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)
Do nothing beyond expanding arguments and performing redirections. The return status is zero.
. (a period)
. filename [arguments]
Read and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the
PATHvariable is used to find filename. When Bash is not in POSIX mode, the current directory is searched if filename is not found in
$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,
sourceinherits any trap on
DEBUG; if it is not, any
DEBUGtrap string is saved and restored around the call to
DEBUGtrap while it executes. If -T is not set, and the sourced file changes the
DEBUGtrap, the new value is retained when
sourcecompletes. The return status is the exit status of the last command executed, 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
Exit from a
selectloop. If n is supplied, the nth enclosing loop is exited. 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 [-L|[-P [-e]] [[email protected]] [directory]
Change the current working directory to directory. If directory is not supplied, the value of the
HOMEshell variable is used. Any additional arguments following directory are ignored. If the shell variable
CDPATHexists, it is used as a search path: each directory name in
CDPATHis searched for directory, with alternative directory names in
CDPATHseparated by a colon (‘:’). If directory begins with a slash,
CDPATHis not used.
The -P option means to not follow symbolic links: symbolic links are resolved while
cdis 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
cdprocesses an instance of ‘..’ in directory.
If ‘..’ appears in directory, it is processed by removing the immediately preceding pathname component, back to a slash or the beginning of directory.
If the -e option is supplied with -P and the current working directory cannot be successfully determined after a successful directory change,
cdwill return an unsuccessful status.
On systems that support it, the [email protected] option presents the extended attributes associated with a file as a directory.
If directory is ‘-’, it is converted to
$OLDPWDbefore the directory change is attempted.
If a non-empty directory name from
CDPATHis used, or if ‘-’ is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output.
The return status is zero if the directory is successfully changed, non-zero otherwise.
Resume the next iteration of an enclosing
selectloop. If n is supplied, the execution of the nth enclosing loop is resumed. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.
The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of
eval. If there are no arguments or only empty arguments, the return status is zero.
exec [-cl] [-a name] [command [arguments]]
If command is supplied, it replaces the shell without creating a new process. 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
loginprogram 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
execfailshell option is enabled. In that case, it returns failure. An interactive shell returns failure if the file cannot be executed. A subshell exits unconditionally if
execfails. If no command is 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 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
EXITis executed before the shell terminates.
export [-fn] [-p] [name[=value]]
Mark each name to be passed to child processes 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 no longer mark each name for export. If no names are supplied, or if the -p option is given, a list of names of all exported variables is displayed. The -p option displays output in a form that may be reused as input. If a variable name is followed by =value, the value of the variable is set to value.
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.
getopts optstring name [args]
getoptsis used by shell scripts to parse positional parameters. 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,
getoptsplaces 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
OPTINDis initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument,
getoptsplaces that argument into the variable
OPTARG. The shell does not reset
OPTINDautomatically; it must be manually reset between multiple calls to
getoptswithin the same shell invocation if a new set of parameters is to be used.
When the end of options is encountered,
getoptsexits with a return value greater than zero.
OPTINDis set to the index of the first non-option argument, and name is set to ‘?’.
getoptsnormally parses the positional parameters, but if more arguments are given in args,
getoptsparses those instead.
getoptscan report errors in two ways. If the first character of optstring is a colon, silent error reporting is used. In normal operation, diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable
OPTERRis set to 0, no error messages will be displayed, even if the first character of
optstringis not a colon.
If an invalid option is seen,
getoptsplaces ‘?’ into name and, if not silent, prints an error message and unsets
getoptsis silent, the option character found is placed in
OPTARGand no diagnostic message is printed.
If a required argument is not found, and
getoptsis not silent, a question mark (‘?’) is placed in name,
OPTARGis unset, and a diagnostic message is printed. If
getoptsis silent, then a colon (‘:’) is placed in name and
OPTARGis set to the option character found.
hash [-r] [-p filename] [-dt] [name]
hashis invoked, it remembers the full pathnames 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 pathname is discarded. The -p option inhibits the path search, and filename is used as the location of name. The -r option causes the shell to forget all remembered locations. The -d option causes the shell to forget the remembered location of each name. If the -t option is supplied, the full pathname to which each name corresponds is printed. If multiple name arguments are supplied with -t, the name is printed before the hashed full pathname. The -l option causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only -l is supplied, information about remembered commands is printed. The return status is zero unless a name is not found or an invalid option is supplied.
Print the absolute pathname of the current working directory. If the -P option is supplied, 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 [-aAf] [-p] [name[=value]] …
Mark each name as readonly. The values of these names may not be changed by subsequent assignment. 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 given, or if the -p option is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =value, the value of the variable is set to value. 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.
Cause a shell function to stop executing 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 in the function. If
returnis executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If
returnis executed during a
DEBUGtrap, the last command used to determine the status is the last command executed by the trap handler before
returnmay also be used to terminate execution of a script being executed with the
source) builtin, returning 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
RETURNtrap is executed before execution resumes after the function or script. The return status is non-zero if
returnis supplied a non-numeric argument or is used outside a function and not during the execution of a script by
Shift the positional parameters to the left by n. The positional parameters from n+1 …
$#are renamed to
$#-n. Parameters represented by the numbers
$#-n+1 are unset. n must be a non-negative number less than or equal to
$#. If n is zero or greater than
$#, the positional parameters are not changed. If n is not supplied, it is assumed to be 1. The return status is zero unless n is greater than
$#or less than zero, non-zero otherwise.
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.
testdoes not accept any options, nor does it accept and ignore an argument of -- as signifying the end of options.
[form is used, 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. Operator precedence is used when there are five or more arguments.
True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
[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
If the first argument is ‘!’, the result is the negation of the three-argument expression composed of the remaining 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.
When used with
testor ‘[’, the ‘<’ and ‘>’ operators sort lexicographically using ASCII ordering.
Print out the user and system times used by the shell and its children. The return status is zero.
trap [-lp] [arg] [sigspec …]
The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent (and there is a single sigspec) or equal to ‘-’, each specified signal’s disposition is reset to the value it had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is not present and -p has been supplied, the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only -p is given,
trapprints the list of commands associated with each signal number in a form that may be reused as shell input. The -l option causes the shell to print 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
SIGprefix is optional.
If a sigspec is
EXIT, arg is executed when the shell exits. If a sigspec is
DEBUG, the command arg is executed before every simple command,
selectcommand, every arithmetic
forcommand, and before the first command executes in a shell function. Refer to the description of the
extdebugoption to the
shoptbuiltin (see The Shopt Builtin) for details of its effect on the
DEBUGtrap. If a sigspec is
RETURN, the command arg is executed each time a shell function or a script executed with the
sourcebuiltins finishes executing.
If a sigspec is
ERR, the command arg 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
ERRtrap is not executed if the failed command is part of the command list immediately following an
whilekeyword, part of the test following the
elifreserved words, part of a command executed in a
||list except the command following the final
||, any command in a pipeline but the last, or if the command’s return status is being inverted using
!. These are the same conditions obeyed by the
Signals ignored upon entry to the shell cannot be trapped or reset. 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.
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
chmodcommand. If mode is omitted, the current value of the mask is printed. If the -S option is supplied without a mode argument, the mask is printed in a symbolic format. 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
022results in permissions of
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, any function with that name is unset. Readonly variables and functions may not be unset. The return status is zero unless a name is readonly.
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Licensed under the GNU Free Documentation License.