source: trunk/third/bash/doc/INTRO @ 12959

                       BASH - The Bourne-Again Shell

Bash is the shell, or command language interpreter, that will appear
in the GNU operating system.  Bash is an sh-compatible shell that
incorporates useful features from the Korn shell (ksh) and C shell
(csh).  It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2
Shell and Tools standard.  It offers functional improvements over sh
for both programming and interactive use.  In addition, most sh scripts
can be run by Bash without modification. 

Bash is quite portable.  It uses a configuration system that discovers
characteristics of the compilation platform at build time, and may
therefore be built on nearly every version of UNIX.  Ports to
UNIX-like systems such as QNX and Minix and to non-UNIX systems such
as OS/2, Windows 95, and Windows NT are available. 

Bash includes the following features:

Editing and Completion

Bash offers a command-line editing facility which permits users to
edit command lines using familiar emacs or vi-style editing commands.
Editing allows corrections to be made without having to erase back
to the point of error or start the command line anew.  The editing
facilities include a feature that allows users to complete command and
file names.

The Bash line editing library is fully customizable.  Users may define
their own key bindings -- the action taken when a key is pressed.  A
number of variables to fine-tune editing behavior are also available.

History and Command Re-entry

The Bash history feature remembers commands entered to the shell and
allows them to be recalled and re-executed.  The history list may be
of unlimited size.  Bash allows users to search for previous commands
and reuse portions of those commands when composing new ones.  The
history list may be saved across shell sessions. 

Bash allows users to control which commands are saved on the history
list.

Job Control

On systems that support it, Bash provides an interface to the
operating system's job control facilities, which allow processes
to be suspended and restarted, and moved between the foreground
and background.  Bash allows users to selectively `forget' about
background jobs. 

Shell Functions and Aliases

These mechanisms are available to bind a user-selected identifier to a
list of commands that will be executed when the identifier is used as
a command name.  Functions allow local variables and recursion, and
have access to the environment of the calling shell.  Aliases may be
used to create a mnemonic for a command name, expand a single word to
a complex command, or ensure that a command is called with a basic set
of options. 

Arrays

Bash-2.0 supports indexed arrays of unlimited size.  The subscript for
an array is an arithmetic expression.  Arrays may be assigned to with
a new compound assignment syntax, and several builtins have options to
operate on array variables.  Bash includes a number of built-in array
variables.

Arithmetic

Bash allows users to perform integer arithmetic in any base from two
to sixty-four.  Nearly all of the C language arithmetic operators are
available with the same syntax and precedence as in C.  Arithmetic
expansion allows an arithmetic expression to be evaluated and the
result substituted into the command line.  Shell variables can be used
as operands, and the value of an expression may be assigned to a
variable. 

An arithmetic expression may be used as a command; the exit status of
the command is the value of the expression.

ANSI-C Quoting

There is a new quoting syntax that allows backslash-escaped characters
in strings to be expanded according to the ANSI C standard.

Tilde Expansion

Users' home directories may be expanded using this feature.  Words
beginning with a tilde may also be expanded to the current or previous
working directory.

Brace Expansion

Brace expansion is a convenient way to generate a list of strings that
share a common prefix or suffix.

Substring Capabilities

Bash allows new strings to be created by removing leading or trailing
substrings from existing variable values, or by specifying a starting
offset and length.  Portions of variable values may be matched against
shell patterns and the matching portion removed or a new value
substituted. 

Indirect Variable Expansion

Bash makes it easy to find the value of a shell variable whose name is
the value of another variable.

Expanded I/O Capabilities

Bash provides several input and output features not available in sh,
including the ability to:

        o specify a file or file descriptor for both input and output
        o read from or write to asynchronous processes using named pipes
        o read lines ending in backslash
        o display a prompt on the terminal before a read
        o format menus and interpret responses to them
        o echo lines exactly as input without escape processing

Control of Builtin Commands

Bash implements several builtin commands to give users more control
over which commands are executed.  The enable builtin allows other
builtin commands to be selectively enabled or disabled.  The command
and builtin builtins change the order in which the shell searches for
commands. 

On systems that provide dynamic loading, new builtins may be loaded
into a running shell from a shared object file.  These new builtins
have access to all of the shell facilities.

Help

Bash includes a built-in help facility.

Shell Optional Behavior

There is a great deal of customizable shell behavior.  The shopt
builtin command provides a unified interface that allows users to
alter shell defaults. 

Prompt Customization

Bash allows the primary and secondary prompts to be customized by
interpreting a number of backslash-escaped special characters. 
Parameter and variable expansion is also performed on the values of
the primary and secondary prompt strings before they are displayed. 

Security

Bash provides a restricted shell environment.  It is also possible to
control the execution of setuid/setgid scripts. 

Directory Stack

Bash provides a `directory stack', to which directories may be added
and removed.  The current directory may be changed to any directory in
the stack.  It is easy to toggle between two directories in the stack. 
The directory stack may be saved and restored across different shell
invocations. 

POSIX Mode

Bash is nearly completely conformant to POSIX.2.  POSIX mode changes
those few areas where the Bash default behavior differs from the
standard to match the standard.  In POSIX mode, Bash is POSIX.2
compliant. 

Internationalization

Bash provides a new quoting syntax that allows strings to be
translated according to the current locale.  The locale in which the
shell itself runs may also be changed, so that the shell messages
themselves may be language-specific. 

The command-line editing facilities allow the input of eight-bit
characters, so most of the ISO-8859 family of character sets are
supported. 

Command Timing

Bash allows external commands, shell builtin commands and shell functions
to be timed.  The format used to display the timing information may be
changed by the user.