Bourne Shell Builtins

The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX standard.

: (a colon)

: [arguments]

Do nothing beyond expanding arguments and performing redirections. The return status is zero.

. (a period)

. [-p path] filename [arguments]

The . command reads and execute commands from the filename argument in the current shell context.

If filename does not contain a slash, . searches for it. If -p is supplied, . treats path as a colon-separated list of directories in which to find filename; otherwise, . uses the directories in PATH to find filename. filename does not need to be executable. When Bash is not in POSIX mode, it searches the current directory if filename is not found in $PATH, but does not search the current directory if -p is supplied. If the sourcepath option (see The Shopt Builtin) is turned off, . does not search PATH.

If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged.

If the -T option is enabled, . inherits any trap on DEBUG; if it is not, any DEBUG trap string is saved and restored around the call to ., and . unsets the DEBUG trap while it executes. If -T is not set, and the sourced file changes the DEBUG trap, the new value persists after . completes. The return status is the exit status of the last command executed from filename, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to source.

break

break [n]

Exit from a for, while, until, or select loop. If n is supplied, break exits the nth enclosing loop. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

cd

cd [-L] [-@] [directory]
cd -P [-e] [-@] [directory]

Change the current working directory to directory. If directory is not supplied, the value of the HOME shell variable is used as directory. If the shell variable CDPATH exists, and directory does not begin with a slash, cd uses it as a search path: cd searches each directory name in CDPATH for directory, with alternative directory names in CDPATH separated by a colon (‘:’). A null directory name in CDPATH means the same thing as the current directory.

The -P option means not to follow symbolic links: symbolic links are resolved while cd is traversing directory and before processing an instance of .. in directory.

By default, or when the -L option is supplied, symbolic links in directory are resolved after cd processes an instance of .. in directory.

If .. appears in directory, cd processes it by removing the immediately preceding pathname component, back to a slash or the beginning of directory, and verifying that the portion of directory it has processed to that point is still a valid directory name after removing the pathname component. If it is not a valid directory name, cd returns a non-zero status.

If the -e option is supplied with -P and cd cannot successfully determine the current working directory after a successful directory change, it returns a non-zero status.

On systems that support it, the -@ option presents the extended attributes associated with a file as a directory.

If directory is ‘-’, it is converted to $OLDPWD before attempting the directory change.

If cd uses a non-empty directory name from CDPATH, or if ‘-’ is the first argument, and the directory change is successful, cd writes the absolute pathname of the new working directory to the standard output.

If the directory change is successful, cd sets the value of the PWD environment variable to the new directory name, and sets the OLDPWD environment variable to the value of the current working directory before the change.

The return status is zero if the directory is successfully changed, non-zero otherwise.

continue

continue [n]

continue resumes the next iteration of an enclosing for, while, until, or select loop. If n is supplied, Bash resumes the execution of the nth enclosing loop. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

eval

eval [arguments]

The arguments are concatenated together into a single command, separated by spaces. Bash then reads and executes this command and returns its exit status as the exit status of eval. If there are no arguments or only empty arguments, the return status is zero.

exec

exec [-cl] [-a name] [command [arguments]]

If command is supplied, it replaces the shell without creating a new process. command cannot be a shell builtin or function. The arguments become the arguments to command If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what the login program does. The -c option causes command to be executed with an empty environment. If -a is supplied, the shell passes name as the zeroth argument to command.

If command cannot be executed for some reason, a non-interactive shell exits, unless the execfail shell option is enabled. In that case, it returns a non-zero status. An interactive shell returns a non-zero status if the file cannot be executed. A subshell exits unconditionally if exec fails.

If command is not specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero.

exit

exit [n]

Exit the shell, returning a status of n to the shell’s parent. If n is omitted, the exit status is that of the last command executed. Any trap on EXIT is executed before the shell terminates.

export

export [-fn] [-p] [name[=value]]

Mark each name to be passed to subsequently executed commands in the environment. If the -f option is supplied, the names refer to shell functions; otherwise the names refer to shell variables.

The -n option means to unexport each name: no longer mark it for export. If no names are supplied, or if only the -p option is given, export displays a list of names of all exported variables on the standard output. Using -p and -f together displays exported functions. The -p option displays output in a form that may be reused as input.

export allows the value of a variable to be set at the same time it is exported or unexported by following the variable name with =value. This sets the value of the variable is to value while modifying the export attribute.

The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a shell function.

false

false

Does nothing; returns a non-zero status.

getopts

getopts optstring name [arg ...]

getopts is used by shell scripts or functions to parse positional parameters and obtain options and their arguments. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by whitespace. The colon (‘:’) and question mark (‘?’) may not be used as option characters.

Each time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG.

The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation to use a new set of parameters.

When it reaches the end of options, getopts exits with a return value greater than zero. OPTIND is set to the index of the first non-option argument, and name is set to ‘?’.

getopts normally parses the positional parameters, but if more arguments are supplied as arg values, getopts parses those instead.

getopts can report errors in two ways. If the first character of optstring is a colon, getopts uses silent error reporting. In normal operation, getopts prints diagnostic messages when it encounters invalid options or missing option arguments. If the variable OPTERR is set to 0, getopts does not display any error messages, even if the first character of optstring is not a colon.

If getopts detects an invalid option, it places ‘?’ into name and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, it assigns the option character found to OPTARG and does not print a diagnostic message.

If a required argument is not found, and getopts is not silent, it sets the value of name to a question mark (‘?’), unsets OPTARG, and prints a diagnostic message. If getopts is silent, it sets the value of name to a colon (‘:’), and sets OPTARG to the option character found.

getopts returns true if an option, specified or unspecified, is found. It returns false when it encounters the end of options or if an error occurs.

hash

hash [-r] [-p filename] [-dt] [name]

Each time hash is invoked, it remembers the full filenames of the commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in $PATH. Any previously-remembered filename associated with name is discarded. The -p option inhibits the path search, and hash uses filename as the location of name.

The -r option causes the shell to forget all remembered locations. Assigning to the PATH variable also clears all hashed filenames. The -d option causes the shell to forget the remembered location of each name.

If the -t option is supplied, hash prints the full pathname corresponding to each name. If multiple name arguments are supplied with -t, hash prints each name before the corresponding hashed full path. The -l option displays output in a format that may be reused as input.

If no arguments are given, or if only -l is supplied, hash prints information about remembered commands. The -t, -d, and -p options (the options that act on the name arguments) are mutually exclusive. Only one will be active. If more than one is supplied, -t has higher priority than -p, and both have higher priority than -d.

The return status is zero unless a name is not found or an invalid option is supplied.

pwd

pwd [-LP]

Print the absolute pathname of the current working directory. If the -P option is supplied, or the -o physical option to the set builtin (see The Set Builtin) is enabled, the pathname printed will not contain symbolic links. If the -L option is supplied, the pathname printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.

readonly

readonly [-aAf] [-p] [name[=value]] ...

Mark each name as readonly. The values of these names may not be changed by subsequent assignment or unset. If the -f option is supplied, each name refers to a shell function. The -a option means each name refers to an indexed array variable; the -A option means each name refers to an associative array variable. If both options are supplied, -A takes precedence. If no name arguments are supplied, or if the -p option is supplied, print a list of all readonly names. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option displays output in a format that may be reused as input.

readonly allows the value of a variable to be set at the same time the readonly attribute is changed by following the variable name with =value. This sets the value of the variable is to value while modifying the readonly attribute.

The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the -f option is supplied with a name that is not a shell function.

return

return [n]

Stop executing a shell function or sourced file and return the value n to its caller. If n is not supplied, the return value is the exit status of the last command executed. If return is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If return is executed during a DEBUG trap, the last command used to determine the status is the last command executed by the trap handler before return was invoked.

When return is used to terminate execution of a script being executed with the . (source) builtin, it returns either n or the exit status of the last command executed within the script as the exit status of the script. If n is supplied, the return value is its least significant 8 bits.

Any command associated with the RETURN trap is executed before execution resumes after the function or script.

The return status is non-zero if return is supplied a non-numeric argument or is used outside a function and not during the execution of a script by . or source.

shift

shift [n]

Shift the positional parameters to the left by n: the positional parameters from n+1 … $# are renamed to $1 … $#-n. Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is not supplied, it is assumed to be 1. If n is zero or greater than $#, the positional parameters are not changed. The return status is zero unless n is greater than $# or less than zero, non-zero otherwise.

test

[

test expr

Evaluate a conditional expression expr and return a status of 0 (true) or 1 (false). Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in Bash Conditional Expressions. test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end of options. When using the [ form, the last argument to the command must be a ].

Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. test uses operator precedence when there are five or more arguments.

! expr: True if expr is false.
( expr ): Returns the value of expr. This may be used to override normal operator precedence.
expr1 -a expr2: True if both expr1 and expr2 are true.
expr1 -o expr2: True if either expr1 or expr2 is true.

The test and [ builtins evaluate conditional expressions using a set of rules based on the number of arguments.

0 arguments

The expression is false.

1 argument

The expression is true if, and only if, the argument is not null.

2 arguments

If the first argument is ‘!’, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (see Bash Conditional Expressions), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false.

3 arguments

The following conditions are applied in the order listed.

If the second argument is one of the binary conditional operators (see Bash Conditional Expressions), the result of the expression is the result of the binary test using the first and third arguments as operands. The ‘-a’ and ‘-o’ operators are considered binary operators when there are three arguments.
If the first argument is ‘!’, the value is the negation of the two-argument test using the second and third arguments.
If the first argument is exactly ‘(’ and the third argument is exactly ‘)’, the result is the one-argument test of the second argument.
Otherwise, the expression is false.

4 arguments

The following conditions are applied in the order listed.

If the first argument is ‘!’, the result is the negation of the three-argument expression composed of the remaining arguments.
If the first argument is exactly ‘(’ and the fourth argument is exactly ‘)’, the result is the two-argument test of the second and third arguments.
Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.

5 or more arguments

The expression is parsed and evaluated according to precedence using the rules listed above.

If the shell is in POSIX mode, or if the expression is part of the [[ command, the ‘<’ and ‘>’ operators sort using the current locale. If the shell is not in POSIX mode, the test and ‘[’ commands sort lexicographically using ASCII ordering.

The historical operator-precedence parsing with 4 or more arguments can lead to ambiguities when it encounters strings that look like primaries. The POSIX standard has deprecated the -a and -o primaries and enclosing expressions within parentheses. Scripts should no longer use them. It’s much more reliable to restrict test invocations to a single primary, and to replace uses of -a and -o with the shell’s && and || list operators. For example, use

test -n string1 && test -n string2

instead of

test -n string1 -a -n string2

times

times

Print out the user and system times used by the shell and its children. The return status is zero.

trap

trap [-lpP] [action] [sigspec ...]

The action is a command that is read and executed when the shell receives any of the signals sigspec. If action is absent (and there is a single sigspec) or equal to ‘-’, each specified sigspec’s disposition is reset to the value it had when the shell was started. If action is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes.

If no arguments are supplied, trap prints the actions associated with each trapped signal as a set of trap commands that can be reused as shell input to restore the current signal dispositions.

If action is not present and -p has been supplied, trap displays the trap commands associated with each sigspec, or, if no sigspecs are supplied, for all trapped signals, as a set of trap commands that can be reused as shell input to restore the current signal dispositions. The -P option behaves similarly, but displays only the actions associated with each sigspec argument. -P requires at least one sigspec argument. The -P or -p options may be used in a subshell environment (e.g., command substitution) and, as long as they are used before trap is used to change a signal’s handling, will display the state of its parent’s traps.

The -l option prints a list of signal names and their corresponding numbers. Each sigspec is either a signal name or a signal number. Signal names are case insensitive and the SIG prefix is optional. If -l is supplied with no sigspec arguments, it prints a list of valid signal names.

If a sigspec is 0 or EXIT, action is executed when the shell exits. If a sigspec is DEBUG, action is executed before every simple command, for command, case command, select command, (( arithmetic command, [[ conditional command, arithmetic for command, and before the first command executes in a shell function. Refer to the description of the extdebug shell option (see The Shopt Builtin) for details of its effect on the DEBUG trap. If a sigspec is RETURN, action is executed each time a shell function or a script executed with the . or source builtins finishes executing.

If a sigspec is ERR, action is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following an until or while reserved word, part of the test following the if or elif reserved words, part of a command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, (subject to the state of the pipefail shell option), or if the command’s return status is being inverted using !. These are the same conditions obeyed by the errexit (-e) option.

When the shell is not interactive, signals ignored upon entry to a non-interactive shell cannot be trapped or reset. Interactive shells permit trapping signals ignored on entry. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created.

The return status is zero unless a sigspec does not specify a valid signal; non-zero otherwise.

true

true

Does nothing, returns a 0 status.

umask

umask [-p] [-S] [mode]

Set the shell process’s file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the chmod command. If mode is omitted, umask prints the current value of the mask. If the -S option is supplied without a mode argument, umask prints the mask in a symbolic format; the default output is an octal number. If the -p option is supplied, and mode is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no mode argument is supplied, and non-zero otherwise.

Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from 7. Thus, a umask of 022 results in permissions of 755.

unset

unset [-fnv] [name]

Remove each variable or function name. If the -v option is given, each name refers to a shell variable and that variable is removed. If the -f option is given, the names refer to shell functions, and the function definition is removed. If the -n option is supplied, and name is a variable with the nameref attribute, name will be unset rather than the variable it references. -n has no effect if the -f option is supplied. If no options are supplied, each name refers to a variable; if there is no variable by that name, a function with that name, if any, is unset. Readonly variables and functions may not be unset. When variables or functions are removed, they are also removed from the environment passed to subsequent commands. Some shell variables may not be unset. Some shell variables lose their special behavior if they are unset; such behavior is noted in the description of the individual variables. The return status is zero unless a name is readonly or may not be unset.