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BASH - The Bourne-Again Shell
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Bash is the shell, or command language interpreter, that will appear
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in the GNU operating system. Bash is an sh-compatible shell that
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incorporates useful features from the Korn shell (ksh) and C shell
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(csh). It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2
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Shell and Tools standard. It offers functional improvements over sh
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for both programming and interactive use. In addition, most sh scripts
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can be run by Bash without modification.
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Bash is quite portable. It uses a configuration system that discovers
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characteristics of the compilation platform at build time, and may
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therefore be built on nearly every version of UNIX. Ports to
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UNIX-like systems such as QNX and Minix and to non-UNIX systems such
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as OS/2, Windows 95, and Windows NT are available.
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Bash includes the following features:
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Editing and Completion
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Bash offers a command-line editing facility which permits users to
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edit command lines using familiar emacs or vi-style editing commands.
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Editing allows corrections to be made without having to erase back
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to the point of error or start the command line anew. The editing
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facilities include a feature that allows users to complete command and
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The Bash line editing library is fully customizable. Users may define
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their own key bindings -- the action taken when a key is pressed. A
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number of variables to fine-tune editing behavior are also available.
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History and Command Re-entry
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The Bash history feature remembers commands entered to the shell and
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allows them to be recalled and re-executed. The history list may be
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of unlimited size. Bash allows users to search for previous commands
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and reuse portions of those commands when composing new ones. The
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history list may be saved across shell sessions.
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Bash allows users to control which commands are saved on the history
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On systems that support it, Bash provides an interface to the
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operating system's job control facilities, which allow processes
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to be suspended and restarted, and moved between the foreground
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and background. Bash allows users to selectively `forget' about
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Shell Functions and Aliases
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These mechanisms are available to bind a user-selected identifier to a
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list of commands that will be executed when the identifier is used as
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a command name. Functions allow local variables and recursion, and
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have access to the environment of the calling shell. Aliases may be
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used to create a mnemonic for a command name, expand a single word to
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a complex command, or ensure that a command is called with a basic set
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Bash-2.0 supports indexed arrays of unlimited size. The subscript for
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an array is an arithmetic expression. Arrays may be assigned to with
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a new compound assignment syntax, and several builtins have options to
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operate on array variables. Bash includes a number of built-in array
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Bash allows users to perform integer arithmetic in any base from two
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to sixty-four. Nearly all of the C language arithmetic operators are
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available with the same syntax and precedence as in C. Arithmetic
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expansion allows an arithmetic expression to be evaluated and the
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result substituted into the command line. Shell variables can be used
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as operands, and the value of an expression may be assigned to a
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An arithmetic expression may be used as a command; the exit status of
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the command is the value of the expression.
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There is a new quoting syntax that allows backslash-escaped characters
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in strings to be expanded according to the ANSI C standard.
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Users' home directories may be expanded using this feature. Words
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beginning with a tilde may also be expanded to the current or previous
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Brace expansion is a convenient way to generate a list of strings that
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share a common prefix or suffix.
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Substring Capabilities
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Bash allows new strings to be created by removing leading or trailing
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substrings from existing variable values, or by specifying a starting
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offset and length. Portions of variable values may be matched against
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shell patterns and the matching portion removed or a new value
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Indirect Variable Expansion
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Bash makes it easy to find the value of a shell variable whose name is
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the value of another variable.
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Expanded I/O Capabilities
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Bash provides several input and output features not available in sh,
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including the ability to:
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o specify a file or file descriptor for both input and output
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o read from or write to asynchronous processes using named pipes
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o read lines ending in backslash
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o display a prompt on the terminal before a read
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o format menus and interpret responses to them
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o echo lines exactly as input without escape processing
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Control of Builtin Commands
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Bash implements several builtin commands to give users more control
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over which commands are executed. The enable builtin allows other
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builtin commands to be selectively enabled or disabled. The command
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and builtin builtins change the order in which the shell searches for
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On systems that provide dynamic loading, new builtins may be loaded
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into a running shell from a shared object file. These new builtins
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have access to all of the shell facilities.
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Bash includes a built-in help facility.
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Shell Optional Behavior
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There is a great deal of customizable shell behavior. The shopt
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builtin command provides a unified interface that allows users to
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alter shell defaults.
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Bash allows the primary and secondary prompts to be customized by
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interpreting a number of backslash-escaped special characters.
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Parameter and variable expansion is also performed on the values of
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the primary and secondary prompt strings before they are displayed.
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Bash provides a restricted shell environment. It is also possible to
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control the execution of setuid/setgid scripts.
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Bash provides a `directory stack', to which directories may be added
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and removed. The current directory may be changed to any directory in
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the stack. It is easy to toggle between two directories in the stack.
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The directory stack may be saved and restored across different shell
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Bash is nearly completely conformant to POSIX.2. POSIX mode changes
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those few areas where the Bash default behavior differs from the
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standard to match the standard. In POSIX mode, Bash is POSIX.2
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Bash provides a new quoting syntax that allows strings to be
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translated according to the current locale. The locale in which the
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shell itself runs may also be changed, so that the shell messages
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themselves may be language-specific.
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The command-line editing facilities allow the input of eight-bit
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characters, so most of the ISO-8859 family of character sets are
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Bash allows external commands, shell builtin commands and shell functions
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to be timed. The format used to display the timing information may be
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removed
doc/INTRO
