4.1 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)

. filename [arguments]

Read 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.
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.
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 source.

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

Change the current working directory to directory.
If directory is not supplied, the value of the HOME
shell variable is used.
Any additional arguments following directory are ignored.
If the shell variable
CDPATH exists, it is used as a search path:
each directory name in CDPATH is searched for
directory, with alternative directory names in CDPATH
separated by a colon (‘:’).
If directory begins with a slash, CDPATH is not used.

The -P option means to not 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, 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, cd will return an unsuccessful
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 the directory change is attempted.

If a non-empty directory name from CDPATH is 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.

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 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 failure.
An interactive shell returns failure if the file cannot be executed.
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.
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 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

getopts optstringname [args]

getopts is 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, 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] [-dt] [name]

Each time hash is 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.

pwd

pwd [-LP]

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

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.

return

return [n]

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.
return may 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 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 $# 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.
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.

test

[

test expr

Evaluate a conditional express
ion 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 the [ 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.

! expr

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.

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

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 test or ‘[’, the ‘<’ and ‘>’
operators sort lexicographically using ASCII ordering.

times

times

Print 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 (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, trap prints 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 SIG prefix is optional.

If a sigspec
is 0 or EXIT, arg is executed when the shell exits.
If a sigspec is DEBUG, the command arg is executed
before every simple command, for command, case command,
select command, every arithmetic for command, and before
the first command executes in a shell function.
Refer to the description of the extdebug option to the
shopt builtin (see The Shopt Builtin) for details of its
effect on the DEBUG trap.
If a sigspec is RETURN, the command arg is executed
each time a shell function or a script executed with the . or
source builtins 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 ERR trap is not executed if the failed command is part of the
command list immediately following an until or while keyword,
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,
or if the command’s return
status is being inverted using !.
These are the same conditions obeyed by the errexit (-e)
option.

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

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.

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 remvoved.
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.