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Main Page|The Alternative Way|Troubleshooting


1    Troubleshooting
1.1 More tools
2    How it all works
2.1 Keyboard Generalities
2.2 Terminfo what and why
2.3 Fiddling with Terminfo entries
2.4 Overview (picture)


The purpose of this page is to provide some more information for people who want or need to know more. While this page is currently not much more than some bits and pieces, it does already contain some things not mentioned other places, check e.g. the terminfo section. See the references for pointers to other sources of related information.


The best way to trouble-shoot is using Ctrl-v <key>. Press Control and v simultaneously, release them, and then type one of the special keys, e.g. <Delete>. It will tell you which sequence will be sent to the terminal (console or xterm). You can then edit ~/.inputrc, ~/.cshrc or ~/.Xdefaults to do the right thing when a particular sequence is sent. For example, if <Delete> doesn't delete, and Ctrl-v <Delete> tells you that ^? is sent, add the following to your ~/.inputrc (note that ~/.inputrc is the initialization file of the readline library, which is used by several applications to read input):

"\C-?": delete-char
(this is equivalent to DEL: delete-char, but a better example...). In kvt, I found that <Delete> sent ^[[3~, so I added the following to ~/.inputrc:

"\e[3~": delete-char

This means that bash (or any application using the readline library) will delete when it receives ^[[3~ as well. As long as this sequence isn't supposed to do other things (and it isn't), it is perfectly O.K. to let bash delete on both occasions. It's the same story for Home and End in kvt, see the main page. Note that \e means ESC. Guess what you'll get when you do Ctrl-v <ESC>? Right, you'll get ^[.

You can use the same sequence to do different things, as long as it happens in different environments, and as long as you tell bash or tcsh about it. You'll just have to make constructions such as the following:

$if TERM=linux
"\e[1~": beginning-of-line
"\e[4~": end-of-line

$if TERM=xterm
"\e[7~": beginning-of-line
"\e[8~": end-of-line
"\e[H": beginning-of-line
"\e[F": end-of-line

$if Bash
"\e[11~": "Function Key 1"
"\e[12~": "Hello"

And so on. I couldn't think of a (useful) case where you need two sequences to do the same thing, but the example serves to show how to set up different settings in different environments.

More tools

With showkey you can see what keycodes are produced on the console. The keycodes are the result of a translation of the scancodes, which are the actual output of the keyboard. The scancodes can be made visible by showkey -s, and by getkeycodes. You can affect the translation from scan- to keycodes, with the utility setkeycodes. You don't need it.

The system keymap is the translation table which converts keycodes to strings. The system keymap is a plain ASCII file which is loaded by loadkeys. In X, you can use xev to show the keycodes the X server receives. The X server's keymap is a translation of the system keymap. This has led to a lot of confusion, since many people didn't realize the X keymap depends on the system keymap for its initialization. The X keymap can however, be overridden with xmodmap or xkeycaps.

There's a lot more about all this in the HOWTO.

A trick you can use to fix the keys in apps that are broken (or apps that you have broken:-) is to use expect to intercept certain keyevents, and change them, as demonstrated in the script below.


eval spawn -noecho $argv

interact {
 \177        {send "\010"}
 "\033\[3~"  {send "\177"}
You could run it as bsdelfix telnet brokenapp. This particular script sends BS to the application if \177 is received (DEL) and DEL if ^[3~ is received. See the page about the alternative way for an example of this method.

How it all works

In this section, an attempt will be made to shed some light on the mechanisms behind all the trouble.

Keyboard Generalities

From the Keyboard-and-Console-HOWTO:
  You press a key, and the keyboard controller sends scancodes to the
  kernel keyboard driver. Some keyboards can be programmed, but usually
  the scancodes corresponding to your keys are fixed.  The kernel
  keyboard driver just transmits whatever it receives to the application
  program when it is in scancode mode, like when X is running.
  Otherwise, it parses the stream of scancodes into keycodes,
  corresponding to key press or key release events.  (A single key press
  can generate up to 6 scancodes.)  These keycodes are transmitted to
  the application program when it is in keycode mode (as used, for
  example, by showkey).  Otherwise, these keycodes are looked up in the
  keymap, and the character or string found there is transmitted to the
  application, or the action described there is performed.  (For
  example, if one presses and releases the a key, then the keyboard
  produces scancodes 0x1e and 0x9e, this is converted to keycodes 30 and
  158, and then transmitted as 0141, the ASCII or latin-1 code for 'a';
  if one presses and releases Delete, then the keyboard produces
  scancodes 0xe0 0x53 0xe0 0xd3, these are converted to keycodes 111 and
  239, and then transmitted as the 4-symbol sequence ESC [ 3 ~, all
  assuming a US keyboard and a default keymap. An example of a key
  combination to which an action is assigned is Ctrl-Alt-Del.)

Terminfo what and why

If you still have problems with certain programs, e.g. ports from other platforms, it's very possible due to broken terminfo data bases. Note that the termcap database has been declared obsolete, it's only present for compatability with very old software. In short, most linux programs have learned to work with the broken entries, which is the reason why most people will not need to modify them. The full and excellent explanation, handed to me by Liviu Daia, follows below (the question leading to this was in which cases it would be necessary to edit terminfo entries):
    Simply put, keyboard input in text-mode applications can be done
at several logical "layers".

    The first (usable) layer is the raw string input: the application
opens /dev/tty, and reads the keystrokes from it ("raw" here means
"without further munging", not the keyboard raw mode).  This method is
used f.i. by login.  With it, the Backspace key works automatically
(basically the tty driver takes care of that), but not the arrow
keys, the Delete key, or anything else.  If an application needs more
functionality, it has to detect the relevant keys (or key combinations)
by itself.

    Now, the exact behaviour of this string input is affected by the
terminfo settings (or termio on older Unices).  This settings can be
changed independently for each tty (or ttyp) line, and control things
like the line speed, whether the keystrokes are echoed to the screen or
not, whether the input is line-buffered or not, and some more exotic
parameters (mostly relevant to serial lines, modems and so on), but also
the "erase" character (used to remove the last character entered), and
the "interrupt" character (normally ^C, used to send an INT signal to
the application).  For the Backspace / Delete topic, the only relevant
setting is "erase", which, of course, has to be in synch with the code
returned by the Backspace key.

    At the shell level, these settings can be changed with the stty
command.  The following script f.i. will read a character without
waiting for the Return key:

	#! /bin/sh
	echo -n "Enter a character: "
	stty cbreak
	readchar=`dd if=/dev/tty bs=1 count=1 2>/dev/null`
	stty -cbreak
	echo "Thank you for typing a $readchar."

Consequently, another typical use for reading /dev/tty directly is when
waiting for a keypress --- but that's another story.

    Now, the method above is nice and effective for reading "normal"
ASCII characters (or even control characters), but it's useless for
things like arrow keys, simply because different terminals will send
different sequences for those keys.  So this is handled at another
layer, which takes into account all (well, most of) terminal-specific
quirks: the termcap.  Basically, termcap has a data base containing
strings specific to each terminal (usually located in /etc/termcap);
when an application needs the value of, say, an arrow key, it finds the
section corresponding to the current terminal in the termcap data base
(according to the value of the TERM environment variable), then looks
for the string corresponding to that arrow.

    Since other terminal-specific settings can be described the same
way, the termcap data base also contains informations related to the
number of colors the terminal supports, the string it expects to change
colors, to clear the screen or to move the cursor, and so on.  Termcap
can be (and it is) used hence for "full screen" applications, but it's
rather inefficient, and the programming interface is pretty awkward to
use.  Typical example of application that use it: less (this is actually
a configuration option, less can also use terminfo).

    The next layer is readline.  This is a library that sits on top of
termcap, and provides fancy functions for string input.  It doesn't
mess with colors, screen movement, and the like.  The typical example
of application that use readline is bash.  Certain parameters can be
configured via the inputrc file, and these parameters are shared among
all applications that use readline.  The one relevant to the Backspace /
Delete discussion is "delete-char".

    Readline provides a much nicer interface for string input than
both reading /dev/tty and plain termcap, but it's also rather bloated
(see "man 3 readline" for a list of functions: some of them are
quite exotic), and the library itself is big, slow, and bad written.
Therefore, despite the ever increasing number of requests, it will never
be included in "small" or security-sensitive programs, like login.

    The next layer (unrelated to readline) is terminfo.  The idea
is the same as for termcap --- a data base is used to describe the
terminals --- but some limitations in termcap have been removed, other
features have been largely expanded (f.i. there are special entries
for line drawing characters, so that the same application can show
nice continuous lines on fancy terminals, and automatically fallback
to ugly "|" and "-" characters on dumb terminals), and generally is
more efficient.  Unlike termcap, the records in the data base are kept
in separate binary files, organized in a hierarchy under a common
directory.  As I said with another occasion, these records can be
decompiled and compiled back with the infocmp and tic commands.

    AFAIK, terminfo is distributed in the source form for Linux only
within the ncurses package.

    Now, terminfo is only the data base containing the descriptions
for various terminals --- it doesn't provide functions to access the
records by itself.  Specialized libraries must be used for accessing
these records, the most popular ones being (n)curses and S-Lang.  These
libraries provide a nice high-level abstraction layer for all screen
operations (reading keyboard, drawing to the screen, colors, line
characters and so on), and, in the absence of specific restrictions
(like small memory footprint etc.), they are the preferred approach
to writing text-mode applications.  They also usually go far beyond
terminfo: both S-Lang and ncurses can fallback to termcap if the current
terminal has no corresponding terminfo entry, they can manipulate
text windows, and so on.  S-Lang has even a (very restrictive) curses
emulation mode, and a few applications are specially designed to
use them interchangeably (f.i. Midnight Commander and Mutt).  On an
unrelated topic, please note however that S-Lang and ncurses are still
fundamentally different both in design and in functionality, so most
applications will only use one of them (f.i. slrn will probably never
use ncurses).

    Ok, getting closer to the initial question (which you probably
almost forgot by now ;-)), about the need to mess with terminfo.  As
usual, a little history can shed some light on the matter first.

    Ncurses, which was intended to be a free and improved replacement
for the curses library found on commercial Unices, used to be extremely
buggy, and the same was true for the associated terminfo.  Version
1.9.9g (the latest official one until a few days ago) was a real
nightmare from this POV, and was pretty much unusable for anything
serious.  As a reaction, some applications (f.i. Vim) migrated to some
incredibly contorted (but "safe") solutions, like reading terminfo by
themselves, and using lower level functions whenever possible.  Other
applications have switched to S-Lang: this one was written specifically
to overcome the bugs in ncurses, and could also use somewhat broken
terminfo entries (it included later a full interpreted language --- but
that's another story).  Finally, a minority of applications (f.i. Elvis)
decided to live with the broken ncurses, and hope for the best from the
future versions.

    Anyway, back to your question now.  Most applications seem to be
happy with the existing terminfo because they have learned how to live
with broken entries one way or another: they either use terminfo for
screen operations only and revert to reading /dev/tty for keyboard
input, or provide hard-coded values for keys on most popular terminals
hoping they will somehow match the actual settings, or even provide
a "learn keys" function to adapt to the current settings on the fly
(f.i. Midnight Commander does that).  So, basically, you don't need to
change anything as long as nothing seems to be broken. :-) But this is
only conjunctural, and the approaches above are contortions, _not_ the
"normal" way to write applications!  If you find a "normal" application
(possibly ported from another Unix system) that (naively) assumes it's
ok to use getch() from ncurses or S-Lang to read the keyboard, you'd
better make sure that terminfo is in synch with the codes sent by your
keyboard.  If it's not, you have to debug it and modify it manually
--- and that operation might turn to a royal pain.  If you change the
keycodes generated by Backspace and Delete, you have to modify terminfo
and termcap too.  It would be nice to have the relevant incantations
described somewhere. :-)

Fiddling with Terminfo entries


This is a bit tricky, do yourself a favour and make backups of all the files you tinker with. First check whether modifications are needed.

The terminfo database is usually located under /usr/lib/terminfo, /usr/share/terminfo, or /etc/terminfo depending on the distribution. The files you'll find are all binary, and have to be "decompiled" before you can edit them. This is done with infocmp (see man infocmp):

infocmp >file
This will decompile the entry corresponding to de current terminal. You can now edit file and change the relevant entries (see man terminfo). The file can be recompiled with tic (see man tic):

tic file
If you run tic as root, this will install the file to the right place. If you do it as an unprivileged user, you need to create a directory named .terminfo in your home directory, and set (or "export" ;) an environment variable named TERMINFO pointing to it prior to running tic. That should instruct tic to create the new file in ~/.terminfo. You also need to set the TERMINFO variable in your ~/.bash_profile, in order to make terminfo look at the new file rather that at the system-wide one.

By far the most tricky part is deciding what exactly you need to change, and the man page is not particularly helpful at that point. For example, if your Backspace sends ^?, you have to set

You need to repeat the above procedure for all terminal types you use.


Termcap entries in /etc/termcap are easier to modify, since it is a plain text file. Now all you have to do is locate your terminal type and edit the value you want to change (see man termcap). You can also add a certain key if it's not present.The values of interest are:

	kb = Backspace
	kh = Home
	@7 = End (don't ask me why "@7"...)
	kD = Delete
	kI = Insert
	ku = Up arrow
	kd = Down arrow
	kl = Left arrow
	kr = Right arrow
	kP = PageUp
	kN = PageDown
You need to get the values returned by the keyboard by pressing Ctrl+v and the relevant key, and make sure they are the same in termcap.


Some notes: the keymap of the X Server is initialized using the system keymap. This means that changes in the keymap affect X. Xmodmap or xkb can overrule this. Thanks to Christian Schwartz for the picture.

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