The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard.
:
: [arguments]Do nothing beyond expanding arguments and performing redirections. The return status is zero.
.
. filenameRead and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the
$PATH
variable is used to find
filename. The current directory is searched if filename
is not found in $PATH
.
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.
break
break [n]Exit from a
for
, while
, until
, or select
loop.
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
cd [-LP] [directory]Change the current working directory to directory. If directory is not given, the value of the
HOME
shell variable is used. If the
shell variable CDPATH
exists, it is used as a search path. If
directory begins with a slash, CDPATH
is not used.
The `-P' option means
to not follow symbolic links; symbolic links are followed by default
or with the `-L' option.
If directory is `-', it is equivalent to $OLDPWD
.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
continue
continue [n]Resume the next iteration of an enclosing
for
, while
,
until
, or select
loop.
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.
eval
eval [arguments]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
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 in the zeroth arg 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 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
exit [n]Exit the shell, returning a status of n to the shell's parent. Any trap on
EXIT
is executed before the shell terminates.
export
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 exported names is displayed. The `-p' option displays output in a form that may be reused as input. 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
getopts optstring name [args]
getopts
is used by shell scripts to parse positional parameters.
optstring contains the option letters to be recognized; if a letter
is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
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 if a new set of parameters is to be used.
When the end of options is encountered, 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
given in args, getopts
parses those instead.
getopts
can 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 OPTERR
is set to 0, no error messages will be displayed, even if the first
character of optstring
is not a colon.
If an invalid option is seen,
getopts
places `?' into name and, if not silent,
prints an error message and unsets OPTARG
.
If getopts
is silent, the option character found is placed in
OPTARG
and no diagnostic message is printed.
If a required argument is not found, and getopts
is not silent, a question mark (`?') is placed in name,
OPTARG
is unset, and a diagnostic message is printed.
If getopts
is silent, then a colon (`:') is placed in
name and OPTARG
is set to the option character found.
hash
hash [-r] [-p filename] [name]Remember the full pathnames of 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
.
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.
If no arguments are given, information about remembered commands is printed.
The return status is zero unless a name is not found or an invalid
option is supplied.
pwd
pwd [-LP]Print the current working directory. If the `-P' option is supplied, the path printed will not contain symbolic links. If the `-L' option is supplied, the path 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 [-apf] [name] ...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 array variable. If no name arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed. The `-p' option causes output to be displayed in a format that may be reused as input. 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]Cause a shell function to exit with the return value n. This may also be used to terminate execution of a script being executed with the
.
builtin, returning either n or the exit status
of the last command executed within the script as the exit status of the
script.
The return status is false if return
is used outside a function
and not during the execution of a script by `.'.
shift
shift [n]Shift the positional parameters to the left by n. The positional parameters from n+1 ...
$#
are
renamed to $1
... $#
-n+1.
Parameters represented by the numbers $#
to 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.
The return status is zero unless n is greater than $#
or
less than zero, non-zero otherwise.
test
[
! expr
( expr )
expr1 -a expr2
expr1 -o expr2
test
and [
builtins evaluate conditional
expressions using a set of rules based on the number of arguments.
times
timesPrint out the user and system times used by the shell and its children. The return status is zero.
trap
trap [-lp] [arg] [sigspec ...]The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent or equal to `-', all specified signals are reset to the values they 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 `-p', the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only `-p' is given,
trap
prints the list of commands
associated with each signal number in a form that may be reused as
shell input.
Each sigspec is either a signal name such as SIGINT
(with
or without the SIG
prefix) or a signal number.
If a sigspec
is 0
or EXIT
, arg is executed when the shell exits.
If a sigspec is DEBUG
, the command arg is executed
after every simple command.
The `-l' option causes the shell to print a list of signal names
and their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child
process when it is created.
The return status is zero unless a sigspec does not specify a
valid signal.
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, 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.
unset
unset [-fv] [name]Each variable or function name is removed. If no options are supplied, or the `-v' option is given, each name refers to a shell variable. If the `-f' option is given, the names refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a name does not exist or is readonly.
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