# man > BASH-BUILTINS(7) [BASH-BUILTINS(7)](https://www.chedong.com/phpMan.php/man/BASH-BUILTINS/7/markdown) Miscellaneous Information Manual [BASH-BUILTINS(7)](https://www.chedong.com/phpMan.php/man/BASH-BUILTINS/7/markdown) ## NAME bash-builtins - bash built-in commands, see [**bash**(1)](https://www.chedong.com/phpMan.php/man/bash/1/markdown) ## SYNOPSIS bash defines the following built-in commands: :, ., [, alias, bg, bind, break, builtin, case, cd, command, compgen, complete, continue, declare, dirs, disown, echo, enable, eval, exec, exit, export, fc, fg, getopts, hash, help, history, if, 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, until, wait, while. ## 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. The **:**, **true**, **false**, and **test**/**[** builtins do not accept options and do not treat **--** specially. The **exit**, **logout**, **return**, **break**, **continue**, **let**, and **shift** builtins accept and process arguments beginning with **-** with‐ out requiring **--**. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with **-** as invalid options and require **--** to prevent this interpretation. **:** [_arguments_] No effect; the command does nothing beyond expanding _arguments_ and performing any specified redirections. The return status is zero. **.** _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 con‐ tain a slash, filenames 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** op‐ tion to the **shopt** builtin command is turned off, the **PATH** is not searched. If any _ar__‐ _guments_ are supplied, they become the positional parameters when _filename_ is executed. Otherwise the positional parameters are unchanged. If the **-T** option is enabled, **source** inherits any trap on **DEBUG**; if it is not, any **DEBUG** trap string is saved and restored around the call to **source**, and **source** unsets the **DEBUG** trap while it exe‐ cutes. If **-T** is not set, and the sourced file changes the **DEBUG** trap, the new value is retained when **source** completes. The return status is the status of the last com‐ mand 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 de‐ fined. **bg** [_jobspec_ ...] Resume each 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 en‐ abled, any specified _jobspec_ was not found or was started without job control. **bind** [**-m** _keymap_] [**-lpsvPSVX**] **bind** [**-m** _keymap_] [**-q** _function_] [**-u** _function_] [**-r** _keyseq_] **bind** [**-m** _keymap_] **-f** _filename_ **bind** [**-m** _keymap_] **-x** _keyseq_:_shell-command_ **bind** [**-m** _keymap_] _keyseq_:_function-name_ **bind** [**-m** _keymap_] _keyseq_:_readline-command_ Display current **readline** key and function bindings, bind a key sequence to a **readline** function or macro, or set a **readline** variable. Each non-option argument is a command as it would appear in _.inputrc_, but each binding or command must be passed as a sepa‐ rate 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_ (_vi-move_ is also a synonym); _emacs_ is equivalent to _emacs-standard_. **-l** List the names of all **readline** functions. **-p** Display **readline** function names and bindings in such a way that they can be re- read. **-P** List current **readline** function names and bindings. **-s** Display **readline** key sequences bound to macros and the strings they output in such a way that they can be re-read. **-S** Display **readline** key sequences bound to macros and the strings they output. **-v** Display **readline** variable names and values in such a way that they can be re- read. **-V** List current **readline** variable names and values. **-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. When _shell-com__‐ _mand_ is executed, the shell sets the **READLINE**___**LINE** variable to the contents of the **readline** line buffer and the **READLINE**___**POINT** and **READLINE**___**MARK** variables to the current location of the insertion point and the saved insertion point (the mark), respectively. If the executed command changes the value of any of **READ**‐‐ **LINE**___**LINE**, **READLINE**___**POINT**, or **READLINE**___**MARK**, those new values will be reflected in the editing state. **-X** List all key sequences bound to shell commands and the associated commands in a format that can be reused as input. The return 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_ lev‐ els. _n_ must be ≥ 1. If _n_ is greater than the number of enclosing loops, all enclos‐ ing loops are exited. The return value is 0 unless _n_ is not greater than or equal to 1. **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, re‐ taining the functionality of the builtin within the function. The **cd** builtin is com‐ monly redefined this way. The return status is false if _shell-builtin_ is not a shell builtin command. **caller** [_expr_] Returns the context of any active subroutine call (a shell function or a script exe‐ cuted with the **.** or **source** builtins). Without _expr_, **caller** displays the line number and source filename of the current subroutine call. If a non-negative integer is sup‐ plied as _expr_, **caller** displays the line number, subroutine name, and source file cor‐ responding to that position in the current execution call stack. This extra informa‐ tion may be used, for example, to print a stack trace. The current frame is frame 0. The return value is 0 unless the shell is not executing a subroutine call or _expr_ does not correspond to a valid position in the call stack. **cd** [**-L**|[**-P** [**-e**]] [-@]] [_dir_] Change the current directory to _dir_. if _dir_ is not supplied, the value of the **HOME** shell variable is the default. Any additional arguments following _dir_ are ignored. The variable **CDPATH** defines the search path for the directory containing _dir_: each di‐ rectory name in **CDPATH** is searched for _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 causes **cd** to use the physical directory structure by resolving symbolic links while traversing _dir_ and before processing instances of _.._ in _dir_ (see also the **-P** option to the **set** builtin command); the **-L** option forces symbolic links to be fol‐ lowed by resolving the link after processing instances of _.._ in _dir_. If _.._ appears in _dir_, it is processed by removing the immediately previous pathname component from _dir_, back to a slash or the beginning of _dir_. If the **-e** option is supplied with **-P**, and the current working directory cannot be successfully determined after a successful di‐ rectory change, **cd** will return an unsuccessful status. On systems that support it, the **-@** option presents the extended attributes associated with a file as a directory. An argument of **-** is converted to **$OLDPWD** before the directory change is attempted. If a non-empty directory name from **CDPATH** is used, or if **-** is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. The return value is true if the directory was suc‐ cessfully changed; false otherwise. **command** [**-pVv**] _command_ [_arg_ ...] Run _command_ with _args_ suppressing the normal shell function lookup. Only builtin com‐ mands 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 de‐ scription of _command_ is printed. The **-v** option causes a single word indicating the command or filename 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 _option_s, which may be any option accepted by the **complete** 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 gen‐ erated. **complete** [**-abcdefgjksuv**] [**-o** _comp-option_] [**-DEI**] [**-A** _action_] [**-G** _globpat_] [**-W** _wordlist_] [**-F** _function_] [**-C** _command_] [**-X** _filterpat_] [**-P** _prefix_] [**-S** _suffix_] _name_ [_name_ _..._] **complete** **-pr** [**-DEI**] [_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 specifica‐ tion for each _name_, or, if no _name_s are supplied, all completion specifications. The **-D** option indicates that other supplied options and actions should apply to the ``de‐ fault'' command completion; that is, completion attempted on a command for which no completion has previously been defined. The **-E** option indicates that other supplied options and actions should apply to ``empty'' command completion; that is, completion attempted on a blank line. The **-I** option indicates that other supplied options and actions should apply to completion on the initial non-assignment word on the line, or after a command delimiter such as **;** or **|**, which is usually command name completion. If multiple options are supplied, the **-D** option takes precedence over **-E**, and both take precedence over **-I**. If any of **-D**, **-E**, or **-I** are supplied, any other _name_ argu‐ ments are ignored; these completions only apply to the case specified by the option. The process of applying these completion specifications when word completion is at‐ tempted 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 pro‐ tect 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: **bashdefault** Perform the rest of the default **bash** completions if the compspec gen‐ erates no matches. **default** Use readline's default filename completion if the compspec generates no matches. **dirnames** Perform directory name completion if the compspec generates no matches. **filenames** Tell readline that the compspec generates filenames, so it can perform any filename-specific processing (like adding a slash to directory names, quoting special characters, or suppressing trailing spaces). Intended to be used with shell functions. **noquote** Tell readline not to quote the completed words if they are filenames (quoting filenames is the default). **nosort** Tell readline not to sort the list of possible completions alphabeti‐ cally. **nospace** Tell readline not to append a space (the default) to words completed at the end of the line. **plusdirs** After any matches defined by the compspec are generated, directory name completion is attempted and any matches are added to the results of the other actions. **-A** _action_ The _action_ may be one of the following to generate a list of possible comple‐ tions: **alias** Alias names. May also be specified as **-a**. **arrayvar** Array variable names. **binding** **Readline** key binding names. **builtin** Names of shell builtin commands. May also be specified as **-b**. **command** Command names. May also be specified as **-c**. **directory** Directory names. May also be specified as **-d**. **disabled** Names of disabled shell builtins. **enabled** Names of enabled shell builtins. **export** Names of exported shell variables. May also be specified as **-e**. **file** File names. May also be specified as **-f**. **function** Names of shell functions. **group** Group names. May also be specified as **-g**. **helptopic** Help topics as accepted by the **help** builtin. **hostname** Hostnames, as taken from the file specified by the **HOSTFILE** shell variable. **job** Job names, if job control is active. May also be specified as **-j**. **keyword** Shell reserved words. May also be specified as **-k**. **running** Names of running jobs, if job control is active. **service** Service names. May also be specified as **-s**. **setopt** Valid arguments for the **-o** option to the **set** builtin. **shopt** Shell option names as accepted by the **shopt** builtin. **signal** Signal names. **stopped** Names of stopped jobs, if job control is active. **user** User names. May also be specified as **-u**. **variable** Names of all shell variables. May also be specified as **-v**. **-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 the function is executed, the first argument (**$1**) is the name of the com‐ mand whose arguments are being completed, the second argument (**$2**) is the word being completed, and the third argument (**$3**) is the word preceding the word being completed on the current command line. When it finishes, the possible completions are retrieved from the value of the **COMPREPLY** array variable. **-G** _globpat_ The pathname expansion pattern _globpat_ is expanded to generate the possible completions. **-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. **-W** _wordlist_ The _wordlist_ is split using the characters in the **IFS** special variable as de‐ limiters, and each resultant word is expanded. Shell quoting is honored within _wordlist_, in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of **IFS**. The possible comple‐ tions are the members of the resultant list which match the word being com‐ pleted. **-X** _filterpat_ _filterpat_ is a pattern as used for pathname 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. 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. **compopt** [**-o** _option_] [**-DEI**] [**+o** _option_] [_name_] Modify completion options for each _name_ according to the _option_s, or for the cur‐ rently-executing completion if no _name_s are supplied. If no _option_s are given, dis‐ play the completion options for each _name_ or the current completion. The possible values of _option_ are those valid for the **complete** builtin described above. The **-D** op‐ tion indicates that other supplied options should apply to the ``default'' command completion; that is, completion attempted on a command for which no completion has previously been defined. The **-E** option indicates that other supplied options should apply to ``empty'' command completion; that is, completion attempted on a blank line. The **-I** option indicates that other supplied options should apply to completion on the initial non-assignment word on the line, or after a command delimiter such as **;** or **|**, which is usually command name completion. The return value is true unless an invalid option is supplied, an attempt is made to modify the options for a _name_ for which no completion specification exists, or an out‐ put error occurs. **continue** [_n_] Resume the next iteration of the enclosing **for**, **while**, **until**, or **select** loop. If _n_ is specified, resume at the _n_th 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 re‐ sumed. The return value is 0 unless _n_ is not greater than or equal to 1. **declare** [**-aAfFgiIlnrtux**] [**-p**] [_name_[=_value_] ...] **typeset** [**-aAfFgiIlnrtux**] [**-p**] [_name_[=_value_] ...] Declare variables and/or give them attributes. If no _name_s are given then display the values of variables. The **-p** option will display the attributes and values of each _name_. When **-p** is used with _name_ arguments, additional options, other than **-f** and **-F**, are ignored. When **-p** is supplied without _name_ arguments, it will display the at‐ tributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with **-p**, **declare** will display the at‐ tributes and values of all shell variables. The **-f** option will restrict the display to shell functions. The **-F** option inhibits the display of function definitions; only the function name and attributes are printed. If the **extdebug** shell option is enabled using **shopt**, the source file name and line number where each _name_ is defined are dis‐ played as well. The **-F** option implies **-f**. The **-g** option forces variables to be cre‐ ated or modified at the global scope, even when **declare** is executed in a shell func‐ tion. It is ignored in all other cases. The **-I** option causes local variables to in‐ herit the attributes (except the _nameref_ attribute) and value of any existing variable with the same _name_ at a surrounding scope. If there is no existing variable, the lo‐ cal variable is initially unset. The following options can be used to restrict output to variables with the specified attribute or to give variables attributes: **-a** Each _name_ is an indexed array variable (see **Arrays** above). **-A** Each _name_ is an associative array variable (see **Arrays** above). **-f** Use function names only. **-i** The variable is treated as an integer; arithmetic evaluation (see **ARITHMETIC** **EVALUATION** above) is performed when the variable is assigned a value. **-l** When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled. **-n** Give each _name_ the _nameref_ attribute, making it a name reference to another variable. That other variable is defined by the value of _name_. All refer‐ ences, assignments, and attribute modifications to _name_, except those using or changing the **-n** attribute itself, are performed on the variable referenced by _name_'s value. The nameref attribute cannot be applied to array variables. **-r** Make _name_s readonly. These names cannot then be assigned values by subsequent assignment statements or unset. **-t** Give each _name_ the _trace_ attribute. Traced functions inherit the **DEBUG** and **RE**‐‐ **TURN** traps from the calling shell. The trace attribute has no special meaning for variables. **-u** When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled. **-x** Mark _name_s for export to subsequent commands via the environment. Using `+' instead of `-' turns off the attribute instead, with the exceptions that **+a** and **+A** may not be used to destroy array variables and **+r** will not remove the readonly attribute. When used in a function, **declare** and **typeset** make each _name_ local, as with the **local** command, unless the **-g** option is supplied. If a variable name is followed by =_value_, the value of the variable is set to _value_. When using **-a** or **-A** and the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments. The return value is 0 unless an invalid op‐ tion 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 by spaces. Directories are added to the list with the **pushd** command; the **popd** command removes entries from the list. The current directory is always the first directory in the stack. **-c** Clears the directory stack by deleting all of the entries. **-l** Produces a listing using full pathnames; the default listing format uses a tilde to denote the home directory. **-p** Print the directory stack with one entry per line. **-v** Print the directory stack with one entry per line, prefixing each entry with its index in the stack. **+**_n_ Displays the _n_th entry counting from the left of the list shown by **dirs** when invoked without options, starting with zero. **-**_n_ Displays the _n_th entry counting from the right of the list shown by **dirs** when invoked without options, starting with zero. 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_ ... | _pid_ ... ] Without options, remove each _jobspec_ from the table of active jobs. If _jobspec_ is not present, and neither the **-a** nor the **-r** option is supplied, the _current_ _job_ is used. 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 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 _arg_s, separated by spaces, followed by a newline. The return status is 0 unless a write error occurs. If **-n** is specified, the trailing newline is suppressed. If the **-e** option is given, interpretation of the following backslash-escaped charac‐ ters is enabled. The **-E** option disables the interpretation of these escape charac‐ ters, even on systems where they are interpreted by default. The **xpg**___**echo** shell op‐ tion may be used to dynamically determine whether or not **echo** expands these escape characters by default. **echo** does not interpret **--** to mean the end of options. **echo** interprets the following escape sequences: **\a** alert (bell) **\b** backspace **\c** suppress further output **\e** **\E** an escape character **\f** form feed **\n** new line **\r** carriage return **\t** horizontal tab **\v** vertical tab **\\** backslash **\0**_nnn_ the eight-bit character whose value is the octal value _nnn_ (zero to three octal digits) **\x**_HH_ the eight-bit character whose value is the hexadecimal value _HH_ (one or two hex digits) **\u**_HHHH_ the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value _HHHH_ (one to four hex digits) **\U**_HHHHHHHH_ the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value _HHH__‐ _HHHHH_ (one to eight hex digits) **enable** [**-a**] [**-dnps**] [**-f** _filename_] [_name_ ...] Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before disk commands. If **-n** is used, 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 ``en‐ able -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** op‐ tion is supplied, a list of shell builtins is printed. With no other option argu‐ ments, the list consists of all enabled shell builtins. If **-n** is supplied, only dis‐ abled 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 _arg_s 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 _argu__‐ _ments_ become the arguments to _command_. If the **-l** option is supplied, the shell places a dash at the beginning of the zeroth argument passed to _command_. This is what _lo__‐ [_gin_(1)](https://www.chedong.com/phpMan.php/man/gin/1/markdown) 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 com‐ mand. If _command_ cannot be executed for some reason, a non-interactive shell exits, unless the **execfail** shell option is enabled. In that case, it returns failure. An interactive shell returns failure if the file cannot be executed. A subshell exits unconditionally if **exec** fails. If _command_ is not specified, any redirections take ef‐ fect in the current shell, and the return status is 0. If there is a redirection er‐ ror, 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 names of all exported variables is printed. The **-n** option causes the export property to be removed from each _name_. If a variable name is followed by =_word_, the value of the variable is set to _word_. **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_] [**-lnr**] [_first_] [_last_] **fc** **-s** [_pat_=_rep_] [_cmd_] The first form selects a range of commands from _first_ to _last_ from the history list and displays or edits and re-executes them. _First_ and _last_ may be specified as a string (to locate the last command beginning with that string) or as a number (an in‐ dex into the history list, where a negative number is used as an offset from the cur‐ rent command number). When listing, a _first_ or _last_ of 0 is equivalent to -1 and -0 is equivalent to the current command (usually the **fc** command); otherwise 0 is equiva‐ lent to -1 and -0 is invalid. If _last_ is not specified, it is set to the current com‐ mand for listing (so that ``fc -l -10'' prints the last 10 commands) and to _first_ oth‐ erwise. If _first_ 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, _vi_ 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_. _Command_ is interpreted the same as _first_ above. 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 encoun‐ tered or _first_ or _last_ specify history lines out of range. If the **-e** option is sup‐ plied, the return value is the value of the last command executed or failure if an er‐ ror occurs with the temporary file of commands. If the second form is used, the re‐ turn 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 _job__‐ _spec_ specifies a job that was started without job control. **getopts** _optstring_ _name_ [_arg_ _..._] **getopts** is used by shell procedures to parse positional parameters. _optstring_ con‐ tains 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 option characters. Each time it is invoked, **getopts** places the next option in the shell variable _name_, initializing _name_ if it does not exist, and the index of the next argument to be pro‐ cessed into the variable **OPTIND**. **OPTIND** is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, **getopts** places that ar‐ gument into the variable **OPTARG**. The shell does not reset **OPTIND** automatically; it must be manually reset between multiple calls to **getopts** within the same shell invoca‐ tion 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 supplied as _arg_ values, **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_] Each time **hash** is invoked, the full pathname of the command _name_ is determined by searching the directories in **$PATH** and remembered. Any previously-remembered pathname is discarded. If the **-p** option is supplied, no path search is performed, and _filename_ is used as the full filename of the command. The **-r** option causes the shell to forget all remembered locations. The **-d** option causes the shell to forget the remembered lo‐ cation 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** [**-dms**] [_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. **-d** Display a short description of each _pattern_ **-m** Display the description of each _pattern_ in a manpage-like format **-s** Display only a short usage synopsis for each _pattern_ The return status is 0 unless no command matches _pattern_. **history** **[**_n_**]** ### history -c **history** **-d** _offset_ **history** **-d** _start_-_end_ **history** **-anrw** [_filename_] **history** **-p** _arg_ [_arg_ _..._] **history** **-s** _arg_ [_arg_ _..._] With no options, display the command history list with line numbers. Lines listed with a ***** have been modified. An argument of _n_ lists only the last _n_ lines. If the shell variable **HISTTIMEFORMAT** is set and not null, it is used as a format string for [_strftime_(3)](https://www.chedong.com/phpMan.php/man/strftime/3/markdown) to display the time stamp associated with each displayed history entry. No intervening blank is printed between the formatted time stamp and the history line. If _filename_ is supplied, it is used as the name of the history file; if not, the value of **HISTFILE** is used. Options, if supplied, have the following meanings: **-c** Clear the history list by deleting all the entries. **-d** _offset_ Delete the history entry at position _offset_. If _offset_ is negative, it is in‐ terpreted as relative to one greater than the last history position, so nega‐ tive indices count back from the end of the history, and an index of -1 refers to the current **history** **-d** command. **-d** _start_-_end_ Delete the history entries between positions _start_ and _end_, inclusive. Posi‐ tive and negative values for _start_ and _end_ are interpreted as described above. **-a** Append the ``new'' history lines to the history file. These are history lines entered since the beginning of the current **bash** session, but not already ap‐ pended to the history file. **-n** 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. **-r** Read the contents of the history file and append them to the current history list. **-w** Write the current history list to the history file, overwriting the history file's contents. **-p** 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. **-s** 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. If the **HISTTIMEFORMAT** variable is set, the time stamp information associated with each history entry is written to the history file, marked with the history comment charac‐ ter. When the history file is read, lines beginning with the history comment charac‐ ter followed immediately by a digit are interpreted as timestamps for the following history entry. The return value is 0 unless an invalid option is encountered, an er‐ ror 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 meanings: **-l** List process IDs in addition to the normal information. **-n** Display information only about jobs that have changed status since the user was last notified of their status. **-p** List only the process ID of the job's process group leader. **-r** Display only running jobs. **-s** Display only 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**|**-L** [_sigspec_ | _exit_status_] Send the signal named by _sigspec_ or _signum_ to the processes named by _pid_ or _jobspec_. _sigspec_ is either a case-insensitive signal name such as **SIGKILL** (with or without the **SIG** prefix) or a signal number; _signum_ is a signal number. 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 sig‐ nal. The **-L** option is equivalent to **-l**. **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 **ARITHMETIC** **EVALUATION** above). 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 func‐ tion and its children. If _name_ is -, the set of shell options is made local to the function in which **local** is invoked: shell options changed using the **set** builtin inside the function are restored to their original values when the function returns. The re‐ store is effected as if a series of **set** commands were executed to restore the values that were in place before the function. With no operands, **local** writes a list of lo‐ cal 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. **logout** Exit a login shell. **mapfile** [**-d** _delim_] [**-n** _count_] [**-O** _origin_] [**-s** _count_] [**-t**] [**-u** _fd_] [**-C** _callback_] [**-c** _quantum_] [_array_] **readarray** [**-d** _delim_] [**-n** _count_] [**-O** _origin_] [**-s** _count_] [**-t**] [**-u** _fd_] [**-C** _callback_] [**-c** _quan__‐ _tum_] [_array_] Read lines from the standard input into the indexed array variable _array_, or from file descriptor _fd_ if the **-u** option is supplied. The variable **MAPFILE** is the default _ar__‐ _ray_. Options, if supplied, have the following meanings: **-d** The first character of _delim_ is used to terminate each input line, rather than newline. If _delim_ is the empty string, **mapfile** will terminate a line when it reads a NUL character. **-n** Copy at most _count_ lines. If _count_ is 0, all lines are copied. **-O** Begin assigning to _array_ at index _origin_. The default index is 0. **-s** Discard the first _count_ lines read. **-t** Remove a trailing _delim_ (default newline) from each line read. **-u** Read lines from file descriptor _fd_ instead of the standard input. **-C** Evaluate _callback_ each time _quantum_ lines are read. The **-c** option specifies _quantum_. **-c** Specify the number of lines read between each call to _callback_. If **-C** is specified without **-c**, the default quantum is 5000. When _callback_ is evalu‐ ated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments. _callback_ is evaluated after the line is read but before the array element is assigned. If not supplied with an explicit origin, **mapfile** will clear _array_ before assigning to it. **mapfile** returns successfully unless an invalid option or option argument is supplied, _array_ is invalid or unassignable, or if _array_ is not an indexed array. **popd** [-**n**] [+_n_] [-_n_] Removes entries from the directory stack. With no arguments, removes the top direc‐ tory from the stack, and performs a **cd** to the new top directory. Arguments, if sup‐ plied, have the following meanings: **-n** Suppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated. **+**_n_ Removes the _n_th entry counting from the left of the list shown by **dirs**, start‐ ing with zero. For example: ``popd +0'' removes the first directory, ``popd +1'' the second. **-**_n_ Removes the _n_th entry counting from the right of the list shown by **dirs**, start‐ ing with zero. For example: ``popd -0'' removes the last directory, ``popd -1'' the next to last. 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** [**-v** _var_] _format_ [_arguments_] Write the formatted _arguments_ to the standard output under the control of the _format_. The **-v** option causes the output to be assigned to the variable _var_ rather than being printed to the standard output. The _format_ is a character string which contains three types of objects: plain charac‐ ters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive _argument_. In addition to the standard [_printf_(1)](https://www.chedong.com/phpMan.php/man/printf/1/markdown) format specifications, **printf** interprets the following extensions: **%b** causes **printf** to expand backslash escape sequences in the corresponding _argu__‐ _ment_ in the same way as **echo** **-e**. **%q** causes **printf** to output the corresponding _argument_ in a format that can be reused as shell input. **%(**_datefmt_**)T** causes **printf** to output the date-time string resulting from using _datefmt_ as a format string for [_strftime_(3)](https://www.chedong.com/phpMan.php/man/strftime/3/markdown). The corresponding _argument_ is an integer repre‐ senting the number of seconds since the epoch. Two special argument values may be used: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been given. This is an exception to the usual **printf** behavior. The %b, %q, and %T directives all use the field width and precision arguments from the format specification and write that many bytes from (or use that wide a field for) the expanded argument, which usually contains more characters than the original. Arguments to non-string format specifiers are treated as C constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the ASCII value of the following character. The _format_ is reused as necessary to consume all of the _arguments_. If the _format_ re‐ quires 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**] [+_n_] [-_n_] **pushd** [**-n**] [_dir_] 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, **pushd** ex‐ changes the top two directories and returns 0, unless the directory stack is empty. Arguments, if supplied, have the following meanings: **-n** Suppresses the normal change of directory when rotating or adding directories to the stack, so that only the stack is manipulated. **+**_n_ Rotates the stack so that the _n_th directory (counting from the left of the list shown by **dirs**, starting with zero) is at the top. **-**_n_ Rotates the stack so that the _n_th directory (counting from the right of the list shown by **dirs**, starting with zero) is at the top. _dir_ Adds _dir_ to the directory stack at the top, making it the new current working directory as if it had been supplied as the argument to the **cd** builtin. 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 spec‐ ified, 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 read‐ ing the name of the current directory or an invalid option is supplied. **read** [**-ers**] [**-a** _aname_] [**-d** _delim_] [**-i** _text_] [**-n** _nchars_] [**-N** _nchars_] [**-p** _prompt_] [**-t** _timeout_] [**-u** _fd_] [_name_ ...] One line is read from the standard input, or from the file descriptor _fd_ supplied as an argument to the **-u** option, split into words as described above under **Word** **Split**‐‐ **ting**, and the first word is assigned to the first _name_, the second word to the second _name_, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the last _name_. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in **IFS** are used to split the line into words using the same rules the shell uses for expansion (described above under **Word** **Splitting**). The backslash character (**\**) may be used to remove any special meaning for the next character read and for line continuation. Options, if supplied, have the following meanings: **-a** _aname_ The words are assigned to sequential indices of the array variable _aname_, starting at 0. _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 _delim_ is the empty string, **read** will terminate a line when it reads a NUL character. **-e** If the standard input is coming from a terminal, **readline** (see **READLINE** above) is used to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Readline's de‐ fault filename completion. **-i** _text_ If **readline** is being used to read the line, _text_ is placed into the editing buffer before editing begins. **-n** _nchars_ **read** returns after reading _nchars_ characters rather than waiting for a complete line of input, but honors a delimiter if fewer than _nchars_ characters are read before the delimiter. **-N** _nchars_ **read** returns after reading exactly _nchars_ characters rather than waiting for a complete line of input, unless EOF is encountered or **read** times out. Delimiter characters encountered in the input are not treated specially and do not cause **read** to return until _nchars_ characters are read. The result is not split on the characters in **IFS**; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the **-r** option below). **-p** _prompt_ Display _prompt_ on standard error, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a ter‐ minal. **-r** Backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not then be used as a line continuation. **-s** Silent mode. If input is coming from a terminal, characters are not echoed. **-t** _timeout_ Cause **read** to time out and return failure if a complete line of input (or a specified number of characters) is not read within _timeout_ seconds. _timeout_ may be a decimal number with a fractional portion following the decimal point. This option is only effective if **read** is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If **read** times out, **read** saves any partial input read into the specified variable _name_. If _timeout_ is 0, **read** returns immediately, without trying to read any data. The exit status is 0 if input is available on the specified file de‐ scriptor, non-zero otherwise. The exit status is greater than 128 if the time‐ out is exceeded. **-u** _fd_ Read input from file descriptor _fd_. If no _names_ are supplied, the line read, without the ending delimiter but otherwise unmodified, is assigned to the variable **REPLY**. The exit status is zero, unless end- of-file is encountered, **read** times out (in which case the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as the argument to **-u**. **readonly** [**-aAf**] [**-p**] [_name_[=_word_] ...] 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 indexed arrays; the **-A** option restricts the variables to associative arrays. If both options are sup‐ plied, **-A** takes precedence. If no _name_ arguments are given, or if the **-p** option is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The **-p** option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =_word_, the value of the variable is set to _word_. The return status is 0 unless an invalid option is encountered, one of the _names_ is not a valid shell vari‐ able name, or **-f** is supplied with a _name_ that is not a function. **return** [_n_] Causes a function to stop executing and return the value specified by _n_ to its caller. If _n_ is omitted, the return status is that of the last command executed in the func‐ tion body. If **return** is executed by a trap handler, the last command used to deter‐ mine the status is the last command executed before the trap handler. If **return** is executed during a **DEBUG** trap, the last command used to determine the status is the last command executed by the trap handler before **return** was invoked. If **return** is 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 sta‐ tus of the last command executed within the script as the exit status of the script. If _n_ is supplied, the return value is its least significant 8 bits. The return status is non-zero if **return** is supplied a non-numeric argument, or is used outside a func‐ tion and not during execution of a script by **.** or **source**. Any command associated with the **RETURN** trap is executed before execution resumes after the function or script. **set** [**--abefhkmnptuvxBCEHPT**] [**-o** _option-name_] [_arg_ ...] **set** [**+abefhkmnptuvxBCEHPT**] [**+o** _option-name_] [_arg_ ...] Without options, the name and value of each shell variable are displayed in a format that can be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In _posix_ _mode_, only shell variables are listed. The output is sorted according to the current locale. When options are specified, they set or unset shell attributes. Any arguments remaining after option processing are treated as values for the positional parameters and are assigned, in order, to **$1**, **$2**, **...** **$**_n_. Options, if specified, have the following meanings: **-a** Each variable or function that is created or modified is given the export at‐ tribute and marked for export to the environment of subsequent commands. **-b** Report the status of terminated background jobs immediately, rather than be‐ fore the next primary prompt. This is effective only when job control is en‐ abled. **-e** Exit immediately if a _pipeline_ (which may consist of a single _simple_ _command_), a _list_, or a _compound_ _command_ (see **SHELL** **GRAMMAR** above), exits with a non-zero status. The shell does not exit if the command that fails is part of the com‐ mand list immediately following a **while** or **until** keyword, part of the test following the **if** or **elif** reserved words, part of any command executed in a **&&** or **||** list except the command following the final **&&** or **||**, any command in a pipeline but the last, or if the command's return value is being inverted with **!**. If a compound command other than a subshell returns a non-zero status be‐ cause a command failed while **-e** was being ignored, the shell does not exit. A trap on **ERR**, if set, is executed before the shell exits. This option applies to the shell environment and each subshell environment separately (see **COMMAND** **EXECUTION** **ENVIRONMENT** above), and may cause subshells to exit before executing all the commands in the subshell. If a compound command or shell function executes in a context where **-e** is be‐ ing ignored, none of the commands executed within the compound command or function body will be affected by the **-e** setting, even if **-e** is set and a com‐ mand returns a failure status. If a compound command or shell function sets **-e** while executing in a context where **-e** is ignored, that setting will not have any effect until the compound command or the command containing the func‐ tion call completes. **-f** Disable pathname expansion. **-h** Remember the location of commands as they are looked up for execution. This is enabled by default. **-k** All arguments in the form of assignment statements are placed in the environ‐ ment for a command, not just those that precede the command name. **-m** Monitor mode. Job control is enabled. This option is on by default for in‐ teractive shells on systems that support it (see **JOB** **CONTROL** above). All pro‐ cesses run in a separate process group. When a background job completes, the shell prints a line containing its exit status. **-n** 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: **allexport** Same as **-a**. **braceexpand** Same as **-B**. **emacs** 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. This also affects the editing interface used for **read** **-e**. **errexit** Same as **-e**. **errtrace** Same as **-E**. **functrace** Same as **-T**. **hashall** Same as **-h**. **histexpand** Same as **-H**. **history** Enable command history, as described above under **HISTORY**. This option is on by default in interactive shells. **ignoreeof** The effect is as if the shell command ``IGNOREEOF=10'' had been exe‐ cuted (see **Shell** **Variables** above). **keyword** Same as **-k**. **monitor** Same as **-m**. **noclobber** Same as **-C**. **noexec** Same as **-n**. **noglob** Same as **-f**. **nolog** Currently ignored. **notify** Same as **-b**. **nounset** Same as **-u**. **onecmd** Same as **-t**. **physical** Same as **-P**. **pipefail** If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default. **posix** Change the behavior of **bash** where the default operation differs from the POSIX standard to match the standard (_posix_ _mode_). See **SEE** **ALSO** below for a reference to a document that details how posix mode af‐ fects bash's behavior. **privileged** Same as **-p**. **verbose** Same as **-v**. **vi** Use a vi-style command line editing interface. This also affects the editing interface used for **read** **-e**. **xtrace** Same as **-x**. 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. **-p** 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**, **BASHOPTS**, **CDPATH**, and **GLOBIGNORE** variables, if they appear in the environment, are 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. **-t** Exit after reading and executing one command. **-u** Treat unset variables and parameters other than the special parameters "@" and "*" as an error when performing parameter expansion. If expansion is at‐ tempted on an unset variable or parameter, the shell prints an error message, and, if not interactive, exits with a non-zero status. **-v** Print shell input lines as they are read. **-x** After expanding each _simple_ _command_, **for** command, **case** command, **select** com‐ mand, or arithmetic **for** command, display the expanded value of **PS4**, followed by the command and its expanded arguments or associated word list. **-B** The shell performs brace expansion (see **Brace** **Expansion** above). This is on by default. **-C** If set, **bash** does not overwrite an existing file with the **>**, **>&**, and **<>** redi‐ rection operators. This may be overridden when creating output files by using the redirection operator **>|** instead of **>**. **-E** If set, any trap on **ERR** is inherited by shell functions, command substitu‐ tions, and commands executed in a subshell environment. The **ERR** trap is nor‐ mally not inherited in such cases. **-H** Enable **!** style history substitution. This option is on by default when the shell is interactive. **-P** If set, the shell does not resolve symbolic links when executing commands such as **cd** that change the current working directory. It uses the physical direc‐ tory structure instead. By default, **bash** follows the logical chain of direc‐ tories when performing commands which change the current directory. **-T** If set, any traps on **DEBUG** and **RETURN** are inherited by shell functions, com‐ mand substitutions, and commands executed in a subshell environment. The **DE**‐‐ **BUG** and **RETURN** traps are normally not inherited in such cases. **--** If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the _arg_s, even if some of them begin with a **-**. **-** Signal the end of options, cause all remaining _arg_s to be assigned to the po‐ sitional parameters. The **-x** and **-v** options are turned off. If there are no _arg_s, 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 as‐ sumed 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; oth‐ erwise 0. **shopt** [**-pqsu**] [**-o**] [_optname_ ...] Toggle the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the **-o** option is used, those available with the **-o** option to the **set** builtin command. With no options, or with the **-p** option, a list of all settable options is displayed, with an indication of whether or not each is set; if _optnames_ are supplied, the output is restricted to those options. The **-p** op‐ tion causes output to be displayed in a form that may be reused as input. Other op‐ tions have the following meanings: **-s** Enable (set) each _optname_. **-u** Disable (unset) each _optname_. **-q** 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. **-o** 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, **shopt** shows only 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: **assoc**___**expand**___**once** If set, the shell suppresses multiple evaluation of associative array sub‐ scripts during arithmetic expression evaluation, while executing builtins that can perform variable assignments, and while executing builtins that perform array dereferencing. **autocd** If set, a command name that is the name of a directory is executed as if it were the argument to the **cd** command. This option is only used by interactive shells. **cdable**___**vars** If set, an argument to the **cd** builtin command that is not a directory is as‐ sumed to be the name of a variable whose value is the directory to change to. **cdspell** If set, minor errors in the spelling of a directory component in a **cd** command will be corrected. The errors checked for are transposed characters, a miss‐ ing character, and one character too many. If a correction is found, the cor‐ rected filename is printed, and the command proceeds. This option is only used by interactive shells. **checkhash** If set, **bash** checks that a command found in the hash table exists before try‐ ing to execute it. If a hashed command no longer exists, a normal path search is performed. **checkjobs** If set, **bash** lists the status of any stopped and running jobs before exiting an interactive shell. If any jobs are running, this causes the exit to be de‐ ferred until a second exit is attempted without an intervening command (see **JOB** **CONTROL** above). The shell always postpones exiting if any jobs are stopped. **checkwinsize** If set, **bash** checks the window size after each external (non-builtin) command and, if necessary, updates the values of **LINES** and **COLUMNS**. This option is enabled by default. **cmdhist** If set, **bash** attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. This op‐ tion is enabled by default, but only has an effect if command history is en‐ abled, as described above under **HISTORY**. **compat31** **compat32** **compat40** **compat41** **compat42** **compat43** **compat44** These control aspects of the shell's compatibility mode (see **SHELL** **COMPATIBIL**‐‐ **ITY** **MODE** below). **complete**___**fullquote** If set, **bash** quotes all shell metacharacters in filenames and directory names when performing completion. If not set, **bash** removes metacharacters such as the dollar sign from the set of characters that will be quoted in completed filenames when these metacharacters appear in shell variable references in words to be completed. This means that dollar signs in variable names that expand to directories will not be quoted; however, any dollar signs appearing in filenames will not be quoted, either. This is active only when bash is us‐ ing backslashes to quote completed filenames. This variable is set by de‐ fault, which is the default bash behavior in versions through 4.2. **direxpand** If set, **bash** replaces directory names with the results of word expansion when performing filename completion. This changes the contents of the readline editing buffer. If not set, **bash** attempts to preserve what the user typed. **dirspell** If set, **bash** attempts spelling correction on directory names during word com‐ pletion if the directory name initially supplied does not exist. **dotglob** If set, **bash** includes filenames beginning with a `.' in the results of path‐ name expansion. The filenames **``.''** and **``..''** must always be matched ex‐ plicitly, even if **dotglob** is set. **execfail** 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. **expand**___**aliases** If set, aliases are expanded as described above under **ALIASES**. This option is enabled by default for interactive shells. **extdebug** If set at shell invocation, or in a shell startup file, arrange to execute the debugger profile before the shell starts, identical to the **--debugger** option. If set after invocation, behavior intended for use by debuggers is enabled: **1.** The **-F** option to the **declare** builtin displays the source file name and line number corresponding to each function name supplied as an argu‐ ment. **2.** If the command run by the **DEBUG** trap returns a non-zero value, the next command is skipped and not executed. **3.** If the command run by the **DEBUG** trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the **.** or **source** builtins), the shell simulates a call to **return**. **4.** **BASH**___**ARGC** and **BASH**___**ARGV** are updated as described in their descriptions above. **5.** Function tracing is enabled: command substitution, shell functions, and subshells invoked with **(** _command_ **)** inherit the **DEBUG** and **RETURN** traps. **6.** Error tracing is enabled: command substitution, shell functions, and subshells invoked with **(** _command_ **)** inherit the **ERR** trap. **extglob** If set, the extended pattern matching features described above under **Pathname** **Expansion** are enabled. **extquote** If set, **$**'_string_' and **$**"_string_" quoting is performed within **${**_parameter_**}** ex‐ pansions enclosed in double quotes. This option is enabled by default. **failglob** If set, patterns which fail to match filenames during pathname expansion re‐ sult in an expansion error. **force**___**fignore** If set, the suffixes specified by the **FIGNORE** shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See **SHELL** **VARIABLES** above for a description of **FIGNORE**. This option is enabled by default. **globasciiranges** If set, range expressions used in pattern matching bracket expressions (see **Pattern** **Matching** above) behave as if in the traditional C locale when perform‐ ing comparisons. That is, the current locale's collating sequence is not taken into account, so **b** will not collate between **A** and **B**, and upper-case and lower-case ASCII characters will collate together. **globstar** If set, the pattern ****** used in a pathname expansion context will match all files and zero or more directories and subdirectories. If the pattern is fol‐ lowed by a **/**, only directories and subdirectories match. **gnu**___**errfmt** If set, shell error messages are written in the standard GNU error message format. **histappend** If set, the history list is appended to the file named by the value of the **HISTFILE** variable when the shell exits, rather than overwriting the file. **histreedit** If set, and **readline** is being used, a user is given the opportunity to re-edit a failed history substitution. **histverify** If set, and **readline** is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the **readline** editing buffer, allowing further modification. **hostcomplete** If set, and **readline** is being used, **bash** will attempt to perform hostname com‐ pletion when a word containing a **@** is being completed (see **Completing** under **READLINE** above). This is enabled by default. **huponexit** If set, **bash** will send **SIGHUP** to all jobs when an interactive login shell ex‐ its. **inherit**___**errexit** If set, command substitution inherits the value of the **errexit** option, instead of unsetting it in the subshell environment. This option is enabled when _posix_ _mode_ is enabled. **interactive**___**comments** 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. **lastpipe** If set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment. **lithist** 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. **localvar**___**inherit** If set, local variables inherit the value and attributes of a variable of the same name that exists at a previous scope before any new value is assigned. The nameref attribute is not inherited. **localvar**___**unset** If set, calling **unset** on local variables in previous function scopes marks them so subsequent lookups find them unset until that function returns. This is identical to the behavior of unsetting local variables at the current func‐ tion scope. **login**___**shell** The shell sets this option if it is started as a login shell (see **INVOCATION** above). The value may not be changed. **mailwarn** If set, and a file that **bash** is checking for mail has been accessed since the last time it was checked, the message ``The mail in _mailfile_ has been read'' is displayed. **no**___**empty**___**cmd**___**completion** If set, and **readline** is being used, **bash** will not attempt to search the **PATH** for possible completions when completion is attempted on an empty line. **nocaseglob** If set, **bash** matches filenames in a case-insensitive fashion when performing pathname expansion (see **Pathname** **Expansion** above). **nocasematch** If set, **bash** matches patterns in a case-insensitive fashion when performing matching while executing **case** or **[[** conditional commands, when performing pat‐ tern substitution word expansions, or when filtering possible completions as part of programmable completion. **nullglob** If set, **bash** allows patterns which match no files (see **Pathname** **Expansion** above) to expand to a null string, rather than themselves. **progcomp** If set, the programmable completion facilities (see **Programmable** **Completion** above) are enabled. This option is enabled by default. **progcomp**___**alias** If set, and programmable completion is enabled, **bash** treats a command name that doesn't have any completions as a possible alias and attempts alias ex‐ pansion. If it has an alias, **bash** attempts programmable completion using the command word resulting from the expanded alias. **promptvars** If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described in **PROMPTING** above. This option is enabled by default. **restricted**___**shell** 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. **shift**___**verbose** If set, the **shift** builtin prints an error message when the shift count exceeds the number of positional parameters. **sourcepath** If set, the **source** (**.**) builtin uses the value of **PATH** to find the directory containing the file supplied as an argument. This option is enabled by de‐ fault. **xpg**___**echo** 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. A login shell cannot be suspended; the **-f** option can be used to override this and force the suspen‐ sion. The return status is 0 unless the shell is a login shell and **-f** is not sup‐ plied, or if job control is not enabled. **test** _expr_ **[** _expr_ **]** Return a status of 0 (true) or 1 (false) depending on the evaluation of the condi‐ tional expression _expr_. Each operator and operand must be a separate argument. Ex‐ pressions are composed of the primaries described in the **bash** manual page under **CONDI**‐‐ **TIONAL** **EXPRESSIONS**. **test** does not accept any options, nor does it accept and ignore an argument of **--** as signifying the end of options. Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. Opera‐ tor precedence is used when there are five or more arguments. **!** _expr_ True if _expr_ is false. **(** _expr_ **)** Returns the value of _expr_. This may be used to override the normal precedence of operators. _expr1_ -**a** _expr2_ True if both _expr1_ and _expr2_ are true. _expr1_ -**o** _expr2_ True if either _expr1_ or _expr2_ is true. **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 opera‐ tors listed above under **CONDITIONAL** **EXPRESSIONS**, the expression is true if the unary test is true. If the first argument is not a valid unary conditional op‐ erator, the expression is false. 3 arguments The following conditions are applied in the order listed. If the second argu‐ ment is one of the binary conditional operators listed above under **CONDITIONAL** **EXPRESSIONS**, the result of the expression is the result of the binary test us‐ ing the first and third arguments as operands. The **-a** and **-o** operators are considered binary operators when there are three arguments. If the first argu‐ ment 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. Oth‐ erwise, the expression is false. 4 arguments If the first argument is **!**, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above. 5 or more arguments The expression is parsed and evaluated according to precedence using the rules listed above. When used with **test** or **[**, the **<** and **>** operators sort lexicographically using ASCII or‐ dering. **times** 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 (and there is a single _sigspec_) or **-**, each specified signal is reset to its original disposition (the value it 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 sup‐ plied or if only **-p** is given, **trap** prints the list of commands associated with each signal. The **-l** option causes the shell to print a list of signal names and their cor‐ responding numbers. Each _sigspec_ is either a signal name defined in <_signal.h_>, or a signal number. Signal names are case insensitive and the **SIG** prefix is optional. 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 before every _simple_ _command_, _for_ com‐ mand, _case_ command, _select_ command, every arithmetic _for_ command, and before the first command executes in a shell function (see **SHELL** **GRAMMAR** above). Refer to the descrip‐ tion of the **extdebug** option to the **shopt** builtin for details of its effect on the **DE**‐‐ **BUG** trap. If a _sigspec_ is **RETURN**, the command _arg_ is executed each time a shell func‐ tion or a script executed with the **.** or **source** builtins finishes executing. If a _sigspec_ is **ERR**, the command _arg_ is executed whenever a pipeline (which may con‐ sist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions. The **ERR** trap is not executed if the failed command is part of the command list immediately following a **while** or **until** key‐ word, part of the test in an _if_ statement, part of a command executed in a **&&** or **||** list except the command following the final **&&** or **||**, any command in a pipeline but the last, or if the command's return value is being inverted using **!**. These are the same conditions obeyed by the **errexit** (**-e**) option. Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored are reset to their original values in a subshell or sub‐ shell environment when one 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** ei‐ ther 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** op‐ tion 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, which is not neces‐ sarily 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 func‐ tions, 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 all of the arguments are found, false if any are not found. **ulimit** [**-HS**] **-a** **ulimit** [**-HS**] [**-bcdefiklmnpqrstuvxPRT** [_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 by a non-root user 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 _limit_ can be a number in the unit specified for the resource or one of the special values **hard**, **soft**, or **unlimited**, which stand for the current hard limit, the current soft limit, and no limit, respectively. If _limit_ 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, if appropriate, are printed before the value. Other options are interpreted as follows: **-a** All current limits are reported; no limits are set **-b** The maximum socket buffer size **-c** The maximum size of core files created **-d** The maximum size of a process's data segment **-e** The maximum scheduling priority ("nice") **-f** The maximum size of files written by the shell and its children **-i** The maximum number of pending signals **-k** The maximum number of kqueues that may be allocated **-l** The maximum size that may be locked into memory **-m** The maximum resident set size (many systems do not honor this limit) **-n** The maximum number of open file descriptors (most systems do not allow this value to be set) **-p** The pipe size in 512-byte blocks (this may not be set) **-q** The maximum number of bytes in POSIX message queues **-r** The maximum real-time scheduling priority **-s** The maximum stack size **-t** The maximum amount of cpu time in seconds **-u** The maximum number of processes available to a single user **-v** The maximum amount of virtual memory available to the shell and, on some sys‐ tems, to its children **-x** The maximum number of file locks **-P** The maximum number of pseudoterminals **-R** The maximum time a real-time process can run before blocking, in microseconds **-T** The maximum number of threads If _limit_ is given, and the **-a** option is not used, _limit_ is the new value of the speci‐ fied resource. If no option is given, then **-f** is assumed. Values are in 1024-byte increments, except for **-t**, which is in seconds; **-R**, which is in microseconds; **-p**, which is in units of 512-byte blocks; **-P**, **-T**, **-b**, **-k**, **-n**, and **-u**, which are unscaled values; and, when in posix mode, **-c** and **-f**, which are in 512-byte increments. The re‐ turn 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 inter‐ preted as an octal number; otherwise it is interpreted as a symbolic mode mask similar to that accepted by [_chmod_(1)](https://www.chedong.com/phpMan.php/man/chmod/1/markdown). 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 defi‐ nitions are removed. The return value is true unless a supplied _name_ is not a defined alias. **unset** [-**fv**] [-**n**] [_name_ ...] For each _name_, remove the corresponding variable or function. If the **-v** option is given, each _name_ refers to a shell variable, and that variable is removed. Read-only variables may not be unset. If **-f** is specified, each _name_ refers to a shell function, and the function definition is removed. If the **-n** option is supplied, and _name_ is a variable with the _nameref_ attribute, _name_ will be unset rather than the variable it references. **-n** has no effect if the **-f** option is supplied. If no options are sup‐ plied, each _name_ refers to a variable; if there is no variable by that name, a func‐ tion with that name, if any, is unset. Each unset variable or function is removed from the environment passed to subsequent commands. If any of **BASH**___**ALIASES**, **BASH**___**ARGV0**, **BASH**___**CMDS**, **BASH**___**COMMAND**, **BASH**___**SUBSHELL**, **BASHPID**, **COMP**___**WORDBREAKS**, **DIRSTACK**, **EPOCHREALTIME**, **EPOCHSECONDS**, **FUNCNAME**, **GROUPS**, **HISTCMD**, **LINENO**, **RANDOM**, **SEC**‐‐ **ONDS**, or **SRANDOM** are unset, they lose their special properties, even if they are sub‐ sequently reset. The exit status is true unless a _name_ is readonly. **wait** [**-fn**] [**-p** _varname_] [_id_ _..._] Wait for each specified child process and return its termination status. Each _id_ 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 _id_ is not given, **wait** waits for all running back‐ ground jobs and the last-executed process substitution, if its process id is the same as **$!**, and the return status is zero. If the **-n** option is supplied, **wait** waits for a single job from the list of _id_s or, if no _id_s are supplied, any job, to complete and returns its exit status. If none of the supplied arguments is a child of the shell, or if no arguments are supplied and the shell has no unwaited-for children, the exit status is 127. If the **-p** option is supplied, the process or job identifier of the job for which the exit status is returned is assigned to the variable _varname_ named by the option argument. The variable will be unset initially, before any assignment. This is useful only when the **-n** option is supplied. Supplying the **-f** option, when job con‐ trol is enabled, forces **wait** to wait for _id_ to terminate before returning its status, instead of returning when it changes status. If _id_ 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. ## SHELL COMPATIBILITY MODE Bash-4.0 introduced the concept of a `shell compatibility level', specified as a set of op‐ tions to the shopt builtin **compat31**, **compat32**, **compat40**, **compat41**, and so on). There is only one current compatibility level -- each option is mutually exclusive. The compatibility level is intended to allow users to select behavior from previous versions that is incompati‐ ble with newer versions while they migrate scripts to use current features and behavior. It's intended to be a temporary solution. This section does not mention behavior that is standard for a particular version (e.g., set‐ ting **compat32** means that quoting the rhs of the regexp matching operator quotes special reg‐ exp characters in the word, which is default behavior in bash-3.2 and above). If a user enables, say, **compat32**, it may affect the behavior of other compatibility levels up to and including the current compatibility level. The idea is that each compatibility level controls behavior that changed in that version of **bash**, but that behavior may have been present in earlier versions. For instance, the change to use locale-based comparisons with the **[[** command came in bash-4.1, and earlier versions used ASCII-based comparisons, so en‐ abling **compat32** will enable ASCII-based comparisons as well. That granularity may not be sufficient for all uses, and as a result users should employ compatibility levels carefully. Read the documentation for a particular feature to find out the current behavior. Bash-4.3 introduced a new shell variable: **BASH**___**COMPAT**. The value assigned to this variable (a decimal version number like 4.2, or an integer corresponding to the **compat**_NN_ option, like 42) determines the compatibility level. Starting with bash-4.4, Bash has begun deprecating older compatibility levels. Eventually, the options will be removed in favor of **BASH**___**COMPAT**. Bash-5.0 is the final version for which there will be an individual shopt option for the pre‐ vious version. Users should use **BASH**___**COMPAT** on bash-5.0 and later versions. The following table describes the behavior changes controlled by each compatibility level setting. The **compat**_NN_ tag is used as shorthand for setting the compatibility level to _NN_ us‐ ing one of the following mechanisms. For versions prior to bash-5.0, the compatibility level may be set using the corresponding **compat**_NN_ shopt option. For bash-4.3 and later versions, the **BASH**___**COMPAT** variable is preferred, and it is required for bash-5.1 and later versions. ### compat31 • quoting the rhs of the **[[** command's regexp matching operator (=~) has no spe‐ cial effect ### compat32 • interrupting a command list such as "a ; b ; c" causes the execution of the next command in the list (in bash-4.0 and later versions, the shell acts as if it received the interrupt, so interrupting one command in a list aborts the ex‐ ecution of the entire list) ### compat40 • the **<** and **>** operators to the **[[** command do not consider the current locale when comparing strings; they use ASCII ordering. Bash versions prior to bash-4.1 use ASCII collation and [_strcmp_(3)](https://www.chedong.com/phpMan.php/man/strcmp/3/markdown); bash-4.1 and later use the current locale's collation sequence and [_strcoll_(3)](https://www.chedong.com/phpMan.php/man/strcoll/3/markdown). ### compat41 • in _posix_ mode, **time** may be followed by options and still be recognized as a re‐ served word (this is POSIX interpretation 267) • in _posix_ mode, the parser requires that an even number of single quotes occur in the _word_ portion of a double-quoted parameter expansion and treats them spe‐ cially, so that characters within the single quotes are considered quoted (this is POSIX interpretation 221) ### compat42 • the replacement string in double-quoted pattern substitution does not undergo quote removal, as it does in versions after bash-4.2 • in posix mode, single quotes are considered special when expanding the _word_ portion of a double-quoted parameter expansion and can be used to quote a clos‐ ing brace or other special character (this is part of POSIX interpretation 221); in later versions, single quotes are not special within double-quoted word expansions ### compat43 • the shell does not print a warning message if an attempt is made to use a quoted compound assignment as an argument to declare (declare -a foo='(1 2)'). Later versions warn that this usage is deprecated • word expansion errors are considered non-fatal errors that cause the current command to fail, even in posix mode (the default behavior is to make them fatal errors that cause the shell to exit) • when executing a shell function, the loop state (while/until/etc.) is not re‐ set, so **break** or **continue** in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this ### compat44 • the shell sets up the values used by **BASH**___**ARGV** and **BASH**___**ARGC** so they can expand to the shell's positional parameters even if extended debugging mode is not en‐ abled • a subshell inherits loops from its parent context, so **break** or **continue** will cause the subshell to exit. Bash-5.0 and later reset the loop state to prevent the exit • variable assignments preceding builtins like **export** and **readonly** that set at‐ tributes continue to affect variables with the same name in the calling envi‐ ronment even if the shell is not in posix mode ### compat50 • Bash-5.1 changed the way **$RANDOM** is generated to introduce slightly more ran‐ domness. If the shell compatibility level is set to 50 or lower, it reverts to the method from bash-5.0 and previous versions, so seeding the random number generator by assigning a value to **RANDOM** will produce the same sequence as in bash-5.0 • If the command hash table is empty, bash versions prior to bash-5.1 printed an informational message to that effect, even when producing output that can be reused as input. Bash-5.1 suppresses that message when the **-l** option is sup‐ plied. ## SEE ALSO [bash(1)](https://www.chedong.com/phpMan.php/man/bash/1/markdown), [sh(1)](https://www.chedong.com/phpMan.php/man/sh/1/markdown) GNU Bash-2.05a 2001 October 29 [BASH-BUILTINS(7)](https://www.chedong.com/phpMan.php/man/BASH-BUILTINS/7/markdown)