Consistent BackSpace and Delete Configuration

The keymapping problem can be traced back to the time when computers used punched tapes. The only way to correct a wrongly punched character was to punch additional bits in the tape. Characters with all the bits punched were treated as deleted characters. See 'man ascii' for a list of the ASCII characterset, you'll see that DEL, (octal 177, decimal 127 and hexadecimal 7F) is the one with all the bits punched in (the ASCII code with the highest value). To be able to overwrite a character it is necessary to undo the feed between the characters first. For this the ASCII BS (octal 010, decimal 8, hexadecimal 08) was used. So to erase the last-typed character, you would have to press first BS and then DEL. In newer computers either BS or DEL was chosen to do it in one press.

The question to use BS or DEL was (and is...) just a matter of choice, just as different operating systems chose different ways to represent newlines, which was done by first issuing a carriage return (CR) and then a line-feed (LF). UNIX chose ASCII LF (hexadecimal 0A) to do both at the same time, Macintosh chose CR and MS-DOS both. Unfortunately, in the BS-DEL case, Linux ended up with two possibilities.

The essential point in this issue is to go for one way consistently. This page describes a way to consistently use DEL to erase the previous character, the page called The Alternative Way describes a way to consistently use BS. On the latter page there are some tips which may be useful regardless of what approach you take.

At this point the best way to fix the keys on your machine is to install Debian from version 2 (Hamm) and up, or SuSE.

If you miss an application on this page, it's either because the keys are ok in the application ("If it ain't broken, don't fix it"), or because I don't know about it. In either case, don't hesitate to mail me about it.

The linux console emulates a vt220 terminal which has the following key-mapping:

Linux console/VT220
Key            KeySymName       Console characters
  --------------------------------------------------
  Ctrl+H   --->  Control_H  --->  ASCII  BS (0x08)
  [<---]   --->  Backspace  --->  ASCII DEL (0x7f)
  [Delete] --->  Delete     --->  "\e[3~"

VT100
--
Key            KeySymName       Console characters
  --------------------------------------------------
  Ctrl+H   --->  Control_H  --->  ASCII  BS (0x08)
  [<---]   --->  Backspace  --->  ASCII DEL (0x7F)

Xterm's emulation of VT100
Key            KeySymName       Console characters
  --------------------------------------------------
  Ctrl+H   --->  Control_H  --->  ASCII  BS (0x08)
  [<---]   --->  Backspace  --->  ASCII BS (0x08)
  [Delete] --->  Delete     --->  ASCII DEL (0x7F)

In my previous attempt at working around the problem, I put most of my effort in trying to map all keys to suit xterm's vt100 emulation. However, I now propose another way to deal with the problem. This basically means getting xterm to work with the consoles' default settings: [<---] will do ASCII DEL (0x7F, or ^?), and [Delete] will do "\e[3~". This way ^H is not used for any terminal specific functions, it can be used by applications.

When you change the keymapping on your own system, you may have problems when accessing other systems. Luckily, this can be worked-around by a script which changes the input it receives to the output that is expected by the application (telnet, rlogin). See the telnet section.

infocmp | grep kbs

infocmp | grep kdch1

export TERMINFO=~/.terminfo

setenv TERMINFO ~/.terminfo

echo $TERM

/etc/terminfo/x/xterm

/usr/lib/terminfo/x/xterm

*VT100.Translations: #override \
              <Key>BackSpace: string(0x7F)\n\
              <Key>Delete:    string("\033[3~")\n\
              <Key>Home:      string("\033[1~")\n\
              <Key>End:       string("\033[4~")
*ttyModes: erase ^? 

Rxvt only uses Xresources if it is compiled with this option. Since rxvt uses readline, to get home and end to work you will have to edit .inputrc, see the Bash section). Redhat users can pick up the SRPM (source) and compile rxvt themselves with this feature.h, ensuring the right keybindings.

Putting xterm or nxterm in front of *vt100.translations makes the settings specific for xterm or nxterm. Sometimes this does not seem to work properly, in that case try using both nxterm*VT100.Translations and xterm*VT100.Translations.

BTW, have I told you that Ctrl-v <key> will tell you what sequence is sent to the shell? Try Ctrl-v [Delete], it should give a ^[[3~ in your xterm or console (try it with [ESC] and you'll find ^[, which corresponds to \e for readline apps, and \033 for Xresources).

1. The syntax of the Xresources files is very strict, make sure you don't leave blank spaces after the backslash on each line. Also watch the newline (\n\) sequences, there shouldn't be one at the last line of an entry.
2. The sections are named, if you only use *VT100.Translations it will work for all xterm and friends.
3. For info about Xresources and other X related stuff refer to 'man X'.

keycode 22 = BackSpace
keycode 107 = Delete

Note that the keycodes in X are not the same as in a VT. Use showkey to see the keycodes generated by the keyboard in a VT, and xev in X. There is an excellent graphical front end for X-keyboard configuration: xkeycaps. Refer to the Netscape section.

Another way of generating special characters in X (without using xmodmap) is by defining a Compose-key (for use as a "dead key") in XF86Config in the Keyboard section:

  RightCtl        Compose

  XkbModel    "microsoft" 

Yet another way to use "dead keys" in X is by installing a modified version of libX11 which includes support for dead keys (transparent for all apps). First, get the modified version of libX11 (make sure you get the right version, if you use glibc replace both libX11, ELF-libc5 and ELF-libc6). Now, deactivate the XKB extension of XFree86 (edit XF86Config and run xmodmap on a Xmodmap file with dead keys like dead_acute, dead_grave, dead_cedilla, dead_ogonek and a Multi_keys one too.

A special case is if you use xdm, you need to deactivate XKB from the /etc/X11/xdm/Xservers file, passing a -kb parameter to the server like this:

:0 local /usr/X11R6/bin/X -kb

if [ -r /etc/X11/xinit/.Xmodmap ]; then
        /usr/X11R6/bin/xmodmap /etc/X11/xinit/.Xmodmap
fi

More Composing Talk

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È É Ê Ë Ì Í Î Ï Ð Ñ Ò Ó Ô Õ Ö × Ø Ù Ú Û Ü Ý Þ ß à á â ã ä å æ ç è é ê ë ì í î
ï ð ñ ò ó ô õ ö ÷ ø ù ú û ü ý þ ÿ 

 

#!/bin/bash 
export LESSCHARSET=latin1 
exec xterm -T Iso_8859_1-TABLE -geometry 77x4 +sb -bg SteelBlue -fg white \
-fn lucidasanstypewriter-bold-12 -e less /usr/local/lib/iso_8859_1

NROFF         /usr/bin/groff -Tascii -mandoc

NROFF         /usr/bin/groff -Tlatin1 -mandoc

Create a file, /etc/inputrc for system wide use or ~/.inputrc for personal use. Actually, this is the readline initialization file, readline is a library that some programs (bash, kvt) use to read input (try bind -v to see a list of readline key and function bindings). Cut and paste the following in the file to make the Delete key delete characters under the cursor, and make Home and End work as well:

"\e[3~": delete-char
# this is actually equivalent to "\C-?": delete-char
# VT
"\e[1~": beginning-of-line
"\e[4~": end-of-line
# kvt
"\e[H":beginning-of-line
"\e[F":end-of-line
# rxvt and konsole (i.e. the KDE-app...)
"\e[7~":beginning-of-line
"\e[8~":end-of-line

export INPUTRC=/etc/inputrc

Make sure that the stty erase character is set to ^?. Type

stty -a | grep erase

and check if it says

erase = ^?;

If it is set to something else (e.g. ^H) then put the following line in both .bashrc and in either .bash_profile or /etc/profile:

  if tty --quiet ; then
    stty erase '^?'
  fi

*ttyModes: erase ^?
            

If you create /etc/inputrc, note that Bash will ignore ~/.inputrc (currently this happens in all distributions except Debian, however, this might change in the future). As an alternative, you can edit ~/.inputrc, and copy this to /etc/skel/, so it's in the home directories of all new users.
Push the key-combination 'Ctrl-x-r' (push the control-key, the x-key. release it, push the r-key, release it, and then release the control-key) to see if the changes in inputrc take effect. Or just login again, and it will work.

You can also change the keybindings on the fly with the bind command, e.g:

[localhost]> bind "\C-?": backward-delete-char

People using keymaps with e.g. Scandinavian characters who would like bash to display these characters (øl;-) have to add the following lines in .inputrc:
set convert-meta off
set output-meta on
set input-meta on

For more info, check the Danish-HOWTO.

#.cshrc
 if ($term == "xterm" || $term == "vt100" \
            || $term == "vt102" || $term !~ "con*") then
          # bind keypad keys for console, vt100, vt102, xterm
          bindkey "\e[1~" beginning-of-line  # Home
          bindkey "\e[7~" beginning-of-line  # Home rxvt
          bindkey "\e[2~" overwrite-mode     # Ins
          bindkey "\e[3~" delete-char        # Delete
          bindkey "\e[4~" end-of-line        # End
          bindkey "\e[8~" end-of-line        # End rxvt
      endif
 

[localhost]> source .cshrc

*Text.translations:    #override \ 
    ~Shift ~Meta <Key>Delete: delete-next-character()

;; Map certain keypad keys into ASCII characters
;; that people usually expect.
(define-key function-key-map [backspace] [127])
(define-key function-key-map [delete] [127])

(define-key function-key-map [delete] [deletechar])

(define-key function-key-map [delete] [deletechar])

(define-key global-map [home] `beginning-of-line)
(define-key global-map [end] `end-of-line)

bol              ^[ [ H          # Standard xterm escape seq.
bol              ^[ [ 1 ~        # Standard VT escape seq.
bol              ^[ [ 7 ~        # rxvt
eol              ^[ [ F          # Standard xterm escape seq.
eol              ^[ [ 4 ~        # Standard VT escape seq.
eol              ^[ [ 8 ~        # rxvt

 
"\e[H":beginning-of-line
"\e[F":end-of-line

 
"\e[H":beginning-of-line
"\e[F":end-of-line

#line-edit
\e[3~          delete
\e[1~          home
\e[4~          end

[localhost]> lesskey -o /etc/less /etc/lesskey

Add the following line to /etc/profile:

export LESS="-MM -k/etc/less"

Now type:

[localhost]> . /etc/profile

By fiddling with these settings you should be able to get less to accept (all of) the holy keycodes, at least, it works on my computer. For some exciting reading, check 'man lesskey'.

 nedit.remapDeleteKey: False

Save the file, and the next time you start nedit the Delete key will do what we want it to: delete under the cursor. Note: you can also put the line in ~/.nedit, but every time you save the default settings, the .nedit file will be overwritten, and you'll have to (n)edit it again.

Note the nedit, as e.g. netscape, is a motif application. This means that it uses the XKeysymDB, see thet netscape section.

If you use xmodmap, you may want to check out the graphical front end for X-keyboard configuration: xkeycaps. Get the .rpm at RedHat Powertools, or get the Debian-package (or plain tarball). It's a nice tool. You can also look for a default Xmodmap file on your system. At any rate, make sure that the following is in the file:

keycode 22 = BackSpace
keycode 107 = Delete

alternatively, you can put it in e.g. your .Xclients file, or in /etc/X11/xdm/Xsetup_0):

# map the [<---] key to the [BackSpace] keysym.
xmodmap -e "keycode 22 = BackSpace"
# map the [Delete] key to the [Delete] keysym.
xmodmap -e "keycode 107 = Delete"

#!/usr/bin/expect

eval spawn -noecho $argv

interact {
 \177        {send "\008"}
 "\033\[3~"  {send "\004"}
}

picofix pico

# Specifies the program invoked by ^_ in the Composer,
# or the "enable-alternate-editor-implicitly" feature.
editor=

You can enter the command for any editor here, e.g. joe or vi.

If you want the editor to be started right away, start pine, enter SETUP and Config and select the enable-alternate-editor-implicitly option. Now the editor of choice will be started automagically when you start writing a message.

Pico fans may want to try jpico, which is basically joe with pico keybindings (and all the extra's joe offers). Others my be even smarter and switch to vim and mutt. Note that the first can be taught to do the Ctrl-j-trick, the latter can use pine keybindings.

bind editor <delete> delete-char

#!/usr/bin/expect

eval spawn -noecho $argv

interact {
 \177        {send "\008"}
 "\033\[3~"  {send "\177"}
}

The logical next step is to automate the trick. Create a file, /etc/broken-backspace-hosts (or a shorter name...). In this file, list the hosts which expect ^H and ^? instead of ^? and ^[[3~. Now you can make a wrapper for telnet as shown below. This particular script assumes that the telnet binary has been renamed telnet.orig, and that the kbdfix script described in the previous section is somewhere in your path):

#!/bin/sh
if grep -wq "$@" /etc/broken-backspace-hosts ; then
 exec kbdfix telnet.orig "$@"
else
 exec telnet.orig "$@"
fi

• The Linux Keyboard and Console HOWTO
• Linux Keyboard Setup Mini-Howto
• Consistent Keyboard Configuration' article (LJ issue #38, June 97)
• Danish-HOWTO
• Tcsh documentation
• Fixkeys Mini Howto
• Bash documentation
• 'man X'
• 'man XF86Config'
• 'man lesskey'
• 'man joe'
• Dead keys under X11

This document was put together by Anne Baretta (anne@ibbnet.org). Please mail me if you find mistakes, tY-pO's, alternative or plain better solutions. A few people have already pointed out many mistakes (there simply cannot be that many left...), which improved the document a lot. I would like to thank all the people who contributed to the page, and Thomas Telkamp for hosting the site.