LDP/LDP/howto/linuxdoc/Visual-Bell.sgml

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<title>Visible bell mini-Howto
<author>Alessandro Rubini, <tt/rubini@linux.it/
<date>v2.3, 2001-12-03
<abstract>
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.
</abstract>

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<sect>Copyright and License
<p>
This document is Copyright (c) 1997, Alessandro Rubini.
<p>
This document is distributed under the terms of the GNU Free Documentation License.
You should have received a copy along with it. If not, it is available from
<url url="http://www.fsf.org/licenses/fdl.html">.

<sect>Introduction
<p>
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.
<p>
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.

<sect>Spekearectomy
<p>
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.
<p>
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.
<p>
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.

<sect>Per-console Beep Configuration
<p>
As of Linux 1.3.43, Martin Mares added the ability to configure the
pitch and duration of the beep, by modifying <tt/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 <tt>~/.profile</tt>
or <tt>~/.login</tt> file to select a different beep sound associated
to each console (or no beep at all, if needed).
<p>
The escape sequences work as follow:
<itemize>
<item>ESC-<tt/[10;/xx<tt/]/ 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-<tt/[10]/.
<item>ESC-<tt/[11;/xx<tt/]/ 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-<tt/[11]/) the default value
will be used.
</itemize>
<p>
To select, for example, a 50Hz pitch for one-second duration, you can
"<tt/echo -e "\\33[10;50]\\33[11;1000]"/" with <tt/bash/ (where "-e"
means `understand Escape sequences'. If you use <tt/tcsh/ the same
command spells "<tt/echo "\\033[10;50]\\033[11;1000]"/".
<p>
Although I don't know of any version of the <tt/setterm/ command that
supports such configuration, a future version of the command might
well support a command-line option to configure the bell sound.
<p>
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.

<sect>Basic Concepts About Termcap and Terminfo
<p>
The file <tt>/etc/termcap</tt> is a text file that lists the
<tt/term/inal <tt/cap/abilities. Several applications use the
<tt/termcap/ information to move the cursor around the screen and do
other screen-oriented tasks.  <tt/tcsh/, <tt/bash/, <tt/vi/ and all
the <tt/curses/-based applications use the <tt/termcap/ database.
<p>
The database describes several terminal types. The 
<tt/TERM/ environment variable selects the right behaviour
at run-time, by naming a <tt/termcap/ entry to be used by
applications.
<P>
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 "<tt/bl=^G/". This sequence
tells that the bell sound is obtained by printing the control-G
character, the ASCII BEL.
<p>
In addition to the <tt/bl/ capability, the <tt/vb/ capability is
recognized. It is used to represent the "visible bell". <tt/vb/ is
usually missing in the <tt/linux/ entry of the <tt>termcap</tt>
file.
<p>
Most modern applications and libraries use the <tt/terminfo/ database
instead of <tt/termcap/. This database uses one file per terminal-type
and lives in <tt>/usr/lib/terminfo</tt>; to avoid using huge
directories, the description of each terminal type is stored in a
directory named after its first letter; the <tt/linux/ entry,
therefore, is <tt>/usr/lib/terminfo/l/linux</tt>. To build a
<tt/terminfo/ entry you'll ``compile'' the <tt/termcap/ description;
refer to the <tt/tic/ program and its manual page.

<sect>Defining a Visible Bell
<p>
You can add the entry for the <tt/vb/ capability in your own termcap
file, if it doesn't already define one.  Dennis Henriksen
(duke@diku.dk) suggested to insert the following line in the
<tt/termcap/ entry for <tt/linux/ (note that the entry is called
<tt/console/ in old distributions):
<tscreen><verb>
:vb=\E7\E[?5h\E[?5l\E[?5h\E[?5l\E[?5h\E[?5l\E[?5h\E[?5l\E8:\
</verb></tscreen>
The trailing backslash is used to escape the newline in the database.
Dennis' code does the following (his own words):
<itemize>
<item>Save the cursor position (uust a safety precaution).
<item>Change the background color several times between normal and reverse.
<item>Restore the cursor position.
</itemize>

<sect>Disabling the Audible Bell on the Text Console
<p>
If you want to force the visible bell on your console you can use the
"<tt/bl/" entry in <tt/termcap/ and define it with the same string
suggested for "<tt/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.

<sect>Telling Applications to Avoid Beeping
<p>
This is an incomplete list of applications that can be instrued
to use the <tt/vb/ entry for the current terminal type (using either
the <tt/termcap/ information or the <tt/terminfo/ one):

<itemize>

<item>The X server: use the "<tt/xset b/" command to select
	the bell's behaviour. The command takes three numeric
	arguments: volume, pitch and duration. "<tt/xset -b/" disables
	the bell altogether. Configuring the X server affects all
	the applications running on the display.

<item><tt/xterm/: xterm can convert each bell to either a visible or
	audible signal. If you use the audible bell, the settings of
	"<tt/xset/" will apply. The bell in <tt/xterm/ defualts to be
	audible, but you can use the "<tt/-vb/" command line option
	and the "<tt/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.


<item><tt/tcsh/ (6.04 and later): "<tt/set visiblebell/".  The
	instruction can be placed in <tt/.cshrc/ or can be issued
	interactively. To reset the audible bell just "<tt/unset
	visiblebell/". To disable any notification issue use "<tt/set
	nobeep/" instead.

<item><tt/bash/ (any bash, as fas as I know): put "<tt/set bell-style
	visible/" in your <tt>~/.bashrc</tt>. Possible bell-style's
	are also "none" or "audible".

<item><tt/bash/ (with <tt/readline/, as well as other <tt/readline/
	based applications): put "<tt/set prefer-visible-bell/" in
	<tt>~/.inputrc</tt>.

<item><tt/nvi/ and <tt/elvis/: put "<tt/set flash/" in
	<tt>~/.exrc</tt> or tell "<tt/:set flash/" interactively (note
	the colon).  To disable the visible bell use <tt/noflash/ in
	place of <tt/flash/.

<item><tt/emacs/: put "<tt/(setq visible-bell t)/" in your
	<tt>~/.emacs</tt>.  It is disabled by "<tt/(setq visible-bell
	nil)/".

<item><tt/less/: use "<tt/-q/" on command line to use the visual bell,
	use "<tt/-Q/" to disable any reporting. Default options can be
	put in your environment variable "<tt/LESS/".

<item><tt/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.

</itemize>

<sect>The Dark Side of the Problem
<p>
The bad news is that not every application uses <tt/termcap/ or
<tt/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.
<p>
The only way to make these programs silent applications is
spekearectomy, or using the escape sequences by Martin Mares.

</article>