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Abell character (sometimesbell code) is a devicecontrol code originally sent to ring a small electromechanical bell ontickers and otherteleprinters and teletypewriters to alert operators at the other end of the line, often of an incoming message. Though tickers punched the bell codes into their tapes,[1] printers generally do not print a character when the bell code is received. Bell codes are usually represented by the label "BEL". They have been used since 1870 (initially in theBaudot code).[2]
To maintain backward compatibility, video displayterminals (VDTs) that replaced teletypewriters included speakers or buzzers to perform the same function, as did the personal computers that followed. Modernterminal emulators often integrate the warnings to the desktop environment (e.g., themacOSTerminal will play the system warning sound) and also often offer a silentvisual bell feature that flashes the terminal window briefly.
InASCII the bell character's value is 7 and is named "BELL" or "BEL".Unicode does not give names to control characters but has assigned it the alias "ALERT" and abbreviation "BEL." It can sometimes be typed asctrl+G and displayed as^G incaret notation. Unicode also includes characters for the visual representation of the character:U+2407 ␇SYMBOL FOR BELL andU+237E ⍾BELL SYMBOL.
In the 5-bit Baudot codes, BEL is represented by the number 11 (0x0B) when in "figures" mode.[3] The code0x2F is used inEBCDIC.
In the programming languageC (created in 1972), and in many languages influenced by it such asPython, the bell character can be placed in a string or character constant with\a. 'a' stands for "alert" or "audible" and was chosen because\b was already used for thebackspace character.[4]
OnUnix-like systems, or onMS-DOS orWindows, a user can cause the equivalent of ringing the bell to happen by typing at thecommand prompt the command:
echo ^G
where the^G is produced by holding downCtrl and typingG. On Unix the user may need to typeCtrl+V first to "quote" the^G.
On POSIX systems, one may also use:
printf '\a'
and in theBash shell, one may use ANSI-C quoting:[5]
echo $'\a'
An alternative is to use thetput command, which as a part of thencurses library is available on most Unix/Linux operating systems:
tput bel
A program can get the same result by printing the BEL character to a terminal.
On modern systems this may not make a noise; it may instead make a visual indication such as flashing the screen, or do nothing at all.