `$PS1`, `$PS2`, and `$PS4` prompt strings. See Controlling the Prompt, for a complete list of prompt string escape sequences.

`--enable-readline`

  Include support for command-line editing and history with the Bash version of the Readline library (see Command Line Editing).

`--enable-restricted`

  Include support for a restricted shell. If this is enabled, Bash enters a restricted mode when called as `rbash`. See The Restricted Shell, for a description of restricted mode.

`--enable-select`

  Include the `select` compound command, which allows generation of simple menus (see Conditional Constructs).

`--enable-single-help-strings`

  Store the text displayed by the `help` builtin as a single string for each help topic. This aids in translating the text to different languages. You may need to disable this if your compiler cannot handle very long string literals.

`--enable-strict-posix-default`

  Make Bash POSIX-conformant by default (see Bash and POSIX).

`--enable-translatable-strings`

  Enable support for `$"string"` translatable strings (see Locale-Specific Translation).

`--enable-usg-echo-default`

  A synonym for `--enable-xpg-echo-default`.

`--enable-xpg-echo-default`

  Make the `echo` builtin expand backslash-escaped characters by default, without requiring the `-e` option. This sets the default value of the `xpg_echo` shell option to `on`, which makes the Bash `echo` behave more like the version specified in the Single Unix Specification, version 3. See Bash Builtin Commands, for a description of the escape sequences that `echo` recognizes.

The file `config-top.h` contains C Preprocessor ‘`#define`’ statements for options which are not settable from `configure`. Some of these are not meant to be changed; beware of the consequences if you do. Read the comments associated with each definition for more information about its effect.

## Appendix A Reporting Bugs ¶

Please report all bugs you find in Bash. But first, you should make sure that it really is a bug, and that it appears in the latest version of Bash. The latest released version of Bash is always available for FTP from ftp://ftp.gnu.org/pub/gnu/bash/ and from http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz.

Once you have determined that a bug actually exists, use the `bashbug` command to submit a bug report or use the form at the Bash project page. If you have a fix, you are encouraged to submit that as well! Suggestions and ‘philosophical’ bug reports may be mailed to bug-bash@gnu.org or help-bash@gnu.org.

All bug reports should include:

- The version number of Bash.
- The hardware and operating system.
- The compiler used to compile Bash.
- A description of the bug behavior.
- A short script or ‘recipe’ which exercises the bug and may be used to reproduce it.

`bashbug` inserts the first three items automatically into the template it provides for filing a bug report.

Please send all reports concerning this manual to bug-bash@gnu.org.

## Appendix B Major Differences From The Bourne Shell ¶

Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the POSIX standard as the specification of how these features are to be implemented and how they should behave. There are some differences between the traditional Bourne shell and Bash; this section quickly details the differences of significance. A number of these differences are explained in greater depth in previous sections. This section uses the version of `sh` included in SVR4.2 (the last version of the historical Bourne shell) as the baseline reference.

- Bash is POSIX-conformant, even where the POSIX specification differs from traditional `sh` behavior (see Bash and POSIX).
- Bash has multi-character invocation options (see Invoking Bash).
- The Bash restricted mode is more useful (see The Restricted Shell); the SVR4.2 shell restricted mode is too limited.
- Bash has command-line editing (see Command Line Editing) and the `bind` builtin.
- Bash provides a programmable word completion mechanism (see Programmable Completion), and builtin commands `complete`, `compgen`, and `compopt`, to manipulate it.
- Bash decodes a number of backslash-escape sequences in the prompt string variables (`PS0`, `PS1`, `PS2`, and `PS4`) (see Controlling the Prompt).
- Bash expands and displays the `PS0` prompt string variable.
- Bash runs commands from the `PROMPT_COMMAND` array variable before issuing each primary prompt.
- Bash has command history (see Bash History Facilities) and the `history` and `fc` builtins to manipulate it. The Bash history list maintains timestamp information and uses the value of the `HISTTIMEFORMAT` variable to display it.
- Bash implements `csh`-like history expansion (see History Expansion).
- Bash supports the `$'…'` quoting syntax, which expands ANSI-C backslash-escaped characters in the text between the single quotes (see ANSI-C Quoting).
- Bash supports the `$"…"` quoting syntax and performs locale-specific translation of the characters between the double quotes. The `-D`, `--dump-strings`, and `--dump-po-strings` invocation options list the translatable strings found in a script (see Locale-Specific Translation).
- Bash includes brace expansion (see Brace Expansion) and tilde expansion (see Tilde Expansion).
- Bash implements command aliases and the `alias` and `unalias` builtins (see Aliases).
- Bash implements the `!` reserved word to negate the return value of a pipeline (see Pipelines). This is very useful when an `if` statement needs to act only if a test fails. The Bash ‘`-o pipefail`’ option to `set` will cause a pipeline to return a failure status if any command fails (see The Set Builtin).
- Bash has the `time` reserved word and command timing (see Pipelines). The display of the timing statistics may be controlled with the `TIMEFORMAT` variable.
- Bash provides coprocesses and the `coproc` reserved word (see Coprocesses).
- Bash implements the `for (( expr1 ; expr2 ; expr3 ))` arithmetic for command, similar to the C language (see Looping Constructs).
- Bash includes the `select` compound command, which allows the generation of simple menus (see Conditional Constructs).
- Bash includes the `[[` compound command, which makes conditional testing part of the shell grammar (see Conditional Constructs), including optional regular expression matching.
- Bash provides optional case-insensitive matching for the `case` and `[[` constructs (see Conditional Constructs).
- Bash provides additional `case` statement action list terminators: ‘`;&`’ and ‘`;;&`’ (see Conditional Constructs).
- Bash provides shell arithmetic, the `((` compound command (see Conditional Constructs), the `let` builtin, and arithmetic expansion (see Shell Arithmetic).
- Bash has one-dimensional array variables (see Arrays), and the appropriate variable expansions and assignment syntax to use them. Several of the Bash builtins take options to act on arrays. Bash provides a number of built-in array variables.
- Variables present in the shell’s initial environment are automatically exported to child processes (see Command Execution Environment). The Bourne shell does not normally do this unless the variables are explicitly marked using the `export` command.
- Bash can expand positional parameters beyond `$9` using `${num}` (see Shell Parameter Expansion).
- Bash supports the ‘`+=`’ assignment operator, which appends to the value of the variable named on the left hand side (see Shell Parameters).
- Bash includes the POSIX pattern removal ‘`%`’, ‘`#`’, ‘`%%`’ and ‘`##`’ expansions to remove leading or trailing substrings from variable values (see Shell Parameter Expansion).
- The expansion `${#xx}`, which returns the length of `${xx}`, is supported (see Shell Parameter Expansion).
- The expansion `${var:` offset`[:`length`]}`, which expands to the substring of `var`’s value of length length, beginning at offset, is present (see Shell Parameter Expansion).
- The expansion `${var/[/]`pattern`[/`replacement`]}`, which matches pattern and replaces it with replacement in the value of var, is available (see Shell Parameter Expansion), with a mechanism to use the matched text in replacement.
- The expansion `${!prefix*}` expansion, which expands to the names of all shell variables whose names begin with prefix, is available (see Shell Parameter Expansion).
- Bash has indirect variable expansion using `${!word}` (see Shell Parameter Expansion) and implements the `nameref` variable attribute for automatic indirect variable expansion.
- Bash includes a set of parameter transformation word expansions of the form `${var@X}`, where ‘`X`’ specifies the transformation (see Shell Parameter Expansion).
- The POSIX `$()` form of command substitution is implemented (see Command Substitution), and preferred to the Bourne shell’s ```` (which is also implemented for backwards compatibility).
- Bash implements a variant of command substitution that runs the enclosed command in the current shell execution environment: `${ command;}` or `${|command;}` (see Command Substitution).
- Bash has process substitution (see Process Substitution).
- Bash automatically assigns variables that provide information about the current user (`UID`, `EUID`, and `GROUPS`), the current host (`HOSTTYPE`, `OSTYPE`, `MACHTYPE`, and `HOSTNAME`), and the instance of Bash that is running (`BASH`, `BASH_VERSION`, and `BASH_VERSINFO`). See Bash Variables, for details.
- Bash uses many variables to provide functionality and customize shell behavior that the Bourne shell does not. Examples include `RANDOM`, `SRANDOM`, `EPOCHSECONDS`, `EPOCHREALTIME`, `TIMEFORMAT`, `BASHPID`, `BASH_XTRACEFD`, `GLOBIGNORE`, `HISTIGNORE`, and `BASH_VERSION`. See Bash Variables, for a complete list.
- Bash uses the `GLOBSORT` shell variable to control how to sort the results of filename expansion (see Filename Expansion).
- Bash uses the `IFS` variable to split only the results of expansion, not all words (see Word Splitting). This closes a longstanding shell security hole.
- The filename expansion bracket expression code uses ‘`!`’ and ‘`^`’ to negate the set of characters between the brackets (see Filename Expansion). The Bourne shell uses only ‘`!`’.
- Bash implements the full set of POSIX filename expansion operators, including character classes, equivalence classes, and collating symbols (see Filename Expansion).
- Bash implements extended pattern matching features when the `extglob` shell option is enabled (see Pattern Matching).
- The `globstar` shell option extends filename expansion to recursively scan directories and subdirectories for matching filenames (see Pattern Matching).
- It is possible to have a variable and a function with the same name; `sh` does not separate the two name spaces.
- Bash functions are permitted to have local variables using the `local` builtin, and thus users can write useful recursive functions (see Bash Builtin Commands).
- Bash performs filename expansion on filenames specified as operands to input and output redirection operators (see Redirections).
- Bash contains the ‘`<>`’ redirection operator, allowing a file to be opened for both reading and writing, and the ‘`&>`’ redirection operator, for directing standard output and standard error to the same file (see Redirections).
- Bash includes the ‘`<<<`’ redirection operator, allowing a string to be used as the standard input to a command (see Redirections).
- Bash implements the ‘`[n]<&word`’ and ‘`[n]>&word`’ redirection operators, which move one file descriptor to another.
- Bash treats a number of filenames specially when they are used in redirection operators (see Redirections).
- Bash provides the {var}<word capability to have the shell allocate file descriptors for redirections and assign them to var (see Redirections). This works with multiple redirection operators.
- Bash can open network connections to arbitrary machines and services with the redirection operators (see Redirections).
- The `noclobber` option is available to avoid overwriting existing files with output redirection (see The Set Builtin). The ‘`>|`’ redirection operator may be used to override `noclobber`.
- Variable assignments preceding commands affect only that command, even builtins and functions (see Environment). In `sh`, all variable assignments preceding commands are global unless the command is executed from the file system.
- Bash includes a number of features to support a separate debugger for shell scripts: variables (`BASH_ARGC`, `BASH_ARGV`, `BASH_LINENO`, `BASH_SOURCE`), the `DEBUG`, `RETURN`, and `ERR` traps, ‘`declare -F`’, and the `caller` builtin.
- Bash implements a `csh`-like directory stack, and provides the `pushd`, `popd`, and `dirs` builtins to manipulate it (see The Directory Stack). Bash also makes the directory stack visible as the value of the `DIRSTACK` shell variable.
- Bash allows a function to override a builtin with the same name, and provides access to that builtin’s functionality within the function via the `builtin` and `command` builtins (see Bash Builtin Commands).
- Bash includes the `caller` builtin (see Bash Builtin Commands), which displays the context of any active subroutine call (a shell function or a script executed with the `.` or `source` builtins). This supports the Bash debugger.
- The Bash `cd` and `pwd` builtins (see Bourne Shell Builtins) each take `-L` and `-P` options to switch between logical and physical modes.
- The `command` builtin allows selectively skipping shell functions when performing command lookup (see Bash Builtin Commands).
- Bash uses the `declare` builtin to modify the full set of variable and function attributes, and to assign values to variables.
- The `disown` builtin can remove a job from the internal shell job table (see Job Control Builtins) or suppress sending `SIGHUP` to a job when the shell exits as the result of a `SIGHUP`.
- The `enable` builtin (see Bash Builtin Commands) can enable or disable individual builtins and implements support for dynamically loading builtin commands from shared objects.
- The Bash `exec` builtin takes additional options that allow users to control the contents of the environment passed to the executed command, and what the zeroth argument to the command is to be (see Bourne Shell Builtins).
- Shell functions may be exported to children via the environment using `export -f` (see Shell Functions).
- The Bash `export` and `readonly` builtins (see Bourne Shell Builtins can take a `-f` option to act on shell functions, a `-p` option to display variables with various attributes set in a format that can be used as shell input, a `-n` option to remove various variable attributes, and ‘`name=value`’ arguments to set variable attributes and values simultaneously.
- The Bash `hash` builtin allows a name to be associated with an arbitrary filename, even when that filename cannot be found by searching the `$PATH`, using ‘`hash -p`’ (see Bourne Shell Builtins).
- Bash includes a `help` builtin for quick reference to shell facilities (see Bash Builtin Commands).
- Bash includes the `mapfile` builtin to quickly read the contents of a file into an indexed array variable (see Bash Builtin Commands).
- The `printf` builtin is available to display formatted output (see Bash Builtin Commands), and has additional custom format specifiers and an option to assign the formatted output directly to a shell variable.
- The Bash `read` builtin (see Bash Builtin Commands) will read a line ending in ‘`\`’ with the `-r` option, and will use the `REPLY` variable as a default if no non-option arguments are supplied.
- The `read` builtin (see Bash Builtin Commands) accepts a prompt string with the `-p` option and will use Readline to obtain the line when given the `-e` or `-E` options, with the ability to insert text into the line using the `-i` option. The `read` builtin also has additional options to control input: the `-s` option will turn off echoing of input characters as they are read, the `-t` option will allow `read` to time out if input does not arrive within a specified number of seconds, the `-n` option will allow reading only a specified number of characters rather than a full line, and the `-d` option will read until a particular character rather than newline.
- The `return` builtin may be used to abort execution of scripts executed with the `.` or `source` builtins (see Bourne Shell Builtins).
- Bash has much more optional behavior controllable with the `set` builtin (see The Set Builtin).
- The `-x` (`xtrace`) option displays commands other than simple commands when performing an execution trace (see The Set Builtin).
- Bash includes the `shopt` builtin, for finer control of shell optional capabilities (see The Shopt Builtin), and allows these options to be set and unset at shell invocation (see Invoking Bash).
- The `test` builtin (see Bourne Shell Builtins) is slightly different, as it implements the POSIX algorithm, which specifies the behavior based on the number of arguments.
- The `trap` builtin (see Bourne Shell Builtins) allows a `DEBUG` pseudo-signal specification, similar to `EXIT`. Commands specified with a `DEBUG` trap are executed before every simple command, `for` command, `case` command, `select` command, every arithmetic `for` command, and before the first command executes in a shell function. The `DEBUG` trap is not inherited by shell functions unless the function has been given the `trace` attribute or the `functrace` option has been enabled using the `shopt` builtin. The `extdebug` shell option has additional effects on the `DEBUG` trap.

  The `trap` builtin (see Bourne Shell Builtins) allows an `ERR` pseudo-signal specification, similar to `EXIT` and `DEBUG`. Commands specified with an `ERR` trap are executed after a simple command fails, with a few exceptions. The `ERR` trap is not inherited by shell functions unless the `-o errtrace` option to the `set` builtin is enabled.

  The `trap` builtin (see Bourne Shell Builtins) allows a `RETURN` pseudo-signal specification, similar to `EXIT` and `DEBUG`. Commands specified with a `RETURN` trap are executed before execution resumes after a shell function or a shell script executed with `.` or `source` returns. The `RETURN` trap is not inherited by shell functions unless the function has been given the `trace` attribute or the `functrace` option has been enabled using the `shopt` builtin.

- The Bash `type` builtin is more extensive and gives more information about the names it finds (see Bash Builtin Commands).
- The `ulimit` builtin provides control over many more per-process resources (see Bash Builtin Commands).
- The Bash `umask` builtin uses the `-p` option to display the output in the form of a `umask` command that may be reused as input (see Bourne Shell Builtins).
- The Bash `wait` builtin has a `-n` option to wait for the next child to exit, possibly selecting from a list of supplied jobs, and the `-p` option to store information about a terminated child process in a shell variable.
- The SVR4.2 shell behaves differently when invoked as `jsh` (it turns on job control).
- The SVR4.2 shell has two privilege-related builtins (`mldmode` and `priv`) not present in Bash.
- Bash does not have the `stop` or `newgrp` builtins.
- Bash does not use the `SHACCT` variable or perform shell accounting.
- The SVR4.2 `sh` uses a `TIMEOUT` variable like Bash uses `TMOUT`.

More features unique to Bash may be found in Bash Features.

### B.1 Implementation Differences From The SVR4.2 Shell ¶

Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance:

- Bash does not fork a subshell when redirecting into or out of a shell control structure such as an `if` or `while` statement.
- Bash does not allow unbalanced quotes. The SVR4.2 shell will silently insert a needed closing quote at `EOF` under certain circumstances. This can be the cause of some hard-to-find errors.
- The SVR4.2 shell uses a baroque memory management scheme based on trapping `SIGSEGV`. If the shell is started from a process with `SIGSEGV` blocked (e.g., by using the `system()` C library function call), it misbehaves badly.
- In a questionable attempt at security, the SVR4.2 shell, when invoked without the `-p` option, will alter its real and effective UID and GID if they are less than some magic threshold value, commonly 100. This can lead to unexpected results.
- The SVR4.2 shell does not allow users to trap `SIGSEGV`, `SIGALRM`, or `SIGCHLD`.
- The SVR4.2 shell does not allow the `IFS`, `MAILCHECK`, `PATH`, `PS1`, or `PS2` variables to be unset.
- The SVR4.2 shell treats ‘`^`’ as the undocumented equivalent of ‘`|`’.
- Bash allows multiple option arguments when it is invoked (`-x -v`); the SVR4.2 shell allows only one option argument (`-xv`). In fact, some versions of the shell dump core if the second argument begins with a ‘`-`’.
- The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the POSIX special builtins fails, and only for certain failures, as enumerated in the POSIX standard.
- If the `lastpipe` option is enabled, and job control is not active, Bash runs the last element of a pipeline in the current shell execution environment.

## Appendix C GNU Free Documentation License ¶

Version 1.3, 3 November 2008

    Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
    http://fsf.org/

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

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   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

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5. COMBINING DOCUMENTS

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   In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

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   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

11. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

### ADDENDUM: How to use this License for your documents ¶

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.3
      or any later version published by the Free Software Foundation;
      with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
      Texts.  A copy of the license is included in the section entitled ``GNU
      Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

        with the Invariant Sections being list their titles, with
        the Front-Cover Texts being list, and with the Back-Cover Texts
        being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

## Appendix D Indexes ¶

### D.1 Index of Shell Builtin Commands ¶

Jump to: | : . [ A B C D E F G H J K L M P R S T U W

| Index Entry | Section
:
| `:` | Bourne Shell Builtins
.
| `.` | Bourne Shell Builtins
[
| `[` | Bourne Shell Builtins
A
| `alias` | Bash Builtins
B
| `bg` | Job Control Builtins
| `bind` | Bash Builtins
| `break` | Bourne Shell Builtins
| `builtin` | Bash Builtins
C
| `caller` | Bash Builtins
| `cd` | Bourne Shell Builtins
| `command` | Bash Builtins
| `compgen` | Programmable Completion Builtins
| `complete` | Programmable Completion Builtins
| `compopt` | Programmable Completion Builtins
| `continue` | Bourne Shell Builtins
D
| `declare` | Bash Builtins
| `dirs` | Directory Stack Builtins
| `disown` | Job Control Builtins
E
| `echo` | Bash Builtins
| `enable` | Bash Builtins
| `eval` | Bourne Shell Builtins
| `exec` | Bourne Shell Builtins
| `exit` | Bourne Shell Builtins
| `export` | Bourne Shell Builtins
F
| `false` | Bourne Shell Builtins
| `fc` | Bash History Builtins
| `fg` | Job Control Builtins
G
| `getopts` | Bourne Shell Builtins
H
| `hash` | Bourne Shell Builtins
| `help` | Bash Builtins
| `history` | Bash History Builtins
J
| `jobs` | Job Control Builtins
K
| `kill` | Job Control Builtins
L
| `let` | Bash Builtins
| `local` | Bash Builtins
| `logout` | Bash Builtins
M
| `mapfile` | Bash Builtins
P
| `popd` | Directory Stack Builtins
| `printf` | Bash Builtins
| `pushd` | Directory Stack Builtins
| `pwd` | Bourne Shell Builtins
R
| `read` | Bash Builtins
| `readarray` | Bash Builtins
| `readonly` | Bourne Shell Builtins
| `return` | Bourne Shell Builtins
S
| `set` | The Set Builtin
| `shift` | Bourne Shell Builtins
| `shopt` | The Shopt Builtin
| `source` | Bash Builtins
| `suspend` | Job Control Builtins
T
| `test` | Bourne Shell Builtins
| `times` | Bourne Shell Builtins
| `trap` | Bourne Shell Builtins
| `true` | Bourne Shell Builtins
| `type` | Bash Builtins
| `typeset` | Bash Builtins
U
| `ulimit` | Bash Builtins
| `umask` | Bourne Shell Builtins
| `unalias` | Bash Builtins
| `unset` | Bourne Shell Builtins
W
| `wait` | Job Control Builtins

### D.2 Index of Shell Reserved Words ¶

Jump to: | ! [ ] { } C D E F I S T U W

| Index Entry | Section
!
| `!` | Pipelines
[
| `[[` | Conditional Constructs
]
| `]]` | Conditional Constructs
{
| `{` | Command Grouping
}
| `}` | Command Grouping
C
| `case` | Conditional Constructs
D
| `do` | Looping Constructs
| `done` | Looping Constructs
E
| `elif` | Conditional Constructs
| `else` | Conditional Constructs
| `esac` | Conditional Constructs
F
| `fi` | Conditional Constructs
| `for` | Looping Constructs
| `function` | Shell Functions
I
| `if` | Conditional Constructs
| `in` | Conditional Constructs
S
| `select` | Conditional Constructs
T
| `then` | Conditional Constructs
| `time` | Pipelines
U
| `until` | Looping Constructs
W
| `while` | Looping Constructs

### D.3 Parameter and Variable Index ¶

Jump to: | _ - ! ? @ * # $ 0 A B C D E F G H I K L M O P R S T U V

| Index Entry | Section
_
| `_` | Bash Variables
-
| `-` | Special Parameters
!
| `!` | Special Parameters
?
| `?` | Special Parameters
@
| `@` | Special Parameters
*
| `*` | Special Parameters
#
| `#` | Special Parameters
$
| `$` | Special Parameters
| `$_` | Bash Variables
| `$-` | Special Parameters
| `$!` | Special Parameters
| `$?` | Special Parameters
| `$@` | Special Parameters
| `$*` | Special Parameters
| `$#` | Special Parameters
| `$$` | Special Parameters
| `$0` | Special Parameters
0
| `0` | Special Parameters
A
| `active-region-end-color` | Readline Init File Syntax
| `active-region-start-color` | Readline Init File Syntax
| `auto_resume` | Job Control Variables
B
| `BASH` | Bash Variables
| `BASH_ALIASES` | Bash Variables
| `BASH_ARGC` | Bash Variables
| `BASH_ARGV` | Bash Variables
| `BASH_ARGV0` | Bash Variables
| `BASH_CMDS` | Bash Variables
| `BASH_COMMAND` | Bash Variables
| `BASH_COMPAT` | Bash Variables
| `BASH_ENV` | Bash Variables
| `BASH_EXECUTION_STRING` | Bash Variables
| `BASH_LINENO` | Bash Variables
| `BASH_LOADABLES_PATH` | Bash Variables
| `BASH_MONOSECONDS` | Bash Variables
| `BASH_REMATCH` | Bash Variables
| `BASH_SOURCE` | Bash Variables
| `BASH_SUBSHELL` | Bash Variables
| `BASH_TRAPSIG` | Bash Variables
| `BASH_VERSINFO` | Bash Variables
| `BASH_VERSION` | Bash Variables
| `BASH_XTRACEFD` | Bash Variables
| `BASHOPTS` | Bash Variables
| `BASHPID` | Bash Variables
| `bell-style` | Readline Init File Syntax
| `bind-tty-special-chars` | Readline Init File Syntax
| `blink-matching-paren` | Readline Init File Syntax
C
| `CDPATH` | Bourne Shell Variables
| `CHILD_MAX` | Bash Variables
| `colored-completion-prefix` | Readline Init File Syntax
| `colored-stats` | Readline Init File Syntax
| `COLUMNS` | Bash Variables
| `comment-begin` | Readline Init File Syntax
| `COMP_CWORD` | Bash Variables
| `COMP_KEY` | Bash Variables
| `COMP_LINE` | Bash Variables
| `COMP_POINT` | Bash Variables
| `COMP_TYPE` | Bash Variables
| `COMP_WORDBREAKS` | Bash Variables
| `COMP_WORDS` | Bash Variables
| `completion-display-width` | Readline Init File Syntax
| `completion-ignore-case` | Readline Init File Syntax
| `completion-map-case` | Readline Init File Syntax
| `completion-prefix-display-length` | Readline Init File Syntax
| `completion-query-items` | Readline Init File Syntax
| `COMPREPLY` | Bash Variables
| `convert-meta` | Readline Init File Syntax
| `COPROC` | Bash Variables
D
| `DIRSTACK` | Bash Variables
| `disable-completion` | Readline Init File Syntax
E
| `echo-control-characters` | Readline Init File Syntax
| `editing-mode` | Readline Init File Syntax
| `EMACS` | Bash Variables
| `emacs-mode-string` | Readline Init File Syntax
| `enable-active-region The` | Readline Init File Syntax
| `enable-bracketed-paste` | Readline Init File Syntax
| `enable-keypad` | Readline Init File Syntax
| `enable-meta-key` | Readline Init File Syntax
| `ENV` | Bash Variables
| `EPOCHREALTIME` | Bash Variables
| `EPOCHSECONDS` | Bash Variables
| `EUID` | Bash Variables
| `EXECIGNORE` | Bash Variables
| `expand-tilde` | Readline Init File Syntax
F
| `FCEDIT` | Bash Variables
| `FIGNORE` | Bash Variables
| `force-meta-prefix` | Readline Init File Syntax
| `FUNCNAME` | Bash Variables
| `FUNCNEST` | Bash Variables
G
| `GLOBIGNORE` | Bash Variables
| `GLOBSORT` | Bash Variables
| `GROUPS` | Bash Variables
H
| `histchars` | Bash Variables
| `HISTCMD` | Bash Variables
| `HISTCONTROL` | Bash Variables
| `HISTFILE` | Bash Variables
| `HISTFILESIZE` | Bash Variables
| `HISTIGNORE` | Bash Variables
| `history-preserve-point` | Readline Init File Syntax
| `history-size` | Readline Init File Syntax
| `HISTSIZE` | Bash Variables
| `HISTTIMEFORMAT` | Bash Variables
| `HOME` | Bourne Shell Variables
| `horizontal-scroll-mode` | Readline Init File Syntax
| `HOSTFILE` | Bash Variables
| `HOSTNAME` | Bash Variables
| `HOSTTYPE` | Bash Variables
I
| `IFS` | Bourne Shell Variables
| `IGNOREEOF` | Bash Variables
| `input-meta` | Readline Init File Syntax
| `INPUTRC` | Bash Variables
| `INSIDE_EMACS` | Bash Variables
| `isearch-terminators` | Readline Init File Syntax
K
| `keymap` | Readline Init File Syntax
L
| `LANG` | Creating Internationalized Scripts
| `LANG` | Bash Variables
| `LC_ALL` | Bash Variables
| `LC_COLLATE` | Bash Variables
| `LC_CTYPE` | Bash Variables
| `LC_MESSAGES` | Creating Internationalized Scripts
| `LC_MESSAGES` | Bash Variables
| `LC_NUMERIC` | Bash Variables
| `LC_TIME` | Bash Variables
| `LINENO` | Bash Variables
| `LINES` | Bash Variables
M
| `MACHTYPE` | Bash Variables
| `MAIL` | Bourne Shell Variables
| `MAILCHECK` | Bash Variables
| `MAILPATH` | Bourne Shell Variables
| `MAPFILE` | Bash Variables
| `mark-modified-lines` | Readline Init File Syntax
| `mark-symlinked-directories` | Readline Init File Syntax
| `match-hidden-files` | Readline Init File Syntax
| `menu-complete-display-prefix` | Readline Init File Syntax
| `meta-flag` | Readline Init File Syntax
O
| `OLDPWD` | Bash Variables
| `OPTARG` | Bourne Shell Variables
| `OPTERR` | Bash Variables
| `OPTIND` | Bourne Shell Variables
| `OSTYPE` | Bash Variables
| `output-meta` | Readline Init File Syntax
P
| `page-completions` | Readline Init File Syntax
| `PATH` | Bourne Shell Variables
| `PIPESTATUS` | Bash Variables
| `POSIXLY_CORRECT` | Bash Variables
| `PPID` | Bash Variables
| `PROMPT_COMMAND` | Bash Variables
| `PROMPT_DIRTRIM` | Bash Variables
| `PS0` | Bash Variables
| `PS1` | Bourne Shell Variables
| `PS2` | Bourne Shell Variables
| `PS3` | Bash Variables
| `PS4` | Bash Variables
| `PWD` | Bash Variables
R
| `RANDOM` | Bash Variables
| `READLINE_ARGUMENT` | Bash Variables
| `READLINE_LINE` | Bash Variables
| `READLINE_MARK` | Bash Variables
| `READLINE_POINT` | Bash Variables
| `REPLY` | Bash Variables
| `revert-all-at-newline` | Readline Init File Syntax
S
| `search-ignore-case` | Readline Init File Syntax
| `SECONDS` | Bash Variables
| `SHELL` | Bash Variables
| `SHELLOPTS` | Bash Variables
| `SHLVL` | Bash Variables
| `show-all-if-ambiguous` | Readline Init File Syntax
| `show-all-if-unmodified` | Readline Init File Syntax
| `show-mode-in-prompt` | Readline Init File Syntax
| `skip-completed-text` | Readline Init File Syntax
| `SRANDOM` | Bash Variables
T
| `TEXTDOMAIN` | Creating Internationalized Scripts
| `TEXTDOMAINDIR` | Creating Internationalized Scripts
| `TIMEFORMAT` | Bash Variables
| `TMOUT` | Bash Variables
| `TMPDIR` | Bash Variables
U
| `UID` | Bash Variables
V
| `vi-cmd-mode-string` | Readline Init File Syntax
| `vi-ins-mode-string` | Readline Init File Syntax
| `visible-stats` | Readline Init File Syntax

### D.4 Function Index ¶

Jump to: | A B C D E F G H I K M N O P Q R S T U Y

| Index Entry | Section
A
| `abort (C-g)` | Miscellaneous Commands
| `accept-line (Newline or Return)` | Commands For History
| `alias-expand-line ()` | Miscellaneous Commands
B
| `backward-char (C-b)` | Commands For Moving
| `backward-delete-char (Rubout)` | Commands For Text
| `backward-kill-line (C-x Rubout)` | Commands For Killing
| `backward-kill-word (M-`DEL`)` | Commands For Killing
| `backward-word (M-b)` | Commands For Moving
| `beginning-of-history (M-<)` | Commands For History
| `beginning-of-line (C-a)` | Commands For Moving
| `bracketed-paste-begin ()` | Commands For Text
C
| `call-last-kbd-macro (C-x e)` | Keyboard Macros
| `capitalize-word (M-c)` | Commands For Text
| `character-search (C-])` | Miscellaneous Commands
| `character-search-backward (M-C-])` | Miscellaneous Commands
| `clear-display (M-C-l)` | Commands For Moving
| `clear-screen (C-l)` | Commands For Moving
| `complete (`TAB`)` | Commands For Completion
| `complete-command (M-!)` | Commands For Completion
| `complete-filename (M-/)` | Commands For Completion
| `complete-hostname (M-@)` | Commands For Completion
| `complete-into-braces (M-{)` | Commands For Completion
| `complete-username (M-~)` | Commands For Completion
| `complete-variable (M-$)` | Commands For Completion
| `copy-backward-word ()` | Commands For Killing
| `copy-forward-word ()` | Commands For Killing
| `copy-region-as-kill ()` | Commands For Killing
D
| `dabbrev-expand ()` | Commands For Completion
| `delete-char (C-d)` | Commands For Text
| `delete-char-or-list ()` | Commands For Completion
| `delete-horizontal-space ()` | Commands For Killing
| `digit-argument (`M-0`, `M-1`, … `M--`)` | Numeric Arguments
| `display-shell-version (C-x C-v)` | Miscellaneous Commands
| `do-lowercase-version (M-A, M-B, M-x, …)` | Miscellaneous Commands
| `downcase-word (M-l)` | Commands For Text
| `dump-functions ()` | Miscellaneous Commands
| `dump-macros ()` | Miscellaneous Commands
| `dump-variables ()` | Miscellaneous Commands
| `dynamic-complete-history (M-`TAB`)` | Commands For Completion
E
| `edit-and-execute-command (C-x C-e)` | Miscellaneous Commands
| `end-kbd-macro (C-x ))` | Keyboard Macros
| `end-of-file (usually C-d)` | Commands For Text
| `end-of-history (M->)` | Commands For History
| `end-of-line (C-e)` | Commands For Moving
| `exchange-point-and-mark (C-x C-x)` | Miscellaneous Commands
| `execute-named-command (M-x)` | Miscellaneous Commands
| `export-completions ()` | Commands For Completion
F
| `fetch-history ()` | Commands For History
| `forward-backward-delete-char ()` | Commands For Text
| `forward-char (C-f)` | Commands For Moving
| `forward-search-history (C-s)` | Commands For History
| `forward-word (M-f)` | Commands For Moving
G
| `glob-complete-word (M-g)` | Miscellaneous Commands
| `glob-expand-word (C-x *)` | Miscellaneous Commands
| `glob-list-expansions (C-x g)` | Miscellaneous Commands
H
| `history-and-alias-expand-line ()` | Miscellaneous Commands
| `history-expand-line (M-^)` | Miscellaneous Commands
| `history-search-backward ()` | Commands For History
| `history-search-forward ()` | Commands For History
| `history-substring-search-backward ()` | Commands For History
| `history-substring-search-forward ()` | Commands For History
I
| `insert-comment (M-#)` | Miscellaneous Commands
| `insert-completions (M-*)` | Commands For Completion
| `insert-last-argument (M-. or M-_)` | Miscellaneous Commands
K
| `kill-line (C-k)` | Commands For Killing
| `kill-region ()` | Commands For Killing
| `kill-whole-line ()` | Commands For Killing
| `kill-word (M-d)` | Commands For Killing
M
| `magic-space ()` | Miscellaneous Commands
| `menu-complete ()` | Commands For Completion
| `menu-complete-backward ()` | Commands For Completion
N
| `next-history (C-n)` | Commands For History
| `next-screen-line ()` | Commands For Moving
| `non-incremental-forward-search-history (M-n)` | Commands For History
| `non-incremental-reverse-search-history (M-p)` | Commands For History
O
| `operate-and-get-next (C-o)` | Commands For History
| `overwrite-mode ()` | Commands For Text
P
| `possible-command-completions (C-x !)` | Commands For Completion
| `possible-completions (M-?)` | Commands For Completion
| `possible-filename-completions (C-x /)` | Commands For Completion
| `possible-hostname-completions (C-x @)` | Commands For Completion
| `possible-username-completions (C-x ~)` | Commands For Completion
| `possible-variable-completions (C-x $)` | Commands For Completion
| `prefix-meta (`ESC`)` | Miscellaneous Commands
| `previous-history (C-p)` | Commands For History
| `previous-screen-line ()` | Commands For Moving
| `print-last-kbd-macro ()` | Keyboard Macros
Q
| `quoted-insert (C-q or C-v)` | Commands For Text
R
| `re-read-init-file (C-x C-r)` | Miscellaneous Commands
| `redraw-current-line ()` | Commands For Moving
| `reverse-search-history (C-r)` | Commands For History
| `revert-line (M-r)` | Miscellaneous Commands
S
| `self-insert (a, b, A, 1, !, …)` | Commands For Text
| `set-mark (C-@)` | Miscellaneous Commands
| `shell-backward-kill-word ()` | Commands For Killing
| `shell-backward-word (M-C-b)` | Commands For Moving
| `shell-expand-line (M-C-e)` | Miscellaneous Commands
| `shell-forward-word (M-C-f)` | Commands For Moving
| `shell-kill-word (M-C-d)` | Commands For Killing
| `shell-transpose-words (M-C-t)` | Commands For Text
| `skip-csi-sequence ()` | Miscellaneous Commands
| `spell-correct-word (C-x s)` | Miscellaneous Commands
| `start-kbd-macro (C-x ()` | Keyboard Macros
T
| `tilde-expand (M-&)` | Miscellaneous Commands
| `transpose-chars (C-t)` | Commands For Text
| `transpose-words (M-t)` | Commands For Text
U
| `undo (C-_ or C-x C-u)` | Miscellaneous Commands
| `universal-argument ()` | Numeric Arguments
| `unix-filename-rubout ()` | Commands For Killing
| `unix-line-discard (C-u)` | Commands For Killing
| `unix-word-rubout (C-w)` | Commands For Killing
| `upcase-word (M-u)` | Commands For Text
Y
| `yank (C-y)` | Commands For Killing
| `yank-last-arg (M-. or M-_)` | Commands For History
| `yank-nth-arg (M-C-y)` | Commands For History
| `yank-pop (M-y)` | Commands For Killing

### D.5 Concept Index ¶

Jump to: | A B C D E F H I J K L M N O P Q R S T U V W Y

| Index Entry | Section
A
| alias expansion | Aliases
| arithmetic evaluation | Shell Arithmetic
| arithmetic expansion | Arithmetic Expansion
| arithmetic operators | Shell Arithmetic
| arithmetic, shell | Shell Arithmetic
| arrays | Arrays
B
| background | Job Control Basics
| Bash configuration | Basic Installation
| Bash installation | Basic Installation
| binary arithmetic operators | Shell Arithmetic
| bitwise arithmetic operators | Shell Arithmetic
| Bourne shell | Basic Shell Features
| brace expansion | Brace Expansion
| builtin | Definitions
C
| command editing | Readline Bare Essentials
| command execution | Command Search and Execution
| command expansion | Simple Command Expansion
| command history | Bash History Facilities
| command search | Command Search and Execution
| command substitution | Command Substitution
| command timing | Pipelines
| commands, compound | Compound Commands
| commands, conditional | Conditional Constructs
| commands, grouping | Command Grouping
| commands, lists | Lists
| commands, looping | Looping Constructs
| commands, pipelines | Pipelines
| commands, shell | Shell Commands
| commands, simple | Simple Commands
| comments, shell | Comments
| Compatibility Level | Shell Compatibility Mode
| Compatibility Mode | Shell Compatibility Mode
| completion builtins | Programmable Completion Builtins
| conditional arithmetic operator | Shell Arithmetic
| configuration | Basic Installation
| control operator | Definitions
| coprocess | Coprocesses
D
| directory stack | The Directory Stack
| dollar-single quote quoting | ANSI-C Quoting
E
| editing command lines | Readline Bare Essentials
| environment | Environment
| evaluation, arithmetic | Shell Arithmetic
| event designators | Event Designators
| execution environment | Command Execution Environment
| exit status | Definitions
| exit status | Exit Status
| expansion | Shell Expansions
| expansion, arithmetic | Arithmetic Expansion
| expansion, brace | Brace Expansion
| expansion, filename | Filename Expansion
| expansion, parameter | Shell Parameter Expansion
| expansion, pathname | Filename Expansion
| expansion, tilde | Tilde Expansion
| expressions, arithmetic | Shell Arithmetic
| expressions, conditional | Bash Conditional Expressions
F
| field | Definitions
| filename | Definitions
| filename expansion | Filename Expansion
| foreground | Job Control Basics
| functions, shell | Shell Functions
H
| history builtins | Bash History Builtins
| history events | Event Designators
| history expansion | History Interaction
| history list | Bash History Facilities
| History, how to use | A Programmable Completion Example
I
| identifier | Definitions
| initialization file, readline | Readline Init File
| installation | Basic Installation
| interaction, readline | Readline Interaction
| interactive shell | Invoking Bash
| interactive shell | Interactive Shells
| internationalization | Locale Translation
| internationalized scripts | Creating Internationalized Scripts
J
| job | Definitions
| job control | Definitions
| job control | Job Control Basics
K
| kill ring | Readline Killing Commands
| killing text | Readline Killing Commands
L
| localization | Locale Translation
| login shell | Invoking Bash
M
| matching, pattern | Pattern Matching
| metacharacter | Definitions
N
| name | Definitions
| native languages | Locale Translation
| notation, readline | Readline Bare Essentials
O
| operator, shell | Definitions
P
| parameter expansion | Shell Parameter Expansion
| parameters | Shell Parameters
| parameters, positional | Positional Parameters
| parameters, special | Special Parameters
| pathname expansion | Filename Expansion
| pattern matching | Pattern Matching
| pipeline | Pipelines
| POSIX | Definitions
| POSIX description | Bash POSIX Mode
| POSIX Mode | Bash POSIX Mode
| process group | Definitions
| process group ID | Definitions
| process substitution | Process Substitution
| programmable completion | Programmable Completion
| prompting | Controlling the Prompt
Q
| quoting | Quoting
| quoting, ANSI | ANSI-C Quoting
R
| Readline, how to use | Job Control Variables
| redirection | Redirections
| reserved word | Definitions
| reserved words | Reserved Words
| restricted shell | The Restricted Shell
| return status | Definitions
S
| shell arithmetic | Shell Arithmetic
| shell function | Shell Functions
| shell script | Shell Scripts
| shell variable | Shell Parameters
| shell, interactive | Interactive Shells
| signal | Definitions
| signal handling | Signals
| special builtin | Definitions
| special builtin | Special Builtins
| startup files | Bash Startup Files
| string translations | Creating Internationalized Scripts
| suspending jobs | Job Control Basics
T
| tilde expansion | Tilde Expansion
| token | Definitions
| translation, native languages | Locale Translation
U
| unary arithmetic operators | Shell Arithmetic
V
| variable, shell | Shell Parameters
| variables, readline | Readline Init File Syntax
W
| word | Definitions
| word splitting | Word Splitting
Y
| yanking text | Readline Killing Commands