Visible bell mini-Howto
 Alessandro Rubini, [email protected]
 v2.1, Aug 1996

 This doc explains something about termcap usage and provides a pointer
 to a kernel patch to completely 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. Pointers to a kernel patch are provided as well. The patch
 is intended as a catch-all approach to avoid mangling with termcap and
 applications' defaults.

 In my opinion the best solution is a hardware one, 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. It is the simplest way to remove beeps, though some
 additional notes are worth.

 PC's are usually equipped with a silly switch to toggle the mainboard
 clock to a lower speed. The switch is completely unneeded when you
 work in a multitasking environment, and even its original function to
 slow down old dos games based on software-loop delays is no more used.
 Unfortunately we can't use the switch to increase processor speed, but
 we can turn it in a speaker enable/disable toggle. It is nice when
 your computer beeps at you to signal the end of a lenghty compilation,
 even for those who prefer a silent console.

 To modify the switch functionality, just detatch it from the main
 board and connect its wires in series with the loudspeaker. That's all
 to it.

 Owners of laptop boxes, unfortunately, don't have easy access to the
 lousspeaker, and neither have a spare switch to turn to a different
 task.  The preferred solution for such users is trying to configure
 their software to avoid beeping.

 3.  Per-console Beep Configuration

 As of Linux 1.3.43, Martin Mares added configurability to the bell
 sound in console.c. You can change the duration and pitch of the
 console beep on a per-console basis, by writing escape sequences to
 the tty. You can apply your configuration in your own ~/.profile or
 ~/.login, to have a different beep (or no beep) associated to each
 console.

 The escape sequences work as follow:

 �  ESC-[10;xx] chooses the bell frequency in Hertz. The value should
    be in the range 21-32766, otherwise the result is undefined (at
    least up to the 2.0.x version -- I can't foresee the future.  If
    the `xx' argument is missing, the default value (750Hz) will apply,
    as in `ESC-[10].

 �  ESC-[11;xx] chooses 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 print the escape sequences, you can try for example (50Hz, 1s)
 "echo -e "\33[10;50]\33[11;1000]"" with bash (where "-e" means
 `understand escape sequences'. With tcsh "echo
 " 33[10;50] 33[11;1000]" will have the same effect.

 Note that a new `setterm' command might support bell configuration of
 command line, as these control codes are marked as `setterm-commands'.
 However, no `setterm' version I know of supports these codes.

 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 (I do
 it myself, on a small 386), or if you want the visual bell, have a
 good reading.

 4.  Basic Concepts About Termcap

 The file /etc/termcap is a text file which lists the terminal
 capabilities. Several applications use the termcap information to move
 the cursor in the screen and do other screen-oriented tasks.  tcsh,
 bash, vi and all the curses-based applications use the
 termcapdatabase.

 The database represents various terminal types, and applications use
 the TERM environment variable to refer to the right entry in termcap.
 Each capability is then represented by a two-letter code associated to
 the character string used to get the desired effect.  The separator
 character between different capabilities is colon (":").  As an
 example, the audible bell, whith code "bl", is usually represented by
 the string "bl=^G", which instructs the applications to use the
 control-G character, the ASCII BEL.

 In addition to the bl capability, there is a vb capability, which
 represents the "visible bell". vb is usually missing in the console
 entry in Linux' /etc/termcap.

 5.  Defining a visible bell

 You can add the entry for the vb capability in your own termcap file.
 Dennis Henriksen ([email protected]) suggested to insert the following line
 in the termcap entry for console:

      :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 (Just a safety precaution)

 �  Change the background color several times between normal and
    reverse

 �  Restore the cursor position.

 Some warnings about modifying termcap:

 �  Check what your TERM variable looks like: it used to be "console",
    but it depends on your distribution and kernel version.  Actually,
    it is due to change to "linux".
 �  Check if the vb field is already there, distributors are reading
    docs and upgrading their software all the time.

 �  Some applications don't use termcap but the terminfo database.
    They won't sense your vb entry. Terminfo is more powerful than
    termcap, as well as more difficult.

 6.  Telling applications about it

 This is an incomplete list of applications that can be instrued to use
 the vb entry for the current terminal type:

 �  tcsh (6.04 and later): "set visiblebell".  The instruction can
    appear in .cshrc or can be issued interactively. To reset the
    audible bell just "unset visiblebell". To disable any notification
    issue "set nobeep".

 �  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 works on all the virtual
    screens. Refer to the man page under "CUSTOMIZATION" for setting
    the default.

 �  xterm: xterm can convert each bell to either a visible or audible
    signal. It defualt to audible, but you can use the "-vb" command
    line option and the "xterm*visualBell: true" resource. You can
    toggle visible/audible signaling on the fly with the control-
    mouse-1 menu.

 �  other X applications: you can tell the X server the volume of the
    bell, with the "-f volume" commandline option. "volume" is between
    0 and 100. Refer to X docs/experts about how to pass command line
    options to the server.

 7.  Disabling the audible bell

 If you want to force the visible bell on your console you can replace
 the "bl" entry in termcap with the same string suggested for "vb"
 above.  This approach can unload you from the task of customizing each
 application.  I use this option on all the machines where I can run
 Linux.

 Note that applications with hardwired bells in their source code won't
 be affected by this change.

 8.  Easier configurability

 If you want the ability to choose between audible and visible bell on
 a console basis, you can use two different terminal types for the
 linux console.  You can name them, for example, console and console-
 vb.  The console entry would be the original one, while the other
 could feature a visual bell string for the "bl" item.  Thus you can
 change the behaviour of your bell on a console basis:

 �  With tcsh: "setenv TERM console-vb" to get a screen flash, and
    "setenv TERM console" to get the audible beep.

 �  With bash: "TERM=console-vb; export TERM" for the flash, and
    "TERM=console; export TERM" for the beep.

    Note that the termcap format allows to define a terminal-type in
    terms of another, so you need to insert in the database only the
    differences.  Refer to the manpages for more information.

 9.  Other solutions

 The bad news is that not all the applications are termcap-aware. Most
 small programs feature 'backslash-a' characters in the C source code.
 Those chars become a literal ASCII BEL in the executable binary.  Real
 application don't usually fall in this category, but be careful of C
 newcomers who give your their own programs; novice computer-science
 students are the worst of all.

 The only way to shut the loudspeaker for these applications is
 spekearectomy, or use of the escape sequences by Martin Mares.

 A long time ago I made a patch for me. I don't support it anymore as
 the escape sequences are there. When I reworked my patch against
 1.3.42 to support a configurable tone, I found that 1.3.43 already had
 it :-)

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