Visible bell mini-Howto
Alessandro Rubini,
[email protected]
v2.2, 11 November 1997
This document explains how to use termcap to configure a visual bell
on one's system and describes how to disable audible bells on demand.
1. Introduction
The Linux console driver beeps the audible bell whenever a BEL char is
output (ASCII code 7). Though this is a right choice for the default
behaviour, many users don't like their computer to beep. This mini-
Howto is meant to explain how to tell applications not to output the
BEL code. It also explain how to instruct the kernel and the X Window
System to avoid beeping when a BEL is output. Note that most of this
document refers to the text console, as configuring the X server is an
easy catch-all for any user who works in a graphic environment.
In my opinion the best way to face a fussy computer is fixing the
hardware, and my own computer doesn't even carry a loudspeaker.
2. Spekearectomy
Speakerectomy is by far the most brilliant solution to the audible
bell problem. As its name implies, it consists in removing the beeps
by removing the beeper. The operation is straightforward and you don't
even need any anesthetic, but if you want there's room for refinement.
PC's are usually equipped with a silly switch to lower CPU clock. The
switch is never used when you work in a multitasking environment, as
you don't even need to slow the computer down to run games based on
software loops. Unfortunately we can't use the switch to increase
processor speed, but we can use it to enable/disable the loudspeaker.
Sometimes the speaker is useful even if you enjoy a silent number
cruncher, for example to signal the end of a lenghty compilation. To
modify the switch functionality, just detatch it from the main board
and connect its wires in series with the loudspeaker.
Owners of laptop boxes, unfortunately, don't have easy access to the
loudspeaker, and neither they have a spare switch to turn to a
different task. The preferred solution for such users is configuring
their software to avoid beeping, as described below.
3. Per-console Beep Configuration
As of Linux 1.3.43, Martin Mares added the ability to configure the
pitch and duration of the beep, by modifying console.c. Each console
can be configured to feature a different duration and/or pitch of the
bell sound; the task is accomplished by using escape sequences to the
console device. You can configure your own ~/.profile or ~/.login
file to select a different beep sound associated to each console (or
no beep at all, if needed).
The escape sequences work as follow:
ESC-[10;xx] selects the bell frequency in Hertz. The value should
be in the range 21-32766, otherwise the result is undefined. If
the `xx' argument is missing, the default value (750Hz) will apply,
as in `ESC-[10].
ESC-[11;xx] selects the bell duration, in milli-seconds. If you
specify more than 2 seconds, the default applies (125ms). Once
again, if the `xx' argument is missing (ESC-[11]) the default value
will be used.
To select, for example, a 50Hz pitch for one-second duration, you can
"echo -e "\33[10;50]\33[11;1000]"" with bash (where "-e" means
`understand Escape sequences'. If you use tcsh the same command spells
"echo "\033[10;50]\033[11;1000]"".
Although I don't know of any version of the setterm command that
supports such configuration, a future version of the command might
well support a command-line option to configure the bell sound.
If you run Linux-1.3.43 or newer, you may be satisfied with the escape
sequences and avoid reading further. If you run an older kernel, or if
you want the visual bell, you'll enjoy the rest of this document.
4. Basic Concepts About Termcap and Terminfo
The file /etc/termcap is a text file that lists the terminal
capabilities. Several applications use the termcap information to move
the cursor around the screen and do other screen-oriented tasks.
tcsh, bash, vi and all the curses-based applications use the termcap
database.
The database describes several terminal types. The TERM environment
variable selects the right behaviour at run-time, by naming a termcap
entry to be used by applications.
Within the database, each capability of the terminal appears as a two-
letter code and a representation of the actual escape sequence used to
get the desired effect. The separator character between different
capabilities is the colon (":"). As an example, the audible bell,
with code "bl", usually appears as "bl=^G". This sequence tells that
the bell sound is obtained by printing the control-G character, the
ASCII BEL.
In addition to the bl capability, the vb capability is recognized. It
is used to represent the "visible bell". vb is usually missing in the
linux entry of the termcap file.
Most modern applications and libraries use the terminfo database
instead of termcap. This database uses one file per terminal-type and
lives in /usr/lib/terminfo; to avoid using huge directories, the
description of each terminal type is stored in a directory named after
its first letter; the linux entry, therefore, is
/usr/lib/terminfo/l/linux. To build a terminfo entry you'll
``compile'' the termcap description; refer to the tic program and its
manual page.
5. Defining a Visible Bell
You can add the entry for the vb capability in your own termcap file,
if it doesn't already define one. Dennis Henriksen (
[email protected])
suggested to insert the following line in the termcap entry for linux
(note that the entry is called console in old distributions):
:vb=\E7\E[?5h\E[?5l\E[?5h\E[?5l\E[?5h\E[?5l\E[?5h\E[?5l\E8:\
The trailing backslash is used to escape the newline in the database.
Dennis' code does the following (his own words):
Save the cursor position (uust a safety precaution).
Change the background color several times between normal and
reverse.
Restore the cursor position.
6. Disabling the Audible Bell on the Text Console
If you want to force the visible bell on your console you can use the
"bl" entry in termcap and define it with the same string suggested for
"vb" above. This approach is handy if you don't want to customize
each application (which is described below, anyway). I use this
option on all the machines where I can run Linux and I can't detach
the speaker.
7. Telling Applications to Avoid Beeping
This is an incomplete list of applications that can be instrued to use
the vb entry for the current terminal type (using either the termcap
information or the terminfo one):
The X server: use the "xset b" command to select the bell's
behaviour. The command takes three numeric arguments: volume, pitch
and duration. "xset -b" disables the bell altogether. Configuring
the X server affects all the applications running on the display.
xterm: xterm can convert each bell to either a visible or audible
signal. If you use the audible bell, the settings of "xset" will
apply. The bell in xterm defualts to be audible, but you can use
the "-vb" command line option and the "xterm*visualBell: true"
resource to turn it to a visible flash. You can toggle
visible/audible signaling at run-time by using the menu invoked by
control--left-mouse-button. If you run X you most likely won't
need the following information.
tcsh (6.04 and later): "set visiblebell". The instruction can be
placed in .cshrc or can be issued interactively. To reset the
audible bell just "unset visiblebell". To disable any notification
issue use "set nobeep" instead.
bash (any bash, as fas as I know): put "set bell-style visible" in
your ~/.bashrc. Possible bell-style's are also "none" or "audible".
bash (with readline, as well as other readline based applications):
put "set prefer-visible-bell" in ~/.inputrc.
nvi and elvis: put "set flash" in ~/.exrc or tell ":set flash"
interactively (note the colon). To disable the visible bell use
noflash in place of flash.
emacs: put "(setq visible-bell t)" in your ~/.emacs. It is
disabled by "(setq visible-bell nil)".
less: use "-q" on command line to use the visual bell, use "-Q" to
disable any reporting. Default options can be put in your
environment variable "LESS".
screen: issue the CtrlA-CtrlG command. It changes the behaviour of
all the virtual screens. Refer to the man page under
"CUSTOMIZATION" for setting the default.
8. The Dark Side of the Problem
The bad news is that not every application uses termcap or terminfo.
Most small programs feature 'backslash-a' (alarm) characters in the C
source code. The "alarm" code becomes a literal ASCII BEL in the
strings as stored in the executable binary. Real application don't
usually fall in this category, but be careful of C newcomers who give
you their own programs. Students of computer science are the worst of
all, granted.
The only way to make these programs silent applications is
spekearectomy, or using the escape sequences by Martin Mares.
