Bash Builtins (Bash Reference Manual)
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4.2 Bash Builtin Commands
This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX standard.
alias
alias [-p] [name[=value] …]
Without arguments or with the
-p
option,alias
prints the list of aliases on the standard output in a form that allows them to be reused as input. If arguments are supplied, an alias is defined for eachname
whosevalue
is given. If novalue
is given, the name and value of the alias is printed. Aliases are described in Aliases.bind
bind [-m keymap] [-lpsvPSVX] bind [-m keymap] [-q function] [-u function] [-r keyseq] bind [-m keymap] -f filename bind [-m keymap] -x keyseq:shell-command bind [-m keymap] keyseq:function-name bind [-m keymap] keyseq:readline-command
Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument; e.g., ‘
"\C-x\C-r":re-read-init-file
’.Options, if supplied, have the following meanings:
-m keymap
Use
keymap
as the keymap to be affected by the subsequent bindings. Acceptablekeymap
names areemacs
,emacs-standard
,emacs-meta
,emacs-ctlx
,vi
,vi-move
,vi-command
, andvi-insert
.vi
is equivalent tovi-command
(vi-move
is also a synonym);emacs
is equivalent toemacs-standard
.-l
List the names of all Readline functions.
-p
Display Readline function names and bindings in such a way that they can be used as input or in a Readline initialization file.
-P
List current Readline function names and bindings.
-v
Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file.
-V
List current Readline variable names and values.
-s
Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as input or in a Readline initialization file.
-S
Display Readline key sequences bound to macros and the strings they output.
-f filename
Read key bindings from
filename
.-q function
Query about which keys invoke the named
function
.-u function
Unbind all keys bound to the named
function
.-r keyseq
Remove any current binding for
keyseq
.-x keyseq:shell-command
Cause
shell-command
to be executed wheneverkeyseq
is entered. Whenshell-command
is executed, the shell sets theREADLINE_LINE
variable to the contents of the Readline line buffer and theREADLINE_POINT
andREADLINE_MARK
variables to the current location of the insertion point and the saved insertion point (themark
), respectively. If the executed command changes the value of any ofREADLINE_LINE
,READLINE_POINT
, orREADLINE_MARK
, those new values will be reflected in the editing state.-X
List all key sequences bound to shell commands and the associated commands in a format that can be reused as input.
The return status is zero unless an invalid option is supplied or an error occurs.
builtin
builtin [shell-builtin [args]]
Run a shell builtin, passing it
args
, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero ifshell-builtin
is not a shell builtin command.caller
caller [expr]
Returns the context of any active subroutine call (a shell function or a script executed with the
.
orsource
builtins).Without
expr
,caller
displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied asexpr
,caller
displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0.The return value is 0 unless the shell is not executing a subroutine call or
expr
does not correspond to a valid position in the call stack.command
command [-pVv] command [arguments …]
Runs
command
witharguments
ignoring any shell function namedcommand
. Only shell builtin commands or commands found by searching thePATH
are executed. If there is a shell function namedls
, running ‘command ls
’ within the function will execute the external commandls
instead of calling the function recursively. The-p
option means to use a default value forPATH
that is guaranteed to find all of the standard utilities. The return status in this case is 127 ifcommand
cannot be found or an error occurred, and the exit status ofcommand
otherwise.If either the
-V
or-v
option is supplied, a description ofcommand
is printed. The-v
option causes a single word indicating the command or file name used to invokecommand
to be displayed; the-V
option produces a more verbose description. In this case, the return status is zero ifcommand
is found, and non-zero if not.declare
declare [-aAfFgiIlnrtux] [-p] [name[=value] …]
Declare variables and give them attributes. If no
name
s are given, then display the values of variables instead.The
-p
option will display the attributes and values of eachname
. When-p
is used withname
arguments, additional options, other than-f
and-F
, are ignored.When
-p
is supplied withoutname
arguments,declare
will display the attributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with-p
,declare
will display the attributes and values of all shell variables. The-f
option will restrict the display to shell functions.The
-F
option inhibits the display of function definitions; only the function name and attributes are printed. If theextdebug
shell option is enabled usingshopt
(see The Shopt Builtin), the source file name and line number where eachname
is defined are displayed as well.-F
implies-f
.The
-g
option forces variables to be created or modified at the global scope, even whendeclare
is executed in a shell function. It is ignored in all other cases.The
-I
option causes local variables to inherit the attributes (except thenameref
attribute) and value of any existing variable with the samename
at a surrounding scope. If there is no existing variable, the local variable is initially unset.The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:
-a
Each
name
is an indexed array variable (see Arrays).-A
Each
name
is an associative array variable (see Arrays).-f
Use function names only.
-i
The variable is to be treated as an integer; arithmetic evaluation (see Shell Arithmetic) is performed when the variable is assigned a value.
-l
When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.
-n
Give each
name
thenameref
attribute, making it a name reference to another variable. That other variable is defined by the value ofname
. All references, assignments, and attribute modifications toname
, except for those using or changing the-n
attribute itself, are performed on the variable referenced byname
’s value. The nameref attribute cannot be applied to array variables.-r
Make
name
s readonly. These names cannot then be assigned values by subsequent assignment statements or unset.-t
Give each
name
thetrace
attribute. Traced functions inherit theDEBUG
andRETURN
traps from the calling shell. The trace attribute has no special meaning for variables.-u
When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled.
-x
Mark each
name
for export to subsequent commands via the environment.
Using ‘
+
’ instead of ‘-
’ turns off the attribute instead, with the exceptions that ‘+a
’ and ‘+A
’ may not be used to destroy array variables and ‘+r
’ will not remove the readonly attribute. When used in a function,declare
makes eachname
local, as with thelocal
command, unless the-g
option is used. If a variable name is followed by =value
, the value of the variable is set tovalue
.When using
-a
or-A
and the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments.The return status is zero unless an invalid option is encountered, an attempt is made to define a function using ‘
-f foo=bar
’, an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see Arrays), one of thenames
is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with-f
.echo
echo [-neE] [arg …]
Output the
arg
s, separated by spaces, terminated with a newline. The return status is 0 unless a write error occurs. If-n
is specified, the trailing newline is suppressed. If the-e
option is given, interpretation of the following backslash-escaped characters is enabled. The-E
option disables the interpretation of these escape characters, even on systems where they are interpreted by default. Thexpg_echo
shell option may be used to dynamically determine whether or notecho
expands these escape characters by default.echo
does not interpret--
to mean the end of options.echo
interprets the following escape sequences:\a
alert (bell)
\b
backspace
\c
suppress further output
\e
\E
escape
\f
form feed
\n
new line
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\0nnn
the eight-bit character whose value is the octal value
nnn
(zero to three octal digits)\xHH
the eight-bit character whose value is the hexadecimal value
HH
(one or two hex digits)\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH
(one to four hex digits)\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH
(one to eight hex digits)
enable
enable [-a] [-dnps] [-f filename] [name …]
Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before disk commands. If
-n
is used, thename
s become disabled. Otherwisename
s are enabled. For example, to use thetest
binary found via$PATH
instead of the shell builtin version, type ‘enable -n test
’.If the
-p
option is supplied, or noname
arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The-a
option means to list each builtin with an indication of whether or not it is enabled.The
-f
option means to load the new builtin commandname
from shared objectfilename
, on systems that support dynamic loading. The-d
option will delete a builtin loaded with-f
.If there are no options, a list of the shell builtins is displayed. The
-s
option restrictsenable
to the POSIX special builtins. If-s
is used with-f
, the new builtin becomes a special builtin (see Special Builtins).The return status is zero unless a
name
is not a shell builtin or there is an error loading a new builtin from a shared object.help
help [-dms] [pattern]
Display helpful information about builtin commands. If
pattern
is specified,help
gives detailed help on all commands matchingpattern
, otherwise a list of the builtins is printed.Options, if supplied, have the following meanings:
-d
Display a short description of each
pattern
-m
Display the description of each
pattern
in a manpage-like format-s
Display only a short usage synopsis for each
pattern
The return status is zero unless no command matches
pattern
.let
let expression [expression …]
The
let
builtin allows arithmetic to be performed on shell variables. Eachexpression
is evaluated according to the rules given below in Shell Arithmetic. If the lastexpression
evaluates to 0,let
returns 1; otherwise 0 is returned.local
local [option] name[=value] …
For each argument, a local variable named
name
is created, and assignedvalue
. Theoption
can be any of the options accepted bydeclare
.local
can only be used within a function; it makes the variablename
have a visible scope restricted to that function and its children. Ifname
is ‘-
’, the set of shell options is made local to the function in whichlocal
is invoked: shell options changed using theset
builtin inside the function are restored to their original values when the function returns. The restore is effected as if a series ofset
commands were executed to restore the values that were in place before the function. The return status is zero unlesslocal
is used outside a function, an invalidname
is supplied, orname
is a readonly variable.logout
logout [n]
Exit a login shell, returning a status of
n
to the shell’s parent.mapfile
mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable
array
, or from file descriptorfd
if the-u
option is supplied. The variableMAPFILE
is the defaultarray
. Options, if supplied, have the following meanings:-d
The first character of
delim
is used to terminate each input line, rather than newline. Ifdelim
is the empty string,mapfile
will terminate a line when it reads a NUL character.-n
Copy at most
count
lines. Ifcount
is 0, all lines are copied.-O
Begin assigning to
array
at indexorigin
. The default index is 0.-s
Discard the first
count
lines read.-t
Remove a trailing
delim
(default newline) from each line read.-u
Read lines from file descriptor
fd
instead of the standard input.-C
Evaluate
callback
each timequantum
lines are read. The-c
option specifiesquantum
.-c
Specify the number of lines read between each call to
callback
.
If
-C
is specified without-c
, the default quantum is 5000. Whencallback
is evaluated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments.callback
is evaluated after the line is read but before the array element is assigned.If not supplied with an explicit origin,
mapfile
will cleararray
before assigning to it.mapfile
returns successfully unless an invalid option or option argument is supplied,array
is invalid or unassignable, orarray
is not an indexed array.printf
printf [-v var] format [arguments]
Write the formatted
arguments
to the standard output under the control of theformat
. The-v
option causes the output to be assigned to the variablevar
rather than being printed to the standard output.The
format
is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successiveargument
. In addition to the standardprintf(1)
formats,printf
interprets the following extensions:%b
Causes
printf
to expand backslash escape sequences in the correspondingargument
in the same way asecho -e
(see Bash Builtins).%q
Causes
printf
to output the correspondingargument
in a format that can be reused as shell input.%(datefmt)T
Causes
printf
to output the date-time string resulting from usingdatefmt
as a format string forstrftime
(3). The correspondingargument
is an integer representing the number of seconds since the epoch. Two special argument values may be used: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been given. This is an exception to the usualprintf
behavior.
The %b, %q, and %T directives all use the field width and precision arguments from the format specification and write that many bytes from (or use that wide a field for) the expanded argument, which usually contains more characters than the original.
Arguments to non-string format specifiers are treated as C language constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the ASCII value of the following character.
The
format
is reused as necessary to consume all of thearguments
. If theformat
requires morearguments
than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero on failure.read
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name …]
One line is read from the standard input, or from the file descriptor
fd
supplied as an argument to the-u
option, split into words as described above in Word Splitting, and the first word is assigned to the firstname
, the second word to the secondname
, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the lastname
. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of theIFS
variable are used to split the line into words using the same rules the shell uses for expansion (described above in Word Splitting). The backslash character ‘\
’ may be used to remove any special meaning for the next character read and for line continuation.Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the array variable
aname
, starting at 0. All elements are removed fromaname
before the assignment. Othername
arguments are ignored.-d delim
The first character of
delim
is used to terminate the input line, rather than newline. Ifdelim
is the empty string,read
will terminate a line when it reads a NUL character.-e
Readline (see Command Line Editing) is used to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Readline’s default filename completion.
-i text
If Readline is being used to read the line,
text
is placed into the editing buffer before editing begins.-n nchars
read
returns after readingnchars
characters rather than waiting for a complete line of input, but honors a delimiter if fewer thannchars
characters are read before the delimiter.-N nchars
read
returns after reading exactlynchars
characters rather than waiting for a complete line of input, unless EOF is encountered orread
times out. Delimiter characters encountered in the input are not treated specially and do not causeread
to return untilnchars
characters are read. The result is not split on the characters inIFS
; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the-r
option below).-p prompt
Display
prompt
, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal.-r
If this option is given, backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not then be used as a line continuation.
-s
Silent mode. If input is coming from a terminal, characters are not echoed.
-t timeout
Cause
read
to time out and return failure if a complete line of input (or a specified number of characters) is not read withintimeout
seconds.timeout
may be a decimal number with a fractional portion following the decimal point. This option is only effective ifread
is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. Ifread
times out,read
saves any partial input read into the specified variablename
. Iftimeout
is 0,read
returns immediately, without trying to read any data. The exit status is 0 if input is available on the specified file descriptor, non-zero otherwise. The exit status is greater than 128 if the timeout is exceeded.-u fd
Read input from file descriptor
fd
.
If no
name
s are supplied, the line read, without the ending delimiter but otherwise unmodified, is assigned to the variableREPLY
. The exit status is zero, unless end-of-file is encountered,read
times out (in which case the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as the argument to-u
.readarray
readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable
array
, or from file descriptorfd
if the-u
option is supplied.A synonym for
mapfile
.source
source filename
A synonym for
.
(see Bourne Shell Builtins).type
type [-afptP] [name …]
For each
name
, indicate how it would be interpreted if used as a command name.If the
-t
option is used,type
prints a single word which is one of ‘alias
’, ‘function
’, ‘builtin
’, ‘file
’ or ‘keyword
’, ifname
is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If thename
is not found, then nothing is printed, andtype
returns a failure status.If the
-p
option is used,type
either returns the name of the disk file that would be executed, or nothing if-t
would not return ‘file
’.The
-P
option forces a path search for eachname
, even if-t
would not return ‘file
’.If a command is hashed,
-p
and-P
print the hashed value, which is not necessarily the file that appears first in$PATH
.If the
-a
option is used,type
returns all of the places that contain an executable namedfile
. This includes aliases and functions, if and only if the-p
option is not also used.If the
-f
option is used,type
does not attempt to find shell functions, as with thecommand
builtin.The return status is zero if all of the
names
are found, non-zero if any are not found.typeset
typeset [-afFgrxilnrtux] [-p] [name[=value] …]
The
typeset
command is supplied for compatibility with the Korn shell. It is a synonym for thedeclare
builtin command.ulimit
ulimit [-HS] -a ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [limit]
ulimit
provides control over the resources available to processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows:-S
Change and report the soft limit associated with a resource.
-H
Change and report the hard limit associated with a resource.
-a
All current limits are reported; no limits are set.
-b
The maximum socket buffer size.
-c
The maximum size of core files created.
-d
The maximum size of a process’s data segment.
-e
The maximum scheduling priority ("nice").
-f
The maximum size of files written by the shell and its children.
-i
The maximum number of pending signals.
-k
The maximum number of kqueues that may be allocated.
-l
The maximum size that may be locked into memory.
-m
The maximum resident set size (many systems do not honor this limit).
-n
The maximum number of open file descriptors (most systems do not allow this value to be set).
-p
The pipe buffer size.
-q
The maximum number of bytes in POSIX message queues.
-r
The maximum real-time scheduling priority.
-s
The maximum stack size.
-t
The maximum amount of cpu time in seconds.
-u
The maximum number of processes available to a single user.
-v
The maximum amount of virtual memory available to the shell, and, on some systems, to its children.
-x
The maximum number of file locks.
-P
The maximum number of pseudoterminals.
-R
The maximum time a real-time process can run before blocking, in microseconds.
-T
The maximum number of threads.
If
limit
is given, and the-a
option is not used,limit
is the new value of the specified resource. The speciallimit
valueshard
,soft
, andunlimited
stand for the current hard limit, the current soft limit, and no limit, respectively. A hard limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the hard limit. Otherwise, the current value of the soft limit for the specified resource is printed, unless the-H
option is supplied. When more than one resource is specified, the limit name and unit, if appropriate, are printed before the value. When setting new limits, if neither-H
nor-S
is supplied, both the hard and soft limits are set. If no option is given, then-f
is assumed. Values are in 1024-byte increments, except for-t
, which is in seconds;-R
, which is in microseconds;-p
, which is in units of 512-byte blocks;-P
,-T
,-b
,-k
,-n
and-u
, which are unscaled values; and, when in POSIX Mode (see Bash POSIX Mode),-c
and-f
, which are in 512-byte increments.The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit.
unalias
unalias [-a] [name … ]
Remove each
name
from the list of aliases. If-a
is supplied, all aliases are removed. Aliases are described in Aliases.
Next: Modifying Shell Behavior, Previous: Bourne Shell Builtins, Up: Shell Builtin Commands [Contents][Index]