7.7 Built-in Commands

Examples to be entered as a command line are shown with the $ prompt. Otherwise, examples should be treated as code fragments that might be included in a shell script. For convenience, some of the reserved words used by multiline commands also are included.

Ignore all text that follows on the same line. # is used in shell scripts as the comment character and is not really a command.

#!shell

Used as the first line of a script to invoke the named shell (with optional arguments) or other program. For example:

#!/bin/bash

Null command. Returns an exit status of 0. Sometimes used as the first character in a file to denote a bash script. Shell variables can be placed after the : to expand them to their values.

Example

To check whether someone is logged in:

if who | grep -w $1 > /dev/null
   then :     # do nothing
   # if pattern is found
   else echo "User $1 is not logged in"
fi

. file [arguments]

Same as source.

alias

alias [-p] [name[=cmd]]

Assign a shorthand name as a synonym for cmd. If =cmd is omitted, print the alias for name; if name is also omitted or if -p is specified, print all aliases. See also unalias.

bg [jobIDs]

Put current job or jobIDs in the background. See Section 7.7.

bind

bind [options]
bind [options] key:function

Print or set the bindings that allow keys to invoke functions such as cursor movement and line editing. Typical syntax choices for keys are "\C-t" for Ctrl-T and "\M-t" or "\et" for Esc-T (quoting is needed to escape the sequences from the shell). Function names can be seen though the -l option.

Options

-f filename: Consult filename for bindings, which should be in the same format as on the bind command line.
-l: Print all Readline functions, which are functions that can be bound to keys.
-m keymap: Specify a keymap for this and further bindings. Possible keymaps are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert.
-p: Display all functions and the keys that invoke them, in the format by which keys can be set.
-q function: Display the key bindings that invoke function.
-r key: Remove the binding attached to key so that it no longer works.
-s: Display all macros and the keys that invoke them, in the format by which keys can be set.
-u function: Remove all the bindings attached to function so that no keys will invoke it.
-v: Display all Readline variables (settings that affect history and line editing) and their current settings, in the format by which variables can be set.
-x key:command: Bind key to a shell command.
-P: Display all bound keys and the functions they invoke.
-S: Display all macros and the keys that invoke them.
-V: Display all Readline variables (settings that affect history and line editing) and their current settings.

Example

Bind Ctrl-T to copy-forward-word, the function that copies the part of the word following the cursor so it can be repasted:

$ bind "\C-t":copy-forward-word

break

break [n]

Exit from the innermost (most deeply nested) for, while, or until loop, or from the nth innermost level of the loop. Also exits from a select list.

builtin

builtin command [arguments]

Execute command, which must be a shell built-in. Useful for invoking built-ins within scripts of the same name.

case

case string  
   in    
    regex)
      commands
      ;;
      ...
  esac

If string matches regular expression regex, perform the following commands. Proceed down the list of regular expressions until one is found. (To catch all remaining strings, use * as regex at the end.)

cd [options] [dir]

With no arguments, change to user's home directory. Otherwise, change working directory to dir. If dir is a relative pathname but is not in the current directory, search the CDPATH variable.

Options

-L: Force symbolic links to be followed.
-P: Don't follow symbolic links, but use the physical directory structure.

command

command [options] command [arguments]

Execute command, but do not perform function lookup (i.e., refuse to run any command that is neither in PATH nor a built-in). Set exit status to that returned by command unless command cannot be found, in which case exit with a status of 127.

Options

-p: Search default path, ignoring the PATH variable's value.
-v: Print the command or filename that invokes the command.
-V: Like -v, but also print a description of the command.
--: Treat everything that follows as an argument, not an option.

compgen

compgen [options] [word]

Generate possible completion matches for word for use with bash's programmable completion feature, and write the matches to standard output. If word is not specified, display all completions. See complete for the options; any except -p and -r can be used with compgen.

complete

complete [options] names

Specify completions for arguments to each name, for use with bash's programmable completion feature. With no options or with -p, print all completion specifications such that they can be reused as input.

Options

-o comp-option

Specify other aspects of the completion specification's behavior besides generating a completion. Possible values of comp-option are:

default: Use readline's default filename completion if the completion specification generates no matches.
dirnames: Use directory name completion if the completion specification generates no matches.
filenames: Tell readline that the completion specification generates filenames so that it can process them accordingly. For use with shell functions.
nospace: Tell readline not to append a space to completions at the end of the line. This is the default.

-p

Print all completion specifications.

-r

Remove completion specification for each name, or all specifications if no names are given.

-A action

Specify an action to generate a list of completions. Possible actions are:

alias: Alias names. May be specified as -a.
arrayvar: Array variable names.
binding: readline key binding names.
builtin: Shell built-in command names. May be specified as -b.
command: Command names. May be specified as -c.
directory: Directory names. May be specified as -d.
disabled: Disabled shell built-in command names.
enabled: Enabled shell built-in command names.
export: Exported shell variable names. May be specified as -e.
file: Filenames. May be specified as -f.
function: Shell function names.
group: Group names. May be specified as -g.
helptopic: Help topic names accepted by the help built-in command.
hostname: Hostnames, from the file specified by HOSTFILE.
job: Job names, if job control is active. May be specified as -j.
keyword: Shell reserved words. May be specified as -k.
running: Names of running jobs, if job control is active.
service: Service names. May be specified as -s.
setopt: Valid arguments for the -o option to the set built-in command.
shopt: Valid shell option names for the shopt built-in command.
signal: Signal names.
stopped: Names of stopped jobs, if job control is active.
user: Usernames. May be specified as -u.
variable: Shell variable names. May be specified as -v.

-C command

Execute the specified command in a subshell and use the output as possible completions.

-F function

Execute the specified function in the current shell and take the possible completions from the COMPREPLY array variable.

-G globpat

Expand the specified filename expansion pattern to generate the possible completions.

-P prefix

Prepend the specified prefix to each possible completion after all other options have been applied.

-S suffix

Append the specified suffix to each possible completion after all other options have been applied.

-W list

Split the specified word list and expand each resulting word. The possible completions are the members of the resulting list that match the word being completed.

-X pattern

Use the specified pattern as a filter and apply it to the list of possible completions generated by all the other options except -P and -S, removing all matches from the list. A leading ! in the pattern negates it so that any completion that does not match the pattern is removed.

continue

continue [n]

Skip remaining commands in a for, while, or until loop, resuming with the next iteration of the loop (or skipping n loops).

declare

declare [options] [name[=value]]
typeset [options] [name[=value]]

Print or set variables. Options prefaced by + instead of - are inverted in meaning.

Options

-a: Treat the following names as array variables.
-f: Treat the following names as functions.
-i: Expect variable to be an integer, and evaluate its assigned value.
-p: Print names and settings of all shell variables and functions; take no other action.
-r: Do not allow variables to be reset later.
-x: Mark variables for subsequent export.
-F: Print names of all shell functions; take no other action.

dirs

dirs [options]

Print directories currently remembered for pushd/popd operations.

Options

+entry: Print entryth entry from start of list (list starts at 0).
-entry: Print entryth entry from end of list.
-c: Clear the directory stack.
-l: Long listing.
-p: Print the directory stack, one entry per line.
-v: Like -p, but prefix each entry with its position in the stack.

disown

disown [options] [jobIDs]

Let job run, but disassociate it from the shell. By default, does not even list the job as an active job; commands like jobs and fg will no longer recognize it. When -h is specified, the job is recognized but is kept from being killed when the shell dies.

Options

-a: Act on all jobs.
-h: Do not pass a SIGHUP signal received by the shell on to the job.

echo

echo [options] [strings]

Write each string to standard output, separated by spaces and terminated by a newline. If no strings are supplied, echo a newline. (See also echo in Chapter 3.)

Options

-e

Enable interpretation of escape characters:

\a: Audible alert
\b: Backspace
\c: Suppress the terminating newline (same as -n)
\e: Escape character
\f: Form feed
\n: Newline
\r: Carriage return
\t: Horizontal tab
\v: Vertical tab
\\: Backslash
\nnn: The character in the ASCII set corresponding to the octal number nnn.
\xnn: The character in the ASCII set corresponding to the hexadecimal number nn (1 or 2 hex digits).

-n

Do not append a newline to the output.

-E

Disable interpretation of escape characters.

enable

enable [options] [built-in ...]

Enable (or when -n is specified, disable) built-in shell commands. Without built-in argument or with -p option, print enabled built-ins. With -a, print the status of all built-ins. You can disable shell commands in order to define your own functions with the same names.

Options

-a: Display all built-ins, both enabled and disabled.
-d: Delete a built-in command that was previously loaded with -f.
-f filename: On systems that support dynamic loading, load the new built-in command built-in from the shared object filename.
-n: Disable each specified built-in.
-p: Display enabled built-ins.
-s: Restrict display to special built-ins defined by the POSIX standard.

eval

eval [command args...]

Perform command, passing args.

exec

exec [options] [command]

Execute command in place of the current shell (instead of creating a new process). exec is also useful for opening, closing, or copying file descriptors.

Options

-a name: Tell command that it was invoked as name.
-c: Remove all environment variables from the process when the new command runs.
-l: Treat the new process as if the user were logging in.

Examples

 trap 'exec 2>&-' 0     Close standard error when shell script 
                                                exits (signal 0)
$ exec /bin/tcsh         Replace current shell with extended C shell
$ exec < infile          Reassign standard input to infile

exit

exit [n]

Exit a shell script with status n (e.g., exit 1). n can be zero (success) or nonzero (failure). If n is not given, exit status will be that of the most recent command. exit can be issued at the command line to close a window (log out).

Example

if [ $# -eq 0 ]; then
   echo "Usage:  $0 [-c] [-d] file(s)"
   exit 1     # Error status
fi

export

export [options] [variables]
export [options] [name=[value]]...

Pass (export) the value of one or more shell variables, giving global meaning to the variables (which are local by default). For example, a variable defined in one shell script must be exported if its value will be used in other programs called by the script. When a shell variable has been exported, you can access its value by referencing the equivalent environment variable. If no variables are given, export lists the variables exported by the current shell. If name and value are specified, export assigns value to a variable name and exports it.

Options

--: Treat all subsequent strings as arguments, not options.
-f: Expect variables to be functions.
-n: Unexport variable.
-p: List variables exported by current shell.

fc [options] [first] [last]
fc -s [oldpattern=newpattern] [command]

Display or edit commands in the history list. (Use only one of -l or -e.) fc provides capabilities similar to the C shell's history and ! syntax. first and last are numbers or strings specifying the range of commands to display or edit. If last is omitted, fc applies to a single command (specified by first). If both first and last are omitted, fc edits the previous command or lists the last 16. A negative number is treated as an offset from the current command. The second form of fc takes a history command, replaces old string with new string, and executes the modified command. If no strings are specified, command is reexecuted. If no command is given either, the previous command is reexecuted. command is a number or string like first. See earlier examples under Section 7.5.

Options

-e [editor]: Invoke editor to edit the specified history commands. The default editor is set by the shell variable FCEDIT. If FCEDIT is not set, the value of EDITOR is used, or vi if neither is set.
-l [first last]: List the specified command or range of commands, or list the last 16.
-n: Suppress command numbering from the -l listing.
-r: Reverse the order of the -l listing.
-s oldpattern=newpattern: Edit command(s), replacing all occurrences of the specified old pattern with the new pattern. Then reexecute.

fg [jobIDs]

Bring current job or jobIDs to the foreground. See Section 7.6.

for

for x [in list]  
  do
    commands
  done

Assign each word in list to x in turn and execute commands. If list is omitted, $@ (positional parameters) is assumed.

Examples

Paginate all files in the current directory and save each result:

for file in *
do
     pr $file > $file.tmp
done

Search chapters for a list of words (like fgrep -f):

for item in `cat program_list`
do
     echo "Checking chapters for"
     echo "references to program $item..."
     grep -c "$item.[co]" chap*
done

function

function command{ ...}

Define a function. Refer to arguments the same way as positional parameters in a shell script ($1, etc.) and terminate with }.

getopts

getopts string name [args]

Process command-line arguments (or args, if specified) and check for legal options. getopts is used in shell script loops and is intended to ensure standard syntax for command-line options. string contains the option letters to be recognized by getopts when running the shell script. Valid options are processed in turn and stored in the shell variable name. If an option letter is followed by a colon, the option must be followed by one or more arguments.

hash

hash [options] [commands]

Search for commands and remember the directory in which each command resides. Hashing causes the shell to remember the association between a name and the absolute pathname of an executable, so that future executions don't require a search of PATH. With no arguments or only -l, hash lists the current hashed commands. The display shows hits (the number of times the command is called by the shell) and command (the full pathname).

Options

-d: Forget the remembered location of each specified command.
-l: Display the output in a format that can be reused as input.
-p filename: Assume filename is the full path to the command and don't do a path search.
-r: Forget the locations of all remembered commands.
-t: Print the full pathname for each command. With more than one command, print the command before each full path.

help

help [-s] [string]

Print help text on all built-in commands or those matching string. With -s, display only brief syntax; otherwise display summary paragraph also.

history

history [options]
history [lines]

Print a numbered command history, denoting modified commands with *. Include commands from previous sessions. You may specify how many lines of history to print.

Options

-a [file]: bash maintains a file called .bash_history in the user's home directory, a record of previous sessions' commands. Ask bash to append the current session's commands to .bash_history or to file.
-c: Clear history list: remove all previously entered commands from the list remembered by the shell.
-d offset: Delete the history entry at the specified offset from the beginning of the history list.
-n [file]: Append to the history list those lines in .bash_history or in file that have not yet been included.
-p args: Perform history substitution on the specified arguments and display the result on standard output. The results are not stored in the history list. Each argument must be quoted to disable normal history expansion.
-r [file]: Use .bash_history or file as the history list, instead of using the working history list.
-s args: Remove the last command in the history list and then add the specified arguments to the list as a single entry (but don't execute the entry).
-w [file]: Overwrite .bash_history or file with the working history list.

if test-cmds

Begin a conditional statement. The possible formats, shown here side by side, are:

if test-cmds    if test-cmds      if test-cmds
   then            then             then
      cmds1          cmds1             cmds1
fi                 else             elif test-cmds
                      cmds2            then
                fi                        cmds2
                                          ...
                                    else
                                       cmdsn
                                  fi

Usually, the initial if and any elif lines execute one test or [ ] command (although any series of commands is permitted). When if succeeds (that is, the last of its test-cmds returns 0), cmds1 are performed; otherwise, each succeeding elif or else line is tried.

jobs

jobs [options] [jobIDs]

List all running or stopped jobs, or those specified by jobIDs. For example, you can check whether a long compilation or text format is still running. Also useful before logging out. See also Section 7.6 earlier in this chapter.

Options

-l: List job IDs and process group IDs.
-n: List only jobs whose status has changed since last notification.
-p: List process group IDs only.
-r: List active, running jobs only.
-s: List stopped jobs only.
-x command [arguments]: Execute command. If jobIDs are specified, replace them with command.

kill

kill [options] IDs

Terminate each specified process ID or job ID. You must own the process or be a privileged user. See also Section 7.6 and the killall command in Chapter 3.

Options

-signal: The signal number (from ps -f) or name (from kill -l). The default is TERM (signal number 15). With a signal number of 9, the kill is unconditional. If nothing else works to kill a process, kill -9 almost always kills it, but does not allow the process any time to clean up.
--: Consider all subsequent strings to be arguments, not options.
-l [arg]: With no argument, list the signal names. (Used by itself.) The argument can be a signal name or a number representing either the signal number or the exit status of a process terminated by a signal. If it is a name, the correspoding number is returned; otherwise, the corresponding name is returned.
-n signum: Specify the signal number to send.
-s signal: Specify signal. May be a signal name or number.

let

let expressions

Perform arithmetic as specified by one or more integer expressions. expressions consist of numbers, operators, and shell variables (which don't need a preceding $), and must be quoted if they contain spaces or other special characters. For more information and examples, see Section 7.4 earlier in this chapter. See also expr in Chapter 3.

Examples

Both of the following examples add 1 to variable i:

let i=i+1
let "i = i + 1"

local

local [options] [variable[=value]] [variable2[=value]] ...

Without arguments, print all local variables. Otherwise, create (and set, if specified) one or more local variables. See the declare built-in command for options. Must be used within a function.

logout

logout [status]

Exit the shell, returning status as exit status to invoking program if specified. Can be used only in a login shell. Otherwise, use exit.

popd

popd [options]

Manipulate the directory stack. By default, remove the top directory and cd to it. If successful, run dirs to show the new directory stack.

Options

+n: Remove the nth directory in the stack, counting from 0.
-n: Remove the nth entry from the bottom of the stack, counting from 0.
-n: Don't do a cd when removing directories from the stack.

printf

printf string [arguments]

Format the arguments according to string. Works like the C library printf function. Standard printf percent-sign formats are recognized in string, such as %i for integer. Escape sequences such as \n can be included in string and are automatically recognized; if you want to include them in arguments, specify a string of %b. You can escape characters in arguments to output a string suitable for input to other commands by specifying a string of %q.

Examples

 printf "Previous command: %i\n" "$(($HISTCMD-1))"
Previous command: 534
$ echo $PAGER
less -E
$ printf "%q\n" "\t$PAGER"
\\tless\ -E

The last command would probably be used to record a setting in a file where it could be read and assigned by another shell script.

pushd

pushd [directory]
pushd [options]

By default, switch top two directories on stack. If specified, add a new directory to the top of the stack instead, and cd to it.

Options

+n: Rotate the stack to place the nth (counting from 0) directory at the top.
-n: Rotate the stack to place the nth directory from the bottom of the stack at the top.
-n: Don't do a cd when adding directories to the stack.

pwd

pwd [option]

Display the current working directory's absolute pathname. By default, any symbolic directories used when reaching the current directory are displayed, but with -P, or if the -o option to the set built-in is set, the real names are displayed instead.

Options

-L: Include any symbolic links in the pathname.
-P: Do not include symbolic links in the pathname.

read

read [options] [variable1 variable2 ...]

Read one line of standard input and assign each word (as defined by IFS) to the corresponding variable, with all leftover words assigned to the last variable. If only one variable is specified, the entire line will be assigned to that variable. The return status is 0 unless EOF is reached, a distinction that is useful for running loops over input files. If no variable names are provided, read the entire string into the environment variable REPLY.

Options

-a var: Read each word into an element of var, which is treated as an array variable.
-d char: Stop reading the line at char instead of at the newline.
-e: Line editing and command history are enabled during input.
-n num: Read only num characters from the line.
-p string: Display the prompt string to the user before reading each line, if input is interactive.
-r: Raw mode; ignore \ as a line continuation character.
-s: Do not echo the characters entered by the user (useful for reading a password).
-t seconds: Time out and return without setting any variables if input is interactive and no input has been entered for seconds seconds.
-u fd: Read input from specified file descriptor fd instead of standard input.

Examples

 read first last address
Sarah Caldwell 123 Main Street
$ echo "$last, $first\n$address"
Caldwell, Sarah
123 Main Street

The following commands, which read a password into the variable $user_pw and then display its value, use recently added options that are not in all versions of bash in current use.

$ read -sp "Enter password (will not appear on screen)" user_pw
Enter password (will not appear on screen)
$ echo $user_pw
You weren't supposed to know!

The following script reads input from the system's password file, which uses colons to delimit fields (making it a popular subject for examples of input parsing).

IFS=:
cat /etc/passwd |
while
read account pw user group gecos home shell
do
echo "Account name $account has user info: $gecos"
done

readonly

readonly [options] [variable1 variable2...]

Prevent the specified shell variables from being assigned new values. Variables can be accessed (read) but not overwritten.

Options

-a: Treat the following names as array variables.
-f: Treat the following names as functions and set them read-only so that they cannot be changed.
-p: Display all read-only variables (default).

return

return [n]

Normally used inside a function to exit the function with status n or with the exit status of the previously executed command. Can be used outside a function during execution of a script by the . command to cause the shell to stop execution of the script. The return status is n or the script's exit status.

select

select name [ in wordlist ; ] 
 do
   commands
 done

Choose a value for name by displaying the words in wordlist to the user and prompting for a choice. Store user input in the variable REPLY and the chosen word in name. Then execute commands repeatedly until they execute a break or return. The default prompt can be changed by setting the PS3 shell variable.

set

set [options] [arg1 arg2 ...]

With no arguments, set prints the values of all variables known to the current shell. Options can be enabled (-option) or disabled (+option). Options can also be set when the shell is invoked, via bash. Arguments are assigned in order to $1, $2, and so on.

Options

-

Turn off -v and -x, and turn off option processing.

--

Used as the last option; turn off option processing so that arguments beginning with - are not misinterpreted as options. (For example, you can set $1 to -1.) If no arguments are given after --, unset the positional parameters.

-a

From now on, automatically mark variables for export after defining or changing them.

-b

Report background job status at termination instead of waiting for next shell prompt.

-e

Exit if a command yields a nonzero exit status.

-f

Do not expand filename metacharacters (e.g., * ? [ ]). Wildcard expansion is sometimes called globbing.

-h

Locate and remember commands as they are defined.

-k

Assignment of environment variables (var=value) will take effect regardless of where they appear on the command line. Normally, assignments must precede the command name.

-m

Monitor mode. Enable job control; background jobs execute in a separate process group. -m usually is set automatically.

-n

Read commands, but don't execute. Useful for checking errors, particularly for shell scripts.

-o [m]

List shell modes, or turn on mode m. Many modes can be set by other options. The modes can be turned off through the +o option. Modes are:

allexport: Same as -a.
braceexpand: Same as -B.
emacs: Enter Emacs editing mode (on by default).
errexit: Same as -e.
hashall: Same as -h.
histexpand: Same as -H.
history: Default. Preserve command history.
ignoreeof: Don't allow use of a single Ctrl-D (the end-of-file or EOF character) to log off; use the exit command to log off. This has the same effect as setting the shell variable IGNOREEOF=1.
interactive-comments: Allow comments to appear in interactive commands.
keyword: Same as -k.
monitor: Same as -m.
noclobber: Same as -C.
noexec: Same as -n.
noglob: Same as -f.
notify: Same as -b.
nounset: Same as -u.
onecmd: Same as -t.
physical: Same as -P.
posix: Match POSIX standard.
privileged: Same as -p.
verbose: Same as -v.
vi: Enable vi-style command-line editing.
xtrace: Same as -x.

+o [m]

Display the set commands that would recreate the current mode settings or turn off mode m. See the -o option for a list of modes.

-p

Start up as a privileged user; don't process $HOME/.profile.

-t

Exit after one command is executed.

-u

Indicate an error when user tries to use a variable that is undefined.

-v

Show each shell command line when read.

-x

Show commands and arguments when executed, preceded by a + or the prompt defined by the PS4 shell variable. This provides step-by-step debugging of shell scripts. (Same as -o xtrace.)

-B

Default. Enable brace expansion.

-C

Don't allow output redirection (>) to overwrite an existing file.

-H

Default. Enable ! and !! commands.

-P

Print absolute pathnames in response to pwd. By default, bash includes symbolic links in its response to pwd.

Examples

set -- "$num" -20 -30    Set $1 to $num, $2 to -20, $3 to -30
set -vx                 Read each command line; show it; execute it;             
                    show it again (with arguments)
set +x                   Stop command tracing
set -o noclobber          Prevent file overwriting
set +o noclobber          Allow file overwriting again

shift

shift [n]

Shift positional arguments (e.g., $2 becomes $1). If n is given, shift to the left n places.

shopt

shopt [options] [optnames]

Set or unset variables that control optional shell behavior. With no options or with -p, display the settable optnames.

Options

-o: Allow only options defined for the set -o built-in to be set or unset.
-p: Display output in a form that can be reused as input.
-q: Quiet mode. Suppress normal output.
-s: Set (enable) each specified option. With no optname, list all set options.
-u: Unset (disable) each specified option. With no optname, list all unset options.

Settable shell options

Unless otherwise noted, options are disabled by default.

cdable_vars: If an argument to the cd built-in is not a directory, assume that it's a variable containing the name of the directory to change to.
cdspell: For interactive shells, check for minor errors in the name of a directory component (transposed characters, a missing character, or an extra character). Print the corrected name and proceed.
checkhash: Check that a command found in the hash table actually exists before trying to execute it; if it is not found, do a path search.
checkwinsize: Check the window size after each command and update LINES and COLUMNS as necessary.
cmdhist: Attempt to save all lines of a multiline command in one history entry to facilitate re-editing.
dotglob: Include filenames beginning with . in the results of pathname expansion.
execfail: For a noninteractive shell, do not exit if the file specified as an argument to exec cannot be executed. For an interactive shell, do not exit from the shell if exec fails.
expand_aliases: Expand aliases. Enabled by default for interactive shells.
extglob: Enable the shell's extended pattern matching features for pathname expansion.
histappend: Append the history list to the file specified by HISTFILE when the shell exits, instead of overwriting the file.
histreedit: Give the user a chance to re-edit a failed history substitution.
histverify: Load a history substitution into the readline editing buffer so it can be further edited, instead of immediately passing it to the shell parser.
hostcomplete: Try to provide hostname completion when a word containing @ is being completed. Set by default.
huponexit: Send SIGHUP to all jobs when an interactive login shell exits.
interactive_comments: In an interactive shell, treat any word beginning with a #, and any subsequent characters, as a comment. Set by default.
lithist: If cmdhist is also enabled, save multiline commands to the history file separated by embedded newlines rather than semicolons (;) when possible.
login_shell: Set by the shell if it is started as a login shell. Cannot be changed by the user.
mailwarn: Warn if a mail file has been accessed since the last time bash checked it.
no_empty_cmd_completion: Don't attempt to search the PATH for possible completions when completion is attempted on an empty line.
nocaseglob: Use case-insensitive filename matching during pathname expansion.
nullglob: Allow patterns that do not match any files to expand to a null string.
progcomp: Enable the programmable completion facilities. Set by default.
promptvars: Perform variable and parameter expansion on prompt strings after performing normal expansion. Set by default.
restricted_shell: Set by the shell if started in restricted mode. This option cannot be changed by the user and is not reset when the startup files are executed.
shift_verbose: Cause the shift built-in to print an error message when the shift count is greater than the number of positional parameters.
sourcepath: Cause the source built-in (.) to search the PATH to find the directory containing a file supplied as an argument. Set by default.
xpg_echo: Cause the echo built-in to expand backslash-escape sequences by default.

source

source file [arguments]

Read and execute lines in file. file does not have to be executable but must reside in a directory searched by PATH. Any arguments are passed as positional parameters to the file when it is executed.

suspend

suspend [-f]

Same as Ctrl-Z.

Option

-f: Force suspend, even if shell is a login shell.

test

test condition
[ condition ]

Evaluate a condition and, if its value is true, return a zero exit status; otherwise, return a nonzero exit status. An alternate form of the command uses [ ] rather than the word test. condition is constructed using the following expressions. Conditions are true if the description holds true.

File conditions

-a file: file exists.
-b file: file is a block special file.
-c file: file is a character special file.
-d file: file is a directory.
-e file: file exists.
-f file: file is a regular file.
-g file: file has the set-group-ID bit set.
-h file: file is a symbolic link.
-k file: file has its sticky bit (no longer used) set.
-p file: file is a named pipe (FIFO).
-r file: file is readable.
-s file: file has a size greater than 0.
-t [n]: The open file descriptor n is associated with a terminal device (default n is 1).
-u file: file has its set-user-ID bit set.
-w file: file is writable.
-x file: file is executable.
-G file: file's group is the process's effective group ID.
-L file: file is a symbolic link.
-N file: file has been modified since its last time of access.
-O file: file's owner is the process's effective user ID.
-S file: file is a socket.
f1 -ef f2: Files f1 and f2 are linked (refer to the same file through a hard link).
f1 -nt f2: File f1 is newer than f2.
f1 -ot f2: File f1 is older than f2.

String conditions

-n s1: String s1 has nonzero length.
-o s1: Shell option s1 is set. Shell options are described under the set built-in command.
-z s1: String s1 has 0 length.
s1 = s2: Strings s1 and s2 are identical.
s1 = = s2: Strings s1 and s2 are identical.
s1 != s2: Strings s1 and s2 are not identical.
s1 < s2: String s1 is lower in the alphabet (or other sort in use) than s2. By default, the check is performed character-by-character against the ASCII character set.
s1 > s2: String s1 is higher in the alphabet (or other sort in use) than s2.
string: string is not null.

Integer comparisons

n1 -eq n2: n1 equals n2.
n1 -ge n2: n1 is greater than or equal to n2.
n1 -gt n2: n1 is greater than n2.
n1 -le n2: n1 is less than or equal to n2.
n1 -lt n2: n1 is less than n2.
n1 -ne n2: n1 does not equal n2.

Combined forms

! condition: True if condition is false.
condition1 -a condition2: True if both conditions are true.
condition1 -o condition2: True if either condition is true.

Examples

Each of the following examples shows the first line of various statements that might use a test condition:

while test $# -gt 0        While there are arguments . . .
while [ -n "$1" ]          While the first argument is nonempty . . .
if [ $count -lt 10 ]             If $count is less than 10 . . .
if [ -d RCS ]                    If the RCS directory exists . . .
if [ "$answer" != "y" ]          If the answer is not y . . .
if [ ! -r "$1" -o ! -f "$1" ]    If the first argument is not a
                                readable file or a regular file . . .

times

times

Print accumulated process times for user and system.

trap

trap [option] [commands] [signals]

Execute commands if any of signals is received. Each signal can be a signal name or number. Common signals include 0, 1, 2, and 15. Multiple commands should be quoted as a group and separated by semicolons internally. If commands is the null string (e.g., trap "" signals), then signals is ignored by the shell. If commands is omitted entirely, reset processing of specified signals to the default action. If both commands and signals are omitted, list current trap assignments. See examples at the end of this entry and under exec.

Options

-l: List signal names and numbers.
-p: Used with no commands to print the trap commands associated with each signal, or all signals if none is specified.

Signals

Signals are listed along with what triggers them.

0: Exit from shell (usually when shell script finishes).
1: Hang up (usually logout).
2: Interrupt (usually through Ctrl-C).
3: Quit.
4: Illegal instruction.
5: Trace trap.
6: Abort.
7: Unused.
8: Floating-point exception.
9: Termination.
10: User-defined.
11: Reference to invalid memory.
12: User-defined.
13: Write to a pipe without a process to read it.
14: Alarm timeout.
15: Software termination (usually via kill).
16: Unused.
17: Termination of child process.
18: Continue (if stopped).
19: Stop process.
20: Process suspended (usually through Ctrl-Z).
21: Background process has tty input.
22: Background process has tty output.
23-28: Unused.
29: I/O possible on a channel.

Examples

trap "" 2       Ignore signal 2 (interrupts)
trap 2          Obey interrupts again

Remove a $tmp file when the shell program exits or if the user logs out, presses Ctrl-C, or does a kill:

trap "rm -f $tmp; exit" 0 1 2 15

type

type [options] commands

Report absolute pathname of programs invoked for commands and whether or not they are hashed.

Options

--: Consider all subsequent strings to be arguments, not options.
-a, -all: Print all occurrences of command, not just that which would be invoked.
-f: Suppress shell function lookup.
-p, -path: Print the hashed value of command, which may differ from the first appearance of command in the PATH.
-t, -type: Determine and state if command is an alias, keyword, function, built-in, or file.
-P: Force a PATH search for each name, even if -t would not return a value of "file" for the name.

Example

 type mv read
mv is /bin/mv
read is a shell built-in

typeset

typeset

Obsolete. See declare.

ulimit

ulimit [options] [n]

Print the value of one or more resource limits or, if n is specified, set a resource limit to n. Resource limits can be either hard (-H) or soft (-S). By default, ulimit sets both limits or prints the soft limit. The options determine which resource is acted on. Values are in 1024-byte increments unless otherwise indicated.

Options

--: Consider all subsequent strings to be arguments, not options.
-a: Print all current limits.
-H: Hard resource limit.
-S: Soft resource limit.

Specific limits

These options limit specific resource sizes.

-c: Core files.
-d: Size of processes' data segments.
-f: Size of shell-created files.
-l: Size of memory that the process can lock.
-m: Resident set size.
-n: Number of file descriptors. On many systems, this cannot be set.
-p: Pipe size, measured in blocks of 512 bytes.
-s: Stack size.
-t: Amount of CPU time, counted in seconds.
-u: Number of processes per user.
-v: Virtual memory used by shell.

umask

umask [options] [nnn]

Display file creation mask or set file creation mask to octal value nnn. The file creation mask determines which permission bits are turned off (e.g., umask 002 produces rw-rw-r--).

Options

-p: Display mask within a umask command so that a caller can read and execute it.
-S: Display umask symbolically rather than in octal.

unalias

unalias [-a] names

Remove names from the alias list. See also alias.

Option

-a: Remove all aliases.

unset

unset [options] names

Erase definitions of functions or variables listed in names.

Options

-f: Expect name to refer to a function.
-v: Expect name to refer to a variable (default).

until

until
  test-commands
 do
  commands
 done

Execute test-commands (usually a test or [ ] command); if the exit status is nonzero (that is, the test fails), perform commands. Repeat.

wait

wait [ID]

Pause in execution until all background jobs complete (exit status 0 will be returned), or until the specified background process ID or job ID completes (exit status of ID is returned). Note that the shell variable $! contains the process ID of the most recent background process. If job control is not in effect, ID can only be a process ID number. See Section 7.6.

Example

ait $!      Wait for last background process to finish

while

while
  test-commands
 do
  commands
 done

Execute test-commands (usually a test or [ ] command); if the exit status is 0, perform commands. Repeat.