Bash

From Wikipedia, the free encyclopedia

The correct title of this article is bash. The initial letter is shown capitalized due to technical restrictions.

This article is about the Unix shell named bash. For other uses, see Bash (disambiguation).

GNU Bourne-Again Shell

Screenshot of a sample bash session
Developer:	Chet Ramey
Latest release:	3.2.9 / December 12, 2006
OS:	Various
Use:	Unix shell
License:	GNU General Public License
Website:	Official website

bash is a Unix shell written for the GNU Project. Its name is an acronym for Bourne-again shell — a pun ("Bourne again" / "born again") on the name of the Bourne shell (sh), an early, 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 Tiger, 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, and to MS-DOS by the DJGPP project.

1 Syntax
2 Startup scripts
3 External links

[edit] Syntax

bash's command syntax is a superset of the Bourne shell's command syntax. The vast majority of Bourne shell scripts can be executed without alteration by bash, with the exception of those Bourne shell scripts that reference a Bourne special variable or use a Bourne built in command. The 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 being used as an interactive command shell, bash automatically completes partly typed program names, filenames, variable names, etc. when the user presses the TAB key.

bash's syntax has many extensions that the Bourne shell lacks. Several of those extensions are enumerated here.

[edit] Integer mathematics

bash can perform integer calculations without spawning an external process, unlike the Bourne shell. bash uses the ((...)) command and the $[...] variable syntax for this purpose:

VAR=55             # Assign integer 55 to variable VAR.
((VAR = VAR + 1))  # Add one to variable VAR.  Note the absence of the '$' character.
((++VAR))          # Another way to add one to VAR.  Performs C-style pre-increment.
((VAR++))          # Another way to add one to VAR.  Performs C-style post-increment.
echo $[VAR * 22]   # Multiply VAR by 22 and substitute the result into the command.
echo $((VAR * 22)) # Another way to do the above.

The ((...)) command can also be used in conditional statements, because its exit status is 0 or non-0 (most times 1) depending on whether the condition is true or false:

if ((VAR == Y * 3 + X * 2))
then
        echo Yes
fi

((Z > 23)) && echo Yes

The ((...)) command supports the following relational operators: '==', '!=', '>', '<', '>=', and '<='.

bash cannot perform in-process floating point calculations. The only Unix command shells currently capable of this are the Korn Shell and the Z shell.

[edit] I/O redirection

bash has several I/O redirection syntaxes that the traditional Bourne shell lacks. bash can redirect standard output and standard error at the same time using this syntax:

command &> file

This is simpler to type than the equivalent Bourne shell syntax, "command > file 2>&1". Since version 2.05b, bash can redirect standard input from a string using the following syntax (called "here strings", as opposed to the traditional "here documents"):

command <<< "string to be read as standard input"

If the string contains whitespace, it must be quoted.

Example: Redirect standard output to a file, write data, close file, reset stdout

# make Filedescriptor(FD) 6 a copy of stdout (FD 1)
exec 6>&1
# open file "test.data" for writing
exec 1>test.data
# produce some content
echo "data:data:data"
# close file "test.data"
exec 1>&-
# make stdout a copy of FD 6 (reset stdout)
exec 1>&6
# close FD6
exec 6>&-

Open and close files

# open file test.data for reading
exec 6<test.data
# read until end of file
while read -u 6 dta
do
  echo "$dta" 
done
# close file test.data
exec 6<&-

Catch output of external commands

 # execute 'find' and store results in VAR
 # search for filenames which end with the letter "h"
 VAR=$(find . -name "*h")

Read and write from network sockets

 # open a tcp socket on port 80 to en.wikipedia.org for read/write
 exec 5<>/dev/tcp/en.wikipedia.org/80
 # send a html request for the index page
 echo -e "GET / HTTP/1.0\n" >&5
 # output the response to the screen
 cat <&5
 # close output redirection for the socket
 exec 5>&-
 # close input redirection for the socket
 exec 5<&-

[edit] In-process regular expressions

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) man page. The exit status of the above command is 0 if the regex matches the string, 1 if it does not match. Parenthesized subexpressions in the regular expression can be accessed using the shell variable BASH_REMATCH, as follows:

if [[ abcfoobarbletch =~ 'foo(bar)bl(.*)' ]]
then
        echo The regex matches\!
        echo $BASH_REMATCH      -- outputs: foobarbletch
        echo ${BASH_REMATCH[1]} -- outputs: bar
        echo ${BASH_REMATCH[2]} -- outputs: etch
fi

This syntax gives performance superior to spawning a separate process to run a grep command, because the regular expression matching takes place within the bash process. If the regular expression or the string contain whitespace or shell metacharacters (such as '*' or '?'), they should be quoted.

[edit] Backslash escapes

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the C programming language. Backslash escape sequences, if present, are decoded as follows:

Backslash Escapes
Backslash escape	Expands to ...
`\a`	An alert (bell) character
`\b`	A backspace character
`\e`	An escape character
`\f`	A form feed character
`\n`	A new line character
`\r`	A carriage return character
`\t`	A horizontal tab character
`\v`	A vertical tab character
`\\`	A backslash character
`\'`	A single quote character
`\nnn`	The eight-bit character whose value is the octal value nnn (one to three digits)
`\xHH`	The eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
`\cx`	A control-X character

The expanded result is single-quoted, as if the dollar sign had not been present.

A double-quoted string preceded by a dollar sign ($"...") will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.

[edit] Startup scripts

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

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.