Linux: bash builtin commands(fedora38-GNU Bash 5.2)

Linux:  bash builtin commands(fedora38-GNU Bash 5.2)

 

 

 

1. bash builtin commands(fedora38-GNU Bash 5.2)

BASH_BUILTINS(1)                                          General Commands Manual                                          BASH_BUILTINS(1)

NAME
       :,  .,  [,  alias, bg, bind, break, builtin, caller, cd, command, compgen, complete, compopt, continue, declare, dirs, disown, echo,
       enable, eval, exec, exit, export, false, fc, fg, getopts, hash, help, history,  jobs,  kill,  let,  local,  logout,  mapfile,  popd,
       printf,  pushd,  pwd, read, readarray, readonly, return, set, shift, shopt, source, suspend, test, times, trap, true, type, typeset,
       ulimit, umask, unalias, unset, wait - bash built-in commands, see bash(1)

BASH BUILTIN COMMANDS
       Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to signify the
       end of the options.  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 - without 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 exe‐
              cuted from filename.  If filename does not contain a slash, filenames in PATH are used to find the directory containing file‐
              name,  but  filename does not need to be executable.  The file searched for in PATH need not be executable.  When bash is not
              in posix mode, it searches the current directory if no file is found in PATH.  If the sourcepath option to the shopt  builtin
              command  is  turned off, the PATH is not searched.  If any arguments are supplied, they become the positional parameters when
              filename is executed.  Otherwise the positional parameters are unchanged.  If the -T option is enabled, . inherits  any  trap
              on  DEBUG;  if it is not, any DEBUG trap string is saved and restored around the call to ., and . unsets the DEBUG trap while
              it executes.  If -T is not set, and the sourced file changes the DEBUG trap, the new value is retained when . completes.  The
              return  status is the status of the last command exited within the script (0 if no commands are executed), and false if file‐
              name is not found or cannot be read.

       alias [-p] [name[=value] ...]
              Alias with no arguments or with the -p option prints the list of aliases in the form alias  name=value  on  standard  output.
              When  arguments  are  supplied, an alias is defined for each name whose value is given.  A trailing space in value causes the
              next word to be checked for alias substitution when the alias is expanded.  For each name in the argument list for  which  no
              value  is supplied, the name and value of the alias is printed.  Alias returns true unless a name is given for which no alias
              has been defined.

       bg [jobspec ...]
              Resume 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 enabled, 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
       bind readline-command-line
              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 a readline initialization file such as .inputrc, but
              each binding or command must be passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'.  Options, if  supplied,
              have the following meanings:
              -m keymap
                     Use  keymap  as  the keymap to be affected by the subsequent bindings.  Acceptable keymap names are emacs, emacs-stan‐
                     dard, 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-command 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.  The shell as‐
                     signs any numeric argument the user supplied to the READLINE_ARGUMENT variable.  If there was no argument, that  vari‐
                     able is not set.  If the executed command changes the value of any of READLINE_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 levels.  n must be ≥ 1.  If  n  is  greater
              than  the  number  of enclosing loops, all enclosing loops are exited.  The return value is 0 unless 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  func‐
              tion  whose  name  is  the  same  as a shell builtin, retaining the functionality of the builtin within the function.  The cd
              builtin is commonly redefined this way.  The return status is false if shell-builtin is not a shell builtin command.

       caller [expr]
              Returns the context of any active subroutine call (a shell function or a script executed with  the  .  or  source  builtins).
              Without  expr, caller displays the line number and source filename of the current subroutine call.  If a non-negative integer
              is supplied as expr, caller displays the line number, subroutine name, and source file corresponding to that position in  the
              current execution call stack.  This extra information may be used, for example, to print a stack trace.  The current frame is
              frame 0.  The return value is 0 unless the shell is not executing a subroutine call or expr does not correspond  to  a  valid
              position in the call stack.

       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.  The vari‐
              able CDPATH defines the search path for the directory containing dir: each directory name in CDPATH is searched for dir.  Al‐
              ternative 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  physi‐
              cal  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 followed by resolving the link  af‐
              ter  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 directory 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.  If the directory change is successful, cd sets the value of the PWD environment variable to
              the new directory name, and sets the OLDPWD environment variable to the value of the current  working  directory  before  the
              change.  The return value is true if the directory was successfully changed; false otherwise.

       command [-pVv] command [arg ...]
              Run  command with args suppressing the normal shell function lookup.  Only builtin commands or commands found in the PATH are
              executed.  If the -p option is given, the search for command is performed using a default value for PATH that  is  guaranteed
              to  find all of the standard utilities.  If either the -V or -v option is supplied, a description of command is printed.  The
              -v option causes a single word indicating the command or filename used to invoke command to be displayed; the -V option  pro‐
              duces  a  more  verbose description.  If the -V or -v option is supplied, the exit status is 0 if command was found, and 1 if
              not.  If neither option is supplied and an error occurred or command cannot be found, the exit status is 127.  Otherwise, the
              exit status of the command builtin is the exit status of command.

       compgen [option] [word]
              Generate  possible  completion  matches  for  word according to the options, which may be any option accepted by the 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 com‐
              pletion specification with the same flags.  If word is specified, only those completions matching word will be displayed.

              The return value is true unless an invalid option is supplied, or no matches were generated.

       complete [-abcdefgjksuv] [-o comp-option] [-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,  exist‐
              ing  completion  specifications are printed in a way that allows them to be reused as input.  The -r option removes a comple‐
              tion specification for each name, or, if no names are supplied, all completion specifications.  The -D option indicates  that
              other  supplied  options  and  actions 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 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 com‐
              mand  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  arguments
              are ignored; these completions only apply to the case specified by the option.

              The process of applying these completion specifications when word completion is attempted is described in bash(1).

              Other  options,  if  specified, have the following meanings.  The arguments to the -G, -W, and -X options (and, if necessary,
              the -P and -S options) should be quoted to protect them from expansion before the complete builtin is invoked.
              -o comp-option
                      The comp-option controls several aspects of the compspec's behavior beyond  the  simple  generation  of  completions.
                      comp-option may be one of:
                      bashdefault
                              Perform the rest of the default bash completions if the compspec generates 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 alphabetically.
                      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 completions:
                      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.   Arguments  are
                      passed as with the -F option.
              -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 command 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 delimiters, and each resultant word is  ex‐
                      panded.   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 completions are the members  of  the  resultant  list
                      which match the word being completed.
              -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 argu‐
              ment, an attempt is made to remove a completion specification for a name for which no specification exists, or an  error  oc‐
              curs adding a completion specification.

       compopt [-o option] [-DEI] [+o option] [name]
              Modify  completion  options for each name according to the options, or for the currently-executing completion if no names are
              supplied.  If no options are given, display 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 option indicates that other supplied op‐
              tions 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 comple‐
              tion; 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 com‐
              mand 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 output error occurs.

       continue [n]
              Resume the next iteration of the enclosing for, while, until, or select loop.  If n is specified, resume at the nth enclosing
              loop.  n must be ≥ 1.  If n is greater than the number of enclosing loops, the last enclosing loop (the  ``top-level''  loop)
              is resumed.  The return value is 0 unless 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 names 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 attributes and values of all variables
              having the attributes specified by the additional options.  If no other options are supplied with -p,  declare  will  display
              the attributes and values of all shell variables.  The -f option will restrict the display to shell functions.  The -F option
              inhibits the display of function definitions; only the function name and attributes are printed.  If the extdebug  shell  op‐
              tion  is  enabled using shopt, the source file name and line number where each name is defined are displayed as well.  The -F
              option implies -f.  The -g option forces variables to be created or modified at the global scope, even when declare  is  exe‐
              cuted in a shell function.  It is ignored in all other cases.  The -I option causes local variables to inherit 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 local 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 in bash(1)).
              -A     Each name is an associative array variable (see Arrays in bash(1)).
              -f     Use function names only.
              -i     The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION in bash(1)) 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 attri‐
                     bute 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 references, assignments, and attribute modifications to name, except those using or chang‐
                     ing 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 names readonly.  These names cannot then be assigned values by subsequent assignment statements or unset.
              -t     Give  each name the trace attribute.  Traced functions inherit the DEBUG and RETURN traps from the calling shell.  The
                     trace attribute has no special meaning for variables.
              -u     When the variable is assigned a value, all lower-case characters are converted to upper-case.  The  lower-case  attri‐
                     bute is disabled.
              -x     Mark names 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  at‐
              tributes  do not take effect until subsequent assignments.  The return value is 0 unless an invalid option is encountered, an
              attempt is made to define a function using ``-f foo=bar'', an attempt is made to assign a value to a  readonly  variable,  an
              attempt  is made to assign a value to an array variable without using the compound assignment syntax (see Arrays in bash(1)),
              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  direc‐
              tory  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 nth entry counting from the left of the list shown by dirs when invoked without  options,  starting  with
                     zero.
              -n     Displays  the  nth 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 args, 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
              characters is enabled.  The -E option disables the interpretation of these escape characters, even on systems where they  are
              interpreted by default.  The xpg_echo shell option may be used to dynamically determine whether or not echo expands these es‐
              cape characters by default.  echo does not interpret -- to mean the end of options.  echo interprets the following escape se‐
              quences:
              \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
              \0nnn  the eight-bit character whose value is the octal value nnn (zero to three octal digits)
              \xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
              \uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
              \UHHHHHHHH
                     the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)

       enable [-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 ``enable -n test''.  The -f option means to load the new  builtin  command
              name  from  shared  object  filename,  on  systems  that  support dynamic loading.  Bash will use the value of the BASH_LOAD‐
              ABLES_PATH variable as a colon-separated list of directories in which to search for filename.  The default  is  system-depen‐
              dent.  The -d option will delete a builtin previously loaded with -f.  If no name arguments are given, or if the -p option is
              supplied, a list of shell builtins is printed.  With no other option arguments,  the  list  consists  of  all  enabled  shell
              builtins.  If -n is supplied, only disabled builtins are printed.  If -a is supplied, the list printed includes all builtins,
              with an indication of whether or not each is enabled.  If -s is supplied, the output  is  restricted  to  the  POSIX  special
              builtins.   If  no options are supplied and a name is not a shell builtin, enable will attempt to load name from a shared ob‐
              ject named name, as if the command were ``enable -f name name .  The return value is 0 unless a name is not a  shell  builtin
              or there is an error loading a new builtin from a shared object.

       eval [arg ...]
              The  args are read and concatenated together into a single command.  This command is then read and executed by the shell, and
              its exit status is returned as the value of eval.  If there are no args, or only null arguments, eval returns 0.

       exec [-cl] [-a name] [command [arguments]]
              If command is specified, it replaces the shell.  No new process is created.  The arguments become the arguments  to  command.
              If  the  -l  option  is supplied, the shell places a dash at the beginning of the zeroth argument passed to command.  This is
              what login(1) does.  The -c option causes command to be executed with an empty environment.  If -a  is  supplied,  the  shell
              passes name as the zeroth argument to the executed command.  If command cannot be executed for some reason, a non-interactive
              shell exits, unless the 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 effect in the current shell, and the return status is 0.  If there is a redirection error, the return  sta‐
              tus 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 encoun‐
              tered, 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
              index into the history list, where a negative number is used as an offset from the current 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
              equivalent to -1 and -0 is invalid.  If last is not specified, it is set to the current command for listing (so that ``fc  -l
              -10''  prints the last 10 commands) and to first otherwise.  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  op‐
              tion  is  given,  the commands are listed on standard output.  Otherwise, the editor given by ename is invoked on a file con‐
              taining 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 encountered or first or last specify history
              lines out of range.  If the -e option is supplied, the return value is the value of the last command executed or  failure  if
              an  error  occurs  with the temporary file of commands.  If the second form is used, the return status is that of the command
              re-executed, unless cmd does not specify a valid history line, in which case fc returns failure.

       fg [jobspec]
              Resume jobspec in the foreground, and make it the current job.  If jobspec is not present, the shell's notion of the  current
              job  is used.  The return value is that of the command placed into the foreground, or failure if run when job control is dis‐
              abled or, when run with job control enabled, if jobspec does not specify a valid job or jobspec  specifies  a  job  that  was
              started without job control.

       getopts optstring name [arg ...]
              getopts  is  used  by shell procedures to parse positional parameters.  optstring contains the option characters to be recog‐
              nized; 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 processed into the variable OPTIND.  OPTIND is initialized to 1 each time the shell or a shell script is in‐
              voked.  When an option requires an argument, getopts places that argument 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 invocation if a new
              set of parameters is to be used.

              When the end of options is encountered, getopts exits with a return value greater than zero.  OPTIND is set to the  index  of
              the first non-option argument, and name is set to ?.

              getopts normally parses the positional parameters, but if more arguments are supplied as arg values, getopts parses those in‐
              stead.

              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  re‐
              membered.   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 location of each name.  If the -t option is supplied, the full path‐
              name 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 argu‐
              ments are given, or if only -l is supplied, information about remembered commands is printed.  The return status is true  un‐
              less 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 match‐
              ing 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) to display the time stamp associated with each displayed history entry.  No intervening blank is printed  be‐
              tween 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 interpreted as relative to one greater than
                     the  last  history  position, so negative 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 range of history entries between positions start and end, inclusive.   Positive  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 cur‐
                     rent bash session, but not already appended 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 his‐
              tory file, marked with the history comment character.  When the history file is read, lines beginning with the  history  com‐
              ment  character  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 error occurs while reading or writing the  history  file,  an  invalid
              offset or range 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 argu‐
              ment  to -l is a number specifying either a signal number or the exit status of a process terminated by a signal.  The -L op‐
              tion 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 in bash(1)).  If the last arg evaluates to 0,
              let returns 1; 0 is returned otherwise.

       local [option] [name[=value] ... | - ]
              For each argument, a local variable named name is created, and assigned value.  The option can be any of the options accepted
              by  declare.   When  local  is used within a function, it causes the variable name to have a visible scope restricted to that
              function and its children.  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 re‐
              turns.  The restore 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 local variables to the standard output.  It is an error to use local
              when not within a function.  The return status is 0 unless local is used outside a function, an invalid name is supplied,  or
              name is a readonly variable.

       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 quantum] [array]
              Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the -u option is sup‐
              plied.  The variable MAPFILE is the default array.  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 evaluated, it is supplied the index of the next
              array element to be assigned and the line to be assigned to that element as additional arguments.  callback is evaluated  af‐
              ter 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.  The elements are numbered from 0 starting at the first directory listed  by  dirs.
              With  no  arguments, popd removes the top directory from the stack, and changes 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  manipu‐
                     lated.
              +n     Removes the nth entry counting from the left of the list shown by dirs, starting with zero, from the stack.  For exam‐
                     ple: ``popd +0'' removes the first directory, ``popd +1'' the second.
              -n     Removes the nth entry counting from the right of the list shown by dirs, starting with zero.  For example: ``popd -0''
                     removes the last directory, ``popd -1'' the next to last.

              If the top element of the directory stack is modified, and the -n option was not supplied, popd uses the cd builtin to change
              to the directory at the top of the stack.  If the cd fails, popd returns a non-zero value.

              Otherwise, popd returns false if an invalid option is encountered, the directory stack is empty, or a non-existent  directory
              stack entry is specified.

              If the popd command is successful, bash runs dirs to show the final contents of the directory stack, and the return status is
              0.

       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 characters, which are simply copied to standard
              output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of
              which  causes  printing of the next successive argument.  In addition to the standard printf(1) format specifications, printf
              interprets the following extensions:
              %b     causes printf to expand backslash escape sequences in the corresponding argument 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.
              %Q     like %q, but applies any supplied precision to the argument before quoting it.
              %(datefmt)T
                     causes printf to output the date-time string resulting from using datefmt as a format  string  for  strftime(3).   The
                     corresponding  argument is an integer representing the number of seconds since the epoch.  Two special argument values
                     may be used: -1 represents the current time, and -2 represents the time the shell was  invoked.   If  no  argument  is
                     specified, conversion behaves as if -1 had been given.  This is an exception to the 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  origi‐
              nal.

              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 requires more arguments than are  supplied,
              the  extra  format  specifications  behave  as if a zero value or null string, as appropriate, had been supplied.  The return
              value is zero on success, non-zero on failure.

       pushd [-n] [+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 exchanges the top two elements of the directory stack.  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 nth 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 nth 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

              After the stack has been modified, if the -n option was not supplied, pushd uses the cd builtin to change to the directory at
              the top of the stack.  If the cd fails, pushd returns a non-zero value.

              Otherwise, if no arguments are supplied, pushd returns 0 unless the directory stack is empty.  When  rotating  the  directory
              stack, pushd returns 0 unless the directory stack is empty or a non-existent directory stack element is specified.

              If the pushd command is successful, bash runs dirs to show the final contents of the directory stack.

       pwd [-LP]
              Print  the absolute pathname of the current working directory.  The pathname printed contains no symbolic links if the -P op‐
              tion is supplied or the -o physical option to the set builtin command is enabled.  If the -L option  is  used,  the  pathname
              printed  may contain symbolic links.  The return status is 0 unless an error occurs while reading the name of the current di‐
              rectory 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 in bash(1) under Word Splitting, 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  as‐
              signed  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  in  bash(1) 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 in bash(1)) is used to obtain the line.  Read‐
                     line  uses  the  current (or default, if line editing was not previously active) editing settings, but uses readline's
                     default filename completion.
              -i text
                     If readline is being used to read the line, text is placed into the editing buffer before editing begins.
              -n nchars
                     read returns after 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  be‐
                     low).
              -p prompt
                     Display prompt on standard error, without a trailing newline, before attempting to read any input.  The prompt is dis‐
                     played only if input is coming from a terminal.
              -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 descriptor, or the read will return EOF,  non-zero  otherwise.   The  exit  status  is
                     greater than 128 if the timeout is exceeded.
              -u fd  Read input from file descriptor fd.

              If  no  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 sup‐
              plied 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 ar‐
              rays; the -A option restricts the variables to associative arrays.  If both options are supplied, -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  for‐
              mat that may be reused as input.  If a variable name is followed by =word, the value of the variable is set to word.  The re‐
              turn status is 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is  sup‐
              plied 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 function body.  If return is executed by a trap handler, the last  command  used  to
              determine  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 re‐
              turn is used outside a function, but during execution of a script by the .  (source) command, it causes the shell to stop ex‐
              ecuting that script and return either n or the exit status of the last command executed within the script as the exit  status
              of  the script.  If 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 function 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, display the name and value of each shell variable in a format that can be reused as input for setting or re‐
              setting 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 attribute and marked for export to the en‐
                      vironment of subsequent commands.
              -b      Report the status of terminated background jobs immediately, rather than before the next primary prompt.  This is ef‐
                      fective only when job control is enabled.
              -e      Exit  immediately  if  a  pipeline (which may consist of a single simple command), a list, or a compound command (see
                      SHELL GRAMMAR in bash(1)), exits with a non-zero status.  The shell does not exit if the command that fails  is  part
                      of  the  command  list  immediately following a while or until keyword, part of the test following the if or elif re‐
                      served 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 because 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 in bash(1)), 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 being ignored, none of the commands exe‐
                      cuted 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  function
                      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 environment 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 interactive shells on systems  that  support
                      it  (see  JOB  CONTROL in bash(1)).  All processes 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 in bash(1) under HISTORY.  This option is on by default in interactive
                              shells.
                      ignoreeof
                              The effect is as if the shell command ``IGNOREEOF=10'' had been executed (see Shell Variables in bash(1)).
                      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 in bash(1) for a reference to a document that  details  how  posix  mode  affects
                              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 op‐
                      tion-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  in‐
                      herited  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.
              -r      Enable restricted shell mode.  This option cannot be unset once it has been set.
              -t      Exit after reading and executing one command.
              -u      Treat  unset  variables  and parameters other than the special parameters "@" and "*", or array variables subscripted
                      with "@" or "*", as an error when performing parameter expansion.  If expansion is attempted 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 command, 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 in bash(1)).  This is on by default.
              -C      If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators.  This may be  overrid‐
                      den when creating output files by using the redirection operator >| instead of >.
              -E      If  set,  any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell
                      environment.  The ERR trap is normally 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 directory structure instead.  By default, bash follows the logical chain of directo‐
                      ries when performing commands which change the current directory.
              -T      If set, any traps on DEBUG and RETURN are inherited by shell functions, command substitutions, and commands  executed
                      in a subshell environment.  The DEBUG 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 args, even if some of them begin with a -.
              -       Signal the end of options, cause all remaining args to be assigned to the positional parameters.  The -x and  -v  op‐
                      tions are turned off.  If there are no args, the positional parameters remain unchanged.

              The options are off by default unless otherwise noted.  Using + rather than - causes these options to be turned off.  The op‐
              tions can also be specified as arguments to an invocation of the shell.  The current set of options may be found in $-.   The
              return status is always true unless an invalid option is encountered.

       shift [n]
              The  positional  parameters  from n+1 ... are renamed to $1 ....  Parameters represented by the numbers $# down to $#-n+1 are
              unset.  n must be a non-negative number less than or equal to $#.  If n is 0, no parameters are changed.  If n is not  given,
              it  is  assumed  to  be 1.  If n is greater than $#, the positional parameters are not changed.  The return status is greater
              than zero if n is greater than $# or less than zero; otherwise 0.

       shopt [-pqsu] [-o] [optname ...]
              Toggle the values of 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 option causes output to be displayed in a form that may be reused as input.
              Other options have the following meanings:
              -s     Enable (set) each optname.
              -u     Disable (unset) each optname.
              -q     Suppresses normal output (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 subscripts during arithmetic expression evalua‐
                      tion, 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 assumed to be the name of a  variable  whose
                      value is the directory to change to.
              cdspell If  set, minor errors in the spelling of a directory component in a cd command will be corrected.  The errors checked
                      for are transposed characters, a missing character, and 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 trying 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 deferred until a second exit is attempted without an intervening command (see JOB
                      CONTROL in bash(1)).  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 option is enabled by default, but only has an effect if command history is  en‐
                      abled, as described in bash(1) under HISTORY.
              compat31
              compat32
              compat40
              compat41
              compat42
              compat43
              compat44
              compat50
                      These control aspects of the shell's compatibility mode (see SHELL COMPATIBILITY MODE in bash(1)).

              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 using backslashes to quote completed file‐
                      names.  This variable is set by default, 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 completion if the directory name initially
                      supplied does not exist.

              dotglob If set, bash includes filenames beginning with a `.' in the results of pathname expansion.  The filenames ``.''   and
                      ``..''  must always be matched explicitly, 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 in bash(1) under ALIASES.  This option is enabled by default  for  interac‐
                      tive 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  en‐
                      abled:

                      1.     The -F option to the declare builtin displays the source file name and line number corresponding to each func‐
                             tion name supplied as an argument.

                      2.     If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.

                      3.     If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell
                             function or a shell script executed by the . or source builtins), the shell simulates a call to return.

                      4.     BASH_ARGC and BASH_ARGV are updated as described in their descriptions in bash(1)).

                      5.     Function tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) in‐
                             herit the DEBUG and RETURN traps.

                      6.     Error tracing is enabled: command substitution, shell functions, and subshells invoked with (  command  )  in‐
                             herit the ERR trap.

              extglob If set, the extended pattern matching features described in bash(1) under Pathname Expansion are enabled.

              extquote
                      If  set, $'string' and $"string" quoting is performed within ${parameter} expansions enclosed in double quotes.  This
                      option is enabled by default.

              failglob
                      If set, patterns which fail to match filenames during pathname expansion result in an expansion error.

              force_fignore
                      If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing  word  comple‐
                      tion  even  if the ignored words are the only possible completions.  See SHELL VARIABLES in bash(1) 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 in bash(1)) behave as if
                      in  the  traditional  C  locale when performing 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  col‐
                      late together.

              globskipdots
                      If set, pathname expansion will never match the filenames ``.''  and ``..'', even if the pattern begins with a ``.''.
                      This option is enabled by default.

              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 followed 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 completion when a word containing a @ is
                      being completed (see Completing under READLINE in bash(1)).  This is enabled by default.

              huponexit
                      If set, bash will send SIGHUP to all jobs when an interactive login shell exits.

              inherit_errexit
                      If set, command substitution inherits the value of the errexit option, instead of unsetting it in the subshell  envi‐
                      ronment.  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 in bash(1)).  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  function
                      scope.

              login_shell
                      The  shell  sets  this  option  if  it is started as a login shell (see INVOCATION in bash(1)).  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  Expan‐
                      sion in bash(1)).

              nocasematch
                      If  set, bash matches patterns in a case-insensitive fashion when performing matching while executing case or [[ con‐
                      ditional commands, when performing pattern substitution word expansions, or when filtering  possible  completions  as
                      part of programmable completion.

              noexpand_translation
                      If  set,  bash  encloses  the translated results of $"..." quoting in single quotes instead of double quotes.  If the
                      string is not translated, this has no effect.

              nullglob
                      If set, bash allows patterns which match no files (see Pathname Expansion in bash(1)) to expand  to  a  null  string,
                      rather than themselves.

              patsub_replacement
                      If  set,  bash  expands occurrences of & in the replacement string of pattern substitution to the text matched by the
                      pattern, as described under Parameter Expansion in bash(1).  This option is enabled by default.

              progcomp
                      If set, the programmable completion facilities (see Programmable Completion in bash(1)) 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 expansion. If it has an alias, bash attempts programmable completion using the com‐
                      mand word resulting from the expanded alias.

              promptvars
                      If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal af‐
                      ter being expanded as described in PROMPTING in bash(1).  This option is enabled by default.

              restricted_shell
                      The shell sets this option if it is started in restricted mode (see RESTRICTED SHELL in bash(1)).  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  argu‐
                      ment.  This option is enabled by default.

              varredir_close
                      If  set,  the  shell automatically closes file descriptors assigned using the {varname} redirection syntax (see REDI‐
                      RECTION in bash(1)) instead of leaving them open when the command completes.

              syslog_history
                      If set, command history is logged to syslog.

              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, or a shell without  job  control  en‐
              abled,  cannot be suspended; the -f option can be used to override this and force the suspension.  The return status is 0 un‐
              less the shell is a login shell or job control is not enabled and -f is not supplied.

       test expr
       [ expr ]
              Return a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression expr.  Each  operator  and
              operand  must  be  a separate argument.  Expressions are composed of the primaries described in bash(1) under CONDITIONAL EX‐
              PRESSIONS.  test does not accept any options, nor does it accept and ignore an argument of -- as signifying the  end  of  op‐
              tions.

              Expressions  may be combined using the following operators, listed in decreasing order of precedence.  The evaluation depends
              on the number of arguments; see below.  Operator 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 operators listed in bash(1) under CONDITIONAL EXPRESSIONS, the expression is true if
                     the unary test is true.  If the first argument is not a valid unary conditional operator, the expression is false.
              3 arguments
                     The following conditions are applied in the order listed.  If the second argument is one of the binary conditional op‐
                     erators listed in bash(1) under CONDITIONAL EXPRESSIONS, the result of the expression is the result of the binary test
                     using the first and third arguments as operands.  The -a and -o operators are considered binary operators  when  there
                     are  three arguments.  If the first argument is !, the value is the negation of the two-argument test using the second
                     and third arguments.  If the first argument is exactly ( and the third argument is exactly ), the result is  the  one-
                     argument test of the second argument.  Otherwise, the expression is false.
              4 arguments
                     The  following  conditions are applied in the order listed.  If the first argument is !, the result is the negation of
                     the three-argument expression composed of the remaining arguments.  the two-argument test using the second  and  third
                     arguments.   If  the  first argument is exactly ( and the fourth argument is exactly ), the result is the two-argument
                     test of the second and third arguments.  Otherwise, the expression is parsed and evaluated according to precedence us‐
                     ing 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 ordering.

       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 sin‐
              gle 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  argu‐
              ments  are  supplied  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 corresponding numbers.  Each sigspec is either a signal  name  de‐
              fined 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 exe‐
              cuted before every simple command, for command, case command, select command, every arithmetic for command,  and  before  the
              first  command  executes in a shell function (see SHELL GRAMMAR in bash(1)).  Refer to the description of the extdebug option
              to the shopt builtin for details of its effect on the DEBUG trap.  If a sigspec is RETURN, the command arg is  executed  each
              time a shell function 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 consist 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 keyword, 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, reset or listed.  Trapped signals that are not being  ignored  are
              reset  to their original values in a subshell or subshell 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, func‐
              tion, builtin, or disk file, respectively.  If the name is not found, then nothing is printed, and an exit status of false is
              returned.  If the -p option is used, type either returns the name of the disk file that would be executed if name were speci‐
              fied as a command name, or nothing if ``type -t name'' would not return file.  The -P option forces a PATH  search  for  each
              name, even if ``type -t name'' would not return file.  If a command is hashed, -p and -P print the hashed value, which is not
              necessarily the file that appears first in PATH.  If the -a option is used, type prints all of the places that contain an ex‐
              ecutable  named  name.   This  includes  aliases  and functions, if and only if the -p option is not also used.  The table of
              hashed commands is not consulted when using -a.  The -f option suppresses shell function lookup, as with the command builtin.
              type returns true if 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 con‐
              trol.  The -H and -S options specify that the hard or soft limit is set for the given resource.  A hard limit cannot  be  in‐
              creased  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,  un‐
              less  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 systems, to its children
              -x     The maximum number of file locks
              -P     The maximum number of pseudoterminals
              -R     The maximum time a real-time process can run before blocking, in microseconds
              -T     The maximum number of threads

              If limit is given, and the -a option is not used, limit is the new value of the specified resource.  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 return status is 0 unless an invalid option or argument is supplied, or an er‐
              ror occurs while setting a new limit.  In POSIX Mode 512-byte blocks are used for the `-c' and `-f' options.

       umask [-p] [-S] [mode]
              The user file-creation mask is set to mode.  If mode begins with a digit, it is interpreted as an octal number; otherwise  it
              is  interpreted  as  a symbolic mode mask similar to that accepted by chmod(1).  If mode is omitted, the current value of the
              mask is printed.  The -S option causes the mask to be printed in symbolic form; the default output is an  octal  number.   If
              the  -p option is supplied, and mode is omitted, the output is in a form that may be reused as input.  The return status is 0
              if the mode was successfully changed or if no mode argument was supplied, and false otherwise.

       unalias [-a] [name ...]
              Remove each name from the list of defined aliases.  If -a is supplied, all alias definitions are removed.  The  return  value
              is true unless a supplied name is not a defined alias.

       unset [-fv] [-n] [name ...]
              For  each  name, remove the corresponding variable or function.  If the -v option is given, each name refers to a shell vari‐
              able, 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 at‐
              tribute, name will be unset rather than the variable it references.  -n has no effect if the -f option is  supplied.   If  no
              options  are  supplied,  each  name refers to a variable; if there is no variable by that name, a function 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, SECONDS, or SRANDOM are unset, they lose their  special  properties,
              even if they are subsequently reset.  The exit status is true unless a name is readonly or may not be unset.

       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
              background  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 ids or, if no ids 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 sup‐
              plied 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 control 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.  If wait  is  in‐
              terrupted by a signal, the return status will be greater than 128, as described under SIGNALS in bash(1).  Otherwise, the re‐
              turn 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 options to the shopt builtin ( compat31,  com‐
       pat32,  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 incompatible  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., setting compat32 means that quoting the rhs
       of the regexp matching operator quotes special regexp characters in the word, which is default behavior in bash-3.2  and  subsequent
       versions).

       If  a user enables, say, compat32, it may affect the behavior of other compatibility levels up to and including the current compati‐
       bility 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 enabling 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 docu‐
       mentation 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 compatNN 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  previous  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 compatNN tag is used as
       shorthand for setting the compatibility level to NN using one of the following mechanisms.  For versions prior to bash-5.0, the com‐
       patibility  level  may be set using the corresponding compatNN shopt option.  For bash-4.3 and later versions, the BASH_COMPAT vari‐
       able 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 special 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 exe‐
                     cution of the entire list)

       compat40
              •      the < and > operators to the [[ command do not consider the current locale when comparing strings; they use ASCII  or‐
                     dering.   Bash  versions  prior  to bash-4.1 use ASCII collation and strcmp(3); bash-4.1 and later use the current lo‐
                     cale's collation sequence and strcoll(3).

       compat41
              •      in posix mode, time may be followed by options and still be recognized as a reserved word (this is  POSIX  interpreta‐
                     tion 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 specially, 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  ex‐
                     pansion  and  can  be  used  to quote a closing 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 (e.g., 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 reset, so break or continue in  that  func‐
                     tion 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 enabled
              •      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 attributes continue to affect variables with
                     the same name in the calling environment even if the shell is not in posix mode

       compat50
              •      Bash-5.1 changed the way $RANDOM is generated to introduce slightly more randomness. 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 gen‐
                     erator 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.

       compat51
              •      The unset builtin treats attempts to unset array subscripts @ and * differently depending on whether the array is  in‐
                     dexed or associative, and differently than in previous versions.

SEE ALSO
       bash(1), sh(1)

GNU Bash 5.2                                                  2021 November 22                                             BASH_BUILTINS(1)

 

 

 

 

2.reference

　　2.1  [wit@fedora null]$ man exec