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5. Delete and Backspace

Getting Delete and Backspace to work just right is nontrivial, especially in a mixed environment, where you talk to console, to X, to bash, to emacs, login remotely, etc. You may have to edit several configuration files to tell all of the programs involved precisely what you want. On the one hand, there is the matter of which keys generate which codes (and how these codes are remapped by e.g. kermit or emacs), and on the other hand the question of what functions are bound to what codes.

People often complain `my backspace key does not work', as if this key had a built-in function `delete previous character'. Unfortunately, all this key, or any key, does is producing a code, and one only can hope that the kernel tty driver and all application programs can be configured such that the backspace key indeed does function as a `delete previous character' key.

Most Unix programs get their tty input via the kernel tty driver in `cooked' mode, and a simple stty command determines the erase character. However, programs like bash and emacs and X do their own input handling, and have to be convinced one-by-one to do the right thing.

5.1 How to tell Unix what character you want to use to delete the last typed character 

        % stty erase ^?
If the character is erased, but in a funny way, then something is wrong with your tty settings. If echoprt is set, then erased characters are enclosed between \ and /. If echoe is not set, then the erase char is echoed (which is reasonable when it is a printing character, like #). Most people will want stty echoe -echoprt. Saying stty sane will do this and more. Saying stty -a shows your current settings. How come this is not right by default? It is, if you use the right getty.

Note that many programs (like bash, emacs etc.) have their own keybindings (defined in ~/.inputrc, ~/.emacs etc.) and are unaffected by the setting of the erase character.

The standard Unix tty driver does not recognize a cursor, or keys (like the arrow keys) to move the current position, and hence does not have a command `delete current character' either. But for example you can get bash on the console to recognize the Delete key by putting 

        set editing-mode emacs
        "\e[3~":delete-char
into ~/.inputrc.

`Getty used to do the right thing with DEL and BS but is broken now?'

Earlier, the console driver would do BS Space BS (\010\040\010) when it got a DEL (\177). Nowadays, DEL's are ignored (as they should be, since the driver emulates a vt100). Get a better getty, i.e., one that does not output DEL.

`Login behaves differently at the first and second login attempts?'

At the first attempt, you are talking to getty. At the second attempt, you are talking to login, a different program.

5.2 How to tell Linux what code to generate when a key is pressed

On the console, or, more precisely, when not in (MEDIUM)RAW mode, use 

        % loadkeys mykeys.map
and under X use 
        % xmodmap mykeys.xmap
Note that (since XFree86-2.1) X reads the Linux settings of the keymaps when initialising the X keymap. Although the two systems are not 100% compatible, this should mean that in many cases the use of xmodmap has become superfluous.

For example, suppose that you would like the Backspace key to send a BackSpace (^H, octal 010) and the grey Delete key a DEL (octal 0177). Add the following to /etc/rc.local (or wherever you keep your local boot-time stuff): 

        /usr/bin/loadkeys << EOF
        keycode 14 = BackSpace
        keycode 111 = Delete
        EOF
Note that this will only change the function of these keys when no modifiers are used. (You need to specify a keymaps line to tell which keymaps should be affected if you want to change bindings on more keymaps.) The Linux kernel default lets Ctrl-Backspace generate BackSpace - this is sometimes useful as emergency escape, when you find you can only generate DELs.

The left Alt key is sometimes called the Meta key, and by default the combinations AltL-X are bound to the symbol MetaX. But what character sequence is MetaX? That is determined (per-tty) by the Meta flag, set by the command setmetamode. The two choices are: ESC X or X or-ed with 0200.

`Why doesn't the Backspace key generate BackSpace by default?'

(i) Because the VT100 had a Delete key above the Enter key.

(ii) Because Linus decided so.

5.3 How to tell X to interchange Delete and Backspace 

        % xmodmap -e "keysym BackSpace = Delete" -e "keysym Delete = BackSpace"
Or, if you just want the Backspace key to generate a BackSpace: 
        % xmodmap -e "keycode 22 = BackSpace"
Or, if you just want the Delete key to generate a Delete: 
        % xmodmap -e "keycode 107 = Delete"
(but usually this is the default binding already).

5.4 How to tell emacs what to do when it receives a Delete or Backspace

Put in your .emacs file lines like 

        (global-set-key "\?" 'help-command)
        (global-set-key "\C-h" 'delete-backward-char)
Of course you can bind other commands to other keys in the same way. Note that various major and minor modes redefine keybindings. For example, in incremental search mode one finds the code 
        (define-key map "\177" 'isearch-delete-char)
        (define-key map "\C-h" 'isearch-mode-help)
This means that it may be a bad idea to use the above two global-set-key commands. There are too many places where there are built-in assumptions about ^H = help and DEL = delete. That doesn't mean that you have to setup keys so that Backspace generates DEL. But if it doesn't then it is easiest to remap them at the lowest possible level in emacs.

5.5 How to tell emacs to interchange Delete and Backspace

Put in your .emacs file lines 

        (setq keyboard-translate-table (make-string 128 0))
        (let ((i 0))
          (while (< i 128)
            (aset keyboard-translate-table i i)
            (setq i (1+ i))))
        (aset keyboard-translate-table ?\b ?\^?)
        (aset keyboard-translate-table ?\^? ?\b)
Recent versions of emacs have a function keyboard-translate and one may simplify the above to 
        (keyboard-translate ?\C-h ?\C-?)
        (keyboard-translate ?\C-? ?\C-h)
Note that under X emacs can distinguish between Ctrl-h and the Backspace key (regardless of what codes these produce on the console), and by default emacs will view the Backspace key as DEL (and do deletion things, as bound to that character, rather than help things, bound to ^H). One can distinguish Backspace and Delete, e.g. by 
        (global-unset-key [backspace] )
        (global-set-key [backspace] 'delete-backward-char)
        (global-unset-key [delete] )
        (global-set-key [delete] 'delete-char)

5.6 How to tell kermit to interchange Delete and Backspace

Put in your .kermrc file the lines 

        set key \127 \8
        set key \8 \127

5.7 How to tell xterm about your favourite tty modes

Normally xterm will inherit the tty modes from its invoker. Under xdm, the default erase and kill characters are # and @, as in good old Unix Version 6. If you don't like that, you might put something like 

        XTerm*ttymodes: erase ^? kill ^U intr ^C quit ^\ eof ^D \ 
                        susp ^Z start ^Q stop ^S eol ^@
in /usr/lib/X11/app-defaults/XTerm or in $HOME/.Xresources, assuming that you have a line 
        xrdb $HOME/.Xresources
in your $HOME/.xinitrc or $HOME/.xsession.

5.8 How to tell xmosaic that the Backspace key generates a DEL

Putting 

        *XmText.translations: #override\n\
           <Key>osfDelete: delete-previous-character()
        *XmTextField.translations: #override\n\
           <Key>osfDelete: delete-previous-character()
in your $HOME/.Xresources helps.

The netscape FAQ, however, says: 

        Why doesn't my Backspace key work in text fields? 
        By default, Linux and XFree86 come with the Backspace and Delete keys
        misconfigured. All Motif programs (including, of course, Netscape
        Navigator) will malfunction in the same way.

        The Motif spec says that Backspace is supposed to delete the previous
        character and Delete is supposed to delete the following character.
        Linux and XFree86 come configured with both the Backspace and Delete
        keys generating Delete.

        You can fix this by using any one of the xmodmap, xkeycaps, or
        loadkeys programs to make the key in question generate the BackSpace
        keysym instead of Delete.

        You can also fix it by having a .motifbind file; see the man page
        for VirtualBindings(3).

        Note: Don't use the *XmText.translations or *XmTextField.translations
        resources to attempt to fix this problem. If you do, you will blow
        away Netscape Navigator's other text-field key bindings.

5.9 A better solution for Motif-using programs, like netscape

Ted Kandell (ted@tcg.net) suggests the following:

Somewhere in your .profile add the following: 

stty erase ^H
If you are using bash, add the following lines to your .inputrc: 
"\C-?": delete-char
"\C-h": backward-delete-char
Add the following lines to your .xinitrc file: 
xmodmap <<-EOF
keycode 22  =  BackSpace osfBackSpace
keycode 107 =  Delete
EOF

# start your window manager here,  for example:
#(fvwm) 2>&1 | tee /dev/tty /dev/console

stty sane
stty erase ^H
loadmap <<-EOF
keycode 14  = BackSpace
keycode 111 = Delete
EOF

This will definitely work for a PC 101 or 102 key keyboard with any Linux/XFree86 layout.

The important part to making Motif apps like Netscape work properly is adding osfBackSpace to keycode 22 in addition to BackSpace.

Note that there must be spaces on either side of the = sign.

5.10 What about termcap and terminfo?

When people have problems with backspace, they tend to look at their termcap (or terminfo) entry for the terminal, and indeed, there does exist a kb (or kbs) capability describing the code generated by the Backspace key. However, not many programs use it, so unless you are having problems with one particular program only, probably the fault is elsewhere. Of course it is a good idea anyway to correct your termcap (terminfo) entry. See also below under "The TERM variable".

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