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.ND "December 21, 1993"
.TL "311466-6713" "61175"  \" charging case filing case 
Introduction to \f5ksh-93\fP
.AU "David G. Korn" DGK MH 11267 7975 3C-526B "(research!dgk)"
.TM  11267-931221-26  \"  technical memo + TM numbers
.MT 1  \"  memo type
.OK Shell "Command interpreter" Language UNIX  \" keyword
.AS 2   \" abstract start for TM 
\f5ksh-93\fP is a
major rewrite of \f5ksh\fP,
a program that serves as a command language
(shell) for the UNIX* 
.FS  *
UNIX is a registered trademark of Novell.
.FE
operating system.
As with \f5ksh\fP, \f5ksh-93\fP
is essentially compatible with the System V version of the Bourne shell\*(Rf,
.RS
S. R. Bourne,
.I "An Introduction to the UNIX
Shell,"
BSTJ - Vol. 57, No. 6 part 2, pages 1947-1972, 1978.
.RF
and compatible with previous versions of \f5ksh\fP.
\f5ksh-93\fP is intended to comply with the IEEE POSIX 1003.2
and ISO 9945-2\*(Rf
.RS
.I "POSIX \- Part 2: Shell and Utilities,"
IEEE Std 1003.2-1992, ISO/IEC 9945-2, IEEE, 1993.
.RF
shell standard.
In addition to changes in the language required
by these standards, the primary focus of \f5ksh-93\fP
is related to shell programming.
\f5ksh-93\fP provides the programming power of several
other interpretive languages such as \f5awk\fP\*(Rf,
.RS
Al Aho,
Brian Kernighan,
and
Peter Weinberger,
.I "The AWK Programming Language,"
Addison Wesley, 1988.
.RF
\f5FIT\fP\*(Rf,
.RS
Lloyd H. Nakatani and Laurence W. Ruedisueli,
.I "The FIT Programming Language Primer",
TM 1126-920301-03, 1992.
.RF
\f5PERL\fP\*(Rf,
.RS
Larry Wall and Randal Schwartz,
.I "Programming perl,"
O'Reilly & Assoc, 1990.
.RF
and
\f5tcl\fP\*(Rf.
.RS
John K. Ousterhout,
.I "Tcl: An Embeddable Command Language",
Proceedings of the Washington USENIX meeting, pp. 133-146, 1990.
.RF
.P
This memo
assumes that the reader is already familiar with the Bourne shell.
It introduces most of the features of \f5ksh-93\fP
relative to the Bourne shell; both
as a command language and as a programming language.
The Appendix contains
a sample script written in \f5ksh-93\fP.
.AE   \" abstract end
.H 1 "INTRODUCTION"
.P
The term "shell" is used to describe a program that provides
a command language
interface.
Because the UNIX*\ 
.FS *
UNIX is a registered trademark of USL
.FE
system shell is a user level program, and not part of
the operating system itself,
anyone can write a new shell or modify an existing one.
This has caused an evolutionary progress
in the design and implementation of shells,
with the better ones surviving.
The most widely available UNIX system shells are the Bourne shell\*(Rf,
.RS
S. R. Bourne,
.IR "An Introduction to the UNIX Shell" ,
Bell System Technical Journal,
Vol. 57, No. 6, Part 2, pp. 1947-1972, July 1978.
.RF
written by Steve Bourne
at AT&T Bell Laboratories,
the C shell\*(Rf,
.RS
W. Joy,
.IR "An Introduction to the C Shell" ,
Unix Programmer's Manual, Berkeley Software Distribution,
University of California, Berkeley, 1980.
.RF
written by Bill Joy at the University of California, Berkeley,
and the KornShell language \*(Rf,
.RS
Morris Bolsky and David Korn,
.IR "The KornShell Command and Programming Language" ,
Prentice Hall, 1989.
.RF
written by David Korn
at AT&T Bell Laboratories.
The Bourne shell is available on almost all versions of the UNIX
system.
The C Shell is available with all Berkeley Software Distribution (BSD) UNIX systems and on many other systems.
The KornShell
is available on System V Release 4 systems. 
In addition, it is available on many other systems.
The source for the KornShell language is available from the AT&T Toolchest,
an electronic software distribution system.
It runs on all known versions of the UNIX system and
on many UNIX system look-alikes.
.P
There have been several articles comparing the UNIX system shells.
Jason Levitt\*(Rf 
.RS
Jason Levitt,
.IR "The Korn Shell: An Emerging Standard" ,
UNIX/World, pp. 74-81, September 1986. 
.RF
highlights some of the new features
introduced by the KornShell language.
Rich Bilancia\*(Rf 
.RS
Rich Bilancia,
.IR "Proficiency and Power are Yours With the Korn Shell" ,
UNIX/World, pp. 103-107, September 1987. 
.RF
explains some of the advantages of using the KornShell language.
John Sebes\*(Rf 
.RS
John Sebes,
.I "Comparing UNIX Shells,"
UNIX Papers,
Edited by the Waite Group, Howard W. Sams & Co., 1987.
.RF
provides a more detailed comparison of the three shells,
both as a command language and as a programming language.
.P
The KornShell language is a superset of the
Bourne shell. The KornShell language has many of the popular C shell features,
plus additional features of its own.
Its initial popularity stems primarily from its improvements as
a command language.
The primary interactive benefit of the KornShell command language
is a visual command line editor that allows you to
make corrections to your current command line
or to earlier command lines,
without having to retype them.
.P
However,
in the long run,
the power of the KornShell language as a high-level programming language,
as described by Dolotta and Mashey\*(Rf,
.RS
T. A. Dolotta and J. R. Mashey,
.I "Using the shell as a Primary Programming Tool,"
Proc. 2nd. Int. Conf. on Software Engineering, 1976,
pages 169-176.
.RF
may prove to be of greater significance.
\f5ksh-93\fP provides the programming power of several
other interpretive languages such as \f5awk\fP,
\f5FIT\fP,
\f5PERL\fP,
and
\f5tcl\fP.
An application that was originally written in the C programming language
was rewritten in the KornShell language.
More than 20,000 lines of C code were replaced with KornShell scripts
totaling fewer than 700 lines.
In most instances there was no perceptible difference in performance
between the two versions of the code.
.P
The KornShell language has been embedded into windowing systems
allowing graphical user interfaces to be developed in shell
rather than having to build applications that need to be
compiled.
The \f5wksh\fP program\*(Rf
.RS
J. S. Pendergrast,
.IR "WKSH - Korn Shell with X-Windows Support",
USL. 1991.
.RF
provides a method of developing OpenLook or Motif
applications as \f5ksh\fP scripts.
.P
This memo is an introduction to \f5ksh-93\fP,
the program that implements an enhanced version
of the KornShell language.
It is referred to as \f5ksh\fP in the rest of this memo.
The memo describes the KornShell language based on the
features of the 12/28/93 release of \f5ksh\fP.
This memo is not a tutorial, only an introduction.
The second edition of reference [9] gives 
a more complete treatment of the KornShell language.
.P
A concerted effort has been made to achieve both System V Bourne shell
compatibility and IEEE POSIX compatibility
so that scripts written for either of these shells
can run without modification with \f5ksh\fP.
In addition, \f5ksh-93\fP attempts to
be compatible with older versions of \f5ksh\fP.
When there are conflicts between versions of the shell,
\f5ksh-93\fP selects the behavior dictated by the IEEE POSIX
standard.
The description of features in this memo assumes
that the reader is already familiar with the Bourne shell.
.H 1 "COMMAND LANGUAGE"
There is no separate command language.
All features of the language, except job control,
can be
used both within a script and interactively from a terminal.
However, features that are more likely to be used
while running commands interactively from a terminal
are presented here.
.H 2 "Setting Options"
By convention, UNIX commands
consist of a command name followed by options and other arguments.
Options are either of the form \f5-\fP\fIletter\fP,
or \f5-\fP\fIletter value\fP.
In the former case, several options may be grouped after a single \f5-\fP.
The argument \f5--\fP signifies an end to the option list and is
only required when the first non-option argument begins with
a \f5-\fP.
Most commands print an error message which
shows which options are permitted
when given incorrect arguments.
In addition, the option sequence \f5-?\fP causes most commands
to print a usage message which lists the valid options. 
.P
Ordinarily, \f5ksh\fP executes a command by 
using the command name to locate a program to run
and by running the program as a separate process.
Some commands, referred to as
.IR built-ins ,
are carried out by \f5ksh\fP itself,
without creating a separate process.
The reasons that some commands are built in are presented later.
In nearly all cases the distinction
between a command that is built in and one that
is not is invisible to the user.
However, nearly
all commands that are built in follow command line conventions.
.P
\f5ksh\fP has several options that can be set by the user
as command line arguments at invocation and as option arguments to the
\f5set\fP command.
Most other options can be set with a single letter option or as a name
that follows the \f5-o\fP option. 
Use
\f5set\ -o\fP
to display the current option settings.
Some of these options, such as
.B interactive
and
.B monitor
(see
.I "Job Control"
below),
are enabled automatically by \f5ksh\fP
when the shell is connected to a terminal device.
Other options, such as
.B noclobber
and
.BR ignoreeof ,
are normally placed in a startup file.
The
.B noclobber
option causes
\f5ksh\fP
to print an error message when you use
.B >
to redirect output to a file that already exists.
If you want to redirect to an existing file, then
you have to use
.B >|
to override
the
.B noclobber
option.
The
.B ignoreeof
option
is used to prevent the
.I end-of-file
character, normally
.B ^D
(Control- d),
from exiting the shell and possibly logging you out.
You must type \f5exit\fP
to log out.
Most of the options are described in this memo as appropriate. 
.H 2 "Command Aliases"
.P
Command aliases provide a mechanism of associating a command name and
arguments with a shorter name.
Aliases are defined with the \f5alias\fP
built-in.
The form of an \f5alias\fP
command definition is:
.ce
\f5alias\fP \fIname\fP\f5=\fP\fIvalue\fP
As with most other shell assignments, no space is allowed before or after
the \f5=\fP.
The characters of an alias name cannot be characters that are
special to the shell. 
The replacement string,
.I value,
can contain any valid shell script,
including meta-characters such as pipe symbols and i/o-redirection
provided that they are quoted.
Unlike
\f5csh\fP,
aliases in
\f5ksh\fP
cannot take arguments.
The equivalent functionality of aliases with arguments can
be achieved with shell functions, described later.
.P
As a command is being read,
the command name is checked against a list of
.I alias
names.
If it is found,
the name is replaced by the alias value associated with the
.I alias
and then rescanned.
When rescanning the value for an alias, alias substitutions
are performed except for an alias that is currently being processed.
This prevents infinite loops in alias substitutions.
For example with the aliases, \f5alias\ l=ls\ 'ls=ls\ -C'\fP,
the command name \f5l\fP becomes \f5ls\fP, which becomes \f5ls\ -C\fP.
Ordinarily, only the command name word is processed for alias substitution.
However, if the value of an alias ends in a space,
then the word following the alias is also checked for alias substitution.
This makes it possible
to define an alias whose first argument is the name of a command
and have alias substitution performed on this argument,
for example
\f5nohup='nohup\ '\fP.
.P
Aliases can be used to redefine built-in commands so that
the alias,
.ce
\f5alias test=./test\fP
can be used to look for \f5test\fP
in your current working directory rather than
using the built-in \f5test\fP command.
Reserved words such as
\f5for\fP and \f5while\fP
cannot be changed by aliasing.
The command \f5alias\fP,
without arguments, generates
a list of aliases and corresponding alias values.
The \f5unalias\fP command removes the name and text of an alias.
.P
Aliases are used to save typing and to improve readability of scripts.
Several aliases are predefined by \f5ksh\fP.
For example, the predefined alias
.ce
\f5alias integer='typeset -i'\fP
allows the integer variables \f5i\fP and \f5j\fP
to be declared and initialized with the command
.ce
\f5integer i=0 j=1\fP
.P
While aliases can be defined in scripts,
it is not recommended.
The location of an alias command can be important
since aliases are only processed when a command is read.
A \fB\s+2.\s-2\fP
procedure (the shell equivalent of an include file)
is read all at once (unlike
start up files
which are read a command at
a time) so that any aliases defined there will not affect any commands
within this script.
Predefined aliases do not have this problem.
.H 2 "Command Re-entry"
.P
When run interactively,
\f5ksh\fP saves the
commands you type at a terminal in a file.
If the variable
\fB\s-1HISTFILE\s+1\fP
is set to the name of a file to which the user
has write access,
then the commands are stored in this
.I history
file.
Otherwise the file
\fB$\s-1HOME\s+1/.sh_history\fP
is checked for write access and if this fails
an unnamed file is used to hold the history lines.
Commands are always appended to this file.
Instances of \f5ksh\fP
that run concurrently and use the same history file
name, share access to the history file so that a command
entered in one shell will be available for editing in another
shell.
The file may be truncated when \f5ksh\fP
determines that no other shell is using the history file.
The number of commands accessible to the user is determined by the value of the
\fB\s-1HISTSIZE\s+1\fP
variable at the time the shell is invoked.
The default value is 256.
Each command may consist of one or more lines since a compound
command is considered one command.
If the character
.B !
is placed within the
.I "primary prompt"
string,
\fB\s-1PS1\s+1\fP,
then it is replaced by the command number each time the prompt is given.
.P
A built-in command named \f5hist\fP
is used to list and/or edit
any of these saved commands.
The option
.B \-l
is used to specify listing of previous commands.
The command can always be specified with
a range of one or more commands.
The range can be specified by giving the command
number, relative or absolute, or by giving
the first character or characters of the command.
When given without specifying the range,
the last 16
commands are listed, each
preceded by the command number.
.P
If the listing option is not selected,
then the range of commands specified,
or the last command if no range is given,
is passed to an editor program before
being re-executed by \f5ksh\fP.
The editor to be used may be specified
with the option
.B \-e
and following it with the editor name.
If this option is not specified, the
value of the shell variable
\fB\s-1HISTEDIT\s+1\fP
is used as the name of the editor,
providing that this variable has a non-null value.
If this variable is not set, or is null,
and the
.B \-e
option has not been selected,
then
\f5/bin/ed\fP
is used.
When editing has been complete,
the edited text automatically becomes
the input for \f5ksh\fP.
As this text is read by \f5ksh\fP, it is echoed onto the terminal.
.P
The 
.B \-s
option causes the editing to be bypassed
and just re-executes the command.
In this case only a single command can be specified as the range
and an optional argument of the form
\fIold\fP\fB=\fP\fInew\fP
may be added which requests a simple string substitution
prior to evaluation.
A convenient alias,
.ce
\f5alias r='hist -s'\fP
has been predefined so that
the single key-stroke
\f5r\fP
can be used to re-execute the previous command
and the key-stroke sequence,
\f5r\ abc=def\ c\fP
can be used to re-execute the last command that starts with
the letter \f5c\fP
with the first occurrence of the string \f5abc\fP
replaced with the string \f5def\fP.
Typing
\f5r\ c\ >\ file\fP
re-executes the most recent command starting with the letter \f5c\fP,
with standard output redirected to
.IR file .
.H 2 "In-line editing"
.P
Lines typed from a terminal frequently need changes made
before entering them.
With the Bourne shell the only method to fix up commands
is by backspacing or killing the whole line.
\f5ksh\fP offers options that allow the user to edit parts of the
current command line before submitting the command.
The in-line edit options make the command line into a single
line screen edit window.
When the command is longer than the width of the terminal,
only a portion of the command is visible.
Moving within the line automatically makes that portion visible.
Editing can be performed on this window until the
.I return
key is pressed.
The editing modes have editing directives that access the history file
in which previous commands are saved.
A user can copy any of the most recent
\fB\s-1HISTSIZE\s+1\fP
commands from this file into the input edit window.
You can locate commands by searching or by position.
.P
The in-line editing options do not use the
.I termcap
or
.I terminfo
databases.
They work on most standard terminals.
They only require that the backspace character moves the cursor left
and the space character overwrites the current character on the screen
and moves the cursor to the right.
Very few terminals or terminal emulators do not have
this behavior.
.P
There is a choice of editor options.
The
.BR emacs ,
.BR gmacs ,
or
.B vi
option is selected by turning on the
corresponding
option of the \f5set\fP
command.
If the value of the
\fB\s-1EDITOR\s+1\fP
or
\fB\s-1VISUAL\s+1\fP
variables ends with any of these suffixes
the corresponding option is turned on.
A large subset of each of these editors'
features is available within the shell.  Additional
functions, such as file name completion, have also been added.
.P
In the
.B emacs
or
.B gmacs
mode the user positions the cursor to the point
needing correction and inserts, deletes, or replaces
characters as needed.
The only difference between these two modes is the
meaning of the directive
.BR ^T .
Control keys and escape sequences are used for cursor
positioning and control functions.
The available editing functions are listed in the manual page.
.P
The
.B vi
editing mode
starts in insert mode and enters control mode when the
user types ESC ( 033 ).
The
.I return
key, which submits the current command for processing,
can be entered from either mode.
The cursor can be anywhere on the line.
A subset of commonly used
.I vi
editing directives are available.
The
.B k
and
.B j
directives that normally move up and down by one
.IR line ,
move up and down one
.I command
in the history file,
copying the command into the input edit window.
For reasons of efficiency,
the terminal is kept in canonical mode until an
ESC
is typed.
On some terminals,
and on earlier versions of the UNIX operating system,
this doesn't work correctly.
The
.B viraw
option,
which always uses
.I raw
or
.I cbreak
mode,
must be used in this case.
.P
Most of the code for the editing options does not rely on the
\f5ksh\fP code and can be used in a stand-alone mode with most any command
to add in-line edit capability.
However,
all versions of the in-line editors have some features that
use some shell specific code.  For example,
with all edit modes, the
ESC-=
directive applied to command words
(the first word on the line,
or the first word after a
.BR ; ,
.BR | ,
.BR ( ,
or
.BR & )
lists all aliases, functions, or commands
that match the portion of the given current word.
When applied to other words, this directive
prints the names of files that match the current
word.
The ESC\fB-*\fP directive
adds the expanded list of matching files to the command line.
A trailing
.B *
is added to the word if it doesn't contain any file pattern matching
characters before the expansion.
In
.B emacs
and
.B gmacs
mode,
ESC-ESC
indicates command completion when applied to
command names, otherwise it indicates pathname completion.
With command or pathname completion,
the list generated by the
ESC-= directive is examined to find
the longest common prefix.
With command completion, only the last component of
the pathname is used to compute the longest command prefix.
If the longest common prefix is a complete match,
then the word is replaced by the pathname, and a
.B /
is appended if
pathname is a directory, otherwise a space is added.
In
.B vi
mode,
.B \e
from control mode gives the same behavior.
.H 2 "Key Binding"
.P
It is possible to intercept keys as they are entered and
apply new meanings or bindings.
A trap named
\fB\s-1KEYBD\s+1\fP
is evaluated each time
\f5ksh\fP processes characters entered
from the keyboard,  
other than those typed
while entering a search string or an argument to an
edit directive such as
.B r
in vi-mode.
The action associated with this trap can change the value of
the entered key to cause the key to perform a different
operation.
.P
When the
\fB\s-1KEYBD\s+1\fP
trap is entered,
the \fB.sh.edtext\fP
variable contains the contents of the current input line
and the \fB.sh.edcol\fP
variable gives the current cursor position within this line.
The \fB.sh.edmode\fP
variable contains the
.B ESC
character when the trap is entered from
.B vi
insert mode.
Otherwise, this value is null.
The \fB.sh.edchar\fP
variable contains the character or
escape sequence that caused the trap.
A key sequence is either a single character,
.B ESC
followed by a single character,
or
.B ESC[
followed by a single character.
In the \fBvi\fP edit mode,
the characters after the
.B ESC
must be entered within half a second after the
.BR ESC .
The value of \fB.sh.edchar\fP
at the end of the trap will be used as
the input sequence. 
.P
Using the associative array facility of \f5ksh\fP described later,
and the function facility of \f5ksh\fP, it is easy to write
a single trap so that keys can be bound dynamically.  For example,
.sp
.nf
.in .5i
.ta 4i
\f5typeset -A Keytable
trap 'eval "${Keytable[${.sh.edchar}]}"' KEYBD
function keybind # key action
{ 
        typeset key=$(print -f "%q" "$2")
        case $# in
        2)      Keytable[$1]='.sh.edchar=${.sh.edmode}'"$key"
                ;;
        1)      unset Keytable[$1]
                ;;
        *)      print -u2 "Usage: $0 key [action]"
                ;;
        esac
}\fP
.ta
.in
.fi
.sp
.H 2 "Job Control"
.P
The job control mechanism 
is almost identical to the version introduced in \f5csh\fP
of the Berkeley UNIX operating system,
version 4.1 and later.
The job control feature allows the user to stop and
restart programs, and to move programs to and from the
foreground and the background.
It will only work on systems that provide support for
these features.
However,
even systems without job control have a
.B monitor
option which, when enabled, will report the progress
of background jobs and enable the user to \f5kill\fP
jobs by job number or job name.
.P
An interactive shell associates a
.I job
with each pipeline typed in from the terminal
and assigns it a small integer number
called the job number.
If the job is run asynchronously,
the job number is printed at the terminal.
At any given time, only one job owns the terminal,
i.e., keyboard signals are only sent to the processes in one job.
When \f5ksh\fP creates a foreground job,
it gives it ownership of the terminal.
If you are running a job and wish to stop
it you hit the key
.B ^Z
(control-\fBZ\fP)
which sends a
\fB\s-1STOP\s+1\fP
signal to all processes in the current job.
The shell receives notification that the processes
have stopped and takes back control of the terminal.
.P
There are commands to continue programs in the foreground
and background.
There are several ways to refer to jobs.
The character
.B %
introduces a job name.
You can refer to jobs by name or number as described in the manual page.
The built-in command \f5bg\fP
allows you to continue a job in the background,
while the built-in command \f5fg\fP
allows you to continue a job in the foreground even
though you may have started it in the background.
.P
A job being run in the background will stop if it tries
to read from the terminal.
It is also possible to stop background jobs that try to write on
the terminal by setting the terminal options
appropriately.
.P
There is a built-in command \f5jobs\fP
that lists the status of all running and stopped jobs.
In addition,
you are informed of the change of state (running or stopped)
of any background
jobs just before each prompt.
If you want to be notified about background job completions
as soon as they occur without waiting for a prompt, then use the
.B notify
option.
When you try to exit the shell while jobs are stopped or running,
you will receive a message from \f5ksh\fP.
If you ignore this message and try to exit again,
all stopped processes will be terminated.
In addition, for login shells, the
\fB\s-1HUP\s+1\fP
signal will be sent to
all background jobs
unless the job has been disowned with the
.B disown
command.
.P
A built-in version of \f5kill\fP
makes it possible to use
.I job
numbers as targets for signals.
Signals can be selected by number or name.
The name of the signal is the name found in the
.I include
file
.B /usr/include/sys/signal.h
with the prefix
.B \s-1SIG\s+1
removed.
The
.B \-l
option of \f5kill\fP 
provides a means to map individual signal names to and from
signal number.
In addition, if no signal name or number is given,
\f5kill\ -l\fP 
generates a list of valid signal names.
.H 2 "Changing Directories"
By default,
\f5ksh\fP
maintains a logical view of the file system hierarchy
which makes symbolic links transparent.
For systems that have symbolic links,
this means that if \f5/bin\fP is a symbolic link to \f5/usr/bin\fP
and you change directory to \f5/bin\fP, \f5pwd\fP will indicate
that you are in \f5/bin\fP, not \f5/usr/bin\fP. 
\f5pwd\ -P\fP
generates the physical pathname of the present working
directory by resolving all the symbolic links.
By default,
the \f5cd\fP
command will take you where you expect to go even if you cross
symbolic links.
A subsequent \f5cd\ ..\fP in the example above
will place you in \f5/\fP, not \f5/usr\fP.
On systems with symbolic links,
\f5cd\ -P\fP
causes
.B ..
to be treated physically.
.P
\f5ksh\fP remembers your last directory
in the variable
\fB\s-1OLDPWD\s+1\fP.
The \f5cd\fP
built-in can be given with argument
.B \-
to return to the previous directory
and print the name of the directory.
Note that \f5cd\ -\fP
done twice returns you to the starting directory,
not the second previous directory.
A directory
.I stack
manager has been written as shell
.I functions
to
.I push
and
.I pop
directories from the stack.
.H 2 "Prompts"
.P
When \f5ksh\fP
reads commands from a terminal,
it issues a prompt whenever it is ready
to accept more input and then
waits for the user to respond.
The
\fB\s-1TMOUT\s+1\fP
variable
can be set to be the number of seconds that the shell will wait for
input before terminating.
A 60 second warning message is printed
before terminating.
.P
The shell uses two prompts.
The primary prompt,
defined by the value of the
\fB\s-1PS1\s+1\fP
variable,
is issued at the start of each command.
The secondary prompt,
defined by the value of the
\fB\s-1PS2\s+1\fP
variable,
is issued when more input is needed to complete a command.
.P
\f5ksh\fP allows the user to specify a list of files or directories
to check before issuing the
\fB\s-1PS1\s+1\fP
prompt.
The variable
\fB\s-1MAILPATH\s+1\fP
is a colon (
.B :
) separated list of file names to be checked for changes
periodically. The user is notified
before the next prompt.
Each of the names in this list can be followed by a
.B ?
and a message to be given when a change has been detected in the file.
The prompt will be evaluated for parameter expansion, command
substitution and arithmetic expansion which are described later.
The parameter
.B $_
within a mail message will evaluate to the name of the file that
has changed.
The parameter
\fB\s-1MAILCHECK\s+1\fP
is used to specify the minimal interval in seconds before
new mail is checked for.
.P
In addition to replacing each
.B !
in the prompt with the command number,
\f5ksh\fP expands
the value of the
.B \s-1PS1\s+1
variable
for parameter expansions, arithmetic expansions,
and command substitutions as described below
to generate the prompt.
The expansion characters that are to be applied when
the prompt is issued must be quoted to prevent the
expansions from occurring when assigning the value to
.B \s-1PS1\s+1.
For example,
\f3\s-1PS1\s+1="$\s-1PWD\s+1"\fP
causes 
.B \s-1PS1\s+1
to be set to the value of
.B \s-1PWD\s+1
at the time of the assignment whereas
.B \s-1PS1\s+1='$\s-1PWD\s+1'
causes
.B \s-1PWD\s+1
to be expanded at the time the prompt is issued.
.P
Command substitution may require a separate process
to execute and cause the prompt display to be somewhat
slow, especially
when the return key is pressed several times in a row.
Therefore, its use
within
.B \s-1PS1\s+1
is discouraged.
Some variables are maintained by \f5ksh\fP
so that their values can be used with
.B \s-1PS1\s+1.
The
.B \s-1PWD\s+1
variable stores the pathname of the current working directory.
The value of 
.B \s-1SECONDS\s+1
variable
is the value of the most
recent assignment plus the elapsed time.
By default, the time is measured in milliseconds,
but since
.B \s-1SECONDS\s+1
is a floating point variable, the
number of places after the decimal point in the expanded
value can be
specified with
\f5typeset\ -F\fP\fIplaces\fP\f5\ SECONDS\fP.
In a roundabout way, this variable
can be used to generate a time stamp into the
.B \s-1PS1\s+1
prompt without creating a process at each prompt.
The following code explains how you can do this on
System V.  On BSD, you need a different command to initialize
the
.B \s-1SECONDS\s+1
variable.
\f5
.sp
.nf
.in .5i
# . this script and use $TIME as part of your PS1 string to
# get the time of day in your prompt
typeset -RZ2  _x1 _x2 _x3
(( SECONDS=$(date  '+3600*%H+60*%M+%S') ))
_s='_x1=(SECONDS/3600)%24,_x2=(SECONDS/60)%60,_x3=SECONDS%60,0'
TIME='"${_d[_s]}$_x1:$_x2:$_x3"'
# PS1=${TIME}whatever
.fi
.ta
.in
.sp
\fP
.H 2 "Tilde substitution"
.P
The character
.B \(ap
at the beginning of a word has special meaning to \f5ksh\fP.
If the characters after the
.B \(ap
up to a
.B /
match a user login name in the password database, then the
.B \(ap
and the name are replaced by
that user's login directory.
If no match is found, the original word
is unchanged.
A
.B \(ap
by itself, or in front of a
.BR / ,
is replaced by the value of the
\fB\s-1HOME\s+1\fP
parameter.
A
.B \(ap
followed by a
.B +
or
.B \-
is replaced by the value of
.B $\s-1PWD\s+1
or
.B $\s-1OLDPWD\s+1
respectively.
.H 2 "Output formats"
The output of built-in commands and traces have values quoted so that they
can be re-input to the shell.
This makes it easy to cut and paste shell output on systems
which use a pointing device such as a mouse.
In addition, output can be saved in a file for reuse.
.P
.H 2 "The \fB\s-1ENV\s+1\fP file"
When an interactive \f5ksh\fP starts, it evaluates the
.B $\s-1ENV\s+1
variable to arrive at a file name.
If this value is not null,
\f5ksh\fP attempts to read and process
commands in a file by this name.
Earlier versions of \f5ksh\fP read the \fB\s-1ENV\s+1\fP file
for all invocations of the shell primarily to allow
function definitions to be available for all shell
invocations.
The function search path, \fB\s-1FPATH\s+1\fP, described later,
eliminated the primary need for this capability and it was
removed because the high performance cost was no longer
deemed acceptable.
.H 1 "PROGRAMMING LANGUAGE"
The KornShell vastly extends the set of applications that
can be implemented efficiently at the shell level.
It does this by providing simple yet powerful mechanisms
to perform arithmetic, pattern matching,
substring generation,
and arrays.
Users can write applications as separate functions that can
be defined in the same file or in a library of functions
stored in a directory and loaded on demand.
.H 2 "String Processing"
The shell is primarily a string processing language.
By default, variables hold variable length strings.
There are no limits to the length of strings.  Storage
management is handled by the shell automatically.
Declarations are not required.
With most programming languages, string constants are designated
by enclosing characters in single quotes or double quotes.
Since most of the words in the language are strings, the shell
requires quotes only when a string contains characters that
are normally processed specially by the shell, but their
literal meaning is intended.
However, since the shell is a string processing language,
and some characters can occur as literals and as language metacharacters,
quoting is an important part of the language.
.P
There are four quoting mechanisms in \f5ksh\fP.
The simplest is to enclose a sequence of characters inside single quotes.
All characters between a pair of single quotes have their literal meaning;
the single quote itself cannot appear.
A
.B $
immediately preceding
a single quoted string
causes all the characters until the matching single quote
to be interpreted as an ANSI C language string.
Thus, \f5'\en'\fP represents characters \f5\e\fP and
\f5n\fP, whereas, \f5$'\en'\fP 
represents the new-line character.
Double quoted strings remove the special meaning of all characters
except
.BR $ ,
.BR \(ga ,
and
.BR \e ,
so that parameter expansion and command substitution (defined below)
are performed.
The final mechanism for quoting a character is by preceding it with the
escape character
.BR \e\^ .
This mechanism works outside of quoted strings and for the characters
.BR $ ,
.BR \(ga ,
\fB"\fP,
and
.B \e
in double quoted strings.
.P
Variables are designated by
one or more
strings of alphanumeric
characters beginning with an alphabetic character
separated by a \fB\s+2.\s-2\fP.
Upper and lower case characters are distinct, so that the variable
.B A
and
.B a
are names of different variables.
There is no
limit to the length of the name of a variable.
You do not have to declare variables.
You can assign a value to a variable by writing the name of the
variable, followed by an equal sign, followed by a character string
that represents its value.
To create a variable whose name
contains a \fB\s+2.\s-2\fP,
the variable whose name consists of
the characters before the last \fB\s+2.\s-2\fP
must already exist.
You reference a variable by
putting the name inside curly braces and
preceding the braces with a dollar sign.
The braces may be omitted when the name
is alphanumeric.
If \f5x\fP and \f5y\fP
are two shell variables, then
to define a new variable,
\f5z\fP,
whose value is
the concatenation of the values of
\f5x\fP and \f5y\fP,
you just say
\f5z=$x$y\fP.
It is that easy.
.P
The
.B $
can be thought of as meaning
"value of."
You can also capture the output of any command with the notation
.BI $( command ) .
This is referred to as command substitution.
For example,
\f5x=$(date)\fP
assigns the output from the \f5date\fP
command to the variable \f5x\fP.
Command substitution in the
Bourne shell is denoted by enclosing the command between
backquotes,
(\fB\(ga\^\(ga\fP).
This notation
suffers from some
complicated quoting rules.
Thus, it is hard to write \f5sed\fP
patterns which contains back slashes within command substitution.
Putting the pattern in single quotes
is of little help.
\f5ksh\fP accepts the Bourne shell command substitution syntax
for backward compatibility.
The
.BI $( command )
notation allows
the \fIcommand\fP itself to contain quoted strings even if the substitution
occurs within double quotes. Nesting is legal.
.P
The special command substitution of the form
\f5$(cat\ file)\fP
can be replaced by
\f5$(<\ file)\fP,
which is faster because
the \f5cat\fP
command doesn't have to run.
.H 2 "Shell Parameters and Variables"
.P
There are three types of parameters used by \f5ksh\fP,
special parameters, positional parameters, and named
parameters which are called variables.
\f5ksh\fP defines the same special parameters,
.BR 0 ,
.BR * ,
.BR @ ,
.BR # ,
.BR ? ,
.BR $ ,
.BR ! ,
and
.BR \- ,
as in the Bourne shell.
.P
Positional parameters are set when the shell is invoked,
as arguments to the \f5set\fP built-in,
and by calls to functions (see below) and \fB\s+2.\s-2\fP
procedures.
They are named by numbers starting at 1.
.P
The third type of parameter is a variable.
As mentioned earlier,
\f5ksh\fP uses variables whose names
consist of one or more
alphanumeric strings separated by a \fB\s+2.\s-2\fP.
There is no need to specify the
.I type
of a variable in the shell because, by default,
variables store strings of arbitrary length
and values will automatically be converted to numbers
when used in an arithmetic context.
However, \f5ksh\fP variables
can have one or more
.I attributes
that control the internal representation of the variable,
the way the variable is printed, and its access or
scope.
In addition,
\f5ksh\fP
allows variables to represent arrays of values
and references to other variables.
The \f5typeset\fP
built-in command of \f5ksh\fP
assigns attributes to variables.
Two of the attributes,
.I readonly
and
.IR export ,
are available in the Bourne shell.
Most of the remaining attributes are discussed here.
The complete list of attributes appears in the manual.
The \f5unset\fP
built-in of \f5ksh\fP removes
values and attributes of variables.
When a variable is exported, certain of its attributes are also exported.
.P
Whenever a value is assigned to a variable,
the value is transformed according to the attributes of the variable.
Changing the attribute of a variable can change its value.
The attributes
.B \-L
and
.B \-R
are for left and right field justification respectively.
They are useful for aligning columns in a report.
For each of these attributes, a width can be defined explicitly or else
it is defined the first time an assignment is made to the variable.
Each assignment causes justification of the field, truncating
if necessary.
Assignment to fixed-size variables
provides one way to generate a substring consisting of
a fixed number of characters from
the beginning or end of a string.
Other methods are discussed later.
.P
The attributes
.B \-u
and
.B \-l
are used for upper case and lower case
formatting, respectively.
Since it makes no sense to have both attributes on simultaneously,
turning on either of these attributes turns the other off.
The following script,
using \f5read\fP and \f5print\fP which are described later,
provides an example of the use of shell variables
with attributes.
This script reads a file of lines each consisting of five fields separated by
.B :
and prints fields 4 and 2 in upper case in columns 1-15, left justified,
and columns 20-25 right-justified respectively.
.sp
.nf
.in .5i
.ta 3.4i
\f5typeset -uL15 f4                # 15 character left justified
typeset -uR6 f2                 # 6 character right justified
IFS=:                           # set field separator to :
while   read -r f1 f2 f3 f4 f5  # read line, split into fields
do      print -r -- "$f4  $f2"  # print fields 4 and 2
done\fP
.fi
.ta
.in
.sp
.P
The
.BR \-i ,
.BR \-E ,
and
.BR \-F ,
attributes are used to represent numbers.
Each can be followed by a decimal number.
The
.B \-i
attribute causes the value to be represented as an integer and it
can be followed by a number representing the numeric base when expanding
its value.
Whenever a value is assigned to an integer variable, it is evaluated
as an arithmetic expression
and then truncated to an integer.
.P
The
.B \-E
attribute causes the value to be represented in scientific
notation whenever its value is expanded.  The number following the 
.B \-E
determines the number of significant figures, and defaults to 6.
The
.B \-F
attribute causes the value to be represented with a fixed number
of places after the decimal point. 
Assignments to variables with the
.B \-E
or
.B \-F
attributes cause the evaluation of the right hand side of the assignment.
.P
\f5ksh\fP allows one-dimensional
.I arrays
in addition to simple variables.
There are two types of arrays; associative arrays
and indexed arrays.
The subscript for an associative array is an arbitrary
string, whereas the subscript for an indexed array is
an arithmetic expression that is evaluated to yield an integer
index.
Any variable can become an indexed array
by referring to it with
an integer
.IR subscript .
All elements of an array need not exist.
Subscripts for arrays
must evaluate to an
integer between 0 and some maximum value, otherwise
an error results.
The maximum value may vary from one machine to another but
is at least 4095.
Evaluation of subscripts is described in
the next section.
Attributes apply to the whole array.
.P
Assignments to array variables can be made to individual elements
via parameter
assignment commands or the
.B typeset
built-in.
Additionally, values can be assigned sequentially with
compound assignment as described below, or by the
.B \-A
.I name
option of the \f5set\fP command.
Referencing of subscripted variables requires the character
.BR $ ,
but also requires braces around the array element name.
The braces are needed to avoid conflicts with the
file name generation mechanism.
The form of any array element reference is:
.ce
.BI ${ name [ subscript ]}
Subscript values of
.B *
and
.B @
can be used to generate all elements of an array,
as they are used for expansion of positional parameters.
The list of currently defined subscripts for a given
variable can be generated with 
.BI ${! name [@]} ,
or
.BI ${! name [*]} .
.P
The
.B \-n
or
.I nameref
attribute causes the variable to be treated
as a reference to the variable defined by its value.
Once this attribute is set, all references to this variable
become references to the variable named by the value
of this variable.
For example, if \f5foo=bar\fP, then setting the reference
attribute on \f5foo\fP will cause all subsequent references
to \f5foo\fP to behave as the variable whose name is \f5$foo\fP
was referenced, which in this case is the variable \f5bar\fP.
Unsetting this attribute breaks the association.
Reference variables are usually used inside functions whose
arguments are the names of shell variables. 
The names for reference variables cannot contain a \fB\s+2.\s-2\fP.
Whenever a shell variable is referenced, the portion of the
variable up to the first \fB\s+2.\s-2\fP
is checked to see whether it matches the name of a reference
variable.
If it does, then the name of the variable actually used
consists of the concatenation of the name of the variable
defined by the reference plus the remaining portion of the
original variable name.
For example, using the predefined alias, \f5alias\ nameref='typeset\ -n'\fP,
.sp
.nf
.in .5i
.ta 3.4i
\f5\^.bar.home.bam="hello world"
nameref foo=.bar.home
print ${foo.bam}
\fBhello world\fP\fP
.fi
.ta
.in
.sp
.H 2 "Compound Assignment"
Compound assignments are used to assign values to arrays
and compound data structures.
The syntax for a compound assignment is
.IB name =( assignment-list )
where
\fIname\fP
is the name of the variable to which you want to assign values.
No space is permitted between the variable name and the \fB=\fP
but can appear between the \fB=\fP and the open parenthesis.
New-lines can appear between the parentheses.
.P
The \fIassignment-list\fP can be in several different forms
yielding different results.
If \fIassignment-list\fP is simply a list of words, then
the words are processed as they are with the \f5for\fP command
and assigned sequentially as an indexed array.
For example,
.ce
\f5foo=( * )\fP
creates an indexed array \f5foo\fP and assigns the
file names in the current directory to each index starting
at zero.
.P
The second form for \fIassignment-list\fP is a list of assignments
of the special form \fB[\fP\fIword\fP\fB]=\fP\fIword\fP.
No space is permitted before or after the \fB=\fP.
In this case, the variable given by \fIname\fP becomes
an associative array with the given arguments as subscripts.
For example,
.ce
\f5bar=( [color]=red [shape]=box )\fP
creates an associative array named \f5bar\fP whose
subscripts are \f5color\fP and \f5shape\fP.
.P
The third form for \fIassignment-list\fP is a list of
normal assignments, including compound assignments.
These assignments cause sub-variables to be assigned
corresponding to the given assignments.
In addition to assignments, the \fIassignment-list\fP
can contain \f5typeset\fP commands.
In addition to creating sub-variables,
the effect of a compound assignment is to make
the value of the original variable be a parenthesized
assignment list of its components.
For example, the assignment
.sp
.nf
.in .5i
.ta 3.4i
\f5foo=(
        left=bar
        typeset -i count=3
        point=(
                x=50
                y=60
        )
        colors=( red green yellow )
        right=bam
) \fP
.ta
.in
.fi
.sp
is equivalent to the assignments
.sp
.nf
.in .5i
.ta 3.4i
\f5foo.left=bar
foo.count=3
foo.point.x=50
foo.point.y=60
foo.colors=( red green yellow )
foo.right=bam\fP
.ta
.in
.fi
.sp
In addition, the value of \f5"$foo"\fP is
.sp
.nf
.in .5i
.ta 3.4i
\f5(
        colors=( red green yellow )
        left=bar
        typeset -i count=3
        point=(
                y=60
                x=50
        )
        right=bam
)\fP
.ta
.in
.fi
.sp
.H 2 "Substring Generation"
The expansion of a variable or parameter can be modified so that
only a portion of the value results. 
It is often necessary to extract a portion of a shell variable or
a portion of an array.
There are several parameter expansion operators that can do this.
One method to generate a substring is with an expansion of
the form \fB${\fP\fIname\fP\fB:\fP\fIoffset\fP\fB:\fP\fIlength\fP\fB}\fP
where \fIoffset\^\fP is an arithmetic expression that defines the
offset of the first character starting from 0, and
\fIlength\^\fP is an arithmetic expression that defines the
length of the substring.
If
.BI : length\^
is omitted,
the length of the value of
.I name\^
starting at
.I offset\^
is used.
The
.BI : offset : length
operators can also be applied to array expansions and to parameters
.B *
and
.B @
to generate portions of an array.
For example, the expansion, \fB${\fP\fIname\fP\fB[@]:\fP\fIoffset\fP\fB:\fP\fIlength\fP\fB}\fP, yields up to \fIlength\fP elements of the array \fIname\fP
starting at the element \fIoffset\fP.
.P
The other parameter expansion modifiers use shell patterns
to describe portions of the string to modify and delete.
A description of shell patterns is contained below.
When these
modifiers are applied to special parameters
.B @
and
.B *
or to array parameters given as
\fIname\fP\fB[@]\fP or \fIname\fP\fB[*]\fP,
the operation is performed on each element.
There are four parameter expansion modifiers that
strip off leading and trailing substrings
during parameter expansion
by removing the characters matching a given pattern.
An expansion of
the form \fB${\fP\fIname\fP\fB#\fP\fIpattern\fP\fB}\fP
causes the smallest matching prefix of the value of
.I name\^
to be removed.
The largest prefix matching
.I pattern\^
is removed by using
.B ##
instead of
.BR # .
Similarly,
an expansion of
the form \fB${\fP\fIname\fP\fB%\fP\fIpattern\fP\fB}\fP
causes the smallest matching substring at the end of
.I name\^
to be removed.
Again, using
.B %%
instead of
.BR % ,
causes the largest matching trailing substring to be deleted.
For example, if the shell variable
.B file
has value
.BR foo.c ,
then the expression
.B ${file%.c}.o
has value
.BR foo.o .
.P
The value of an expansion can be changed by
specifying a pattern that matches the part that needs to be changed
after the
the parameter expansion modifier
.BR / .
An expansion of the form
\fB${\fP\fIname\fP\fB/\fP\fIpattern\fP\fB/\fP\fIstring\fP\fB}\fP
replaces the first match of \fIpattern\fP with
the value of variable \fIname\fP to \fIstring\fP.
The second
.B /
is not necessary when \fIstring\fP is null.
The expansion
\fB${\fP\fIname\fP\fB//\fP\fIpattern\fP\fB/\fP\fIstring\fP\fB}\fP
changes all occurrences of the \fIpattern\fP into \fIstring\fP.
The parameter expansion modifiers
.B /#
and
.B /%
cause the matching pattern to be anchored to the beginning and
end respectively.
.P
Finally, there are parameter expansion modifiers that yield
the name of the variable, the string length of the value, or the number
of elements of an array.
\fB${!\fP\fIname\fP\fB}\fP
yields the name of the variable which will be \fIname\fP itself
except when \fIname\fP is a reference variable.  In this case
it will yield the name of the variable it refers to.
When applied to an array variable,
\fB${!\fP\fIname\fP\fB[@]}\fP and
\fB${!\fP\fIname\fP\fB[*]}\fP 
generate the names of all subscripts.
\fB${#\fP\fIname\fP\fB}\fP
will be the length in bytes of
\fB$\fP\fIname\fP.
For an array variable
\fB${#\fP\fIname\fP\fB[*]}\fP
gives the number of elements in the array.
.H 2 "Arithmetic Evaluation"
.P
For the most part, the shell is a string processing
language.  However, the need for arithmetic has
long been obvious.
Many of the characters that are special to the
Bourne shell are needed as arithmetic operators.
To make arithmetic easy to use, and to maintain
compatibility with the Bourne shell, \f5ksh\fP uses matching
.B ((
and
.B ))
to delineate arithmetic expressions.
While single parentheses might have been
more desirable, these already mean
.I subshell\^
so that another notation was required.
The arithmetic expression
inside the double parentheses
follows the same syntax, associativity and precedence
as the ANSI C\*(Rf
.RS
American National Standard for Information Systems \- Programming
Language \- C, ANSI X3.159-1989.
.RF
programming language.
The characters between the matching double parentheses
are processed with the same rules used for double
quotes so that spaces can be used to aid readability
without additional quoting.
.P
All arithmetic evaluations are performed using
double precision floating point arithmetic.
Floating point constants follow the same rules as
the ANSI C programming language.
Integer arithmetic constants are written as
.ce
.IB base # number,
where
.I base\^
is a decimal integer between
two and sixty-four and
.I number\^
is any non-negative number.
Base ten is used
when no base is specified.
The digits are represented by the characters
.BR 0-9a-zA-Z_@ .
For bases less than or equal to 36,
upper and lower case characters can
be used interchangeably to represent the digits
from 10 through 35.
.P
Arithmetic expressions are made from constants,
variables, and operators.
Parentheses may be used for grouping.
The contents inside the double parentheses
are processed with the same expansions as occurs in a double quoted string,
so that all
.B $
expansions are performed before the expression is evaluated.
However, there is usually no need to use the
.B $
to get the value of a variable
because the arithmetic evaluator replaces the name of the variable
by its value within an arithmetic expression.
The
.B $
cannot be used when the variable is the subject of assignment
or an increment operation.
As a rule it is better not to use
.B $
in front of variables in an arithmetic expression.
.P
An arithmetic command of the form
.B
(( ... ))
.R
is a command that evaluates the enclosed arithmetic expression.
For example, the command
.ce
\f5(( x++ ))\fP
can be used to
increment the variable \f5x\fP,
assuming that \f5x\fP contains some numerical value.
The arithmetic command is true (return value 0), when the resulting
expression is non-zero, and false (return value 1) when the
expression evaluates to zero.
This makes the command easy to use with the \f5if\fP and \f5while\fP
compound commands.
.P
The \f5for\fP compound command
has been extended for use in arithmetic contexts. 
The syntax,
.ce
\f5for\fP \fB((\fP \fIexpr1\fP\fB;\fP \fIexpr2\fP \fB;\fP \fIexpr3 \fP\fB))\fP
can be used as the first line of a \f5for\fP loop with the same semantics
as the \f5for\fP statement in the ANSI C programming language.
.P
Arithmetic evaluations can also be performed as part of the evaluation
of a command line.
The syntax
.B
$((\ ...\ ))
.R
expands to the value of the enclosed arithmetic expression.
This expansion can occur wherever parameter expansion is performed.
For example using the \f5ksh\fP command \f5print\fP (described
later)
.ce
\f5print $((2+2))\fP
prints the number 4.
.P
The following script prints the first
.I n
lines of its standard input onto its standard output,
where
.I n
can be supplied as an optional argument whose default value is 20.
.sp
.nf
.in .5i
.ta 4i
\f5integer n=${1-20}                       # set n
while   (( n-- >=0 )) && read -r line   # at most n lines
do      print -r -- "$line"
done\fP
.fi
.ta
.in
.sp
.H 2 "Shell Expansions"
.P
The commands you enter from the terminal or from a script
are divided into words and each word undergoes several
expansions to generate the command name and its arguments.
This is done in two phases.
The first phase recognizes reserved words, spaces and operators
to decide where command boundaries lie.
Alias substitutions take place during this phase.
The second phase performs expansions in the following order:
.BL
.LI
Tilde substitution,
parameter expansion,
arithmetic expansion,
and command substitution
are performed from left to right.
The option
.B \-u
or
.BR nounset ,
will cause an error to occur when any variable 
that is not set is expanded. 
.LI
The characters that result from parameter expansion and
command substitution above are checked with the characters
in the
\fB\s-1IFS\s+1\fP variable
for possible
field splitting.
(See a description of \f5read\fP below to see how
\fB\s-1IFS\s+1\fP is used.)
Setting
\fB\s-1IFS\s+1\fP to a null
value causes field splitting to be skipped.
.LI
Pathname generation (as described below)
is performed on each of the fields.
Any field that doesn't match a pathname is left alone.
The option,
.B \-f
or
.BR noglob ,
is used to disable pathname generation.
.LE
.H 2 "Pattern Matching"
The shell is primarily a string processing language and uses
patterns for matching file names as well as for matching strings.
The characters
.BR ? ,
.BR * ,
and
.B [
are processed specially 
by the shell when not quoted.
These characters are used to form patterns that
match strings.
Patterns are used by the shell to match pathnames,
to specify substrings,
and for
.B case
commands.
The character
.B ?
matches any one character.
The character
.B *
matches zero or more characters.
The character sequence
.BR [ ... ]
defines a character class
that matches any character contained within
.BR [\^] .
A range of characters can be specified by putting a
.B \-
between the first and last character of the range.
An exclamation mark,
.BR ! ,
immediately after the
.BR [ ,
means match all characters except the characters specified.
For example, the pattern
\f5a?c*.[!a-z]\fP
matches any string beginning with an
.BR a ,
whose third character is a
.BR c ,
and that ends in
.B .
(dot) followed by any character except the lower case letters,
.BR a\-z .
The sequence \f5[:alpha:]\fP
inside a character class, matches any set of characters in
the ANSI C
.B alpha
class.
Similarly, \f5[:\fP\fIclass\fP\f5:]\fP matches
each of the characters in the given \fIclass\fP
for all the ANSI C character classes.
For example, \f5[[:alnum:]_]\fP
matches any alphanumeric character or the character
.BR _ .
.P
\f5ksh\fP
treats
strings of the form
.BI ( pattern-list
.BR ) ,
where
.I pattern-list
is a list of one or more patterns separated by a
.BR \(bv ,
specially when preceded by
.BR * ,
.BR ? ,
.BR + ,
.BR @ ,
or
.BR ! .
A
.B ?
preceding
.BI ( pattern-list )
means that the pattern list enclosed in
.B (\^)
is optional.
An
.BI @( pattern-list )
matches any pattern in the list of patterns enclosed in
.BR () .
A
.BI *( pattern-list )
matches any string that contains zero or more of each of the enclosed
patterns,
whereas
.BI +( pattern-list )
requires a match of one or more of any of the given patterns.
For instance, the pattern
.B +([0\-9])?(.)
matches one or more digits optionally followed by a
.BR . (dot).
A
.BI !( pattern-list )
matches anything except any of the given patterns.
For example,
\f5print\ !(*.o)\fP
displays all file names in the current directory that do not end in
.BR .o .
.P
When patterns are used to generate pathnames when expanding
commands several other rules apply.
A separate match is made
for each file name component of the pathname. 
Read permission is required for
any portion of the pathname that contains any special
pattern character. 
Search permission is required for every component except
possibly the last.
.P
By default,
file names in each directory that begin with \fB\s+2.\s-2\fP
are skipped when performing a match.
If the pattern to be matched starts with a leading \fB\s+2.\s-2\fP,
then only files beginning with a \fB\s+2.\s-2\fP,
are examined when reading each directory to find matching files.
If the
\fB\s-1FIGNORE\s+1\fP variable
is set,
then only files that do not match this pattern
are considered.
This overrides the special meaning of \fB\s+2.\s-2\fP
in a pattern and in a file name.
.P
If the
.B markdirs
option is set,
each matching pathname that is the name
of a directory has a trailing 
.B /
appended to the name.
.P
.H 2 "Conditional Expressions"
The Bourne shell uses the \f5test\fP
command, or the equivalent \f5[\fP
command, to test files for attributes
and to compare strings or numbers.
The problem with \f5test\fP
is that the shell has expanded the words of the \f5test\fP
command and
split them into arguments before \f5test\fP begins execution.
\f5test\fP
cannot distinguish between operators and operands.
In most cases
\f5test\ "$1"\fP
will test whether argument 1 is non-null.
However,
if argument 1 is
.BR \-f ,
then \f5test\fP
will treat
.B \-f
as an operator and
yield a syntax error.
One of the most frequent errors with
\f5test\fP
occurs when its operands are not within double quotes.
In this case, the argument may expand to more than a single
argument or to no argument at all.  In either case this
will likely cause a syntax error.
What makes this most insidious is that these errors are frequently
data dependent.  A script that appears to run correctly may abort
if given unexpected data.
.P
To get around these problems,
\f5ksh\fP
has a compound command for conditional expression testing
as part of the language.
The reserved words
.B [[
and
.B ]]
delimit the range of the command. 
Because they are reserved words, not operator characters,
they require spaces to separate them
from arguments.
The words between
.B [[
and
.B ]]
are not processed for field splitting or for pathname generation.
In addition, since \f5ksh\fP
determines the operators before parameter expansion,
expansions that yield no argument cause no problem.
The operators within
.BR [[ ... ]]
are almost the same as those for the \f5test\fP
command.
All unary operators are of the form
.BI \- letter
and are followed by a single operand.
Instead of
.B \-a
and
.BR \-o ,
.BR [[ ... ]]
uses
.B &&
and
.B \(bv\(bv
to indicate "and" and "or".
Parentheses are used without quoting for grouping.
.P
The right hand side of the string comparison operators
.B ==
and
.B !=
takes a pattern and tests whether the left hand operand
matches this pattern.  Quoting the pattern results
is a string comparison rather than the pattern match. 
The operators
.B <
and
.B >
within
.BR [[ ... ]]
designate lexicographical comparison.
.P
In addition there are several other new comparison primitives.
The binary operators
.B \-ot
and
.B \-nt
compare the modification times
of two files to see which file is
.I "older than"
or
.I "newer than"
the other.
The binary operator
.B \-ef
tests whether two files
have the same device and i-node number,
i.\ e., a link to the same file.
.P
The unary operator
.B \-L
returns true if its operand is a symbolic link.
The unary operator
.B \-O
(\fB\-G\fP)
returns true if the owner (or group) of the file operand matches
that of the caller.
The unary operator
.B \-o
returns true when its operand is the name of an option that is
currently on.
.P
The following script illustrates some of the uses of
.BR [[ ... ]] . 
The reference manual contains the complete list of operators.
.sp
.nf
.in .5i
.ta 4i
\f5for i
do      # execute foo for numeric directory
        if      [[ \-d $i && $i == +([0\-9]) ]]
        then    foo
        # otherwise if writable or executable file and not mine
        elif    [[ (\-w $i\(bv\(bv\-x $i) && ! \-O $i ]]
        then    bar
        fi
done\fP
.fi
.ta
.in
.sp
.H 2 "Input and Output"
\f5ksh\fP has
extended I/O capabilities to enhance the
use of the shell as a programming language.
As with the Bourne shell,
you use the I/O redirection operator,
.BR < ,
to control where input comes from,
and the I/O redirection operator,
.BR > ,
to control where output goes to. 
Each of these operators can be preceded with a single digit that
specifies a file unit number to associate with the file stream.
Ordinarily you specify these I/O redirection operators with a specific
command to which it applies.
However, if you specify I/O redirections with the \f5exec\fP
command,
and don't specify arguments to \f5exec\fP,
then the I/O redirection applies to the current program.
For example, the command
\f5exec\ <\ foobar\fP
opens file \f5foobar\fP
for reading.
The \f5exec\fP
command is also used to close files.
A file descriptor unit can be opened as a copy of an existing
file descriptor unit by using either of the
.B <&
or
.B >&
operators and putting the file descriptor unit of the original file
after the
.BR & .
Thus, \f52>&1\fP means open standard error (file descriptor 2)
as a copy of standard output (file descriptor 1).
A file descriptor value of
.B \-
after the
.B &
indicates that the file should be closed.
To close file unit 5, specify
\f5exec\ 5<&-\fP.
There are two additional redirection operators with \f5ksh\fP
and the POSIX shell that are not part of the Bourne shell.
The
.B >|
operator overrides the effect of the
.B noclobber
option described earlier.
The
.B <\^>
operator causes a file to be opened for both reading and writing.
.P
\f5ksh\fP recognizes certain pathnames and treats them
specially.
Pathnames of the form
.BI /dev/fd/ n\^
are treated as equivalent to the file defined by file descriptor
.IR n .
These name can be used as the script argument to \f5ksh\fP
and in conditional testing as described above.
On underlying systems that support
.B /dev/fd
in the file system, these names can be passed to other commands.
Pathnames of the form
.BI /dev/tcp/ hostid / port
and
.BI /dev/udp/ hostid / port
can be used to create
.B tcp
and
.B udp
connections to services given by the
.I hostid\^
number and
.I port\^
number.
The
.I hostid\^
cannot use symbolic values. In practice these
numbers are typically generated by command substitution.
For example,
\f5exec\ 5>\ /dev/tcp/$(service\ name)\fP
would open file descriptor 5 for sending messages
to hostid and port number defined by the output of \f5service\ name\fP.
.P
The Bourne shell has a built-in command \f5read\fP
for reading lines from standard input (file descriptor 0)
and splitting it into fields based on the value of the
.B \s-1IFS\s+1
variable, and a command \f5echo\fP
to write strings to standard output.
(On some systems, \f5echo\fP
is not a built-in command and incurs considerable overhead to use.)
Unfortunately, neither of these commands
is able to perform some very basic tasks.
For example.
with the Bourne shell,
the \f5read\fP
built-in cannot read a single line that ends in
.BR \e .
With \f5ksh\fP
the \f5read\fP
built-in has a
.B \-r
option to remove the special meaning for
.B \e
which allows it to be
treated as a regular
character rather than the line continuation character.
With the Bourne shell,
there is no simple way to have more than one file open
at any time for reading.
\f5ksh\fP has options on the \f5read\fP
command to specify the file
descriptor for the input.
The fields that are read from a line can be stored into an indexed
array with the
.B \-A
option to read.
This allows a line to be split into an arbitrary number of fields.
.P
The way the Bourne shell uses the
\fB\s-1IFS\s+1\fP variable to
split lines into fields greatly limits its utility.
Often data files consist of lines that use a character such
as
.B :
to delimit fields with two adjacent delimiters that denote
a null field.
The Bourne shell treats adjacent delimiters as a single
field delimiter.
With \f5ksh\fP,
delimiters that are considered white space characters
have the behavior of the Bourne shell, but other
adjacent delimiters separate
null fields.
.P
The \f5read\fP command is often used in scripts that interact
with the user by prompting the user and then requesting some
input.
With the Bourne shell two commands are needed; one to
prompt the user, the other to read the reply.
\f5ksh\fP allows these two commands to be combined.
The first argument of the \f5read\fP
command can be followed by a
.B ?
and a prompt string which is used whenever the input
device is a terminal.
Because the prompt is associated with the \f5read\fP built-in,
the built-in command line editors will be able to re-output
the prompt whenever the line needs to be refreshed when
reading from a terminal device.
.P
With the Bourne shell,
there is no way to set a time limit for waiting for the user
response to read.
The
.B \-t
option to \f5read\fP takes a floating
point argument that gives the time in seconds,
or fractions of seconds that the shell should wait for a reply.
.P
The version of the \f5echo\fP command in System V
treats certain sequences beginning with
.B \e
as control sequences.
This makes it hard to output strings without interpretation.
Most BSD derived systems do not interpret
.B \e
control sequences.
Unfortunately, the BSD versions of \f5echo\fP accepts a 
.B \-n
option to prevent a trailing new-line, but has no way to
cause the string
.B \-n
to be printed.
Neither of these versions is adequate. Also, because they
are incompatible, it is very hard to write portable shell scripts
using \f5echo\fP.
The \f5ksh\fP built-in, \f5print\fP,
outputs characters to the terminal or to a file and
subsumes the functions of all versions of \f5echo\fP.
Ordinarily, escape sequences in arguments beginning with
.B \e
are processed the same as for the System V \f5echo\fP command.
However \f5print\fP follows the standard conventions for
options and has options that make \f5print\fP very versatile.
The
.B \-r
option can be used to output the arguments without any special meaning.
The
.B \-n
option can be used here to suppress the trailing new-line
that is ordinarily appended.
As with \f5read\fP, it is possible to specify the file descriptor number
as an option to the command to avoid having to use
redirection operators with each occurrence of the command.
.P
The IEEE POSIX shell and utilities standard committee was unable
to reconcile the differences between the System V and BSD
versions of \f5echo\fP.
They introduced a new command named \f5printf\fP
which takes an ANSI C format string and a list of options
and outputs the strings using the ANSI C formatting rules.
Since \f5ksh\fP is POSIX conforming, it accepts \f5printf\fP.
However, there is a
.B \-f
options to \f5print\fP that can be used to specify
a format string which processes the arguments the same way that
\f5printf\fP does.
.P
The format processing for \f5print\fP and \f5printf\fP has
been extended slightly.
There are three additional formatting directives.
The
.B %b
format causes the
.B \e
escape sequences to be expanded as they are with the System V \f5echo\fP
command.
The
.B %q
format causes quotes to
be placed on the output as required
so that it can be used as shell input.
Special characters in the output of most \f5ksh\fP built-in commands
and in the output from an execution trace
are quoted in an equivalent fashion. 
The
.B %P
format causes an extended regular expression string to
be converted into a shell pattern.
This is useful for writing shell applications that have
to accept regular expressions as input.
Finally, the escape sequence
.B \e\^E
which expands to the terminal escape character (octal 033)
has been added.
.P
The shell is frequently used as a programming language for
interactive dialogues.
The
\f5select\fP
statement has been added to the language
to make it easier to
present menu selection alternatives to the
user and evaluate the reply.
The list of alternatives is numbered and put in columns.
A user settable prompt,
\fB\s-1PS3\s+1\fP,
is issued and if the answer is
a number corresponding to one of the alternatives,
the select loop variable is set to this value.
In any case, the
.B \s-1REPLY\s+1
variable is used to store the user entered reply.
The shell variables
.B \s-1LINES\s+1
and
.B \s-1COLUMNS\s+1
are used to control the layout of select lists.
.H 2 "Option Parsing"
The \f5getopts\fP built-in command can be used
to process command arguments in a manner consistent
with the way \f5ksh\fP does for its own built-in commands.
.P
The \f5getopts\fP built-in allows users to specify options
as separate arguments or to group options that do not
take arguments together.  Options that require arguments
do not require space to separate them from the option argument.
The
.B \s-1OPTARG\s+1
variable stores the value of the option argument
after finding a variable that takes an argument.
The
.B \s-1OPTIND\s+1
variable holds the index of the current options argument.
After processing options, the arguments should be
shifted by
.B \s-1OPTIND\s+1\-1
to make the
remaining arguments be \f5"$@"\fP.
.P
The \f5getopts\fP argument description allows additional
information to be specified along with the options
that is used to generate \fIusage\fP messages for
incorrect arguments and for the option argument \fB\-?\fP.
The example in the APPENDIX uses \f5getopts\fP to process
its arguments.
.H 2 "Co-process"
\f5ksh\fP can spawn a
.I co-process
by adding a
.B "|&"
after a command.
This process will be run with its standard input and its
standard output connected to the shell.  The built-in command \f5print\fP
with the
.B \-p
option will write into the standard input of this
process and
the built-in command \f5read\fP
with the
.B \-p
option will read from the output of this process.
.P
In addition, the I/O redirection operators \fB<&\fP and \fB>&\fP can
be used to move the input or output pipe of the co-process
to a numbered file descriptor.
Use \f5exec\ 3>&\ p\fP to move the input of the co-process
to file descriptor \fB3\fP.
After you have connected to file descriptor \fB3\fP, you
can direct the output of any command to the co-process
by running \fIcommand\fP\f5\ >&3\fP.
Also, by moving the input of the co-process to a numbered descriptor,
it is possible to run a second co-process.
The output of both co-processes will be the file descriptor
associated with \f5read\ -p\fP.
You can use \f5exec\ 4<&\ p\fP to cause the output of these
co-processes to go to file descriptor \fB4\fP of the shell.
Once you have moved the pipe to descriptor \fB4\fP, it is possible
to connect a server to the co-process by running \fIcommand\fP\f5\ 4<&\ p\fP
or to close the co-process pipe with \f5exec\ 4<&\ -\fP.
.H 2 "Functions"
.P
Function definitions are of the form
.sp
.in +.5i
.nf
\f5function\fP \fIname\fP
.br
.B {
.br
        any shell script
.br
.B }
.fi
.sp
.in
A function whose name contains a \fB\s+2.\s-2\fP
is called a \fIdiscipline\fP function.
The portion of the name after the last \fB\s+2.\s-2\fP
is the name of the discipline.
Discipline functions named \f5get\fP, \f5set\fP, and \f5unset\fP
can be assigned to any variable to intercept lookups,
assignments and unsetting of the variable
defined by the portion of the name before the last \fB\s+2.\s-2\fP.
Applications can create additional disciplines for variables
that are created as part of user defined built-ins.
The portion of the name before the last \fB\s+2.\s-2\fP
must refer to the name of an existing variable.
Thus, if \f5p\fP is a reference to \f5PATH\fP, then
the function name \f5p.get\fP and \f5PATH.get\fP
refer to the same function.
.P
The function is invoked either
by specifying
.I name
as the command name
and optionally following it with arguments
or by using it as an option to the \fB\s+2.\s-2\fP
built-in command.
Positional parameters are saved before each
function call and restored when completed.
The arguments that follow the function name on the calling
line become positional parameters inside the function.
The \f5return\fP
built-in can be used to cause the function to return to
the statement following
the point of invocation.
.P
Functions can also be defined with the System V notation,
.sp
.in +.5i
.nf
\fIname\fP \f5()\fP
.br
.B {
.br
        any shell script
.br
.B }
.fi
.sp
.in
Functions defined with this syntax cannot be used as the first
argument to a \fB\s+2.\s-2\fP procedure.
\f5ksh\fP accepts this notation for compatibility only.
There is no need to use this notation when writing
\f5ksh\fP scripts.
.P
Functions defined with the \f5function\fP\ \fIname\fP syntax
and invoked by name
are executed in the current shell environment
and can share named variables with the calling program.
Options, other than execution trace
.BR \-x ,
set by the calling program are
passed down to a function.
The options are
not shared with
the function so that any options set within a function are
restored when the function exits.
Traps ignored by the caller are ignored within the function
and cannot be enabled.
Traps caught by the calling program are reset to their
default action within the function.
In most instances, the default action is
to cause the function to terminate.
A trap on
\fB\s-1EXIT\s+1\fP
defined within a function executes after the function
completes but
before the caller resumes.
Therefore,
any variable assignments and
any options set as part of a trap action will be effective
after the caller resumes.
.P
By default, variables are inherited by the function and shared
by the calling program.
However,
for functions defined with the \f5function\fP\ \fIname\fP syntax
that are invoked by name,
environment substitutions preceding the function call
apply only to the scope of the function call.
Also, variables whose names do not contain a \fB\s+2.\s-2\fP
that are defined with the \f5typeset\fP
built-in command are local to the function that they are declared in.
Thus, for the function defined
.sp
.nf
.in .5i
\f5function  name
{
     typeset -i x=10
     let z=x+y
     print $z
}\fP
.fi
.ta
.in
.sp
invoked as
\f5y=13\ name\fP,
\f5x\fP and \f5y\fP
are local variables with respect to the function
\f5name\fP
while
\f5z\fP
is global.
.P
Functions defined with the \fIname\fP\f5()\fP syntax,
and functions invoked as an argument to the \fB\s+2.\s-2\fP
command,
share everything other than positional parameters with the caller.
Assignments that precede the call remain in effect after the
function completes.
.P
Alias and function names are not passed down to shell scripts
or carried across separate
invocations of \f5ksh\fP.
The
.B $\s-1FPATH\s+1
variable gives a colon separated list of directories that
is searched for function definitions when trying to resolve
the command name.
Whenever a file name contained in
.B $\s-1FPATH\s+1
is found, the complete file is read and all functions
contained within become defined.
.P
Calls that reference functions can be recursive.
Except for special built-ins,
function names take precedence over built-in names and names
of programs when used as command names.
To write a replacement function that invokes the command that
you wish to replace,
you can use the \f5command\fP built-in command.
The arguments to \f5command\fP are the name and arguments
of the program you want to execute.
For example to write a
.B cd
function which changes the directory and prints out the directory name,
you can write
.sp
.nf
.in .5i
\f5function  cd
{
     if      command cd  "$@"
     then    print  -r -- $PWD
     fi
}\fP
.fi
.ta
.in
.sp
.P
The
\fB\s-1FPATH\s+1\fP
variable is a colon separated list that \f5ksh\fP
uses to search for function definitions.
When
\f5ksh\fP
encounters an autoload function,
it runs the
.B .
command on the script containing the function,
and then executes the function.
.P
For interactive shells,
function definitions may also be placed in the
\fB\s-1ENV\s+1\fP
file.
However, this
causes the shell to take longer to begin executing.
.H 2 "Process Substitution"
.P
This feature is only available
on versions of the UNIX operating system which support the
.B /dev/fd
directory for naming open files.
Each command argument of the form
\fB<(\fP\fIlist\^\fP\fB)\fP
or
\fB>(\fP\fIlist\^\fP\fB)\fP
will run process
.I list
asynchronously connected to some file in the
.B /dev/fd
directory.
The name of this file will become the argument to the command.
If the form with
.B >
is selected then writing on this file will provide input for
.IR list .
If
.B <
is used,
then the file passed as an argument will contain the output of the
.I list
process.
For example,
.sp
.nf
.in .5i
\f5paste  <(cut \-f1 \fP\fIfile1\fP\f5)  <(cut \-f2 \fP\fIfile2\fP\f5) | tee >(\fP\fIprocess1\fP\f5)  >(\fP\fIprocess2\fP\f5)\fP
.fi
.ta
.in
.sp
extracts
fields 1 and 3 from
the files
.I file1
and
.I file2
respectively,
places the
results side by side, and
sends it
to the processes
.I process1
and
.IR process2 ,
as well as putting it onto the standard output.
Note that the file which is passed as an argument to the command is
a UNIX system
.IR pipe (2)
so that the programs that expect to
.IR lseek (2)
on the file will not work.
.H 2 "Finding Commands"
.P
The addition of aliases, functions,
and more built-ins
has made it substantially more difficult to know what
a given command name really means.
.P
Commands that begin with reserved words
are an integral part of the shell language itself
and typically define the control flow of the language.
Some control flow commands are not reserved words in
the language but are \fIspecial\fP built-ins.
Special built-ins are built-ins that are considered a
part of the language rather than user definable commands.
The best examples of commands that fit this description
are \f5break\fP and \f5continue\fP.
Because they are not reserved words, they can be the
result of shell expansions and are not affected by quoting.
These commands have the following special properties:
.BL
.LI
Assignments that precede them apply to the current shell process,
not just to the given command.
.LI
An error in the format of these commands cause a shell script
or function that contains them to abort.
.LI
They cannot be overridden by shell functions.
.LE
.P
Other commands are built in because they perform side effects
on the current environment that would be nearly impossible
to implement otherwise.
Built-ins such as \f5cd\fP and \f5read\fP
are examples of such built-ins.
These built-ins are semantically equivalent to commands that
are not built-in except that they don't take a path search
to locate.
.P
A third reason to have a command built-in is so that
it will be unaffected by the setting of the
.B \s-1PATH\s+1
variable.
The \f5print\fP command fits this category.
Scripts that use \f5print\fP will be portable
to all sites that run \f5ksh\fP.
.P
The final reason for having a command be a built-in is
for performance.
On most systems it is more than an order of magnitude
faster to initiate a command that is built in than
to create a separate process to run the command.
Examples that fit this category are \f5test\fP
and \f5pwd\fP.
.P
Given a command name \f5ksh\fP decides what it means using
the following order:
.BL
.LI
Reserved words define commands that form part of the shell
grammar.
They cannot be quoted.
.LI
Alias substitutions occur first as part of the reading of commands.
Using quotes in the command name will prevent alias substitutions.
.LI
Special built-ins.
.LI
Functions.
.LI
Commands that are built in that are not associated with a pathname
such as \f5cd\fP and \f5print\fP.
.LI
If the command name contains a
.BR / ,
the program or script corresponding to the given name is executed.
.LI
A path search locates the pathname corresponding to the command.
If the pathname where it is found matches the pathname associated
with a built-in command, the built-in command is executed.
If the directory where the command is found is listed in the
.B \s-1FPATH\s+1
variable, the file is read into the shell
like a dot script, and a function by that name is invoked.
Once a pathname is found, \f5ksh\fP remembers its location
and only checks relative directories in \fB\s-1PATH\s+1\fP
the next time the command name is used.
Assigning a value to \fB\s-1PATH\s+1\fP
causes \f5ksh\fP to forget the location of all command names.
.LI
The
.B \s-1FPATH\s+1
variable is searched and files found are treated as described above.
.LE
.P
The first argument of the \f5command\fP built-in, described earlier,
skips the checks for reserved words and for function definitions.
In all other ways, \f5command\fP behaves like a built-in
that is not associated with a pathname.
As a result, if the first argument of \f5command\fP is
a special built-in, the special properties of this built-in
do not apply.  
For example, whereas, \f5exec\ 3<\ foo\fP will cause a script containing
it to abort if the open fails, \f5command\ exec\ 3<\ foo\fP
results in a non-zero exit status but does not abort the script.
.P
You can get a complete list of the special built-in commands
with \f5builtin\ -s\fP. 
In addition \f5builtin\fP without arguments gives a list of
the current built-ins and the pathname that they are associated with.
A built-in can be bound to another pathname by giving
the pathname for the built-in.  The basename of this path must
be the name of an existing built-in for this to succeed.
Specifying the name of the built-in without a pathname causes
this built-in to be found before a path search.
A built-in can be deleted with the \fB\-d\fP option.
.P
On systems with run time loading of libraries, built-in commands
can be added with the \f5builtin\fP command.
Each command that is to be built-in must be written as a
C function whose name is of the form \f5b_\fP\fIname\fP, where
\fIname\fP is the name of the built-in that is to be added.
The function has the same argument calling convention as
\f5main\fP.  The lower eight bits of the return value become
the exit status for this built-in.
Builtins are added by specifying the pathname of the library
as an argument to the \fB\-f\fP option of \f5builtin\fP.
.P
The built-in command,
\f5whence\fP,
when used with the
.B \-v
option, tells how a given command is bound.
A line is printed for each argument to \f5whence\fP
telling what would happen if this argument were used as a command name.
It reports on reserved words, aliases, built-ins, and
functions.
If the command is none of the above,
it follows the path search rules and prints the full path-name,
if any, otherwise it prints an error message.
.H 2 "Symbolic Names"
To avoid implementation dependencies, \f5ksh\fP
accepts and generates symbolic names
for built-ins that use numerical values in the Bourne shell.
The
.B \-S
option of the
\f5umask\fP built-in command
accepts and displays
default file creation permissions
symbolically.
It uses the same symbolic notation as the \f5chmod\fP command.
.P
The \f5trap\fP and \f5kill\fP built-in commands
allows the signal names to be given symbolically.
The names of signals and traps
corresponding to signals are the same as the signal name with
the
.B \s-1SIG\s+1
prefix removed.
The trap
.B 0
is named
\fB\s-1EXIT\s+1\fP.
.H 2 "Additional Variables"
In addition to the variables discussed earlier, \f5ksh\fP
has other variables that it handles specially. 
The variable \fB\s-1RANDOM\s+1\fP
produces a random number in the range 0 to 32767 each time it is referenced.
Assignment to this variable sets the seed for the
random number generator.
.P
The parameter \fB\s-1PPID\s+1\fP
is used to generate the process ID of the process which invoked this shell.
.H 2 "Added Traps"
A new trap named
\fB\s-1ERR\s+1\fP
has been added.
This trap is invoked whenever the shell would exit if the
.B \-e
option were set.
This trap is used by
Fourth Generation Make\*(Rf
.RS
G. S. Fowler,
.I "The Fourth Generation Make,"
Proceedings of the Portland USENIX meeting, pp. 159-174, 1985.
.RF
which runs \f5ksh\fP
as a co-process.
.P
A trap named
\fB\s-1DEBUG\s+1\fP
gets executed after each command.
This trap can be used for debugging and other purposes.
.P
The
\fB\s-1KEYBD\s+1\fP
trap was described earlier.
.H 2 Debugging
The primary method for debugging Bourne shell scripts is to
use the
.B \-x
option to enable the execution trace.
After all
the expansions have been performed,
but before each command is executed,
the trace writes to standard error the name and arguments
of each command preceded by a
.BR + .
While the trace is very useful, there is no way
to find out what line of source a given trace line
corresponds to.
With
\f5ksh\fP
the
\fB\s-1PS4\s+1\fP
variable
is evaluated for parameter expansion and
is displayed before each command,
instead of the
.BR + .
.P
The
\fB\s-1LINENO\s+1\fP
variable is set to the current line number relative to the
beginning of the current script or function. 
It is most useful as part of the
\fB\s-1PS4\s+1\fP
prompt.
.P
The
\fB\s-1DEBUG\s+1\fP
trap can be used to write a break point shell
debugger in \f5ksh\fP.
An example of such a debugger is \f5kshdb\fP.\*(Rf
.RS
Bill Rosenblatt,
.IR "Debugging Shell Scripts with \f5kshdb\fP" ,
Unix World, Volume X, No. 5, 1993.
.RF
.H 2 "Timing Commands"
.P
Finding the time it takes to execute commands
has been a serious problem with the Bourne shell.
Since the \f5time\fP command is not part of the
language, it is necessary to write a script
in order to time a \f5for\fP or \f5while\fP loop.
The extra time in invoking the shell and processing
the script is accumulated along with the time
to execute the script.
.P
More seriously, the Bourne shell does not give correct
times for pipelines.
The reason for this is that the times for some members
of a pipeline are not counted when computing the time.
As an extreme example,
running \f5time\fP on the script
.ce
\f5cat < /dev/null | sort -u bigfile | wc\fP
with the Bourne shell will show very little
user and system time no matter how
large \f5bigfile\fP is.
.P
To correct these problems,
a reserved word \f5time\fP
has been added to replace
the \f5time\fP
command.
Any function, command or pipeline can be preceded by this reserved word
to obtain information about the elapsed, user, and system times.
Since I/O redirections bind to the command, not to
\f5time\fP,
parentheses should be used to redirect the timing information which
is normally printed on file descriptor 2.
.H 1 SECURITY
There are several documented problems associated with the security of
shell procedures\*(Rf.
.RS
F. T. Grampp and R. H. Morris,
.I "UNIX Operating System Security,"
AT&T Bell Labs Tech. Journal, Vol. 63, No. 8, Part 2, pp. 1649-1671, 1984.
.RF
These security holes occur primarily because a user can manipulate the
.I environment
to subvert the intent of a
.I setuid
shell procedure.
Sometimes, shell procedures are initiated from 
binary programs, without the author's
awareness, by library routines which invoke shells to carry out
their tasks.
When the binary program is run
.I setuid
then the shell procedure runs with the permissions afforded to the
owner of the binary file.
.P
In the Bourne shell,
the
.B \s-1IFS\s+1
parameter is used to split each word into separate command arguments.
If a user knows that some
.I setuid
program will run
\f5sh\ -c\ /bin/pwd\fP
(or any other command in
.BR /bin )
then the user sets and exports
.BR \s-1IFS\s+1=\^/ .
Instead of running
.B /bin/pwd
the shell will run
.B bin
with
.B pwd
as an argument.
The user puts his or her own \f5bin\fP
program into the current directory.
This program can
create a copy of the shell,
make this shell
.IR setuid ,
and then run the \f5/bin/pwd\fP
program so that the original program continues to run successfully.
This kind of penetration is not possible with
\f5ksh\fP
since the
.B \s-1IFS\s+1
parameter only splits arguments that result from command or parameter
substitution.
.P
Some
.I setuid
programs run programs using
.I system()
without giving the full pathname.
If the
user sets the
.B \s-1PATH\s+1
variable so that the desired command will be found
in his or her local bin, then the same technique described above can
be employed to compromise the security of the system.
To close up this and other security holes,
\f5ksh\fP
resets the effective user ID to the real user ID and the effective
group ID to the real group ID unless the
.I privileged
option
.RB ( \-p\^ )
is specified at invocation.
In
this mode, the
.B privileged
mode, the
.B .profile
and
.B \s-1ENV\s+1
files are not processed.
Instead, the file
.B /etc/suid_profile
is read and executed.
This gives an administrator control over the
environment to set the
.B \s-1PATH\s+1
variable or to log setuid shell invocations.
Clearly security of the system is compromised if
.B /etc
or this file is publicly writable.
.P
Some versions of the UNIX operating system look for the characters
\f5#!\fP
as the first two characters of an executable file.
If these characters are found, then the next word on this line is taken
as the interpreter to
invoke
for this command and the interpreter is
.IR exec ed
with the name of the script as argument zero and argument one.
If the
.I setuid
or
.I setgid
bits are on for this file, then the interpreter
is run with the effective UID and/or GID set accordingly.
This scheme has three major drawbacks.
First of all,
putting the pathname of the interpreter into the script
makes the script less portable since the interpreter
may be installed in a different directory on another system.
Secondly, using the
\f5#!\fP
notation forces an
.B exec
of the interpreter even when the call is invoked from the interpreter
which it must exec.  This is inefficient since
\f5ksh\fP can handle a failed exec much faster than starting up
again.
More importantly,
.I setuid
and
.I setgid
procedures provide an easy target for intrusion.
By linking a
.I setuid
or
.I setgid
procedure to a name beginning with a
.B \-
the interpreter is fooled into thinking that it is being invoked with
a command line option rather than the name of a file.
When the interpreter is the shell, the user gets a privileged
interactive shell.
There is code in
\f5ksh\fP
to guard against this simple form of intrusion.
.P
A more reliable way to handle
.I setuid
and
.I setgid
procedures is provided with
\f5ksh\fP.
The technique does not require any changes to the operating system
and provides better security.
Another advantage to this method is that it also allows scripts which
have execute permission but no read permission to run.  Taking away read
permission makes scripts more secure.
.P
The method relies on a setuid
.B root
program to authenticate the
request and exec the shell with the correct mode bits to carry out
the task.  This shell is invoked with the requested file already open
for reading.  A script which cannot be opened for reading or which
has its setuid and/or setgid bits turned on causes this setuid
.B root
program to get \fBexec\fPed.
For security reasons, this program is given the full
pathname
\f5/etc/suid_exec\fP.
A description of the implementation of the
\f5/etc/suid_exec\fP
program can be found in
a separate paper\*(Rf.
.RS
D. G Korn
.I "Parlez-vous Kanji?"
TM-59554-860602-03, 1986.
.RF
.H 1 "CODE CHANGES"
\f5ksh\fP is written in ANSI C as a reusable library.
The code can be compiled with C++ and older K&R C as well.
The code uses the IEEE POSIX 1003.1 and ISO 9945-1 standard\*(Rf
.RS
.I "POSIX \- Part 1: System Application Program Interface,"
IEEE Std 1003.1-1990, ISO/IEC 9945-1:1990.
.RF
wherever possible so that \f5ksh\fP should be able to run
on any POSIX compliant system.  In addition, it is possible
to compile \f5ksh\fP for older systems.
.P
Unlike earlier version of the Bourne shell,
\f5ksh\fP treats eight bit characters transparently
without stripping off the
leading bit.
There is also a compile time switch to enable handling multi-byte
and multi-width characters sets.
.P
On systems with dynamic libraries, it is possible to add built-in
commands at run time  with the built-in command \f5builtin\fP 
described earlier.
It is also possible to embed \f5ksh\fP in applications in
a manner analogous to \f5tcl\fP.
.H 1 "EXAMPLE"
.P
An example of a \f5ksh\fP script is included
in the Appendix.
This one page program is a variant of the UNIX system
\f5grep\fP(1) program.
Pattern matching for this version of \f5grep\fP
means shell patterns.
.P
The first half uses the \f5getopts\fP command to
find the option flags.
Nearly all options have been implemented.
The second half goes through each line of each file
to look for a pattern match.
.P
This program is not intended to serve as a
replacement for \f5grep\fP
which has been highly tuned for performance.
It does
illustrate the programming power of \f5ksh\fP.
Note that no auxiliary processes are spawned by this script.
It was written and debugged in under two hours.
While performance is acceptable for small files,
this program runs at only one tenth
the speed of \f5grep\fP
for large files.
.H 1 "PERFORMANCE"
.P
\f5ksh\fP executes many scripts faster than the System V Bourne shell;
in some cases more than 10 times as fast.
The primary reason for this is that \f5ksh\fP creates fewer
processes.
The time to execute a built-in command or a function is one or two
orders of magnitude faster than performing a \f5fork\fP() and
\f5exec\fP() to create a separate process.
Command substitution and commands inside parentheses
are performed without creating another process, unless necessary
to preserve correct behavior.
.P
Another reason for improved performance is the use of the \fBsfio\fP\*(Rf,
.RS
David Korn and Kiem-Phong Vo,
.IR "SFIO - A Safe/Fast String/File I/O,"
Proceedings of the Summer Usenix,
pp. 235-255, 1991.
.RF
library for I/O.  The \fBsfio\fP library buffers all I/O
and buffers are flushed only when required.
The algorithms used in \fBsfio\fP perform better than
traditional versions of standard I/O so that programs that
spend most of their time
formatting output may actually perform better
than versions written in C.
.P
Several of the internal algorithms have been changed
so that the number of subroutine calls has been
substantially reduced.
\f5ksh\fP uses variable sized hash tables for variables.
Scripts that rely heavily on referencing variables execute faster.
More processing is performed while reading the script
so that execution time is saved while running loops.
These changes are not noticeable for scripts that \f5fork()\fP
and run processes,
but they reduce the time that it takes to interpret commands by
more than a factor of two.
.P
Most importantly, \f5ksh\fP provide mechanisms to write applications
that do not require as many processes.
The arithmetic provided by the shell eliminates the need for the
\f5expr\fP command.
The pattern matching and substring capabilities eliminate the
need to use \f5sed\fP or \f5awk\fP to process strings.
.P
The architecture of \f5ksh\fP makes it easy to make commands
built-ins without changing the semantics at all.
Systems that have run-time binding of libraries allow
applications to be sped up by supplying the critical
programs as shell built-in commands.
Implementations on other systems can add built-in commands
at compile time.
The procedure for writing built-in commands that can be loaded
at run time is in a separate document.\*(Rf,
.RS
David Korn,
.IR "Guidelines for writing \f5ksh-93\fP built-in commands,"
to be published, 1994.
.RF
.H 1  "CONCLUSION"
.P
The 1988 version of \f5ksh\fP has tens of thousands of regular users
and is a suitable replacement for the Bourne shell.
The 1993 version of \f5ksh\fP is essentially upward compatible with
both the 1988 version of \f5ksh\fP and with the recent IEEE POSIX
and ISO shell standard.
The 1993 version offers many advantages for programming applications,
and it has been rewritten so that it can be used in embedded applications.
It also offers improved performance.
.SG dgk \"  signature typist initials 
\" .CS 14 24 38 0 0 16  \" cover sheet for TM
.bp
.ce
\fIAPPENDIX\fP
.nf
\f5
.ta .66i 1.33i 2i 2.66i 3.33i 4i 4.66i 5.33i 6i 6.66i 7.33i 8i
.so grep.mm
.fi
\fP


