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Bash includes features from the Korn Shell for both interactive use and shell programming. For programming, Bash provides variables such as RANDOM and REPLY ,the typeset builtin, the & 8230; Bash › The GNU shell* Chet Ramey Case Western Reserve University [email protected] 1. Introduction Bash is the shell, or command language interpreter,that will appear in the GNU operating system. The name is an acronym for the & 8220;Bourne-Again SHell& 8221;, a pun on Steve Bourne, the author of the direct ancestor of the current UNIX®shell /bin/sh ,which & 8230;

3. BasicBash features Since the Bourne shell provides Bash with most of its philosophical underpinnings, Bash inherits most of its features and functionality from sh. Bash implements all of the traditional sh çowcontrol constructs ( for , if , while ,etc.). Allof the Bourne shell builtins, including those not speciåed in the POSIX.2 standard, appear in Bash. Shell functions ,introduced in the SVR2 version of the Bourne shell, are similar to shell scripts, but are deåned using a special syntax and are executed in the same process as the calling shell. Bash has shell functions which behave inafashion upward-compatible with sh functions. There are certain shell variables that Bash interprets in the same way as sh, such as PS1 , IFS ,and PATH .Bash implements essentially the same grammar,parameter and variable expansion semantics, redirection, and quoting as the Bourne shell. Where differences appear between the POSIX.2 standard and traditional sh behavior,Bash follows POSIX. The Korn Shell ( ksh )isadescendent of the Bourne shell written at AT& 038;T Bell Laboratories by David Korná. Itprovides a number of useful features that POSIX and Bash have adopted. Manyofthe interactive facilities in POSIX.2 have their roots in the ksh: for example, the POSIX and ksh job control facilities are nearly identical. Bash includes features from the Korn Shell for both interactive use and shell programming. For programming, Bash provides variables such as RANDOM and REPLY ,the typeset builtin, the ability to remove substrings from variables based on patterns, and shell arithmetic. RANDOM expands to a random number each time it is referenced; assigning a value to RANDOM seeds the random number generator. REPLY is the default variable used by the read builtin when no variable names are supplied as arguments. The typeset builtin is used to deåne variables and give them attributes such as read- only .Bash arithmetic allows the evaluation of an expression and the substitution of the result. Shell variables may be used as operands, and the result of an expression may be assigned to a variable. Nearlyall of the operators from the C language are available, with the same precedence rules: $echo $((3 + 5 * 32)) 163 Forinteractive use, Bash implements ksh-style aliases and builtins such as fc (discussed below) and jobs . Bash aliases allowastring to be substituted for a command name. Theycan be used to create a mnemonic for a UNIX command name (alias del=rm), to expand a single word to a complexcommand (alias news=& 8217;xterm -g 80& 215;45 -title trn -e trn -e -S1 -N & 038;& 8217;), or to ensure that a command is invokedwith a basic set of options (alias ls=& 8221;/bin/ls -F& 8221;). The C shell ( csh )á, originally written by Bill Joywhile at Berkeley, iswidely used and quite popular for its interactive facilities. Bashincludes a csh-compatible history expansion mechanism (ô! historyö), brace expansion, access to a stack of directories via the pushd , popd ,and dirs builtins, and tilde expansion, to generate users& 8217; home directories. Tilde expansion has also been adopted by both the Korn Shell and áMorris Bolskyand David Korn, The KornShell Command and Programming Language ,Prentice Hall, 1989

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