bash(1) - Waikato Linux Users Group

Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and csh).

Bash is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2).

OPTIONS

In addition to the single-character shell options documented in the description of the set builtin command, bash interprets the following options when it is invoked:

-c string: If the -c option is present, then commands are read from string . If there are arguments after the string , they are assigned to the positional parameters, starting with $0 .
-r: If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below).
-i: If the -i option is present, the shell is interactive .
-s: If the -s option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell.
-D: A list of all double-quoted strings preceded by $ is printed on the standard ouput. These are the strings that are subject to language translation when the current locale is not C or POSIX. This implies the -n option; no commands will be executed.
-: A - signals the end of options and disables further option processing. Any arguments after the - are treated as filenames and arguments. An argument of - is equivalent to -.

Bash also interprets a number of multi-character options. These options must appear on the command line before the single-character options in order for them to be recognized.

-dump-po-strings: Equivalent to -D, but the output is in the GNU gettext po (portable object) file format.
-dump-strings: Equivalent to -D.
-help: Display a usage message on standard output and exit successfully.

;-init-file file: ;-rcfile file :

Execute commands from file instead of the standard personal initialization file /.bashrc if the shell is interactive (see INVOCATION below).

-login: Make bash act as if it had been invoked as a login shell (see INVOCATION below).
-noediting: Do not use the GNU readline library to read command lines when the shell is interactive.
-noprofile: Do not read either the system-wide startup file /etc/profile or any of the personal initialization files /.bash_profile , /.bash_login , or /.profile . By default, bash reads these files when it is invoked as a login shell (see INVOCATION below).
-norc: Do not read and execute the personal initialization file /.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh .
-posix: Change the behavior of bash where the default operation differs from the POSIX 1003.2 standard to match the standard (posix mode).
-restricted: The shell becomes restricted (see RESTRICTED SHELL below).
-verbose: Equivalent to -v.
-version: Show version information for this instance of bash on the standard output and exit successfully.

ARGUMENTS

If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is assumed to be the name of a file containing shell commands. If bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0.

INVOCATION

A login shell is one whose first character of argument zero is a - , or one started with the -login option.

An interactive shell is one started without non-option arguments and without the -c option whose standard input and output are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.

The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in file names as described below under Tilde Expansion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non-interactive shell with the -login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for /.bash_profile, /.bash_login, and /.profile, in that order, and reads and executes commands from the first one that exists and is readable. The -noprofile option may be used when the shell is started to inhibit this behavior.

When a login shell exits, bash reads and executes commands from the file /.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash reads and executes commands from /.bashrc, if that file exists. This may be inhibited by using the -norc option. The -rcfile file option will force bash to read and execute commands from file instead of /.bashrc.

When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed: if [ -n "$BASH_ENV"?; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the file name.

If bash is invoked with the name sh , it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive shell with the -login option, it first attempts to read and execute commands from /etc/profile and /.profile , in that order. The -noprofile option may be used to inhibit this behavior. When invoked as an interactive shell with the name sh , bash looks for the variable ENV , expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the -rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files. When invoked as sh , bash enters posix mode after the startup files are read.

When bash is started in posix mode, as with the -posix command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.

Bash attempts to determine when it is being run by the remote shell daemon, usually rshd. If bash determines it is being run by rshd, it reads and executes commands from /.bashrc, if that file exists and is readable. It will not do this if invoked as sh. The -norc option may be used to inhibit this behavior, and the -rcfile option may be used to force another file to be read, but rshd does not generally invoke the shell with those options or allow them to be specified.

If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS variable, if it appears in the environment, is ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.

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Last edited on Friday, November 21, 2003 1:55:47 pm by JohnMcPherson

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