BackspaceDelete(1).pdf

Linux Backspace/Delete mini−HOWTO

Sebastiano Vigna

vigna@acm.org

Revision History

Revision v1.6 19 Jan 2002

Included many comments from Alex Boldt and Chung−Rui Kao.

Revision v1.5 3 May 2000

Updated for new distros and the tput trick.

Revision v1.4 7 December 2000

Updated for Red Hat 7.0 and Helix Gnome conflicts.

Revision v1.3

18 October 2000

Name change.

Revision v1.2 15 October 2000

Updated. Added "What If Nothing Works" section.

Revision v1.1

13 September 2000

Added tcsh fixes

Revision v1.0

5 September 2000

First release

Linux Backspace/Delete mini−HOWTO

Table of Contents

1. Introduction. ....................................................................................................................................................1

2. How Keys Are Turned Into Actions. .............................................................................................................2

3. Why It Doesn't (Always) Work .....................................................................................................................3

4. X. .......................................................................................................................................................................4

5. What You Should Do When Writing Applications. .....................................................................................5

6. What You Should Do On Your System.. .......................................................................................................6

6.1. What Needs to Be Done. ...................................................................................................................6

6.1.1. Detecting Deviance. ..........................................................................................................6

6.1.2. Distinguishing Between Emulators. .................................................................................6

6.1.3. Fixing the Terminal Database. ..........................................................................................7

6.1.4. Fixing the Shell Behaviour. ..............................................................................................7

6.2. How to Do It. ....................................................................................................................................7

6.3. Fixing for tcsh. ..................................................................................................................................9

Notes. ........................................................................................................................................10

7. What If Nothing Works. ...............................................................................................................................11

8. More Hacking. ...............................................................................................................................................12

9. Conclusions. ...................................................................................................................................................12

1. Introduction

Every Linux user has been sooner or later trapped in a situation in which having working Backspace and

Delete keys on the console and on X seemed impossible. This paper explains why this happens and suggests

solutions. The notions given here are essentially distribution−independent: due to the widely different content

of system configuration files in each distribution, I will try to give the reader enough knowledge to think up

his/her own fixes, if necessary.

I assume that the Backspace key should go back one character and then erase the character under the cursor.

On the other hand, the Delete key should delete the character under the cursor, without moving it. If you

think that the function of the two keys should be exchanged, in spite of the fact that most keyboards feature

an arrow pointing to the left ( ) on the Backspace key, then this paper will not give you immediate solutions,

but certainly you may find the explanations given here useful.

Another assumption is that the fixes should alter only local (user) files. No standard part of the distribution

should be altered. Finally, this document discusses how to set up your system so that applications get the

right events. If an application decides to interpret such events in an idiosyncratic way, the only possible fix is

to reconfigure the application.

Since the first release of this Mini−HOWTO things have

become even more entangled. Different distributions of the

same terminal emulator (e.g., gnome−terminal as provided by

Red Hat 7.0, Helix Code/Ximian or even Red Hate7.1)

generate different ASCII sequences. Due to this mismatch,

now the terminal databases correspond even less to the

terminal emulators they are supposed to describe. To set a

firm ground for the following discussion, we assume basically

as correct settings the ones proposed in the Debian keyboard

policy .

1. Introduction

2. How Keys Are Turned Into Actions

When a key is pressed on the keyboard, a number of hardware and software components cooperate so as to

guarantee that the intended meaning of the key (e.g., emitting a certain character) matches the actual

behaviour of the key. I will concentrate on the software side (as our control on the hardware part is

nonexistent), and in particular, for the time being, on the events related to console output.

Hitting a key causes raw keyboard scancodes to be generated; these scancodes are then transformed

in a keycode . On an i386 system, usually the key Backspace emits 14 and the key Delete emits 111.

The keycodes are translated by the keyboard library into a keyboard symbol (keysym) using the

keyboard definition loaded by the user. If you look into your keyboard database (e.g., in

/lib/kbd/ ), you'll discover several definitions for different computers, different layouts and

possibly different interpretations of the same keys (e.g., one could desire that the two Alt keys really

behave as distinct modifiers). The Linux console keyboard layout assigns keysym Delete to keycode

14 and keysym Remove to keycode 111. This may seem strange, but the Linux console emulates a

VT100 terminal, and this is the way things work in that realm. [1]

Our journey has still to come to an end. Console applications read ASCII sequences, not keysyms. So

the console must read keysyms and translate them into ASCII sequences that suitably encode the

keys. Of course, this operation must be performed in a way that is understandable by applications.

For instance, on the Linux console the Delete keysym is mapped to the ASCII code 127 (DEL), the

Remove keysym on a suitable escape sequence, and the BackSpace keysym to ASCII code 8 (BS).

Finally, we must in a sense roll back to what we had before and translate the ASCII sequences

generated by each key into a key capability . This goal is reached by a terminal database , which

contains, for each kind of terminal, a reverse mapping from sequences of characters to key

capabilities (which are essentially a subset of the keysyms). [2]

Unfortunately, there are two "standard" terminal

databases, termcap and terminfo. Depending on your

distribution, you could be using either one of them,

or the database could even depend on the

application. Our discussion will concentrate on the

more modern terminfo database, but the suggested

fixes take both into consideration.

For instance, on the Linux console F1 generates an escape followed by [[A , which can be translated

to the capability key_f1 by looking into the terminal−database entry of the console (try infocmp

linux if you want to have a look at the entry). A very good and thorough discussion of terminal

databases can be found in GNU's termcap manual. Usually Linux applications use the newer

terminfo database, contained in the ncurses package.

Maybe at this point not surprisingly, the Linux console terminfo entry maps DEL to the

kbs (backspace key) capability, and escape followed by [3~ to the kdch1 ("delete−one−char" key)

capability. Even if you could find strange that the Backspace key emits a DEL, the terminal database

puts everything back into its right place, and correctly behaving applications will interpret DEL as

the capability kbs , thus deleting the character to the left of the cursor.

2. How Keys Are Turned Into Actions

3. Why It Doesn't (Always) Work

I hope the basic problem is clear at this point: there is a bottleneck between the keyboard and console

applications, that is, the fact that they can only communicate by ASCII sequences. So special keys must be

first translated from keysyms to sequences, and then from sequences to key capabilities. Since different

consoles have different ideas about what this translation can look like, we need a terminal database. The

system would work flawlessly, except for a small problem: it is not always set up correctly, and not everyone

uses it.

Applications must have a way to know which database entry to use: this is accomplished by suitably setting

the TERM environment variable. In some cases, there is a mismatch between the terminal emulator and the

content of the database entry suggested by TERM .

Moreover, many applications do not use the terminal database (or at least not all of it), and consider BS and

DEL ASCII codes with an intended meaning: thus, without looking at the database, they assign them

semantics (usually, of course, the semantics is removing the character before or under the cursor). So now our

beautiful scheme is completely broken (as every Linux user is bitterly aware). For instance, the bash assumes

that DEL should do a backward−delete−char, that is, backspace. Hence, on a fresh install the Backspace key

works on the console as expected, but just because of two twists in a row! Of course, the Delete key does not

work. This happens because the bash does not look into the terminal database for the kdch1 capability.

Just to illustrate how things have become entangled, consider the fix_bs_and_del script provided with the

Red Hat distribution (and maybe others). It assigns on−the−fly the BackSpace keysym to the Backspace key,

and the Delete keysym to the Delete key. Now the shell works! Unfortunately, all programs relying on the

correct coupling of keysym generation and terminal database mappings are now not working at all, as the

Delete keysym is mapped to DEL, and the latter to the kbs key capability by the terminfo database, so in

such programs both keys produce backspacing.

3. Why It Doesn't (Always) Work

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