Visible bell mini-Howto <author>Alessandro Rubini, <tt/rubini@ipvvis.unipv.it/ <date>v2.00, May 1995 <abstract> This doc explains something about termcap usage and provides a pointer to a kernel patch to completely disable audible bells on demand. </abstract>  <toc>  <sect>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 embed a loudspeaker. <sect>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. 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 in an useful toggle. The preferred solution for such users is trying to configure their software to avoid beeping. <sect>Basic Concepts about termcap The file <tt>/etc/termcap</tt> is a text file which lists the <tt/term/inal <tt/cap/abilities. Several applications use the <tt/termcap/ information to move the cursor in 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. The database represents various terminal types, and applications use the <tt/TERM/ environment variable to refer to the right entry in <tt/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 "<tt/bl=^G/", which instructs the applications to use the control-G character, the ASCII BEL. In addition to the <tt/bl/ capability, there is a <tt/vb/ capability, which represents the "visible bell". <tt/vb/ is usually missing in the <tt/console/ entry in Linux' <tt>/etc/termcap</tt>. <sect>Defining a visible bell You can add the entry for the <tt/vb/ capability in your own termcap file. Dennis Henriksen (duke@diku.dk) suggested to insert the following line in the <tt/termcap/ entry for <tt/console/: <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 (Just a safety precaution) <item>Change the background color several times between normal and reverse <item>Restore the cursor position. </itemize> Some warnings about modifying termcap: <itemize> <item>Check what your <tt/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". <item>Check if the <tt/vb/ field is already there, distributors are reading docs and upgrading their software all the time. <item>Some applications don't use <tt/termcap/ but the <tt/terminfo/ database. They won't sense your <tt/vb/ entry. Terminfo is more powerful than termcap, as well as more difficult. </itemize> <sect>Telling applications about it This is an incomplete list of applications that can be instrued to use the <tt/vb/ entry for the current terminal type: <itemize> <item><tt/tcsh/ (6.04 and later): "<tt/set visiblebell/". The instruction can appear in <tt/.cshrc/ or can be issued interactively. To reset the audible bell just "<tt/unset visiblebell/". To disable any notification issue "<tt/set nobeep/". <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 works on all the virtual screens. Refer to the man page under "CUSTOMIZATION" for setting the default. <item><tt/xterm/: xterm can convert each bell to either a visible or audible signal. It defualt to audible, but you can use the "<tt/-vb/" command line option and the "<tt/xterm*visualBell: true/" resource. You can toggle visible/audible signaling on the fly with the control-mouse-1 menu. <item>other X applications: you can tell the X server the volume of the bell, with the "<tt/-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. </itemize> <sect>Disabling the audible bell If you want to force the visible bell on your console you can replace the "<tt/bl/" entry in <tt/termcap/ with the same string suggested for "<tt/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 theis source code won't be affected by this change. <sect>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, <tt/console/ and <tt/console-vb/. The <tt/console/ entry would be the original one, while the other could feature a visual bell string for the "<tt/bl/" item. Thus you can change the behaviour of your bell on a console basis: <itemize> <item>With <tt/tcsh/: "<tt/setenv TERM console-vb/" to get a screen flash, and "<tt/setenv TERM console/" to get the audible beep. <item>With <tt/bash/: "<tt/TERM=console-vb; export TERM/" for the flash, and "<tt/TERM=console; export TERM/" for the beep. </itemize> Note that the <tt/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. <sect>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 a modification of the console driver in the kernel. An old patch to the kernel is available by ftp from <tt>iride.unipv.it:/pub/linux/</tt>. It was taken against the 1.1.31 kernel sources, but it applies to older kernels as well. The patch comes with a small user program to set/unset visible and audible bells on a per-console basis. I'm not currently mantaining the patch, but if you are interested in it drop me a note. If there's enough interest we could even make it in the mainstream kernel. </article>