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GNU Bash Reference Manual
by Chet Ramey and Brian Fox
Paperback (6"x9"), 180 pages
ISBN 0954161777
RRP £19.95 ($29.95)

"An essential resource .... the most detailed coverage available for all aspects of Bash" --- Linux User and Developer Magazine (Issue 37, Mar 2004) Get a printed copy>>>

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 each name whose value is given. If no value is given, the name and value of the alias is printed. Aliases are described in section 6.6 Aliases.
bind
bind [-m keymap] [-lpsvPSV]
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 readline-command
Display current Readline (see section 8 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 section 8.3 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. Acceptable keymap names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-command; emacs is equivalent to emacs-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 whenever keyseq is entered.
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 if shell-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 . or source builtins). Without expr, caller displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as expr, 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 with arguments ignoring any shell function named command. Only shell builtin commands or commands found by searching the PATH are executed. If there is a shell function named ls, running ‘command ls’ within the function will execute the external command ls instead of calling the function recursively. The -p option means to use a default value for PATH that is guaranteed to find all of the standard utilities. The return status in this case is 127 if command cannot be found or an error occurred, and the exit status of command otherwise. If either the -V or -v option is supplied, a description of command is printed. The -v option causes a single word indicating the command or file name used to invoke command to be displayed; the -V option produces a more verbose description. In this case, the return status is zero if command is found, and non-zero if not.
declare
declare [-afFirtx] [-p] [name[=value] ...]
Declare variables and give them attributes. If no names are given, then display the values of variables instead. The -p option will display the attributes and values of each name. When -p is used, additional options are ignored. The -F option inhibits the display of function definitions; only the function name and attributes are printed. If the extdebug shell option is enabled using shopt (see section 4.2 Bash Builtin Commands), the source file name and line number where the function is defined are displayed as well. -F implies -f. 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 array variable (see section 6.7 Arrays).
-f
Use function names only.
-i
The variable is to be treated as an integer; arithmetic evaluation (see section 6.5 Shell Arithmetic) is performed when the variable is assigned a value.
-r
Make names readonly. These names cannot then be assigned values by subsequent assignment statements or unset.
-t
Give each name the trace attribute. Traced functions inherit the DEBUG and RETURN traps from the calling shell. The trace attribute has no special meaning for variables.
-x
Mark each name for export to subsequent commands via the environment.
Using ‘+’ instead of ‘-’ turns off the attribute instead. When used in a function, declare makes each name local, as with the local command. 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 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 section 6.7 Arrays), one of the names 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 args, separated by spaces, terminated with a newline. The return status is always 0. 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. The xpg_echo shell option may be used to dynamically determine whether or not echo 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 trailing newline
\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)
enable
enable [-n] [-p] [-f filename] [-ads] [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, the names become disabled. Otherwise names are enabled. For example, to use the test binary found via $PATH instead of the shell builtin version, type ‘enable -n test’. If the -p option is supplied, or no name 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 command name from shared object filename, 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 restricts enable to the POSIX special builtins. If -s is used with -f, the new builtin becomes a special builtin (see section 4.4 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 [-s] [pattern]
Display helpful information about builtin commands. If pattern is specified, help gives detailed help on all commands matching pattern, otherwise a list of the builtins is printed. The -s option restricts the information displayed to a short usage synopsis. 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. Each expression is evaluated according to the rules given below in section 6.5 Shell Arithmetic. If the last expression 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 assigned value. The option can be any of the options accepted by declare. local can only be used within a function; it makes the variable name have a visible scope restricted to that function and its children. The return status is zero unless local is used outside a function, an invalid name is supplied, or name is a readonly variable.
logout
logout [n]
Exit a login shell, returning a status of n to the shell's parent.
printf
printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the control of the format. 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 successive argument. In addition to the standard printf(1) formats, ‘%b’ causes printf to expand backslash escape sequences in the corresponding argument, (except that ‘\c’ terminates output, backslashes in ‘\'’, ‘\"’, and ‘\?’ are not removed, and octal escapes beginning with ‘\0’ may contain up to four digits), and ‘%q’ causes printf to output the corresponding argument in a format that can be reused as shell input. The -v option causes the output to be assigned to the variable var rather than being printed to the standard output. The format is reused as necessary to consume all of the arguments. If the format requires more arguments 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] [-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, and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. 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 the IFS variable are used to split the line into words. The backslash character ‘\’ may be used to remove any special meaning for the next character read and for line continuation. If no names are supplied, the line read is assigned to the variable REPLY. The return code is zero, unless end-of-file is encountered, read times out, or an invalid file descriptor is supplied as the argument to -u. 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 from aname before the assignment. Other name arguments are ignored.
-d delim
The first character of delim is used to terminate the input line, rather than newline.
-e
Readline (see section 8 Command Line Editing) is used to obtain the line.
-n nchars
read returns after reading nchars characters rather than waiting for a complete line of input.
-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 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 is not read within timeout seconds. This option has no effect if read is not reading input from the terminal or a pipe.
-u fd
Read input from file descriptor fd.
shopt
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior. With no options, or with the -p option, a list of all settable options is displayed, with an indication of whether or not each is set. The -p option causes output to be displayed in a form that may be reused as input. Other options have the following meanings:
-s
Enable (set) each optname.
-u
Disable (unset) each optname.
-q
Suppresses normal output; the return status indicates whether the optname is set or unset. If multiple optname arguments are given with -q, the return status is zero if all optnames are enabled; non-zero otherwise.
-o
Restricts the values of optname to be those defined for the -o option to the set builtin (see section 4.3 The Set Builtin).
If either -s or -u is used with no optname arguments, the display is limited to those options which are set or unset, respectively. Unless otherwise noted, the shopt options are disabled (off) by default. The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option. The list of shopt options is:
cdable_vars
If this is set, an argument to the cd builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to.
cdspell
If set, minor errors in the spelling of a directory component in a cd command will be corrected. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. This option is only used by interactive shells.
checkhash
If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed.
checkwinsize
If set, Bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS.
cmdhist
If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands.
dotglob
If set, Bash includes filenames beginning with a `.' in the results of filename expansion.
execfail
If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin command. An interactive shell does not exit if exec fails.
expand_aliases
If set, aliases are expanded as described below under Aliases, section 6.6 Aliases. This option is enabled by default for interactive shells.
extdebug
If set, behavior intended for use by debuggers is enabled:
  1. The -F option to the declare builtin (see section 4.2 Bash Builtin Commands) displays the source file name and line number corresponding to each function name supplied as an argument.
  2. If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.
  3. If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the . or source builtins), a call to return is simulated.
  4. BASH_ARGC and BASH_ARGV are updated as described in their descriptions (see section 5.2 Bash Variables).
  5. Function tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) inherit the DEBUG and RETURN traps.
  6. Error tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) inherit the ERROR trap.
extglob
If set, the extended pattern matching features described above (see section 3.5.8.1 Pattern Matching) are enabled.
extquote
If set, $'string' and $"string" quoting is performed within ${parameter} expansions enclosed in double quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See section 5.2 Bash Variables, for a description of FIGNORE. This option is enabled by default.
gnu_errfmt
If set, shell error messages are written in the standard GNU error message format.
histappend
If set, the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file.
histreedit
If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution.
histverify
If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification.
hostcomplete
If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a ‘@’ is being completed (see section 8.4.6 Letting Readline Type For You). This option is enabled by default.
huponexit
If set, Bash will send SIGHUP to all jobs when an interactive login shell exits (see section 3.7.6 Signals).
interactive_comments
Allow a word beginning with ‘#’ to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default.
lithist
If enabled, and the cmdhist option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible.
login_shell
The shell sets this option if it is started as a login shell (see section 6.1 Invoking Bash). The value may not be changed.
mailwarn
If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message "The mail in mailfile has been read" is displayed.
no_empty_cmd_completion
If set, and Readline is being used, Bash will not attempt to search the PATH for possible completions when completion is attempted on an empty line.
nocaseglob
If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion.
nocasematch
If set, Bash matches patterns in a case-insensitive fashion when performing matching while executing case or [[ conditional commands.
nullglob
If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves.
progcomp
If set, the programmable completion facilities (see section 8.6 Programmable Completion) are enabled. This option is enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described below (see section 6.9 Controlling the Prompt). This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted mode (see section 6.10 The Restricted Shell). The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted.
shift_verbose
If this is set, the shift builtin prints an error message when the shift count exceeds the number of positional parameters.
sourcepath
If set, the source builtin uses the value of PATH to find the directory containing the file supplied as an argument. This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape sequences by default.
The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.
source
source filename
A synonym for . (see section 4.1 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’, if name is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the name is not found, then nothing is printed, and type 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 each name, even if -t would not return ‘file’. If a command is hashed, -p and -P print the hashed value, 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 named file. 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 the command builtin. The return status is zero if any of the names are found, non-zero if none are found.
typeset
typeset [-afFrxi] [-p] [name[=value] ...]
The typeset command is supplied for compatibility with the Korn shell; however, it has been deprecated in favor of the declare builtin command.
ulimit
ulimit [-acdefilmnpqrstuvxSH] [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.
-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.
-l
The maximum size that may be locked into memory.
-m
The maximum resident set size.
-n
The maximum number of open file descriptors.
-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 process.
-x
The maximum number of file locks.
If limit is given, it is the new value of the specified resource; the special limit values hard, soft, and unlimited stand for the current hard limit, the current soft limit, and no limit, respectively. Otherwise, the current value of the soft limit for the specified resource is printed, unless the -H option is supplied. 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, -p, which is in units of 512-byte blocks, and -n and -u, which are unscaled values. 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 section 6.6 Aliases.
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