|Linux / Unix Command: trap|
NAMEbash, :, ., [, alias, bg, bind, break, builtin, cd, command, compgen, complete, continue, declare, dirs, disown, echo, enable, eval, exec, exit, export, fc, fg, getopts, hash, help, history, jobs, kill, let, local, logout, popd, printf, pushd, pwd, read, readonly, return, set, shift, shopt, source, suspend, test, times, trap, type, typeset, ulimit, umask, unalias, unset, wait - bash built-in commands, see bash(1)
BASH BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to signify the end of the options.
- : [arguments]
- No effect; the command does nothing beyond expanding arguments and performing any specified redirections. A zero exit code is returned.
- . filename [arguments]
- source filename [arguments]
- Read and execute commands from filename in the current shell environment and return the exit status of the last command executed from filename. If filename does not contain a slash, file names in PATH are used to find the directory containing filename. The file searched for in PATH need not be executable. When bash is not in posix mode, the current directory is searched if no file is found in PATH. If the sourcepath option to the shopt builtin command is turned off, the PATH is not searched. 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 status of the last command exited within the script (0 if no commands are executed), and false if filename is not found or cannot be read.
- alias [-p] [name[=value] ...]
- Alias with no arguments or with the -p option prints the list of aliases in the form alias name=value on standard output. When arguments are supplied, an alias is defined for each name whose value is given. A trailing space in value causes the next word to be checked for alias substitution when the alias is expanded. For each name in the argument list for which no value is supplied, the name and value of the alias is printed. Alias returns true unless a name is given for which no alias has been defined.
- bg [jobspec]
- Resume the suspended job jobspec in the background, as if it had been started with &. If jobspec is not present, the shell's notion of the current job is used. bg jobspec returns 0 unless run when job control is disabled or, when run with job control enabled, if jobspec was not found or started without job control.
- 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
key and function bindings, bind a key sequence to a
function or macro, or set a
Each non-option argument is a command as it would appear in
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.
- List the names of all readline functions.
- Display readline function names and bindings in such a way that they can be re-read.
- List current readline function names and bindings.
- Display readline variable names and values in such a way that they can be re-read.
- List current readline variable names and values.
- Display readline key sequences bound to macros and the strings they output in such a way that they can be re-read.
- 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 value is 0 unless an unrecognized option is given or an error occurred.
- break [n]
- Exit from within a for, while, until, or select loop. If n is specified, break n levels. n must be > 1. If n is greater than the number of enclosing loops, all enclosing loops are exited. The return value is 0 unless the shell is not executing a loop when break is executed.
- builtin shell-builtin [arguments]
- Execute the specified shell builtin, passing it arguments, and return its exit status. This is useful when defining a function whose name is the same as a shell builtin, retaining the functionality of the builtin within the function. The cd builtin is commonly redefined this way. The return status is false if shell-builtin is not a shell builtin command.
- cd [-L|-P] [dir]
- Change the current directory to dir. The variable HOME is the default dir. The variable CDPATH defines the search path for the directory containing dir. Alternative directory names in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, i.e., ``.''. If dir begins with a slash (/), then CDPATH is not used. The -P option says to use the physical directory structure instead of following symbolic links (see also the -P option to the set builtin command); the -L option forces symbolic links to be followed. An argument of - is equivalent to $OLDPWD. The return value is true if the directory was successfully changed; false otherwise.
- command [-pVv] command [arg ...]
- Run command with args suppressing the normal shell function lookup. Only builtin commands or commands found in the PATH are executed. If the -p option is given, the search for command is performed using a default value for PATH that is guaranteed to find all of the standard utilities. 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. If the -V or -v option is supplied, the exit status is 0 if command was found, and 1 if not. If neither option is supplied and an error occurred or command cannot be found, the exit status is 127. Otherwise, the exit status of the command builtin is the exit status of command.
- compgen [option] [word]
Generate possible completion matches for word according to
the options, which may be any option accepted by the
builtin with the exception of -p and -r, and write
the matches to the standard output.
When using the -F or -C options, the various shell variables
set by the programmable completion facilities, while available, will not
have useful values.
The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. If word is specified, only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied, or no matches were generated.
- complete [-abcdefgjksuv] [-o comp-option] [-A action] [-G globpat] [-W wordlist] [-P prefix] [-S suffix]
[-X filterpat] [-F function] [-C command] name [name ...]
- complete -pr [name ...]
Specify how arguments to each name should be completed.
If the -p option is supplied, or if no options are supplied,
existing completion specifications are printed in a way that allows
them to be reused as input.
The -r option removes a completion specification for
each name, or, if no names are supplied, all
The process of applying these completion specifications when word completion is attempted is described above under Programmable Completion.
Other options, if specified, have the following meanings. The arguments to the -G, -W, and -X options (and, if necessary, the -P and -S options) should be quoted to protect them from expansion before the complete builtin is invoked.
- -o comp-option
The comp-option controls several aspects of the compspec's behavior
beyond the simple generation of completions.
comp-option may be one of:
- Use readline's default filename completion if the compspec generates no matches.
- Perform directory name completion if the compspec generates no matches.
- Tell readline that the compspec generates filenames, so it can perform any filename-specific processing (like adding a slash to directory names or suppressing trailing spaces). Intended to be used with shell functions.
- Tell readline not to append a space (the default) to words completed at the end of the line.
- -A action
The action may be one of the following to generate a list of possible
- Alias names. May also be specified as -a.
- Array variable names.
- Readline key binding names.
- Names of shell builtin commands. May also be specified as -b.
- Command names. May also be specified as -c.
- Directory names. May also be specified as -d.
- Names of disabled shell builtins.
- Names of enabled shell builtins.
- Names of exported shell variables. May also be specified as -e.
- File names. May also be specified as -f.
- Names of shell functions.
- Group names. May also be specified as -g.
- Help topics as accepted by the help builtin.
- Hostnames, as taken from the file specified by the HOSTFILE shell variable.
- Job names, if job control is active. May also be specified as -j.
- Shell reserved words. May also be specified as -k.
- Names of running jobs, if job control is active.
- Service names. May also be specified as -s.
- Valid arguments for the -o option to the set builtin.
- Shell option names as accepted by the shopt builtin.
- Signal names.
- Names of stopped jobs, if job control is active.
- User names. May also be specified as -u.
- Names of all shell variables. May also be specified as -v.
- -G globpat
- The filename expansion pattern globpat is expanded to generate the possible completions.
- -W wordlist
- The wordlist is split using the characters in the IFS special variable as delimiters, and each resultant word is expanded. The possible completions are the members of the resultant list which match the word being completed.
- -C command
- command is executed in a subshell environment, and its output is used as the possible completions.
- -F function
- The shell function function is executed in the current shell environment. When it finishes, the possible completions are retrieved from the value of the COMPREPLY array variable.
- -X filterpat
- filterpat is a pattern as used for filename expansion. It is applied to the list of possible completions generated by the preceding options and arguments, and each completion matching filterpat is removed from the list. A leading ! in filterpat negates the pattern; in this case, any completion not matching filterpat is removed.
- -P prefix
- prefix is added at the beginning of each possible completion after all other options have been applied.
- -S suffix
- suffix is appended to each possible completion after all other options have been applied.
The return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a name argument, an attempt is made to remove a completion specification for a name for which no specification exists, or an error occurs adding a completion specification.
- continue [n]
- Resume the next iteration of the enclosing for, while, until, or select loop. If n is specified, resume at the nth enclosing loop. n must be > 1. If n is greater than the number of enclosing loops, the last enclosing loop (the ``top-level'' loop) is resumed. The return value is 0 unless the shell is not executing a loop when continue is executed.
- declare [-afFirtx] [-p] [name[=value]]
- typeset [-afFirtx] [-p] [name[=value]]
Declare variables and/or give them attributes.
If no names are given then display the values of variables.
option will display the attributes and values of each
is used, additional options are ignored.
option inhibits the display of function definitions; only the
function name and attributes are printed.
The following options can
be used to restrict output to variables with the specified attribute or
to give variables attributes:
- Each name is an array variable (see Arrays above).
- Use function names only.
- The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION ) is performed when the variable is assigned a value.
- Make names readonly. These names cannot then be assigned values by subsequent assignment statements or unset.
- Give each name the trace attribute. Traced functions inherit the DEBUG trap from the calling shell. The trace attribute has no special meaning for variables.
- Mark names for export to subsequent commands via the environment.
Using `+' instead of `-' turns off the attribute instead, with the exception that +a may not be used to destroy an array variable. When used in a function, makes each name local, as with the local command. The return value is 0 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 above), 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.
- dirs [-clpv] [+n] [-n]
Without options, displays the list of currently remembered directories.
The default display is on a single line with directory names separated
Directories are added to the list with the
command removes entries from the list.
- Displays the nth entry counting from the left of the list shown by dirs when invoked without options, starting with zero.
- Displays the nth entry counting from the right of the list shown by dirs when invoked without options, starting with zero.
- Clears the directory stack by deleting all of the entries.
- Produces a longer listing; the default listing format uses a tilde to denote the home directory.
- Print the directory stack with one entry per line.
- Print the directory stack with one entry per line, prefixing each entry with its index in the stack.
The return value is 0 unless an invalid option is supplied or n indexes beyond the end of the directory stack.
- disown [-ar] [-h] [jobspec ...]
- Without options, each jobspec is removed from the table of active jobs. If the -h option is given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If no jobspec is present, and neither the -a nor the -r option is supplied, the current job is used. If no jobspec is supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts operation to running jobs. The return value is 0 unless a jobspec does not specify a valid job.
- echo [-neE] [arg ...]
Output the args, separated by spaces, followed by 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
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.
does not interpret
to mean the end of options.
interprets the following escape sequences:
- alert (bell)
- suppress trailing newline
- an escape character
- form feed
- new line
- carriage return
- horizontal tab
- vertical tab
- the eight-bit character whose value is the octal value nnn (zero to three octal digits)
- the eight-bit character whose value is the octal value nnn (one to three octal digits)
- the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
- enable [-adnps] [-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, each name is disabled; otherwise, names are enabled. For example, to use the test binary found via the PATH instead of the shell builtin version, run ``enable -n test''. 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 previously loaded with -f. If no name arguments are given, or if the -p option is supplied, a list of shell builtins is printed. With no other option arguments, the list consists of all enabled shell builtins. If -n is supplied, only disabled builtins are printed. If -a is supplied, the list printed includes all builtins, with an indication of whether or not each is enabled. If -s is supplied, the output is restricted to the POSIX special builtins. The return value is 0 unless a name is not a shell builtin or there is an error loading a new builtin from a shared object.
- eval [arg ...]
- The args are read and concatenated together into a single command. This command is then read and executed by the shell, and its exit status is returned as the value of eval. If there are no args, or only null arguments, eval returns 0.
- exec [-cl] [-a name] [command [arguments]]
- If command is specified, it replaces the shell. No new process is created. The arguments become the arguments to command. If the -l option is supplied, the shell places a dash at the beginning of the zeroth arg passed to command. This is what login(1) 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 the executed command. If command cannot be executed for some reason, a non-interactive shell exits, unless the shell option execfail is enabled, in which case it returns failure. An interactive shell returns failure if the file cannot be executed. If command is not specified, any redirections take effect in the current shell, and the return status is 0. If there is a redirection error, the return status is 1.
- exit [n]
- Cause the shell to exit with a status of n. If n is omitted, the exit status is that of the last command executed. A trap on EXIT is executed before the shell terminates.
- export [-fn] [name[=word]] ...
- export -p
- The supplied names are marked for automatic export to the environment of subsequently executed commands. If the -f option is given, the names refer to functions. If no names are given, or if the -p option is supplied, a list of all names that are exported in this shell is printed. The -n option causes the export property to be removed from the named variables. export returns an exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.
- fc [-e ename] [-nlr] [first] [last]
- fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands from
is selected from the history list.
may be specified as a string (to locate the last command beginning
with that string) or as a number (an index into the history list,
where a negative number is used as an offset from the current
command number). If
is not specified it is set to
the current command for listing (so that
``fc -l -10''
prints the last 10 commands) and to
is not specified it is set to the previous
command for editing and -16 for listing.
The -n option suppresses the command numbers when listing. The -r option reverses the order of the commands. If the -l option is given, the commands are listed on standard output. Otherwise, the editor given by ename is invoked on a file containing those commands. If ename is not given, the value of the FCEDIT variable is used, and the value of EDITOR if FCEDIT is not set. If neither variable is set, is used. When editing is complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance of pat is replaced by rep. A useful alias to use with this is ``r=fc -s'', so that typing ``r cc'' runs the last command beginning with ``cc'' and typing ``r'' re-executes the last command.
If the first form is used, the return value is 0 unless an invalid option is encountered or first or last specify history lines out of range. If the -e option is supplied, the return value is the value of the last command executed or failure if an error occurs with the temporary file of commands. If the second form is used, the return status is that of the command re-executed, unless cmd does not specify a valid history line, in which case fc returns failure.
- fg [jobspec]
- Resume jobspec in the foreground, and make it the current job. If jobspec is not present, the shell's notion of the current job is used. The return value is that of the command placed into the foreground, or failure if run when job control is disabled or, when run with job control enabled, if jobspec does not specify a valid job or jobspec specifies a job that was started without job control.
- getopts optstring name [args]
is used by shell procedures to parse positional parameters.
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 white space.
The colon and question mark characters may not be used as
Each time it is invoked,
places the next option in the shell variable
if it does not exist,
and the index of the next argument to be processed into the
is initialized to 1 each time the shell or a shell script
is invoked. When an option requires an argument,
places that argument into the variable
The shell does not reset
automatically; it must be manually reset between multiple
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.
getopts returns true if an option, specified or unspecified, is found. It returns false if the end of options is encountered or an error occurs.
- hash [-lr] [-p filename] [-dt] [name]
- For each name, the full file name of the command is determined by searching the directories in $PATH and remembered. If the -p option is supplied, no path search is performed, and filename is used as the full file name of the command. 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 true unless a name is not found or an invalid option is supplied.
- help [-s] [pattern]
- Display helpful information about builtin commands. If pattern is specified, help gives detailed help on all commands matching pattern; otherwise help for all the builtins and shell control structures is printed. The -s option restricts the information displayed to a short usage synopsis. The return status is 0 unless no command matches pattern.
- history [n]
- history -c
- history -d offset
- history -anrw [filename]
- history -p arg [arg ...]
- history -s arg [arg ...]
With no options, display the command
history list with line numbers. Lines listed
have been modified. An argument of
lists only the last
lines. If filename is supplied, it is used as the
name of the history file; if not, the value of
is used. Options, if supplied, have the following meanings:
- Clear the history list by deleting all the entries.
- -d offset
- Delete the history entry at position offset.
- Append the ``new'' history lines (history lines entered since the beginning of the current bash session) to the history file.
- Read the history lines not already read from the history file into the current history list. These are lines appended to the history file since the beginning of the current bash session.
- Read the contents of the history file and use them as the current history.
- Write the current history to the history file, overwriting the history file's contents.
- Perform history substitution on the following args and display the result on the standard output. Does not store the results in the history list. Each arg must be quoted to disable normal history expansion.
- Store the args in the history list as a single entry. The last command in the history list is removed before the args are added.
The return value is 0 unless an invalid option is encountered, an error occurs while reading or writing the history file, an invalid offset is supplied as an argument to -d, or the history expansion supplied as an argument to -p fails.
- jobs [-lnprs] [ jobspec ... ]
- jobs -x command [ args ... ]
The first form lists the active jobs. The options have the following
- List process IDs in addition to the normal information.
- List only the process ID of the job's process group leader.
- Display information only about jobs that have changed status since the user was last notified of their status.
- Restrict output to running jobs.
- Restrict output to stopped jobs.
If jobspec is given, output is restricted to information about that job. The return status is 0 unless an invalid option is encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec found in command or args with the corresponding process group ID, and executes command passing it args, returning its exit status.
- kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
- kill -l [sigspec | exit_status]
- Send the signal named by sigspec or signum to the processes named by pid or jobspec. sigspec is either a signal name such as SIGKILL or a signal number; signum is a signal number. If sigspec is a signal name, the name may be given with or without the SIG prefix. If sigspec is not present, then SIGTERM is assumed. An argument of -l lists the signal names. If any arguments are supplied when -l is given, the names of the signals corresponding to the arguments are listed, and the return status is 0. The exit_status argument to -l is a number specifying either a signal number or the exit status of a process terminated by a signal. kill returns true if at least one signal was successfully sent, or false if an error occurs or an invalid option is encountered.
- let arg [arg ...]
- Each arg is an arithmetic expression to be evaluated (see ARITHMETICEVALUATION). If the last arg evaluates to 0, let returns 1; 0 is returned otherwise.
- 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. When local is used within a function, it causes the variable name to have a visible scope restricted to that function and its children. With no operands, local writes a list of local variables to the standard output. It is an error to use local when not within a function. The return status is 0 unless local is used outside a function, an invalid name is supplied, or name is a readonly variable.
- Exit a login shell.
- popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments,
removes the top directory from the stack, and performs a
to the new top directory.
Arguments, if supplied, have the following meanings:
- Removes the nth entry counting from the left of the list shown by dirs, starting with zero. For example: ``popd +0'' removes the first directory, ``popd +1'' the second.
- Removes the nth entry counting from the right of the list shown by dirs, starting with zero. For example: ``popd -0'' removes the last directory, ``popd -1'' the next to last.
- Suppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated.
If the popd command is successful, a dirs is performed as well, and the return status is 0. popd returns false if an invalid option is encountered, the directory stack is empty, a non-existent directory stack entry is specified, or the directory change fails.
- printf 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
In addition to the standard printf(1) formats, %b causes
printf to expand backslash escape sequences in the corresponding
argument, and %q causes printf to output the corresponding
argument in a format that can be reused as shell input.
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.
- pushd [-n] [dir]
- pushd [-n] [+n] [-n]
Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory. With no arguments, exchanges the top two directories
and returns 0, unless the directory stack is empty.
Arguments, if supplied, have the following meanings:
- Rotates the stack so that the nth directory (counting from the left of the list shown by dirs, starting with zero) is at the top.
- Rotates the stack so that the nth directory (counting from the right of the list shown by dirs, starting with zero) is at the top.
- Suppresses the normal change of directory when adding directories to the stack, so that only the stack is manipulated.
- Adds dir to the directory stack at the top, making it the new current working directory.
If the pushd command is successful, a dirs is performed as well. If the first form is used, pushd returns 0 unless the cd to dir fails. With the second form, pushd returns 0 unless the directory stack is empty, a non-existent directory stack element is specified, or the directory change to the specified new current directory fails.
- pwd [-LP]
- Print the absolute pathname of the current working directory. The pathname printed contains no symbolic links if the -P option is supplied or the -o physical option to the set builtin command is enabled. If the -L option is used, the pathname printed may contain symbolic links. The return status is 0 unless an error occurs while reading the name of the current directory or an invalid option is supplied.
- read [-ers] [-u fd] [-t timeout] [-a aname] [-p prompt] [-n nchars] [-d delim] [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
the second word to the second
and so on, with leftover words and their intervening separators assigned
to the last
If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in
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.
Options, if supplied, have the following meanings:
- -a aname
- The words are assigned to sequential indices of the array variable aname, starting at 0. aname is unset before any new values are assigned. Other name arguments are ignored.
- -d delim
- The first character of delim is used to terminate the input line, rather than newline.
- If the standard input is coming from a terminal, readline (see READLINE above) 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 on standard error, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal.
- 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.
- 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 fdFP
- Read input from file descriptor fd.
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.
- readonly [-apf] [name ...]
- The given names are marked readonly; the values of these names may not be changed by subsequent assignment. If the -f option is supplied, the functions corresponding to the names are so marked. The -a option restricts the variables to arrays. If no name arguments are given, or if the -p option is supplied, a list of all readonly names is printed. The -p option causes output to be displayed in a format that may be reused as input. The return status is 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.
- return [n]
- Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed in the function body. If used outside a function, but during execution of a script by the . (source) command, it causes the shell to stop executing that script and return either n or the exit status of the last command executed within the script as the exit status of the script. If used outside a function and not during execution of a script by ., the return status is false.
- set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
Without options, the name and value of each shell variable are displayed
in a format that can be reused as input.
The output is sorted according to the current locale.
When options are specified, they set or unset shell attributes.
Any arguments remaining after the options are processed are treated
as values for the positional parameters and are assigned, in order, to
Options, if specified, have the following meanings:
- Automatically mark variables and functions which are modified or created for export to the environment of subsequent commands.
- Report the status of terminated background jobs immediately, rather than before the next primary prompt. This is effective only when job control is enabled.
- Exit immediately if a simple command (see SHELL GRAMMAR above) exits with a non-zero status. The shell does not exit if the command that fails is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return value is being inverted via !. A trap on ERR, if set, is executed before the shell exits.
- Disable pathname expansion.
- Remember the location of commands as they are looked up for execution. This is enabled by default.
- All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.
- Monitor mode. Job control is enabled. This option is on by default for interactive shells on systems that support it (see JOB CONTROL above). Background processes run in a separate process group and a line containing their exit status is printed upon their completion.
- Read commands but do not execute them. This may be used to check a shell script for syntax errors. This is ignored by interactive shells.
- -o option-name
The option-name can be one of the following:
- Same as -a.
- Same as -B.
- Use an emacs-style command line editing interface. This is enabled by default when the shell is interactive, unless the shell is started with the --noediting option.
- Same as -e.
- Same as -h.
- Same as -H.
- Enable command history, as described above under HISTORY. This option is on by default in interactive shells.
- The effect is as if the shell command ``IGNOREEOF=10'' had been executed (see Shell Variables above).
- Same as -k.
- Same as -m.
- Same as -C.
- Same as -n.
- Same as -f. nolog Currently ignored.
- Same as -b.
- Same as -u.
- Same as -t.
- Same as -P.
- Change the behavior of bash where the default operation differs from the POSIX 1003.2 standard to match the standard (posix mode).
- Same as -p.
- Same as -v.
- Use a vi-style command line editing interface.
If -o is supplied with no option-name, the values of the current options are printed. If +o is supplied with no option-name, a series of set commands to recreate the current option settings is displayed on the standard output.
- Turn on privileged mode. In this mode, the $ENV and $BASH_ENV files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS variable, if it appears in the environment, is ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.
- Exit after reading and executing one command.
- Treat unset variables as an error when performing parameter expansion. If expansion is attempted on an unset variable, the shell prints an error message, and, if not interactive, exits with a non-zero status.
- Print shell input lines as they are read.
- After expanding each simple command, display the expanded value of PS4, followed by the command and its expanded arguments.
- The shell performs brace expansion (see Brace Expansion above). This is on by default.
- If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. This may be overridden when creating output files by using the redirection operator >| instead of >.
- Enable ! style history substitution. This option is on by default when the shell is interactive.
- If set, the shell does not follow symbolic links when executing commands such as cd that change the current working directory. It uses the physical directory structure instead. By default, bash follows the logical chain of directories when performing commands which change the current directory.
- If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the args, even if some of them begin with a -.
- Signal the end of options, cause all remaining args to be assigned to the positional parameters. The -x and -v options are turned off. If there are no args, the positional parameters remain unchanged.
The options are off by default unless otherwise noted. Using + rather than - causes these options to be turned off. The options can also be specified as arguments to an invocation of the shell. The current set of options may be found in $-. The return status is always true unless an invalid option is encountered.
- shift [n]
- The positional parameters from n+1 ... are renamed to $1 .... Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The return status is greater than zero if n is greater than $# or less than zero; otherwise 0.
- shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior.
With no options, or with the
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:
- Enable (set) each optname.
- Disable (unset) each optname.
- Suppresses normal output (quiet mode); 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.
- Restricts the values of optname to be those defined for the -o option to 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 (unset) 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:
- If 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.
- 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 one character too many. If a correction is found, the corrected file name is printed, and the command proceeds. This option is only used by interactive shells.
- If 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.
- If set, bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS.
- 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.
- If set, bash includes filenames beginning with a `.' in the results of pathname expansion.
- If 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.
- If set, aliases are expanded as described above under ALIASES. This option is enabled by default for interactive shells.
- If set, the extended pattern matching features described above under Pathname Expansion are enabled.
- 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.
- If set, and readline is being used, a user is given the opportunity to re-edit a failed history substitution.
- 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.
- If set, and readline is being used, bash will attempt to perform hostname completion when a word containing a @ is being completed (see Completing under READLINE above). This is enabled by default.
- If set, bash will send SIGHUP to all jobs when an interactive login shell exits.
- If set, allow a word beginning with # to cause that word and all remaining characters on that line to be ignored in an interactive shell (see COMMENTS above). This option is enabled by default.
- If set, and the cmdhist option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible.
- The shell sets this option if it is started as a login shell (see INVOCATION above). The value may not be changed.
- 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.
- 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.
- If set, bash matches filenames in a case-insensitive fashion when performing pathname expansion (see Pathname Expansion above).
- If set, bash allows patterns which match no files (see Pathname Expansion above) to expand to a null string, rather than themselves.
- If set, the programmable completion facilities (see Programmable Completion above) are enabled. This option is enabled by default.
- If set, prompt strings undergo variable and parameter expansion after being expanded as described in PROMPTING above. This option is enabled by default.
- The shell sets this option if it is started in restricted mode (see RESTRICTED SHELL below). 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.
- If set, the shift builtin prints an error message when the shift count exceeds the number of positional parameters.
- 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.
- If set, the echo builtin expands backslash-escape sequences by default.
- suspend [-f]
- Suspend the execution of this shell until it receives a SIGCONT signal. The -f option says not to complain if this is a login shell; just suspend anyway. The return status is 0 unless the shell is a login shell and -f is not supplied, or if job control is not enabled.
- test expr
- [ expr ]
Return a status of 0 or 1 depending on
the evaluation of the conditional expression
Each operator and operand must be a separate argument.
Expressions are composed of the primaries described above under
Expressions may be combined using the following operators, listed in decreasing order of precedence.
- ! 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.
test and [ 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 listed above under CONDITIONALEXPRESSIONS, the expression is true if the unary test is true. If the first argument is not a valid unary conditional operator, the expression is false.
- 3 arguments
- If the second argument is one of the binary conditional operators listed above under CONDITIONALEXPRESSIONS, the result of the expression is the result of the binary test using the first and third arguments as operands. 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. The -a and -o operators are considered binary operators in this case.
- 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.
- Print the accumulated user and system times for the shell and for processes run from the shell. The return status is 0.
- trap [-lp] [arg] [sigspec ...]
- The command arg is to be read and executed when the shell receives signal(s) sigspec. If arg is absent or -, all specified signals are reset to their original values (the values they had upon entrance to the shell). If arg is the null string the signal specified by each sigspec is ignored by the shell and by the commands it invokes. If arg is not present and -p has been supplied, then the trap commands associated with each sigspec are displayed. If no arguments are supplied or if only -p is given, trap prints the list of commands associated with each signal number. Each sigspec is either a signal name defined in <signal.h>, or a signal number. If a sigspec is EXIT (0) the command arg is executed on exit from the shell. If a sigspec is DEBUG, the command arg is executed after every simple command (see SHELL GRAMMAR above). If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status. The ERR trap is not executed if the failed command is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return value is being inverted via !. The -l option causes the shell to print a list of signal names and their corresponding numbers. Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals are reset to their original values in a child process when it is created. The return status is false if any sigspec is invalid; otherwise trap returns true.
- type [-aftpP] name [name ...]
- With no options, indicate how each name would be interpreted if used as a command name. If the -t option is used, type prints a string which is one of alias, keyword, function, builtin, or file if name is an alias, shell reserved word, function, builtin, or disk file, respectively. If the name is not found, then nothing is printed, and an exit status of false is returned. If the -p option is used, type either returns the name of the disk file that would be executed if name were specified as a command name, or nothing if ``type -t name'' would not return file. The -P option forces a PATH search for each name, even if ``type -t name'' 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 prints all of the places that contain an executable named name. This includes aliases and functions, if and only if the -p option is not also used. The table of hashed commands is not consulted when using -a. The -f option suppresses shell function lookup, as with the command builtin. type returns true if any of the arguments are found, false if none are found.
- ulimit [-SHacdflmnpstuv [limit]]
Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control.
The -H and -S options specify that the hard or soft limit is
set for the given resource. A hard limit cannot be increased once it
is set; a soft limit may be increased up to the value of the hard limit.
If neither -H nor -S is specified, both the soft and hard
limits are set.
The value of
can be a number in the unit specified for the resource
or one of the special values
which stand for the current hard limit, the current soft limit, and
no limit, respectively.
is omitted, the current value of the soft limit of the resource is
printed, unless the -H option is given. When more than one
resource is specified, the limit name and unit are printed before the value.
Other options are interpreted as follows:
- All current limits are reported
- The maximum size of core files created
- The maximum size of a process's data segment
- The maximum size of files created by the shell
- The maximum size that may be locked into memory
- The maximum resident set size
- The maximum number of open file descriptors (most systems do not allow this value to be set)
- The pipe size in 512-byte blocks (this may not be set)
- The maximum stack size
- The maximum amount of cpu time in seconds
- The maximum number of processes available to a single user
- The maximum amount of virtual memory available to the shell
If limit is given, it is the new value of the specified resource (the -a option is display only). 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 0 unless an invalid option or argument is supplied, or an error occurs while setting a new limit.
- umask [-p] [-S] [mode]
- The user file-creation mask is set to mode. If mode begins with a digit, it is interpreted as an octal number; otherwise it is interpreted as a symbolic mode mask similar to that accepted by chmod(1). If mode is omitted, the current value of the mask is printed. The -S option causes the mask to be printed in symbolic form; the default output is an octal number. 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 0 if the mode was successfully changed or if no mode argument was supplied, and false otherwise.
- unalias [-a] [name ...]
- Remove each name from the list of defined aliases. If -a is supplied, all alias definitions are removed. The return value is true unless a supplied name is not a defined alias.
- unset [-fv] [name ...]
- For each name, remove the corresponding variable or function. If no options are supplied, or the -v option is given, each name refers to a shell variable. Read-only variables may not be unset. If -f is specifed, each name refers to a shell function, and the function definition is removed. Each unset variable or function is removed from the environment passed to subsequent commands. If any of RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are unset, they lose their special properties, even if they are subsequently reset. The exit status is true unless a name does not exist or is readonly.
- wait [n]
- Wait for the specified process and return its termination status. n may be a process ID or a job specification; if a job spec is given, all processes in that job's pipeline are waited for. If n is not given, all currently active child processes are waited for, and the return status is zero. If n specifies a non-existent process or job, the return status is 127. Otherwise, the return status is the exit status of the last process or job waited for.
SEE ALSObash(1), sh(1)
Important: Use the man command (% man) to see how a command is used on your particular computer.