What You Should Do On Your System

Linux Backspace/Delete mini-HOWTO
Prev |  | Next

# 6. What You Should Do On Your System

Note again that the main issue that confuses people trying to fix their system is that usually they are fixing thing in the wrong place. Since the parts that work often just work by chance, trying to fix the system assuming something is broken will often lead to change correct settings into incorrect settings.

## 6.1. What Needs to Be Done

### 6.1.1. Detecting Deviance

The first step towards a clean solution is to know exactly which terminals are deviant and which not. Usually they all behave like the console, and in this case the modifications to get everything working are minimal. If, however, you have some deviant terminal (e.g., a deviant version of gnome-terminal), you will have to treat it in a special way.

The following C one-liner

    void main(void) {int c; while(c = getchar()) printf("%d 0x%02X\n", c, c);}

may help you. Put the line into a file named `ascii.c`, compile it with gcc ascii.c -o ascii, type ./ascii and press a key followed by RETURN. The program will display the decimal and hexadecimal codes of the ASCII sequence produced (you may want to do a stty erase ^- first to get really all the codes). Now you can easily see what Backspace key does: if it emits a DEL (127), you have a standard emulator, if it emits a BS (8) you have a deviant one.

### 6.1.2. Distinguishing Between Emulators

If you have some deviant terminal emulator, you must distinguish it from the standard ones. Theoretically, this should not be a problem because there are different entries in the terminal database for terminals with different sequences (the entry used depends on the value of the `TERM` variable).

Here we take the approach that the `gnome` entry should be used for all deviant VT100 emulators, and the `xterm` entry for the standard ones. This is in line with several distributions (except a few cases like RedHat ≤5.0, where the `xterm` entry is deviant).

However, gnome-terminal uses by default the same entry as xterm, so if one is deviant and the other one is not you will need to find a way to tell them apart. The option `termname` of gnome-terminal allows the user to set the `TERM` variable to a more sensible name. However, in older versions of gnome-terminal the option does not work. Moreover, sometimes it is not easy to modify the way gnome-terminal is started.

A good idea here is to exploit the fact that gnome-terminal sets the `COLORTERM` variable to `gnome-terminal`. Thus, by adding a simple test to the shell configuration files we can fix the `TERM` variable.

### 6.1.3. Fixing the Terminal Database

Our problem now is that the terminal database could lack a `gnome` entry for deviant terminals (this happens on a number of termcap and terminfo versions). Recent terminfo databases have an entry `gnome`, but, in any case, since gnome-terminal behaves essentially like xterm modulo our famous two keys, it is possible to automagically generate a brand new correct entry.

### 6.1.4. Fixing the Shell Behaviour

The readline library used by the bash and by many other programs to read the input line can be customized so to recognize specific sequences of characters. The customization can also depend on the `TERM` variable, so once we can distinguish terminals we can do fine tuning of the keyboard.

Moreover, if you want less and other application that do raw line input to work correctly, you must convince the shell that under a deviant terminal emulator the erase character is BS, and not DEL (in the other case the Backspace key is already emitting DEL, so we do not have to do anything). This can be done using the command stty.

## 6.2. How to Do It

| These fixes have some drawbacks. First, they work only for the specified terminals. Second, in theory (but this is unlikely to happen) they could confuse the readline library on other terminals. Both limitations are however mostly harmless.

First of all, check with infocmp gnome whether you already have a `gnome` entry in your terminfo database (we will fix termcap later). If the entry does not exist, the following command

    bash$ tic <(infocmp xterm |\
            sed 's/xterm|/gnome|/' |\
            sed 's/kbs=\\177,/kbs=^H,/' |\
            sed 's/kdch1=\\E\[3~,/kdch1=\\177,/')

will create a correct one in `~/.terminfo`. If the same command is launched by the root, it will generate the entry in the global database (you can override this behaviour by setting `TERMINFO` to `~/.terminfo`). Note that if your `xterm` entry is already deviant (e.g., you have a Red Hat ≤5.0) the script will copy it unchanged, which is exactly what we want.

Now, add the following snippet to `~/.inputrc`[1]:

    "\e[3~": delete-char

This line teaches the readline library how to manage your standard Delete key for standard emulators, and with a bit of luck it should not interfere with other terminals. However, now we must also explain to the library the meaning of the DEL character on deviant terminals, for instance by adding

    $if term=gnome
    DEL: delete-char
    Meta-DEL: kill-word
    "\M-\C-?": kill-word
    $endif

to `~/.inputrc`. If xterm is deviant, too, you must add other three lines for it. On the other hand, if no terminal emulator is deviant this part is not needed. All these changes can be made global by altering the `/etc/inputrc` file.

Note that the conditional assignments make deviant terminal emulators work given that the `TERM` variable is set correctly. To guarantee this, there are a number of techniques. First of all, since the default value of the `TERM` variable for gnome-terminal is `xterm`, if all terminals are not deviant then we do nothing. If, however, a terminal that by default uses the `xterm` entry is deviant you must find a way to set the `TERM` variable correctly; assume for instance this is true of gnome-terminal.

The simplest way to obtain this effect is to start gnome-terminal with the argument `--termname=gnome`, for instance by suitably setting the command line in the launcher on the GNOME panel. If however you have an old version, and this method does not work, you can add the lines

    if [ "$COLORTERM" = "gnome-terminal" ]
    then
        export TERM=gnome
    fi

to your `~/.bashrc` configuration file[2]. The assignment is executed only under gnome-terminal, and sets correctly the `TERM` variable.

| Setting the terminal to `gnome` could prevent ls from using colours, as many versions of ls do not know that gnome-terminal is colour capable. To avoid this problem, create a configuration file `~/.dircolors` with dircolors --print-database >~/.dircolors, and add a line `TERM=gnome` to the configuration file.

We will now generate on-the-fly a suitable termcap entry for deviant terminal emulators; this can be done as follows, always in `~/.bashrc`:

    if [ "$TERM" = "gnome" ]
    then
        export TERMCAP=$(infocmp -C gnome | grep -v '^#' | \
                        tr '\n\t' '  ' | sed 's/\\  //g' | sed s/::/:/g)
    fi

Finally, we must explain to the terminal device which character is generated by the erase key. Since usually the erase key is expected to backspace, there is a nice trick taken from the Red Hat `/etc/bashrc` that works: add this to `~/.bashrc`:

    KBS=$(tput kbs)
    if [ ${#KBS} -eq 1 ]; then stty erase $KBS; fi

It's a simple idea: we read from the terminal database the capability `kbs`, and set the erase character to its value if it is a single character (which happens in both standard and deviant terminals).

| Certain distributions could have fixes already in place in the system-wide `/etc/inputrc` configuration file. In this case you can eliminate redundant lines from your `~/.inputrc`.

## 6.3. Fixing for tcsh

In the case of the tcsh, the fixes go all in `~/.tcshrc`, and follow the same rationale as the ones for the bash:

    bindkey "^[[3~" delete-char

    if ($?COLORTERM) then
       if ($COLORTERM == "gnome-terminal") then
          setenv TERM gnome
       endif
    endif

    if ($?TERM) then
       if ($TERM == "gnome") then
          setenv TERMCAP \
           "`infocmp -C gnome | grep -v '^#' | tr '\n\t' '  ' | sed 's/\\  //g' | sed s/::/:/g`"
          bindkey "^?" delete-char
          bindkey "^[^?" delete-word
          bindkey "\377" delete-word
       endif
    endif

    set KBS=`tput kbs`
    if (${%KBS} == 1) then
       stty erase $KBS
    endif

The second part must be replicated for every deviant terminal. Of course, if a termcap entry already exists it is not necessary to generate it.

### Notes

[1] | On older version of the bash, you must remember to set `INPUTRC` suitably, for instance adding
    export INPUTRC=~/.inputrc
to your `~/.profile` (or whichever file is read just by login shells).
[2] | More precisely, to the shell configuration file that is read in every shell, not only in login shells. The right file depend on startup sequence of your bash.

Prev | Home | Next
What You Should Do When Writing Applications |  | What If Nothing Works