Posts Tagged ‘bash shell’

Bash Shell

Saturday, May 8th, 2010

Bash is a Unix shell written for the GNU Project. The name of the actual executable is bash. Its name is an acronym for Bourne-again shell, a pun on the name of the Bourne shell (sh) (i.e. “Bourne again” or “born again”), an early and important Unix shell written by Stephen Bourne and distributed with Version 7 Unix circa 1978. Bash was created in 1987 by Brian Fox. In 1990 Chet Ramey became the primary maintainer.

Bash is the default shell on most Linux systems as well as on Mac OS X and it can be run on most Unix-like operating systems. It has also been ported to Microsoft Windows within the Cygwin POSIX emulation environment for Windows, to MS-DOS by the DJGPP project and to Novell NetWare. Released under the GNU General Public License, Bash is free software.

Features
The Bash command syntax is a superset of the Bourne shell command syntax. The vast majority of Bourne shell scripts can be executed by Bash without modification, with the exception of Bourne shell scripts referencing a Bourne special variable or those using builtin Bourne commands. Bash command syntax includes ideas drawn from the Korn shell (ksh) and the C shell (csh) such as command line editing, command history, the directory stack, the $RANDOM and $PPID variables, and POSIX command substitution syntax: $(…). When used as an interactive command shell and pressing the tab key, Bash automatically completes partly typed program names, filenames, variable names, etc.

Bash’s syntax has many extensions which the Bourne shell lacks. Several are enumerated here.

Bash can perform integer calculations without spawning external processes, unlike the Bourne shell. Bash uses the ((…)) command and the $[…] variable syntax for this purpose.

Bash syntax simplifies I/O redirection in ways which are not possible in the traditional Bourne shell. For example, Bash can redirect standard output and standard error at the same time using the following syntax.

Startup scripts
When Bash starts, it executes the commands in a variety of different scripts.

When Bash is invoked as an interactive login shell, 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. One can force this behavior for a non-interactive shell with the –login option. The –noprofile option may be used 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.

command >& file
This is simpler to type than the Bourne shell equivalent ‘command > file 2>&1’.

Bash supports here documents just as the Bourne shell always has. However, since version 2.05b Bash can redirect standard input from a “here string” using the following syntax.

command <<< "string to be read as standard input" Bash 3.0 supports in-process regular expression matching using the following syntax, reminiscent of Perl: [[ string =~ regex ]] The regular expression syntax is the same as that documented by the regex(7)