Bash

This article is about the UNIX shell named Bash. For information about the popular website with humorous quotes, see Bash.org.

Bash syntax highlights

Bash's command syntax is a superset of the Bourne shell's command syntax. The definitive specification of Bash's command syntax is the man page that's installed along with Bash. This section highlights some of Bash's unique syntax features.

The vast majority of Bourne shell scripts can be executed without alteration by Bash, with the exception of those Bourne shell scripts that happen to reference a Bash special variable or to use a Bash builtin 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 supports completion of partly typed-in program names, filenames, variable names, etc. when the user presses the TAB key.

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

Integer mathematics

A major limitation of the Bourne shell is that it cannot perform integer calculations without spawning an external process. Bash can perform in-process integer calculations using the ((...)) command and the $[...] variable syntax, as follows:

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] # Multiple 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 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 shell capable of this is Korn Shell (1993 version).

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 which is simpler to type than the equivalent Bourne shell syntax, "command > file 2>&1". Bash can redirect standard input from a string using the following syntax:

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 6Catch Output of external commands

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

In-process regular expressions

[[ string =~ regex The regular expression syntax is the same as that documented by the regex(3) 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.

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:

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.

Bash 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.

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