Bash

Tartalom

 

Adatok

Licenc: GNU GPLv3+
Verziószám: 5.0.3 (Debian 10-ben)
Fejlesztő/tulajdonos: Free Software Foundation Inc.

Rövid leírás:

A Bash parancsfeldolgozó man oldala és súgója. A Bash egy sh-kompatibilis parancsnyelvi feldolgozó, amely végrehajtja a szabványos bemenetből vagy egy fájlból olvasott parancsokat. A Bash a Korn és a C héj hasznos tulajdonságait is tartalmazza (ksh és csh). A Bash célja az IEEE POSIX specifikáció (IEEE 1003.1 szabvány) Shell és segédprogramok részének megfelelő megvalósítása. A Bash alapértelmezés szerint úgy konfigurálható, hogy POSIX-kompatibilis legyen.

 

 

Man oldal kimenet

man bash
BASH(1)                              General Commands Manual                              BASH(1)

NAME
       bash - GNU Bourne-Again SHell

SYNOPSIS
       bash [options] [command_string | file]

COPYRIGHT
       Bash is Copyright (C) 1989-2018 by the Free Software Foundation, Inc.

DESCRIPTION
       Bash is an sh-compatible command language interpreter that executes commands read from the
       standard input or from a file.  Bash also incorporates useful features from the Korn and C
       shells (ksh and csh).

       Bash  is  intended to be a conformant implementation of the Shell and Utilities portion of
       the IEEE POSIX specification (IEEE Standard 1003.1).  Bash can be configured to be  POSIX-
       conformant by default.

OPTIONS
       All of the single-character shell options documented in the description of the set builtin
       command, including -o, can be used as options when the shell  is  invoked.   In  addition,
       bash interprets the following options when it is invoked:

       -c        If  the  -c  option is present, then commands are read from the first non-option
                 argument command_string.  If there are arguments after the  command_string,  the
                 first argument is assigned to $0 and any remaining arguments are assigned to the
                 positional parameters.  The assignment to $0 sets the name of the  shell,  which
                 is used in warning and error messages.
       -i        If the -i option is present, the shell is interactive.
       -l        Make bash act as if it had been invoked as a login shell (see INVOCATION below).
       -r        If  the -r option is present, the shell becomes restricted (see RESTRICTED SHELL
                 below).
       -s        If the -s option is present, or if no arguments remain after option  processing,
                 then  commands  are  read from the standard input.  This option allows the posi‐
                 tional parameters to be set when invoking an interactive shell or  when  reading
                 input through a pipe.
       -v        Print shell input lines as they are read.
       -x        Print commands and their arguments as they are executed.
       -D        A  list  of  all  double-quoted strings preceded by $ is printed on the standard
                 output.  These are the strings that are subject to language translation when the
                 current  locale is not C or POSIX.  This implies the -n option; no commands will
                 be executed.
       [-+]O [shopt_option]
                 shopt_option is one of the shell options accepted  by  the  shopt  builtin  (see
                 SHELL BUILTIN COMMANDS below).  If shopt_option is present, -O sets the value of
                 that option; +O unsets it.  If shopt_option is not supplied, the names and  val‐
                 ues  of  the shell options accepted by shopt are printed on the standard output.
                 If the invocation option is +O, the output is displayed in a format that may  be
                 reused as input.
       --        A -- signals the end of options and disables further option processing.  Any ar‐
                 guments after the -- are treated as filenames and arguments.  An argument  of  -
                 is equivalent to --.

       Bash  also  interprets  a number of multi-character options.  These options must appear on
       the command line before the single-character options to be recognized.

       --debugger
              Arrange for the debugger profile to be executed before the shell starts.  Turns  on
              extended  debugging  mode  (see the description of the extdebug option to the shopt
              builtin below).
       --dump-po-strings
              Equivalent to -D, but the output is in the GNU gettext po  (portable  object)  file
              format.
       --dump-strings
              Equivalent to -D.
       --help Display a usage message on standard output and exit successfully.
       --init-file file
       --rcfile file
              Execute  commands  from  file  instead  of  the  system  wide  initialization  file
              /etc/bash.bashrc and the standard personal initialization  file  ~/.bashrc  if  the
              shell is interactive (see INVOCATION below).

       --login
              Equivalent to -l.

       --noediting
              Do  not use the GNU readline library to read command lines when the shell is inter‐
              active.

       --noprofile
              Do not read either the system-wide startup file /etc/profile or any of the personal
              initialization  files  ~/.bash_profile,  ~/.bash_login, or ~/.profile.  By default,
              bash reads these files when it is invoked as a login shell (see INVOCATION below).

       --norc Do not read and execute the system wide initialization  file  /etc/bash.bashrc  and
              the  personal  initialization file ~/.bashrc if the shell is interactive.  This op‐
              tion is on by default if the shell is invoked as sh.

       --posix
              Change the behavior of bash where the default  operation  differs  from  the  POSIX
              standard to match the standard (posix mode).  See SEE ALSO below for a reference to
              a document that details how posix mode affects bash's behavior.

       --restricted
              The shell becomes restricted (see RESTRICTED SHELL below).

       --verbose
              Equivalent to -v.

       --version
              Show version information for this instance of bash on the standard output and  exit
              successfully.

ARGUMENTS
       If arguments remain after option processing, and neither the -c nor the -s option has been
       supplied, the first argument is assumed to be the name of a  file  containing  shell  com‐
       mands.  If bash is invoked in this fashion, $0 is set to the name of the file, and the po‐
       sitional parameters are set to the remaining arguments.  Bash reads and executes  commands
       from this file, then exits.  Bash's exit status is the exit status of the last command ex‐
       ecuted in the script.  If no commands are executed, the exit status is 0.  An  attempt  is
       first  made  to open the file in the current directory, and, if no file is found, then the
       shell searches the directories in PATH for the script.

INVOCATION
       A login shell is one whose first character of argument zero is a -, or  one  started  with
       the --login option.

       An  interactive shell is one started without non-option arguments (unless -s is specified)
       and without the -c option whose standard input and error are both connected  to  terminals
       (as  determined  by  isatty(3)), or one started with the -i option.  PS1 is set and $- in‐
       cludes i if bash is interactive, allowing a shell script or a startup file  to  test  this
       state.

       The  following  paragraphs  describe  how  bash executes its startup files.  If any of the
       files exist but cannot be read, bash reports an error.  Tildes are expanded  in  filenames
       as described below under Tilde Expansion in the EXPANSION section.

       When bash is invoked as an interactive login shell, or as a non-interactive shell with the
       --login option, it first reads and executes commands from the file /etc/profile,  if  that
       file  exists.   After  reading that file, it looks for ~/.bash_profile, ~/.bash_login, and
       ~/.profile, in that order, and reads and executes commands from the first one that  exists
       and  is readable.  The --noprofile option may be used when the shell is started to inhibit
       this behavior.

       When an interactive login shell exits, or a non-interactive login shell executes the  exit
       builtin  command, bash reads and executes commands from the file ~/.bash_logout, if it ex‐
       ists.

       When an interactive shell that is not a login shell is started, bash  reads  and  executes
       commands from /etc/bash.bashrc and ~/.bashrc, if these files exist.  This may be inhibited
       by using the --norc option.  The --rcfile file option will force bash to read and  execute
       commands from file instead of /etc/bash.bashrc and ~/.bashrc.

       When  bash  is started non-interactively, to run a shell script, for example, it looks for
       the variable BASH_ENV in the environment, expands its value if it appears there, and  uses
       the expanded value as the name of a file to read and execute.  Bash behaves as if the fol‐
       lowing command were executed:
              if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
       but the value of the PATH variable is not used to search for the filename.

       If bash is invoked with the name sh, it tries to mimic the startup behavior of  historical
       versions  of  sh  as  closely as possible, while conforming to the POSIX standard as well.
       When invoked as an interactive login shell, or a non-interactive shell  with  the  --login
       option,  it  first attempts to read and execute commands from /etc/profile and ~/.profile,
       in that order.  The --noprofile option may be used to inhibit this behavior.  When invoked
       as  an  interactive  shell  with the name sh, bash looks for the variable ENV, expands its
       value if it is defined, and uses the expanded value as the name of a file to read and exe‐
       cute.   Since a shell invoked as sh does not attempt to read and execute commands from any
       other startup files, the --rcfile option has no effect.  A non-interactive  shell  invoked
       with  the  name  sh does not attempt to read any other startup files.  When invoked as sh,
       bash enters posix mode after the startup files are read.

       When bash is started in posix mode, as with the --posix command line  option,  it  follows
       the  POSIX  standard  for  startup files.  In this mode, interactive shells expand the ENV
       variable and commands are read and executed from the  file  whose  name  is  the  expanded
       value.  No other startup files are read.

       Bash  attempts  to  determine  when it is being run with its standard input connected to a
       network connection, as when executed by the remote shell daemon, usually rshd, or the  se‐
       cure  shell daemon sshd.  If bash determines it is being run in this fashion, it reads and
       executes commands from ~/.bashrc and ~/.bashrc, if these files exist and are readable.  It
       will  not  do this if invoked as sh.  The --norc option may be used to inhibit this behav‐
       ior, and the --rcfile option may be used to force another file to  be  read,  but  neither
       rshd nor sshd generally invoke the shell with those options or allow them to be specified.

       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, no startup files are read, shell  functions
       are  not  inherited  from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
       variables, if they appear in the environment, are ignored, and the effective  user  id  is
       set to the real user id.  If the -p option is supplied at invocation, the startup behavior
       is the same, but the effective user id is not reset.

DEFINITIONS
       The following definitions are used throughout the rest of this document.
       blank  A space or tab.
       word   A sequence of characters considered as a single unit by the shell.  Also known as a
              token.
       name   A  word  consisting  only of alphanumeric characters and underscores, and beginning
              with an alphabetic character or an underscore.  Also referred to as an identifier.
       metacharacter
              A character that, when unquoted, separates words.  One of the following:
              |  & ; ( ) < > space tab newline
       control operator
              A token that performs a control function.  It is one of the following symbols:
              || & && ; ;; ;& ;;& ( ) | |& <newline>

RESERVED WORDS
       Reserved words are words that have a special meaning to the shell.   The  following  words
       are  recognized  as  reserved  when unquoted and either the first word of a simple command
       (see SHELL GRAMMAR below) or the third word of a case or for command:

       ! case  coproc  do done elif else esac fi for function if in select then until while  {  }
       time [[ ]]

SHELL GRAMMAR
   Simple Commands
       A  simple  command  is a sequence of optional variable assignments followed by blank-sepa‐
       rated words and redirections, and terminated by a control operator.  The first word speci‐
       fies  the command to be executed, and is passed as argument zero.  The remaining words are
       passed as arguments to the invoked command.

       The return value of a simple command is its exit status, or 128+n if the command is termi‐
       nated by signal n.

   Pipelines
       A pipeline is a sequence of one or more commands separated by one of the control operators
       | or |&.  The format for a pipeline is:

              [time [-p]] [ ! ] command [ [|⎪|&] command2 ... ]

       The standard output of command is connected via a pipe to the standard input of  command2.
       This  connection  is performed before any redirections specified by the command (see REDI‐
       RECTION below).  If |& is used, command's standard error, in addition to its standard out‐
       put,  is connected to command2's standard input through the pipe; it is shorthand for 2>&1
       |.  This implicit redirection of the standard error to the standard  output  is  performed
       after any redirections specified by the command.

       The  return  status  of  a  pipeline  is  the  exit status of the last command, unless the
       pipefail option is enabled.  If pipefail is enabled, the pipeline's return status  is  the
       value  of the last (rightmost) command to exit with a non-zero status, or zero if all com‐
       mands exit successfully.  If the reserved word !  precedes a pipeline, the exit status  of
       that  pipeline  is  the logical negation of the exit status as described above.  The shell
       waits for all commands in the pipeline to terminate before returning a value.

       If the time reserved word precedes a pipeline, the elapsed as well as user and system time
       consumed  by  its  execution  are  reported  when  the pipeline terminates.  The -p option
       changes the output format to that specified by POSIX.  When the shell is in posix mode, it
       does not recognize time as a reserved word if the next token begins with a `-'.  The TIME‐
       FORMAT variable may be set to a format string that specifies how  the  timing  information
       should be displayed; see the description of TIMEFORMAT under Shell Variables below.

       When  the  shell  is  in posix mode, time may be followed by a newline.  In this case, the
       shell displays the total user and system time consumed by the shell and its children.  The
       TIMEFORMAT variable may be used to specify the format of the time information.

       Each  command  in a pipeline is executed as a separate process (i.e., in a subshell).  See
       COMMAND EXECUTION ENVIRONMENT for a description of a subshell environment.  If  the  last‐
       pipe  option  is enabled using the shopt builtin (see the description of shopt below), the
       last element of a pipeline may be run by the shell process.

   Lists
       A list is a sequence of one or more pipelines separated by one of the operators ;, &,  &&,
       or ||, and optionally terminated by one of ;, &, or <newline>.

       Of  these list operators, && and || have equal precedence, followed by ; and &, which have
       equal precedence.

       A sequence of one or more newlines may appear in a list instead of a semicolon to  delimit
       commands.

       If  a  command  is terminated by the control operator &, the shell executes the command in
       the background in a subshell.  The shell does not wait for the command to finish, and  the
       return  status  is 0.  These are referred to as asynchronous commands.  Commands separated
       by a ; are executed sequentially; the shell waits for each command to terminate  in  turn.
       The return status is the exit status of the last command executed.

       AND and OR lists are sequences of one or more pipelines separated by the && and || control
       operators, respectively.  AND and OR lists are executed with left associativity.   An  AND
       list has the form

              command1 && command2

       command2 is executed if, and only if, command1 returns an exit status of zero (success).

       An OR list has the form

              command1 || command2

       command2 is executed if, and only if, command1 returns a non-zero exit status.  The return
       status of AND and OR lists is the exit status of the last command executed in the list.

   Compound Commands
       A compound command is one of the following.  In most cases a list in a command's  descrip‐
       tion  may  be  separated  from the rest of the command by one or more newlines, and may be
       followed by a newline in place of a semicolon.

       (list) list is executed in a subshell environment (see COMMAND EXECUTION  ENVIRONMENT  be‐
              low).   Variable  assignments and builtin commands that affect the shell's environ‐
              ment do not remain in effect after the command completes.  The return status is the
              exit status of list.

       { list; }
              list  is simply executed in the current shell environment.  list must be terminated
              with a newline or semicolon.  This is known as a group command.  The return  status
              is  the  exit status of list.  Note that unlike the metacharacters ( and ), { and }
              are reserved words and must occur where a reserved word is permitted to  be  recog‐
              nized.   Since  they do not cause a word break, they must be separated from list by
              whitespace or another shell metacharacter.

       ((expression))
              The expression is evaluated according to the rules described below under ARITHMETIC
              EVALUATION.   If  the  value of the expression is non-zero, the return status is 0;
              otherwise the return status is 1.  This is exactly equivalent to let "expression".

       [[ expression ]]
              Return a status of 0 or 1 depending on the evaluation of the conditional expression
              expression.  Expressions are composed of the primaries described below under CONDI‐
              TIONAL EXPRESSIONS.  Word splitting and pathname expansion are not performed on the
              words  between  the  [[  and ]]; tilde expansion, parameter and variable expansion,
              arithmetic expansion, command substitution, process substitution, and quote removal
              are  performed.  Conditional operators such as -f must be unquoted to be recognized
              as primaries.

              When used with [[, the < and > operators sort lexicographically using  the  current
              locale.

       See the description of the test builtin command (section SHELL BUILTIN COMMANDS below) for
       the handling of parameters (i.e.  missing parameters).

       When the == and != operators are used, the string to the right of the operator is  consid‐
       ered  a pattern and matched according to the rules described below under Pattern Matching,
       as if the extglob shell option were enabled.  The = operator is equivalent to ==.  If  the
       nocasematch  shell option is enabled, the match is performed without regard to the case of
       alphabetic characters.  The return value is 0 if the string matches (==) or does not match
       (!=)  the  pattern,  and  1 otherwise.  Any part of the pattern may be quoted to force the
       quoted portion to be matched as a string.

       An additional binary operator, =~, is available, with the same precedence as  ==  and  !=.
       When  it  is  used, the string to the right of the operator is considered a POSIX extended
       regular expression and matched accordingly (as in regex(3)).  The return value is 0 if the
       string  matches  the pattern, and 1 otherwise.  If the regular expression is syntactically
       incorrect, the conditional expression's return value is 2.  If the nocasematch  shell  op‐
       tion  is  enabled, the match is performed without regard to the case of alphabetic charac‐
       ters.  Any part of the pattern may be quoted to force the quoted portion to be matched  as
       a  string.   Bracket  expressions  in regular expressions must be treated carefully, since
       normal quoting characters lose their meanings between brackets.  If the pattern is  stored
       in  a  shell  variable,  quoting  the  variable  expansion forces the entire pattern to be
       matched as a string.  Substrings matched by parenthesized subexpressions within the  regu‐
       lar  expression are saved in the array variable BASH_REMATCH.  The element of BASH_REMATCH
       with index 0 is the portion of the string matching the entire regular expression.  The el‐
       ement of BASH_REMATCH with index n is the portion of the string matching the nth parenthe‐
       sized subexpression.

       Expressions may be combined using the following operators, listed in decreasing  order  of
       precedence:

              ( expression )
                     Returns  the  value  of expression.  This may be used to override the normal
                     precedence of operators.
              ! expression
                     True if expression is false.
              expression1 && expression2
                     True if both expression1 and expression2 are true.
              expression1 || expression2
                     True if either expression1 or expression2 is true.

              The && and || operators do not evaluate expression2 if the value of expression1  is
              sufficient to determine the return value of the entire conditional expression.

       for name [ [ in [ word ... ] ] ; ] do list ; done
              The  list of words following in is expanded, generating a list of items.  The vari‐
              able name is set to each element of this list in turn, and list  is  executed  each
              time.  If the in word is omitted, the for command executes list once for each posi‐
              tional parameter that is set (see PARAMETERS below).  The return status is the exit
              status  of the last command that executes.  If the expansion of the items following
              in results in an empty list, no commands are executed, and the return status is 0.

       for (( expr1 ; expr2 ; expr3 )) ; do list ; done
              First, the arithmetic expression expr1 is evaluated  according  to  the  rules  de‐
              scribed below under ARITHMETIC EVALUATION.  The arithmetic expression expr2 is then
              evaluated repeatedly until it evaluates to zero.  Each time expr2  evaluates  to  a
              non-zero  value, list is executed and the arithmetic expression expr3 is evaluated.
              If any expression is omitted, it behaves as if it evaluates to 1.  The return value
              is the exit status of the last command in list that is executed, or false if any of
              the expressions is invalid.

       select name [ in word ] ; do list ; done
              The list of words following in is expanded, generating a list of items.  The set of
              expanded words is printed on the standard error, each preceded by a number.  If the
              in word is omitted, the positional parameters are printed (see  PARAMETERS  below).
              The  PS3  prompt is then displayed and a line read from the standard input.  If the
              line consists of a number corresponding to one of the  displayed  words,  then  the
              value  of name is set to that word.  If the line is empty, the words and prompt are
              displayed again.  If EOF is read, the command  completes.   Any  other  value  read
              causes  name to be set to null.  The line read is saved in the variable REPLY.  The
              list is executed after each selection until a break command is executed.  The  exit
              status  of  select is the exit status of the last command executed in list, or zero
              if no commands were executed.

       case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
              A case command first expands word, and tries to match it against  each  pattern  in
              turn, using the matching rules described under Pattern Matching below.  The word is
              expanded using tilde expansion, parameter and variable expansion, arithmetic expan‐
              sion,  command  substitution, process substitution and quote removal.  Each pattern
              examined is expanded using  tilde  expansion,  parameter  and  variable  expansion,
              arithmetic  expansion,  command substitution, and process substitution.  If the no‐
              casematch shell option is enabled, the match is performed  without  regard  to  the
              case  of  alphabetic  characters.  When a match is found, the corresponding list is
              executed.  If the ;; operator is used, no subsequent matches  are  attempted  after
              the first pattern match.  Using ;& in place of ;; causes execution to continue with
              the list associated with the next set of patterns.  Using ;;& in place of ;; causes
              the  shell  to test the next pattern list in the statement, if any, and execute any
              associated list on a successful match.  The exit  status  is  zero  if  no  pattern
              matches.  Otherwise, it is the exit status of the last command executed in list.

       if list; then list; [ elif list; then list; ] ... [ else list; ] fi
              The  if  list  is executed.  If its exit status is zero, the then list is executed.
              Otherwise, each elif list is executed in turn, and if its exit status is zero,  the
              corresponding then list is executed and the command completes.  Otherwise, the else
              list is executed, if present.  The exit status is the exit status of the last  com‐
              mand executed, or zero if no condition tested true.

       while list-1; do list-2; done
       until list-1; do list-2; done
              The while command continuously executes the list list-2 as long as the last command
              in the list list-1 returns an exit status of zero.  The until command is  identical
              to  the  while command, except that the test is negated: list-2 is executed as long
              as the last command in list-1 returns a non-zero exit status.  The exit  status  of
              the  while  and  until  commands is the exit status of the last command executed in
              list-2, or zero if none was executed.

   Coprocesses
       A coprocess is a shell command preceded by the coproc reserved word.  A coprocess is  exe‐
       cuted  asynchronously in a subshell, as if the command had been terminated with the & con‐
       trol operator, with a two-way pipe established between the executing shell and the  copro‐
       cess.

       The format for a coprocess is:

              coproc [NAME] command [redirections]

       This creates a coprocess named NAME.  If NAME is not supplied, the default name is COPROC.
       NAME must not be supplied if command is a simple command (see above); otherwise, it is in‐
       terpreted  as  the  first word of the simple command.  When the coprocess is executed, the
       shell creates an array variable (see Arrays below) named NAME in the context of  the  exe‐
       cuting shell.  The standard output of command is connected via a pipe to a file descriptor
       in the executing shell, and that file descriptor is assigned to NAME[0].  The standard in‐
       put  of  command  is connected via a pipe to a file descriptor in the executing shell, and
       that file descriptor is assigned to NAME[1].  This pipe is established before any redirec‐
       tions  specified by the command (see REDIRECTION below).  The file descriptors can be uti‐
       lized as arguments to shell commands and  redirections  using  standard  word  expansions.
       Other  than  those created to execute command and process substitutions, the file descrip‐
       tors are not available in subshells.  The process ID of the shell spawned to  execute  the
       coprocess  is  available  as the value of the variable NAME_PID.  The wait builtin command
       may be used to wait for the coprocess to terminate.

       Since the coprocess is created as an asynchronous command, the coproc command  always  re‐
       turns success.  The return status of a coprocess is the exit status of command.

   Shell Function Definitions
       A shell function is an object that is called like a simple command and executes a compound
       command with a new set of positional parameters.  Shell functions are declared as follows:

       name () compound-command [redirection]
       function name [()] compound-command [redirection]
              This defines a function named name.  The reserved word function  is  optional.   If
              the  function reserved word is supplied, the parentheses are optional.  The body of
              the function is  the  compound  command  compound-command  (see  Compound  Commands
              above).  That command is usually a list of commands between { and }, but may be any
              command listed under Compound Commands above, with one exception: If  the  function
              reserved  word  is  used,  but the parentheses are not supplied, the braces are re‐
              quired.  compound-command is executed whenever name is specified as the name  of  a
              simple  command.   When in posix mode, name may not be the name of one of the POSIX
              special builtins.  Any redirections (see REDIRECTION below) specified when a  func‐
              tion  is defined are performed when the function is executed.  The exit status of a
              function definition is zero unless a syntax error occurs  or  a  readonly  function
              with the same name already exists.  When executed, the exit status of a function is
              the exit status of the last command executed in the body.  (See FUNCTIONS below.)

COMMENTS
       In a non-interactive shell, or an interactive shell in which the interactive_comments  op‐
       tion  to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word beginning
       with # causes that word and all remaining characters on that line to be ignored.   An  in‐
       teractive  shell  without the interactive_comments option enabled does not allow comments.
       The interactive_comments option is on by default in interactive shells.

QUOTING
       Quoting is used to remove the special meaning of certain characters or words to the shell.
       Quoting  can  be  used to disable special treatment for special characters, to prevent re‐
       served words from being recognized as such, and to prevent parameter expansion.

       Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell
       and must be quoted if it is to represent itself.

       When  the  command  history expansion facilities are being used (see HISTORY EXPANSION be‐
       low), the history expansion character, usually !, must be quoted to prevent history expan‐
       sion.

       There  are  three  quoting  mechanisms:  the  escape  character, single quotes, and double
       quotes.

       A non-quoted backslash (\) is the escape character.  It preserves the literal value of the
       next  character  that  follows, with the exception of <newline>.  If a \<newline> pair ap‐
       pears, and the backslash is not itself quoted, the \<newline> is treated as a line contin‐
       uation (that is, it is removed from the input stream and effectively ignored).

       Enclosing characters in single quotes preserves the literal value of each character within
       the quotes.  A single quote may not occur between single quotes, even when preceded  by  a
       backslash.

       Enclosing characters in double quotes preserves the literal value of all characters within
       the quotes, with the exception of $, `, \, and, when  history  expansion  is  enabled,  !.
       When  the  shell is in posix mode, the ! has no special meaning within double quotes, even
       when history expansion is enabled.  The characters $ and ` retain  their  special  meaning
       within double quotes.  The backslash retains its special meaning only when followed by one
       of the following characters: $, `, ", \, or <newline>.   A  double  quote  may  be  quoted
       within double quotes by preceding it with a backslash.  If enabled, history expansion will
       be performed unless an !  appearing in double quotes is escaped using  a  backslash.   The
       backslash preceding the !  is not removed.

       The  special parameters * and @ have special meaning when in double quotes (see PARAMETERS
       below).

       Words of the form $'string' are treated specially.  The word expands to string, with back‐
       slash-escaped  characters  replaced as specified by the ANSI C standard.  Backslash escape
       sequences, if present, are decoded as follows:
              \a     alert (bell)
              \b     backspace
              \e
              \E     an escape character
              \f     form feed
              \n     new line
              \r     carriage return
              \t     horizontal tab
              \v     vertical tab
              \\     backslash
              \'     single quote
              \"     double quote
              \?     question mark
              \nnn   the eight-bit character whose value is the octal value nnn (one to three oc‐
                     tal 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)
              \cx    a control-x character

       The expanded result is single-quoted, as if the dollar sign had not been present.

       A double-quoted string preceded by a dollar sign ($"string") will cause the string  to  be
       translated according to the current locale.  If the current locale is C or POSIX, the dol‐
       lar sign is ignored.  If the string is translated and replaced, the replacement is double-
       quoted.

PARAMETERS
       A  parameter  is  an entity that stores values.  It can be a name, a number, or one of the
       special characters listed below under Special Parameters.  A variable is a  parameter  de‐
       noted  by a name.  A variable has a value and zero or more attributes.  Attributes are as‐
       signed using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS).

       A parameter is set if it has been assigned a value.  The null string  is  a  valid  value.
       Once a variable is set, it may be unset only by using the unset builtin command (see SHELL
       BUILTIN COMMANDS below).

       A variable may be assigned to by a statement of the form

              name=[value]

       If value is not given, the variable is assigned the null string.  All values undergo tilde
       expansion,  parameter  and variable expansion, command substitution, arithmetic expansion,
       and quote removal (see EXPANSION below).  If the variable has its integer  attribute  set,
       then  value is evaluated as an arithmetic expression even if the $((...)) expansion is not
       used (see Arithmetic Expansion below).  Word splitting is not performed, with  the  excep‐
       tion  of "$@" as explained below under Special Parameters.  Pathname expansion is not per‐
       formed.  Assignment statements may also appear as arguments to the alias,  declare,  type‐
       set,  export,  readonly, and local builtin commands (declaration commands).  When in posix
       mode, these builtins may appear in a command after one or more instances  of  the  command
       builtin and retain these assignment statement properties.

       In  the  context where an assignment statement is assigning a value to a shell variable or
       array index, the += operator can be used to append to or add to  the  variable's  previous
       value.  This includes arguments to builtin commands such as declare that accept assignment
       statements (declaration commands).  When += is applied to a variable for which the integer
       attribute  has  been  set, value is evaluated as an arithmetic expression and added to the
       variable's current value, which is also evaluated.  When += is applied to an  array  vari‐
       able  using  compound assignment (see Arrays below), the variable's value is not unset (as
       it is when using =), and new values are appended to the array  beginning  at  one  greater
       than the array's maximum index (for indexed arrays) or added as additional key-value pairs
       in an associative array.  When applied to a string-valued variable, value is expanded  and
       appended to the variable's value.

       A variable can be assigned the nameref attribute using the -n option to the declare or lo‐
       cal builtin commands (see the descriptions  of  declare  and  local  below)  to  create  a
       nameref,  or a reference to another variable.  This allows variables to be manipulated in‐
       directly.  Whenever the nameref variable is referenced, assigned to, unset, or has its at‐
       tributes  modified (other than using or changing the nameref attribute itself), the opera‐
       tion is actually performed on the variable specified by the nameref variable's  value.   A
       nameref  is  commonly  used  within  shell  functions to refer to a variable whose name is
       passed as an argument to the function.  For instance, if a variable name is  passed  to  a
       shell function as its first argument, running
              declare -n ref=$1
       inside the function creates a nameref variable ref whose value is the variable name passed
       as the first argument.  References and assignments to ref, and changes to its  attributes,
       are  treated as references, assignments, and attribute modifications to the variable whose
       name was passed as $1.  If the control variable in a for loop has the  nameref  attribute,
       the  list  of  words can be a list of shell variables, and a name reference will be estab‐
       lished for each word in the list, in turn, when the loop  is  executed.   Array  variables
       cannot  be  given  the  nameref attribute.  However, nameref variables can reference array
       variables and subscripted array variables.  Namerefs can be unset using the -n  option  to
       the unset builtin.  Otherwise, if unset is executed with the name of a nameref variable as
       an argument, the variable referenced by the nameref variable will be unset.

   Positional Parameters
       A positional parameter is a parameter denoted by one or more digits, other than the single
       digit  0.   Positional  parameters  are assigned from the shell's arguments when it is in‐
       voked, and may be reassigned using the set builtin command.  Positional parameters may not
       be  assigned to with assignment statements.  The positional parameters are temporarily re‐
       placed when a shell function is executed (see FUNCTIONS below).

       When a positional parameter consisting of more than a single digit is expanded, it must be
       enclosed in braces (see EXPANSION below).

   Special Parameters
       The  shell  treats several parameters specially.  These parameters may only be referenced;
       assignment to them is not allowed.
       *      Expands to the positional parameters, starting from one.  When the expansion is not
              within  double  quotes,  each  positional parameter expands to a separate word.  In
              contexts where it is performed, those words are subject to further  word  splitting
              and pathname expansion.  When the expansion occurs within double quotes, it expands
              to a single word with the value of each parameter separated by the first  character
              of  the  IFS special variable.  That is, "$*" is equivalent to "$1c$2c...", where c
              is the first character of the value of the IFS variable.  If IFS is unset, the  pa‐
              rameters  are separated by spaces.  If IFS is null, the parameters are joined with‐
              out intervening separators.
       @      Expands to the positional parameters, starting from one.  In  contexts  where  word
              splitting  is performed, this expands each positional parameter to a separate word;
              if not within double quotes, these words are subject to word  splitting.   In  con‐
              texts  where  word  splitting  is not performed, this expands to a single word with
              each positional parameter separated by a space.  When the expansion  occurs  within
              double quotes, each parameter expands to a separate word.  That is, "$@" is equiva‐
              lent to "$1" "$2" ...  If the double-quoted expansion occurs within a word, the ex‐
              pansion  of  the  first parameter is joined with the beginning part of the original
              word, and the expansion of the last parameter is joined with the last part  of  the
              original  word.   When  there  are  no positional parameters, "$@" and $@ expand to
              nothing (i.e., they are removed).
       #      Expands to the number of positional parameters in decimal.
       ?      Expands to the exit status of the most recently executed foreground pipeline.
       -      Expands to the current option flags  as  specified  upon  invocation,  by  the  set
              builtin command, or those set by the shell itself (such as the -i option).
       $      Expands  to  the  process  ID  of  the  shell.  In a () subshell, it expands to the
              process ID of the current shell, not the subshell.
       !      Expands to the process ID of the job most  recently  placed  into  the  background,
              whether  executed  as an asynchronous command or using the bg builtin (see JOB CON‐
              TROL below).
       0      Expands to the name of the shell or shell script.  This is set at shell initializa‐
              tion.   If  bash  is invoked with a file of commands, $0 is set to the name of that
              file.  If bash is started with the -c option, then $0 is set to the first  argument
              after  the  string  to be executed, if one is present.  Otherwise, it is set to the
              filename used to invoke bash, as given by argument zero.
       _      At shell startup, set to the absolute pathname used to invoke the  shell  or  shell
              script being executed as passed in the environment or argument list.  Subsequently,
              expands to the last argument to the previous simple command executed in  the  fore‐
              ground, after expansion.  Also set to the full pathname used to invoke each command
              executed and placed in the environment exported to  that  command.   When  checking
              mail, this parameter holds the name of the mail file currently being checked.

   Shell Variables
       The following variables are set by the shell:

       BASH   Expands to the full filename used to invoke this instance of bash.
       BASHOPTS
              A  colon-separated list of enabled shell options.  Each word in the list is a valid
              argument for the -s option to the shopt builtin command (see SHELL BUILTIN COMMANDS
              below).   The  options appearing in BASHOPTS are those reported as on by shopt.  If
              this variable is in the environment when bash starts up, each shell option  in  the
              list will be enabled before reading any startup files.  This variable is read-only.
       BASHPID
              Expands  to the process ID of the current bash process.  This differs from $$ under
              certain circumstances, such as subshells that do not require bash to be re-initial‐
              ized.   Assignments  to  BASHPID have no effect.  If BASHPID is unset, it loses its
              special properties, even if it is subsequently reset.
       BASH_ALIASES
              An associative array variable whose members correspond  to  the  internal  list  of
              aliases as maintained by the alias builtin.  Elements added to this array appear in
              the alias list; however, unsetting array elements currently does not cause  aliases
              to  be removed from the alias list.  If BASH_ALIASES is unset, it loses its special
              properties, even if it is subsequently reset.
       BASH_ARGC
              An array variable whose values are the number of parameters in each  frame  of  the
              current bash execution call stack.  The number of parameters to the current subrou‐
              tine (shell function or script executed with . or source) is  at  the  top  of  the
              stack.   When  a  subroutine is executed, the number of parameters passed is pushed
              onto BASH_ARGC.  The shell sets BASH_ARGC only when in extended debugging mode (see
              the description of the extdebug option to the shopt builtin below).  Setting extde‐
              bug after the shell has started to execute a script, or referencing  this  variable
              when extdebug is not set, may result in inconsistent values.
       BASH_ARGV
              An  array  variable  containing all of the parameters in the current bash execution
              call stack.  The final parameter of the last subroutine call is at the top  of  the
              stack; the first parameter of the initial call is at the bottom.  When a subroutine
              is executed, the parameters supplied are pushed onto  BASH_ARGV.   The  shell  sets
              BASH_ARGV only when in extended debugging mode (see the description of the extdebug
              option to the shopt builtin below).  Setting extdebug after the shell  has  started
              to execute a script, or referencing this variable when extdebug is not set, may re‐
              sult in inconsistent values.
       BASH_ARGV0
              When referenced, this variable expands to the name of the  shell  or  shell  script
              (identical to $0; see the description of special parameter 0 above).  Assignment to
              BASH_ARGV0 causes the value assigned to also be assigned to $0.  If  BASH_ARGV0  is
              unset, it loses its special properties, even if it is subsequently reset.
       BASH_CMDS
              An  associative  array variable whose members correspond to the internal hash table
              of commands as maintained by the hash builtin.  Elements added to this array appear
              in  the hash table; however, unsetting array elements currently does not cause com‐
              mand names to be removed from the hash table.  If BASH_CMDS is unset, it loses  its
              special properties, even if it is subsequently reset.
       BASH_COMMAND
              The  command  currently being executed or about to be executed, unless the shell is
              executing a command as the result of a trap, in which case it is the  command  exe‐
              cuting at the time of the trap.
       BASH_EXECUTION_STRING
              The command argument to the -c invocation option.
       BASH_LINENO
              An  array  variable  whose  members are the line numbers in source files where each
              corresponding member of FUNCNAME was invoked.  ${BASH_LINENO[$i]} is the line  num‐
              ber  in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or
              ${BASH_LINENO[$i-1]} if referenced within another shell function).  Use  LINENO  to
              obtain the current line number.
       BASH_LOADABLES_PATH
              A  colon-separated  list  of  directories  in which the shell looks for dynamically
              loadable builtins specified by the enable command.
       BASH_REMATCH
              An array variable whose members are assigned by the =~ binary operator  to  the  [[
              conditional  command.  The element with index 0 is the portion of the string match‐
              ing the entire regular expression.  The element with index n is the portion of  the
              string matching the nth parenthesized subexpression.  This variable is read-only.
       BASH_SOURCE
              An  array  variable  whose members are the source filenames where the corresponding
              shell function names in the FUNCNAME array variable are defined.  The  shell  func‐
              tion  ${FUNCNAME[$i]}  is  defined  in  the file ${BASH_SOURCE[$i]} and called from
              ${BASH_SOURCE[$i+1]}.
       BASH_SUBSHELL
              Incremented by one within each subshell or subshell environment when the shell  be‐
              gins executing in that environment.  The initial value is 0.
       BASH_VERSINFO
              A  readonly array variable whose members hold version information for this instance
              of bash.  The values assigned to the array members are as follows:
              BASH_VERSINFO[0]        The major version number (the release).
              BASH_VERSINFO[1]        The minor version number (the version).
              BASH_VERSINFO[2]        The patch level.
              BASH_VERSINFO[3]        The build version.
              BASH_VERSINFO[4]        The release status (e.g., beta1).
              BASH_VERSINFO[5]        The value of MACHTYPE.
       BASH_VERSION
              Expands to a string describing the version of this instance of bash.
       COMP_CWORD
              An index into ${COMP_WORDS} of the word containing  the  current  cursor  position.
              This variable is available only in shell functions invoked by the programmable com‐
              pletion facilities (see Programmable Completion below).
       COMP_KEY
              The key (or final key of a key sequence) used  to  invoke  the  current  completion
              function.
       COMP_LINE
              The  current  command line.  This variable is available only in shell functions and
              external commands invoked by the programmable completion facilities  (see  Program‐
              mable Completion below).
       COMP_POINT
              The  index  of the current cursor position relative to the beginning of the current
              command.  If the current cursor position is at the end of the current command,  the
              value  of this variable is equal to ${#COMP_LINE}.  This variable is available only
              in shell functions and external commands invoked by the programmable completion fa‐
              cilities (see Programmable Completion below).
       COMP_TYPE
              Set  to  an  integer  value  corresponding to the type of completion attempted that
              caused a completion function to be called: TAB, for normal completion, ?, for list‐
              ing  completions after successive tabs, !, for listing alternatives on partial word
              completion, @, to list completions if the word is not unmodified, or  %,  for  menu
              completion.   This  variable is available only in shell functions and external com‐
              mands invoked by the programmable completion facilities (see  Programmable  Comple‐
              tion below).
       COMP_WORDBREAKS
              The set of characters that the readline library treats as word separators when per‐
              forming word completion.  If COMP_WORDBREAKS is unset, it loses its special proper‐
              ties, even if it is subsequently reset.
       COMP_WORDS
              An array variable (see Arrays below) consisting of the individual words in the cur‐
              rent command line.  The line is split into words as readline would split it,  using
              COMP_WORDBREAKS as described above.  This variable is available only in shell func‐
              tions invoked by the programmable completion facilities (see  Programmable  Comple‐
              tion below).
       COPROC An  array variable (see Arrays below) created to hold the file descriptors for out‐
              put from and input to an unnamed coprocess (see Coprocesses above).
       DIRSTACK
              An array variable (see Arrays below) containing the current contents of the  direc‐
              tory stack.  Directories appear in the stack in the order they are displayed by the
              dirs builtin.  Assigning to members of this array variable may be  used  to  modify
              directories  already  in the stack, but the pushd and popd builtins must be used to
              add and remove directories.  Assignment to this variable will not change  the  cur‐
              rent  directory.  If DIRSTACK is unset, it loses its special properties, even if it
              is subsequently reset.
       EPOCHREALTIME
              Each time this parameter is referenced, it expands to the number of  seconds  since
              the  Unix Epoch (see time(3)) as a floating point value with micro-second granular‐
              ity.  Assignments to EPOCHREALTIME are ignored.   If  EPOCHREALTIME  is  unset,  it
              loses its special properties, even if it is subsequently reset.
       EPOCHSECONDS
              Each  time  this parameter is referenced, it expands to the number of seconds since
              the Unix Epoch  (see  time(3)).   Assignments  to  EPOCHSECONDS  are  ignored.   If
              EPOCHSECONDS  is unset, it loses its special properties, even if it is subsequently
              reset.
       EUID   Expands to the effective user ID of the current user, initialized at shell startup.
              This variable is readonly.
       FUNCNAME
              An array variable containing the names of all shell functions currently in the exe‐
              cution call stack.  The element with index 0 is the name of any currently-executing
              shell  function.   The  bottom-most  element  (the  one  with the highest index) is
              "main".  This variable exists only when a shell function is executing.  Assignments
              to FUNCNAME have no effect.  If FUNCNAME is unset, it loses its special properties,
              even if it is subsequently reset.

              This variable can be used with BASH_LINENO and BASH_SOURCE.  Each element of  FUNC‐
              NAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call
              stack.  For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]}
              at  line  number  ${BASH_LINENO[$i]}.  The caller builtin displays the current call
              stack using this information.
       GROUPS An array variable containing the list of groups of which the current user is a mem‐
              ber.   Assignments to GROUPS have no effect.  If GROUPS is unset, it loses its spe‐
              cial properties, even if it is subsequently reset.
       HISTCMD
              The history number, or index in the history  list,  of  the  current  command.   If
              HISTCMD  is  unset, it loses its special properties, even if it is subsequently re‐
              set.
       HOSTNAME
              Automatically set to the name of the current host.
       HOSTTYPE
              Automatically set to a string that uniquely describes the type of machine on  which
              bash is executing.  The default is system-dependent.
       LINENO Each time this parameter is referenced, the shell substitutes a decimal number rep‐
              resenting the current sequential line number (starting with 1) within a  script  or
              function.   When  not in a script or function, the value substituted is not guaran‐
              teed to be meaningful.  If LINENO is unset, it loses its special  properties,  even
              if it is subsequently reset.
       MACHTYPE
              Automatically set to a string that fully describes the system type on which bash is
              executing, in the standard GNU cpu-company-system format.  The default  is  system-
              dependent.
       MAPFILE
              An  array  variable (see Arrays below) created to hold the text read by the mapfile
              builtin when no variable name is supplied.
       OLDPWD The previous working directory as set by the cd command.
       OPTARG The value of the last option argument processed by the getopts builtin command (see
              SHELL BUILTIN COMMANDS below).
       OPTIND The  index of the next argument to be processed by the getopts builtin command (see
              SHELL BUILTIN COMMANDS below).
       OSTYPE Automatically set to a string that describes the operating system on which bash  is
              executing.  The default is system-dependent.
       PIPESTATUS
              An  array  variable (see Arrays below) containing a list of exit status values from
              the processes in the most-recently-executed foreground pipeline (which may  contain
              only a single command).
       PPID   The process ID of the shell's parent.  This variable is readonly.
       PWD    The current working directory as set by the cd command.
       RANDOM Each  time  this  parameter  is referenced, a random integer between 0 and 32767 is
              generated.  The sequence of random numbers may be initialized by assigning a  value
              to RANDOM.  If RANDOM is unset, it loses its special properties, even if it is sub‐
              sequently reset.
       READLINE_LINE
              The contents of the readline line buffer, for use with "bind -x" (see SHELL BUILTIN
              COMMANDS below).
       READLINE_POINT
              The position of the insertion point in the readline line buffer, for use with "bind
              -x" (see SHELL BUILTIN COMMANDS below).
       REPLY  Set to the line of input read by the read builtin command  when  no  arguments  are
              supplied.
       SECONDS
              Each  time  this parameter is referenced, the number of seconds since shell invoca‐
              tion is returned.  If a value is assigned to SECONDS, the value returned upon  sub‐
              sequent references is the number of seconds since the assignment plus the value as‐
              signed.  If SECONDS is unset, it loses its special properties, even if it is subse‐
              quently reset.
       SHELLOPTS
              A  colon-separated list of enabled shell options.  Each word in the list is a valid
              argument for the -o option to the set builtin command (see SHELL  BUILTIN  COMMANDS
              below).  The options appearing in SHELLOPTS are those reported as on by set -o.  If
              this variable is in the environment when bash starts up, each shell option  in  the
              list will be enabled before reading any startup files.  This variable is read-only.
       SHLVL  Incremented by one each time an instance of bash is started.
       UID    Expands  to  the  user  ID of the current user, initialized at shell startup.  This
              variable is readonly.

       The following variables are used by the shell.  In some  cases,  bash  assigns  a  default
       value to a variable; these cases are noted below.

       BASH_COMPAT
              The  value  is used to set the shell's compatibility level.  See the description of
              the shopt builtin below under SHELL BUILTIN COMMANDS for a description of the vari‐
              ous  compatibility  levels  and  their  effects.  The value may be a decimal number
              (e.g., 4.2) or an integer (e.g., 42) corresponding  to  the  desired  compatibility
              level.  If BASH_COMPAT is unset or set to the empty string, the compatibility level
              is set to the default for the current version.  If BASH_COMPAT is set  to  a  value
              that  is  not one of the valid compatibility levels, the shell prints an error mes‐
              sage and sets the compatibility level to the default for the current version.   The
              valid  compatibility levels correspond to the compatibility options accepted by the
              shopt builtin described below (for example, compat42 means  that  4.2  and  42  are
              valid values).  The current version is also a valid value.
       BASH_ENV
              If this parameter is set when bash is executing a shell script, its value is inter‐
              preted as a filename containing commands to initialize the shell, as in  ~/.bashrc.
              The  value  of  BASH_ENV is subjected to parameter expansion, command substitution,
              and arithmetic expansion before being interpreted as a filename.  PATH is not  used
              to search for the resultant filename.
       BASH_XTRACEFD
              If  set to an integer corresponding to a valid file descriptor, bash will write the
              trace output generated when set -x is enabled to that file  descriptor.   The  file
              descriptor  is  closed when BASH_XTRACEFD is unset or assigned a new value.  Unset‐
              ting BASH_XTRACEFD or assigning it the empty string causes the trace output  to  be
              sent to the standard error.  Note that setting BASH_XTRACEFD to 2 (the standard er‐
              ror file descriptor) and then unsetting it will result in the standard error  being
              closed.
       CDPATH The  search path for the cd command.  This is a colon-separated list of directories
              in which the shell looks for destination directories specified by the  cd  command.
              A sample value is ".:~:/usr".
       CHILD_MAX
              Set  the number of exited child status values for the shell to remember.  Bash will
              not allow this value to be decreased below a POSIX-mandated minimum, and there is a
              maximum value (currently 8192) that this may not exceed.  The minimum value is sys‐
              tem-dependent.
       COLUMNS
              Used by the select compound command to determine the terminal width  when  printing
              selection  lists.  Automatically set if the checkwinsize option is enabled or in an
              interactive shell upon receipt of a SIGWINCH.
       COMPREPLY
              An array variable from which bash reads the possible  completions  generated  by  a
              shell  function  invoked  by the programmable completion facility (see Programmable
              Completion below).  Each array element contains one possible completion.
       EMACS  If bash finds this variable in the environment when the  shell  starts  with  value
              "t",  it  assumes  that  the shell is running in an Emacs shell buffer and disables
              line editing.
       ENV    Similar to BASH_ENV; used when the shell is invoked in posix mode.
       EXECIGNORE
              A colon-separated list of shell patterns (see Pattern Matching) defining  the  list
              of  filenames  to  be ignored by command search using PATH.  Files whose full path‐
              names match one of these patterns are not considered executable files for the  pur‐
              poses  of  completion  and command execution via PATH lookup.  This does not affect
              the behavior of the [, test, and [[ commands.  Full pathnames in the  command  hash
              table  are  not  subject to EXECIGNORE.  Use this variable to ignore shared library
              files that have the executable bit set, but are not executable files.  The  pattern
              matching honors the setting of the extglob shell option.
       FCEDIT The default editor for the fc builtin command.
       FIGNORE
              A  colon-separated  list  of suffixes to ignore when performing filename completion
              (see READLINE below).  A filename whose suffix matches one of the entries  in  FIG‐
              NORE  is  excluded  from  the  list of matched filenames.  A sample value is ".o:~"
              (Quoting is needed when assigning a value to this variable, which contains tildes).
       FUNCNEST
              If set to a numeric value greater than 0, defines a maximum function nesting level.
              Function  invocations that exceed this nesting level will cause the current command
              to abort.
       GLOBIGNORE
              A colon-separated list of patterns defining the set of file names to be ignored  by
              pathname  expansion.   If  a file name matched by a pathname expansion pattern also
              matches one of the patterns in GLOBIGNORE, it is removed from the list of matches.
       HISTCONTROL
              A colon-separated list of values controlling how commands are saved on the  history
              list.   If  the list of values includes ignorespace, lines which begin with a space
              character are not saved in the history list.  A value of  ignoredups  causes  lines
              matching  the  previous  history  entry  to not be saved.  A value of ignoreboth is
              shorthand for ignorespace and ignoredups.  A value of erasedups causes all previous
              lines  matching  the  current  line to be removed from the history list before that
              line is saved.  Any value not in the above list is ignored.  If HISTCONTROL is  un‐
              set,  or  does  not  include  a valid value, all lines read by the shell parser are
              saved on the history list, subject to the value of HISTIGNORE.  The second and sub‐
              sequent lines of a multi-line compound command are not tested, and are added to the
              history regardless of the value of HISTCONTROL.
       HISTFILE
              The name of the file in which command history is saved (see  HISTORY  below).   The
              default  value is ~/.bash_history.  If unset, the command history is not saved when
              a shell exits.
       HISTFILESIZE
              The maximum number of lines contained in the history file.  When this  variable  is
              assigned  a  value, the history file is truncated, if necessary, to contain no more
              than that number of lines by removing the oldest entries.  The history file is also
              truncated to this size after writing it when a shell exits.  If the value is 0, the
              history file is truncated to zero size.  Non-numeric values and numeric values less
              than  zero  inhibit  truncation.   The shell sets the default value to the value of
              HISTSIZE after reading any startup files.
       HISTIGNORE
              A colon-separated list of patterns used to decide which  command  lines  should  be
              saved  on  the history list.  Each pattern is anchored at the beginning of the line
              and must match the complete line (no implicit `*' is appended).   Each  pattern  is
              tested  against the line after the checks specified by HISTCONTROL are applied.  In
              addition to the normal shell pattern matching characters, `&' matches the  previous
              history  line.   `&' may be escaped using a backslash; the backslash is removed be‐
              fore attempting a match.  The second and subsequent lines of a multi-line  compound
              command  are  not  tested,  and are added to the history regardless of the value of
              HISTIGNORE.  The pattern matching honors the setting of the extglob shell option.
       HISTSIZE
              The number of commands to remember in the command history (see HISTORY below).   If
              the  value  is  0, commands are not saved in the history list.  Numeric values less
              than zero result in every command being saved on the  history  list  (there  is  no
              limit).  The shell sets the default value to 500 after reading any startup files.
       HISTTIMEFORMAT
              If  this  variable  is  set  and not null, its value is used as a format string for
              strftime(3) to print the time stamp associated with each history entry displayed by
              the  history builtin.  If this variable is set, time stamps are written to the his‐
              tory file so they may be preserved across shell sessions.  This  uses  the  history
              comment character to distinguish timestamps from other history lines.
       HOME   The  home  directory  of  the current user; the default argument for the cd builtin
              command.  The value of this variable is also used when performing tilde expansion.
       HOSTFILE
              Contains the name of a file in the same format as /etc/hosts that  should  be  read
              when the shell needs to complete a hostname.  The list of possible hostname comple‐
              tions may be changed while the shell is running; the next time hostname  completion
              is  attempted after the value is changed, bash adds the contents of the new file to
              the existing list.  If HOSTFILE is set, but has no value, or does not name a  read‐
              able file, bash attempts to read /etc/hosts to obtain the list of possible hostname
              completions.  When HOSTFILE is unset, the hostname list is cleared.
       IFS    The Internal Field Separator that is used for word splitting after expansion and to
              split  lines  into  words  with  the  read  builtin  command.  The default value is
              ``<space><tab><newline>''.
       IGNOREEOF
              Controls the action of an interactive shell on receipt of an EOF character  as  the
              sole  input.   If  set, the value is the number of consecutive EOF characters which
              must be typed as the first characters on an input line before bash exits.   If  the
              variable  exists  but  does  not have a numeric value, or has no value, the default
              value is 10.  If it does not exist, EOF signifies the end of input to the shell.
       INPUTRC
              The filename for the readline startup file, overriding the  default  of  ~/.inputrc
              (see READLINE below).
       INSIDE_EMACS
              If  this  variable  appears  in the environment when the shell starts, bash assumes
              that it is running inside an Emacs shell buffer and may disable line  editing,  de‐
              pending on the value of TERM.
       LANG   Used  to  determine  the locale category for any category not specifically selected
              with a variable starting with LC_.
       LC_ALL This variable overrides the value of LANG and any other LC_ variable  specifying  a
              locale category.
       LC_COLLATE
              This variable determines the collation order used when sorting the results of path‐
              name expansion, and determines  the  behavior  of  range  expressions,  equivalence
              classes, and collating sequences within pathname expansion and pattern matching.
       LC_CTYPE
              This variable determines the interpretation of characters and the behavior of char‐
              acter classes within pathname expansion and pattern matching.
       LC_MESSAGES
              This variable determines the locale used to translate  double-quoted  strings  pre‐
              ceded by a $.
       LC_NUMERIC
              This variable determines the locale category used for number formatting.
       LC_TIME
              This variable determines the locale category used for data and time formatting.
       LINES  Used by the select compound command to determine the column length for printing se‐
              lection lists.  Automatically set if the checkwinsize option is enabled  or  in  an
              interactive shell upon receipt of a SIGWINCH.
       MAIL   If  this  parameter is set to a file or directory name and the MAILPATH variable is
              not set, bash informs the user of the arrival of mail  in  the  specified  file  or
              Maildir-format directory.
       MAILCHECK
              Specifies  how often (in seconds) bash checks for mail.  The default is 60 seconds.
              When it is time to check for mail, the shell does so before displaying the  primary
              prompt.   If this variable is unset, or set to a value that is not a number greater
              than or equal to zero, the shell disables mail checking.
       MAILPATH
              A colon-separated list of filenames to be checked for  mail.   The  message  to  be
              printed  when  mail arrives in a particular file may be specified by separating the
              filename from the message with a `?'.  When used in the text of the message, $_ ex‐
              pands to the name of the current mailfile.  Example:
              MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
              Bash  can  be  configured  to supply a default value for this variable (there is no
              value by default), but the location of the user mail files that it uses  is  system
              dependent (e.g., /var/mail/$USER).
       OPTERR If  set  to  the  value  1,  bash  displays error messages generated by the getopts
              builtin command (see SHELL BUILTIN COMMANDS below).  OPTERR  is  initialized  to  1
              each time the shell is invoked or a shell script is executed.
       PATH   The search path for commands.  It is a colon-separated list of directories in which
              the shell looks for commands (see COMMAND EXECUTION below).  A  zero-length  (null)
              directory name in the value of PATH indicates the current directory.  A null direc‐
              tory name may appear as two adjacent colons, or as an initial  or  trailing  colon.
              The  default path is system-dependent, and is set by the administrator who installs
              bash.  A common value is ``/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:
              /sbin''.
       POSIXLY_CORRECT
              If  this  variable  is  in the environment when bash starts, the shell enters posix
              mode before reading the startup files, as if the --posix invocation option had been
              supplied.   If it is set while the shell is running, bash enables posix mode, as if
              the command set -o posix had been executed.  When the shell enters posix  mode,  it
              sets this variable if it was not already set.
       PROMPT_COMMAND
              If set, the value is executed as a command prior to issuing each primary prompt.
       PROMPT_DIRTRIM
              If  set  to a number greater than zero, the value is used as the number of trailing
              directory components to retain when expanding the \w and \W prompt  string  escapes
              (see PROMPTING below).  Characters removed are replaced with an ellipsis.
       PS0    The  value of this parameter is expanded (see PROMPTING below) and displayed by in‐
              teractive shells after reading a command and before the command is executed.
       PS1    The value of this parameter is expanded (see PROMPTING below) and used as the  pri‐
              mary prompt string.  The default value is ``\s-\v\$ ''.
       PS2    The  value  of  this  parameter  is  expanded as with PS1 and used as the secondary
              prompt string.  The default is ``> ''.
       PS3    The value of this parameter is used as the prompt for the select command (see SHELL
              GRAMMAR above).
       PS4    The value of this parameter is expanded as with PS1 and the value is printed before
              each command bash displays during an execution trace.  The first character  of  the
              expanded  value of PS4 is replicated multiple times, as necessary, to indicate mul‐
              tiple levels of indirection.  The default is ``+ ''.
       SHELL  The full pathname to the shell is kept in this environment variable.  If it is  not
              set  when  the  shell  starts,  bash assigns to it the full pathname of the current
              user's login shell.
       TIMEFORMAT
              The value of this parameter is used as a format string specifying  how  the  timing
              information for pipelines prefixed with the time reserved word should be displayed.
              The % character introduces an escape sequence that is expanded to a time  value  or
              other  information.   The  escape  sequences and their meanings are as follows; the
              braces denote optional portions.
              %%        A literal %.
              %[p][l]R  The elapsed time in seconds.
              %[p][l]U  The number of CPU seconds spent in user mode.
              %[p][l]S  The number of CPU seconds spent in system mode.
              %P        The CPU percentage, computed as (%U + %S) / %R.

              The optional p is a digit specifying the precision, the number of fractional digits
              after a decimal point.  A value of 0 causes no decimal point or fraction to be out‐
              put.  At most three places after the decimal point may be specified;  values  of  p
              greater than 3 are changed to 3.  If p is not specified, the value 3 is used.

              The optional l specifies a longer format, including minutes, of the form MMmSS.FFs.
              The value of p determines whether or not the fraction is included.

              If  this  variable  is  not   set,   bash   acts   as   if   it   had   the   value
              $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'.   If  the value is null, no timing informa‐
              tion is displayed.  A trailing newline is added when  the  format  string  is  dis‐
              played.
       TMOUT  If  set  to  a value greater than zero, TMOUT is treated as the default timeout for
              the read builtin.  The select command terminates if input  does  not  arrive  after
              TMOUT  seconds  when input is coming from a terminal.  In an interactive shell, the
              value is interpreted as the number of seconds to wait for a line of input after is‐
              suing the primary prompt.  Bash terminates after waiting for that number of seconds
              if a complete line of input does not arrive.
       TMPDIR If set, bash uses its value as the name of a directory in which bash creates tempo‐
              rary files for the shell's use.
       auto_resume
              This  variable  controls how the shell interacts with the user and job control.  If
              this variable is set, single word simple commands without redirections are  treated
              as candidates for resumption of an existing stopped job.  There is no ambiguity al‐
              lowed; if there is more than one job beginning with the string typed, the job  most
              recently  accessed is selected.  The name of a stopped job, in this context, is the
              command line used to start it.  If set to the value exact, the string supplied must
              match  the  name of a stopped job exactly; if set to substring, the string supplied
              needs to match a substring of the name of a stopped job.  The substring value  pro‐
              vides  functionality  analogous  to the %?  job identifier (see JOB CONTROL below).
              If set to any other value, the supplied string must be a prefix of a stopped  job's
              name; this provides functionality analogous to the %string job identifier.
       histchars
              The  two  or three characters which control history expansion and tokenization (see
              HISTORY EXPANSION below).  The first character is the history expansion  character,
              the  character  which  signals the start of a history expansion, normally `!'.  The
              second character is the quick substitution character, which is  used  as  shorthand
              for re-running the previous command entered, substituting one string for another in
              the command.  The default is `^'.  The optional third character  is  the  character
              which indicates that the remainder of the line is a comment when found as the first
              character of a word, normally `#'.  The history comment  character  causes  history
              substitution to be skipped for the remaining words on the line.  It does not neces‐
              sarily cause the shell parser to treat the rest of the line as a comment.

   Arrays
       Bash provides one-dimensional indexed and associative array variables.  Any  variable  may
       be  used as an indexed array; the declare builtin will explicitly declare an array.  There
       is no maximum limit on the size of an array, nor any requirement that members  be  indexed
       or  assigned contiguously.  Indexed arrays are referenced using integers (including arith‐
       metic expressions) and are zero-based; associative arrays are referenced  using  arbitrary
       strings.  Unless otherwise noted, indexed array indices must be non-negative integers.

       An  indexed array is created automatically if any variable is assigned to using the syntax
       name[subscript]=value.  The subscript is treated as an  arithmetic  expression  that  must
       evaluate  to  a  number.  To explicitly declare an indexed array, use declare -a name (see
       SHELL BUILTIN COMMANDS below).  declare -a name[subscript] is also accepted; the subscript
       is ignored.

       Associative arrays are created using declare -A name.

       Attributes may be specified for an array variable using the declare and readonly builtins.
       Each attribute applies to all members of an array.

       Arrays are assigned to using compound assignments of the form  name=(value1  ...  valuen),
       where  each value is of the form [subscript]=string.  Indexed array assignments do not re‐
       quire anything but string.  When assigning to indexed arrays, if the optional brackets and
       subscript  are supplied, that index is assigned to; otherwise the index of the element as‐
       signed is the last index assigned to by the statement plus one.  Indexing starts at zero.

       When assigning to an associative array, the subscript is required.

       This syntax is also accepted by the declare builtin.  Individual array elements may be as‐
       signed  to  using the name[subscript]=value syntax introduced above.  When assigning to an
       indexed array, if name is subscripted by a negative number, that number is interpreted  as
       relative  to  one  greater  than the maximum index of name, so negative indices count back
       from the end of the array, and an index of -1 references the last element.

       Any element of an array may be referenced using ${name[subscript]}.  The  braces  are  re‐
       quired  to  avoid conflicts with pathname expansion.  If subscript is @ or *, the word ex‐
       pands to all members of name.  These subscripts differ only when the word  appears  within
       double quotes.  If the word is double-quoted, ${name[*]} expands to a single word with the
       value of each array member separated by the first character of the IFS  special  variable,
       and  ${name[@]}  expands each element of name to a separate word.  When there are no array
       members, ${name[@]} expands to nothing.  If the double-quoted expansion  occurs  within  a
       word, the expansion of the first parameter is joined with the beginning part of the origi‐
       nal word, and the expansion of the last parameter is joined with  the  last  part  of  the
       original  word.  This is analogous to the expansion of the special parameters * and @ (see
       Special Parameters above).  ${#name[subscript]}  expands  to  the  length  of  ${name[sub‐
       script]}.   If  subscript is * or @, the expansion is the number of elements in the array.
       If the subscript used to reference an element of an indexed array evaluates  to  a  number
       less than zero, it is interpreted as relative to one greater than the maximum index of the
       array, so negative indices count back from the end of the array, and an index of -1 refer‐
       ences the last element.

       Referencing  an  array variable without a subscript is equivalent to referencing the array
       with a subscript of 0.  Any reference to a variable using a valid subscript is legal,  and
       bash will create an array if necessary.

       An  array  variable  is considered set if a subscript has been assigned a value.  The null
       string is a valid value.

       It is possible to  obtain  the  keys  (indices)  of  an  array  as  well  as  the  values.
       ${!name[@]}  and  ${!name[*]}  expand to the indices assigned in array variable name.  The
       treatment when in double quotes is similar to the expansion of the  special  parameters  @
       and * within double quotes.

       The unset builtin is used to destroy arrays.  unset name[subscript] destroys the array el‐
       ement at index subscript, for both indexed and associative arrays.  Negative subscripts to
       indexed arrays are interpreted as described above.  Unsetting the last element of an array
       variable does not unset the variable.  unset name,  where  name  is  an  array,  or  unset
       name[subscript], where subscript is * or @, removes the entire array.

       When  using a variable name with a subscript as an argument to a command, such as with un‐
       set, without using the word expansion syntax described above, the argument is  subject  to
       pathname expansion.  If pathname expansion is not desired, the argument should be quoted.

       The  declare,  local,  and readonly builtins each accept a -a option to specify an indexed
       array and a -A option to specify an associative array.  If both options are  supplied,  -A
       takes  precedence.   The  read  builtin accepts a -a option to assign a list of words read
       from the standard input to an array.  The set and declare builtins display array values in
       a way that allows them to be reused as assignments.

EXPANSION
       Expansion  is performed on the command line after it has been split into words.  There are
       seven kinds of expansion performed: brace expansion, tilde expansion, parameter and  vari‐
       able  expansion,  command substitution, arithmetic expansion, word splitting, and pathname
       expansion.

       The order of expansions is: brace expansion; tilde expansion, parameter and  variable  ex‐
       pansion, arithmetic expansion, and command substitution (done in a left-to-right fashion);
       word splitting; and pathname expansion.

       On systems that can support it, there is an additional expansion available:  process  sub‐
       stitution.   This  is performed at the same time as tilde, parameter, variable, and arith‐
       metic expansion and command substitution.

       After these expansions are performed, quote characters present in the  original  word  are
       removed unless they have been quoted themselves (quote removal).

       Only  brace  expansion,  word splitting, and pathname expansion can increase the number of
       words of the expansion; other expansions expand a single word to a single word.  The  only
       exceptions to this are the expansions of "$@" and "${name[@]}", and, in most cases, $* and
       ${name[*]} as explained above (see PARAMETERS).

   Brace Expansion
       Brace expansion is a mechanism by which arbitrary strings may be generated.   This  mecha‐
       nism  is  similar to pathname expansion, but the filenames generated need not exist.  Pat‐
       terns to be brace expanded take the form of an optional preamble, followed by either a se‐
       ries  of  comma-separated  strings or a sequence expression between a pair of braces, fol‐
       lowed by an optional postscript.  The preamble is prefixed to each string contained within
       the  braces,  and the postscript is then appended to each resulting string, expanding left
       to right.

       Brace expansions may be nested.  The results of each expanded string are not sorted;  left
       to right order is preserved.  For example, a{d,c,b}e expands into `ade ace abe'.

       A  sequence expression takes the form {x..y[..incr]}, where x and y are either integers or
       single characters, and incr, an optional increment, is an integer.  When integers are sup‐
       plied,  the  expression expands to each number between x and y, inclusive.  Supplied inte‐
       gers may be prefixed with 0 to force each term to have the same width.  When either x or y
       begins  with  a  zero, the shell attempts to force all generated terms to contain the same
       number of digits, zero-padding where necessary.  When characters are supplied, the expres‐
       sion expands to each character lexicographically between x and y, inclusive, using the de‐
       fault C locale.  Note that both x and y must be of the same type.  When the  increment  is
       supplied,  it  is used as the difference between each term.  The default increment is 1 or
       -1 as appropriate.

       Brace expansion is performed before any other expansions, and any  characters  special  to
       other  expansions are preserved in the result.  It is strictly textual.  Bash does not ap‐
       ply any syntactic interpretation to the context of the expansion or the text  between  the
       braces.

       A  correctly-formed  brace expansion must contain unquoted opening and closing braces, and
       at least one unquoted comma or a valid sequence expression.  Any incorrectly formed  brace
       expansion is left unchanged.  A { or , may be quoted with a backslash to prevent its being
       considered part of a brace expression.  To avoid conflicts with parameter  expansion,  the
       string ${ is not considered eligible for brace expansion, and inhibits brace expansion un‐
       til the closing }.

       This construct is typically used as shorthand when the common prefix of the strings to  be
       generated is longer than in the above example:

              mkdir /usr/local/src/bash/{old,new,dist,bugs}
       or
              chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

       Brace  expansion  introduces  a slight incompatibility with historical versions of sh.  sh
       does not treat opening or closing braces specially when they appear as part of a word, and
       preserves  them  in  the output.  Bash removes braces from words as a consequence of brace
       expansion.  For example, a word entered to sh as file{1,2} appears identically in the out‐
       put.   The same word is output as file1 file2 after expansion by bash.  If strict compati‐
       bility with sh is desired, start bash with the +B option or disable brace  expansion  with
       the +B option to the set command (see SHELL BUILTIN COMMANDS below).

   Tilde Expansion
       If  a  word begins with an unquoted tilde character (`~'), all of the characters preceding
       the first unquoted slash (or all characters, if there is no unquoted slash) are considered
       a  tilde-prefix.  If none of the characters in the tilde-prefix are quoted, the characters
       in the tilde-prefix following the tilde are treated as a possible login name.  If this lo‐
       gin  name  is the null string, the tilde is replaced with the value of the shell parameter
       HOME.  If HOME is unset, the home directory of the user executing the shell is substituted
       instead.   Otherwise, the tilde-prefix is replaced with the home directory associated with
       the specified login name.

       If the tilde-prefix is a `~+', the value of the shell variable PWD replaces the tilde-pre‐
       fix.  If the tilde-prefix is a `~-', the value of the shell variable OLDPWD, if it is set,
       is substituted.  If the characters following the tilde in the tilde-prefix  consist  of  a
       number  N,  optionally  prefixed  by a `+' or a `-', the tilde-prefix is replaced with the
       corresponding element from the directory stack, as it  would  be  displayed  by  the  dirs
       builtin  invoked  with  the  tilde-prefix as an argument.  If the characters following the
       tilde in the tilde-prefix consist of a number without a leading `+' or  `-',  `+'  is  as‐
       sumed.

       If the login name is invalid, or the tilde expansion fails, the word is unchanged.

       Each  variable assignment is checked for unquoted tilde-prefixes immediately following a :
       or the first =.  In these cases, tilde expansion is also performed.  Consequently, one may
       use  filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell as‐
       signs the expanded value.

       Bash also performs tilde expansion on words satisfying the conditions of variable  assign‐
       ments  (as  described above under PARAMETERS) when they appear as arguments to simple com‐
       mands.  Bash does not do this, except for the declaration commands listed above,  when  in
       posix mode.

   Parameter Expansion
       The  `$' character introduces parameter expansion, command substitution, or arithmetic ex‐
       pansion.  The parameter name or symbol to be expanded may be enclosed in braces, which are
       optional but serve to protect the variable to be expanded from characters immediately fol‐
       lowing it which could be interpreted as part of the name.

       When braces are used, the matching ending brace is the first `}' not escaped  by  a  back‐
       slash  or within a quoted string, and not within an embedded arithmetic expansion, command
       substitution, or parameter expansion.

       ${parameter}
              The value of parameter is substituted.  The braces are required when parameter is a
              positional  parameter  with more than one digit, or when parameter is followed by a
              character which is not to be interpreted as part of its name.  The parameter  is  a
              shell parameter as described above PARAMETERS) or an array reference (Arrays).

       If  the  first  character of parameter is an exclamation point (!), and parameter is not a
       nameref, it introduces a level of indirection.  Bash uses the value  formed  by  expanding
       the  rest  of parameter as the new parameter; this is then expanded and that value is used
       in the rest of the expansion, rather than the expansion of the original  parameter.   This
       is known as indirect expansion.  The value is subject to tilde expansion, parameter expan‐
       sion, command substitution, and arithmetic expansion.  If parameter is a nameref, this ex‐
       pands  to the name of the parameter referenced by parameter instead of performing the com‐
       plete indirect expansion.  The exceptions to this are the expansions  of  ${!prefix*}  and
       ${!name[@]} described below.  The exclamation point must immediately follow the left brace
       in order to introduce indirection.

       In each of the cases below, word is subject to tilde expansion, parameter expansion,  com‐
       mand substitution, and arithmetic expansion.

       When not performing substring expansion, using the forms documented below (e.g., :-), bash
       tests for a parameter that is unset or null.  Omitting the colon results in  a  test  only
       for a parameter that is unset.

       ${parameter:-word}
              Use  Default  Values.  If parameter is unset or null, the expansion of word is sub‐
              stituted.  Otherwise, the value of parameter is substituted.
       ${parameter:=word}
              Assign Default Values.  If parameter is unset or null, the expansion of word is as‐
              signed  to  parameter.  The value of parameter is then substituted.  Positional pa‐
              rameters and special parameters may not be assigned to in this way.
       ${parameter:?word}
              Display Error if Null or Unset.  If parameter is null or unset,  the  expansion  of
              word  (or  a message to that effect if word is not present) is written to the stan‐
              dard error and the shell, if it is not interactive, exits.  Otherwise, the value of
              parameter is substituted.
       ${parameter:+word}
              Use Alternate Value.  If parameter is null or unset, nothing is substituted, other‐
              wise the expansion of word is substituted.
       ${parameter:offset}
       ${parameter:offset:length}
              Substring Expansion.  Expands to up to length characters of the value of  parameter
              starting at the character specified by offset.  If parameter is @, an indexed array
              subscripted by @ or *, or an associative array name,  the  results  differ  as  de‐
              scribed  below.  If length is omitted, expands to the substring of the value of pa‐
              rameter starting at the character specified by offset and extending to the  end  of
              the value.  length and offset are arithmetic expressions (see ARITHMETIC EVALUATION
              below).

              If offset evaluates to a number less than zero, the value is used as an  offset  in
              characters from the end of the value of parameter.  If length evaluates to a number
              less than zero, it is interpreted as an offset in characters from the  end  of  the
              value  of  parameter  rather  than a number of characters, and the expansion is the
              characters between offset and that result.  Note that a  negative  offset  must  be
              separated  from the colon by at least one space to avoid being confused with the :-
              expansion.

              If parameter is @, the result is length positional parameters beginning at  offset.
              A negative offset is taken relative to one greater than the greatest positional pa‐
              rameter, so an offset of -1 evaluates to the last positional parameter.  It  is  an
              expansion error if length evaluates to a number less than zero.

              If  parameter  is  an  indexed  array name subscripted by @ or *, the result is the
              length members of the array beginning with ${parameter[offset]}.  A negative offset
              is taken relative to one greater than the maximum index of the specified array.  It
              is an expansion error if length evaluates to a number less than zero.

              Substring expansion applied to an associative array produces undefined results.

              Substring indexing is zero-based unless the  positional  parameters  are  used,  in
              which case the indexing starts at 1 by default.  If offset is 0, and the positional
              parameters are used, $0 is prefixed to the list.

       ${!prefix*}
       ${!prefix@}
              Names matching prefix.  Expands to the names of variables whose  names  begin  with
              prefix,  separated  by  the first character of the IFS special variable.  When @ is
              used and the expansion appears within double quotes, each variable name expands  to
              a separate word.

       ${!name[@]}
       ${!name[*]}
              List of array keys.  If name is an array variable, expands to the list of array in‐
              dices (keys) assigned in name.  If name is not an array, expands to 0  if  name  is
              set  and  null  otherwise.   When @ is used and the expansion appears within double
              quotes, each key expands to a separate word.

       ${#parameter}
              Parameter length.  The length in characters of the value of  parameter  is  substi‐
              tuted.   If  parameter is * or @, the value substituted is the number of positional
              parameters.  If parameter is an array name subscripted by * or @, the value substi‐
              tuted  is  the  number  of elements in the array.  If parameter is an indexed array
              name subscripted by a negative number, that number is interpreted  as  relative  to
              one  greater  than  the  maximum index of parameter, so negative indices count back
              from the end of the array, and an index of -1 references the last element.

       ${parameter#word}
       ${parameter##word}
              Remove matching prefix pattern.  The word is expanded to produce a pattern just  as
              in  pathname  expansion,  and matched against the expanded value of parameter using
              the rules described under Pattern Matching below.  If the pattern matches  the  be‐
              ginning of the value of parameter, then the result of the expansion is the expanded
              value of parameter with the shortest matching pattern (the ``#'' case) or the long‐
              est  matching  pattern (the ``##'' case) deleted.  If parameter is @ or *, the pat‐
              tern removal operation is applied to each positional parameter in turn, and the ex‐
              pansion  is the resultant list.  If parameter is an array variable subscripted with
              @ or *, the pattern removal operation is applied to each member  of  the  array  in
              turn, and the expansion is the resultant list.

       ${parameter%word}
       ${parameter%%word}
              Remove  matching suffix pattern.  The word is expanded to produce a pattern just as
              in pathname expansion, and matched against the expanded value  of  parameter  using
              the  rules described under Pattern Matching below.  If the pattern matches a trail‐
              ing portion of the expanded value of parameter, then the result of the expansion is
              the expanded value of parameter with the shortest matching pattern (the ``%'' case)
              or the longest matching pattern (the ``%%'' case) deleted.  If parameter is @ or *,
              the  pattern removal operation is applied to each positional parameter in turn, and
              the expansion is the resultant list.  If parameter is an array variable subscripted
              with  @  or *, the pattern removal operation is applied to each member of the array
              in turn, and the expansion is the resultant list.

       ${parameter/pattern/string}
              Pattern substitution.  The pattern is expanded to produce  a  pattern  just  as  in
              pathname  expansion, Parameter is expanded and the longest match of pattern against
              its value is replaced with string.  The match is  performed  using  the  rules  de‐
              scribed  under  Pattern  Matching  below.  If pattern begins with /, all matches of
              pattern are replaced with string.  Normally only the first match is  replaced.   If
              pattern  begins with #, it must match at the beginning of the expanded value of pa‐
              rameter.  If pattern begins with %, it must match at the end of the expanded  value
              of  parameter.  If string is null, matches of pattern are deleted and the / follow‐
              ing pattern may be omitted.  If the nocasematch shell option is enabled, the  match
              is  performed without regard to the case of alphabetic characters.  If parameter is
              @ or *, the substitution operation is applied to each positional parameter in turn,
              and  the  expansion  is the resultant list.  If parameter is an array variable sub‐
              scripted with @ or *, the substitution operation is applied to each member  of  the
              array in turn, and the expansion is the resultant list.

       ${parameter^pattern}
       ${parameter^^pattern}
       ${parameter,pattern}
       ${parameter,,pattern}
              Case  modification.   This  expansion modifies the case of alphabetic characters in
              parameter.  The pattern is expanded to produce a pattern just as in pathname expan‐
              sion.  Each character in the expanded value of parameter is tested against pattern,
              and, if it matches the pattern, its case is converted.  The pattern should not  at‐
              tempt  to match more than one character.  The ^ operator converts lowercase letters
              matching pattern to uppercase; the , operator converts matching  uppercase  letters
              to  lowercase.   The ^^ and ,, expansions convert each matched character in the ex‐
              panded value; the ^ and , expansions match and convert only the first character  in
              the  expanded  value.  If pattern is omitted, it is treated like a ?, which matches
              every character.  If parameter is @ or *, the case modification  operation  is  ap‐
              plied  to  each  positional  parameter  in turn, and the expansion is the resultant
              list.  If parameter is an array variable subscripted with @ or *, the case  modifi‐
              cation  operation is applied to each member of the array in turn, and the expansion
              is the resultant list.

       ${parameter@operator}
              Parameter transformation.  The expansion is either a transformation of the value of
              parameter  or  information about parameter itself, depending on the value of opera‐
              tor.  Each operator is a single letter:

              Q      The expansion is a string that is the value of parameter quoted in a  format
                     that can be reused as input.
              E      The  expansion is a string that is the value of parameter with backslash es‐
                     cape sequences expanded as with the $'...' quoting mechanism.
              P      The expansion is a string that is the result of expanding the value  of  pa‐
                     rameter as if it were a prompt string (see PROMPTING below).
              A      The  expansion is a string in the form of an assignment statement or declare
                     command that, if evaluated, will recreate parameter with its attributes  and
                     value.
              a      The expansion is a string consisting of flag values representing parameter's
                     attributes.

              If parameter is @ or *, the operation is applied to each  positional  parameter  in
              turn,  and  the expansion is the resultant list.  If parameter is an array variable
              subscripted with @ or *, the operation is applied to each member of  the  array  in
              turn, and the expansion is the resultant list.

              The  result of the expansion is subject to word splitting and pathname expansion as
              described below.

   Command Substitution
       Command substitution allows the output of a command to replace the  command  name.   There
       are two forms:

              $(command)
       or
              `command`

       Bash  performs  the expansion by executing command in a subshell environment and replacing
       the command substitution with the standard output of the command, with any  trailing  new‐
       lines  deleted.   Embedded  newlines  are not deleted, but they may be removed during word
       splitting.  The command substitution $(cat file) can be replaced  by  the  equivalent  but
       faster $(< file).

       When  the  old-style backquote form of substitution is used, backslash retains its literal
       meaning except when followed by $, `, or \.  The first backquote not preceded by  a  back‐
       slash terminates the command substitution.  When using the $(command) form, all characters
       between the parentheses make up the command; none are treated specially.

       Command substitutions may be nested.  To nest when using the backquoted form,  escape  the
       inner backquotes with backslashes.

       If  the  substitution  appears within double quotes, word splitting and pathname expansion
       are not performed on the results.

   Arithmetic Expansion
       Arithmetic expansion allows the evaluation of an arithmetic expression and  the  substitu‐
       tion of the result.  The format for arithmetic expansion is:

              $((expression))

       The  old  format  $[expression]  is deprecated and will be removed in upcoming versions of
       bash.

       The expression is treated as if it were within double quotes, but a  double  quote  inside
       the  parentheses is not treated specially.  All tokens in the expression undergo parameter
       and variable expansion, command substitution, and quote removal.  The result is treated as
       the arithmetic expression to be evaluated.  Arithmetic expansions may be nested.

       The  evaluation  is performed according to the rules listed below under ARITHMETIC EVALUA‐
       TION.  If expression is invalid, bash prints a message indicating failure and no substitu‐
       tion occurs.

   Process Substitution
       Process  substitution  allows  a process's input or output to be referred to using a file‐
       name.  It takes the form of <(list) or >(list).  The process list is  run  asynchronously,
       and  its input or output appears as a filename.  This filename is passed as an argument to
       the current command as the result of the expansion.  If the >(list) form is used,  writing
       to  the file will provide input for list.  If the <(list) form is used, the file passed as
       an argument should be read to obtain the output of list.   Process  substitution  is  sup‐
       ported  on  systems  that support named pipes (FIFOs) or the /dev/fd method of naming open
       files.

       When available, process substitution is performed simultaneously with parameter and  vari‐
       able expansion, command substitution, and arithmetic expansion.

   Word Splitting
       The  shell  scans the results of parameter expansion, command substitution, and arithmetic
       expansion that did not occur within double quotes for word splitting.

       The shell treats each character of IFS as a delimiter, and splits the results of the other
       expansions  into  words  using these characters as field terminators.  If IFS is unset, or
       its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>,
       and  <newline>  at the beginning and end of the results of the previous expansions are ig‐
       nored, and any sequence of IFS characters not at the beginning or end  serves  to  delimit
       words.   If IFS has a value other than the default, then sequences of the whitespace char‐
       acters space, tab, and newline are ignored at the beginning and end of the word,  as  long
       as  the  whitespace  character  is in the value of IFS (an IFS whitespace character).  Any
       character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace  char‐
       acters,  delimits  a  field.  A sequence of IFS whitespace characters is also treated as a
       delimiter.  If the value of IFS is null, no word splitting occurs.

       Explicit null arguments ("" or '') are retained and passed to commands as  empty  strings.
       Unquoted  implicit null arguments, resulting from the expansion of parameters that have no
       values, are removed.  If a parameter with no value is expanded  within  double  quotes,  a
       null  argument results and is retained and passed to a command as an empty string.  When a
       quoted null argument appears as part of a word whose expansion is non-null, the null argu‐
       ment is removed.  That is, the word -d'' becomes -d after word splitting and null argument
       removal.

       Note that if no expansion occurs, no splitting is performed.

   Pathname Expansion
       After word splitting, unless the -f option has been set, bash  scans  each  word  for  the
       characters  *, ?, and [.  If one of these characters appears, then the word is regarded as
       a pattern, and replaced with an alphabetically sorted list of filenames matching the  pat‐
       tern  (see Pattern Matching below).  If no matching filenames are found, and the shell op‐
       tion nullglob is not enabled, the word is left unchanged.  If the nullglob option is  set,
       and  no  matches are found, the word is removed.  If the failglob shell option is set, and
       no matches are found, an error message is printed and the command is not executed.  If the
       shell  option  nocaseglob is enabled, the match is performed without regard to the case of
       alphabetic characters.  Note that when using range expressions  like  [a-z]  (see  below),
       letters of the other case may be included, depending on the setting of LC_COLLATE.  When a
       pattern is used for pathname expansion, the character ``.''  at the start of a name or im‐
       mediately following a slash must be matched explicitly, unless the shell option dotglob is
       set.  The filenames ``.''  and ``..''  must always be matched explicitly, even if  dotglob
       is  set.   In other cases, the ``.''  character is not treated specially.  When matching a
       pathname, the slash character must always be matched explicitly by a slash in the pattern,
       but  in  other  matching  contexts it can be matched by a special pattern character as de‐
       scribed below under Pattern Matching.  See the description  of  shopt  below  under  SHELL
       BUILTIN  COMMANDS  for  a  description  of the nocaseglob, nullglob, failglob, and dotglob
       shell options.

       The GLOBIGNORE shell variable may be used to restrict the set of  file  names  matching  a
       pattern.   If GLOBIGNORE is set, each matching file name that also matches one of the pat‐
       terns in GLOBIGNORE is removed from the list of matches.  If the nocaseglob option is set,
       the  matching against the patterns in GLOBIGNORE is performed without regard to case.  The
       filenames ``.''  and ``..''  are always ignored when GLOBIGNORE is set and not null.  How‐
       ever,  setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell
       option, so all other filenames beginning with a ``.''  will match.  To get the old  behav‐
       ior of ignoring filenames beginning with a ``.'', make ``.*''  one of the patterns in GLO‐
       BIGNORE.  The dotglob option is disabled when GLOBIGNORE is unset.  The  pattern  matching
       honors the setting of the extglob shell option.

       Pattern Matching

       Any  character  that  appears  in a pattern, other than the special pattern characters de‐
       scribed below, matches itself.  The NUL character may not occur in a pattern.  A backslash
       escapes  the  following character; the escaping backslash is discarded when matching.  The
       special pattern characters must be quoted if they are to be matched literally.

       The special pattern characters have the following meanings:

              *      Matches any string, including the null string.  When the globstar shell  op‐
                     tion is enabled, and * is used in a pathname expansion context, two adjacent
                     *s used as a single pattern will match all files and zero or  more  directo‐
                     ries  and  subdirectories.   If  followed by a /, two adjacent *s will match
                     only directories and subdirectories.
              ?      Matches any single character.
              [...]  Matches any one of the enclosed characters.  A pair of characters  separated
                     by  a  hyphen  denotes  a range expression; any character that falls between
                     those two characters, inclusive, using the current  locale's  collating  se‐
                     quence  and character set, is matched.  If the first character following the
                     [ is a !  or a ^ then any character not enclosed is  matched.   The  sorting
                     order of characters in range expressions is determined by the current locale
                     and the values of the LC_COLLATE or LC_ALL shell variables, if set.  To  ob‐
                     tain  the  traditional  interpretation  of range expressions, where [a-d] is
                     equivalent to [abcd], set value of the LC_ALL shell variable to C, or enable
                     the globasciiranges shell option.  A - may be matched by including it as the
                     first or last character in the set.  A ] may be matched by including  it  as
                     the first character in the set.

                     Within  [  and  ],  character  classes  can  be  specified  using the syntax
                     [:class:], where class is one of the following classes defined in the  POSIX
                     standard:
                     alnum alpha ascii blank cntrl digit graph lower print punct space upper word
                     xdigit
                     A character class matches any character belonging to that class.   The  word
                     character class matches letters, digits, and the character _.

                     Within  [  and  ],  an  equivalence  class can be specified using the syntax
                     [=c=], which matches all characters with the same collation weight  (as  de‐
                     fined by the current locale) as the character c.

                     Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.

       If  the  extglob shell option is enabled using the shopt builtin, several extended pattern
       matching operators are recognized.  In the following description, a pattern-list is a list
       of  one  or more patterns separated by a |.  Composite patterns may be formed using one or
       more of the following sub-patterns:

              ?(pattern-list)
                     Matches zero or one occurrence of the given patterns
              *(pattern-list)
                     Matches zero or more occurrences of the given patterns
              +(pattern-list)
                     Matches one or more occurrences of the given patterns
              @(pattern-list)
                     Matches one of the given patterns
              !(pattern-list)
                     Matches anything except one of the given patterns

       Complicated extended pattern matching against long strings is slow,  especially  when  the
       patterns  contain  alternations  and the strings contain multiple matches.  Using separate
       matches against shorter strings, or using arrays of  strings  instead  of  a  single  long
       string, may be faster.

   Quote Removal
       After  the  preceding  expansions,  all unquoted occurrences of the characters \, ', and "
       that did not result from one of the above expansions are removed.

REDIRECTION
       Before a command is executed, its input and output may be redirected using a special nota‐
       tion  interpreted  by  the  shell.  Redirection allows commands' file handles to be dupli‐
       cated, opened, closed, made to refer to different files, and can change the files the com‐
       mand reads from and writes to.  Redirection may also be used to modify file handles in the
       current shell execution environment.  The following redirection operators may  precede  or
       appear  anywhere  within  a simple command or may follow a command.  Redirections are pro‐
       cessed in the order they appear, from left to right.

       Each redirection that may be preceded by a file descriptor number may instead be  preceded
       by  a  word of the form {varname}.  In this case, for each redirection operator except >&-
       and <&-, the shell will allocate a file descriptor greater than or equal to 10 and  assign
       it  to  varname.  If >&- or <&- is preceded by {varname}, the value of varname defines the
       file descriptor to close.  If {varname} is supplied, the redirection persists  beyond  the
       scope of the command, allowing the shell programmer to manage the file descriptor himself.

       In  the  following  descriptions,  if the file descriptor number is omitted, and the first
       character of the redirection operator is <, the redirection refers to the  standard  input
       (file  descriptor  0).  If the first character of the redirection operator is >, the redi‐
       rection refers to the standard output (file descriptor 1).

       The word following the redirection operator in the following descriptions,  unless  other‐
       wise  noted,  is subjected to brace expansion, tilde expansion, parameter and variable ex‐
       pansion, command substitution, arithmetic expansion, quote  removal,  pathname  expansion,
       and word splitting.  If it expands to more than one word, bash reports an error.

       Note that the order of redirections is significant.  For example, the command

              ls > dirlist 2>&1

       directs both standard output and standard error to the file dirlist, while the command

              ls 2>&1 > dirlist

       directs  only  the  standard output to file dirlist, because the standard error was dupli‐
       cated from the standard output before the standard output was redirected to dirlist.

       Bash handles several filenames specially when they are used in redirections, as  described
       in  the  following table.  If the operating system on which bash is running provides these
       special files, bash will use them; otherwise it will emulate them internally with the  be‐
       havior described below.

              /dev/fd/fd
                     If fd is a valid integer, file descriptor fd is duplicated.
              /dev/stdin
                     File descriptor 0 is duplicated.
              /dev/stdout
                     File descriptor 1 is duplicated.
              /dev/stderr
                     File descriptor 2 is duplicated.
              /dev/tcp/host/port
                     If host is a valid hostname or Internet address, and port is an integer port
                     number or service name, bash attempts to open the corresponding TCP socket.
              /dev/udp/host/port
                     If host is a valid hostname or Internet address, and port is an integer port
                     number or service name, bash attempts to open the corresponding UDP socket.

       A failure to open or create a file causes the redirection to fail.

       Redirections  using  file descriptors greater than 9 should be used with care, as they may
       conflict with file descriptors the shell uses internally.

       Note that the exec builtin command can make redirections take effect in the current shell.

   Redirecting Input
       Redirection of input causes the file whose name results from the expansion of word  to  be
       opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is
       not specified.

       The general format for redirecting input is:

              [n]<word

   Redirecting Output
       Redirection of output causes the file whose name results from the expansion of word to  be
       opened  for  writing on file descriptor n, or the standard output (file descriptor 1) if n
       is not specified.  If the file does not exist it is created; if it does exist it is  trun‐
       cated to zero size.

       The general format for redirecting output is:

              [n]>word

       If the redirection operator is >, and the noclobber option to the set builtin has been en‐
       abled, the redirection will fail if the file whose name results from the expansion of word
       exists and is a regular file.  If the redirection operator is >|, or the redirection oper‐
       ator is > and the noclobber option to the set builtin command is not  enabled,  the  redi‐
       rection is attempted even if the file named by word exists.

   Appending Redirected Output
       Redirection  of  output in this fashion causes the file whose name results from the expan‐
       sion of word to be opened for appending on file descriptor n, or the standard output (file
       descriptor 1) if n is not specified.  If the file does not exist it is created.

       The general format for appending output is:

              [n]>>word

   Redirecting Standard Output and Standard Error
       This  construct allows both the standard output (file descriptor 1) and the standard error
       output (file descriptor 2) to be redirected to the file whose name  is  the  expansion  of
       word.

       There are two formats for redirecting standard output and standard error:

              &>word
       and
              >&word

       Of the two forms, the first is preferred.  This is semantically equivalent to

              >word 2>&1

       When using the second form, word may not expand to a number or -.  If it does, other redi‐
       rection operators apply (see Duplicating File Descriptors below)  for  compatibility  rea‐
       sons.

   Appending Standard Output and Standard Error
       This  construct allows both the standard output (file descriptor 1) and the standard error
       output (file descriptor 2) to be appended to the file whose name is the expansion of word.

       The format for appending standard output and standard error is:

              &>>word

       This is semantically equivalent to

              >>word 2>&1

       (see Duplicating File Descriptors below).

   Here Documents
       This type of redirection instructs the shell to read input from the current source until a
       line  containing  only delimiter (with no trailing blanks) is seen.  All of the lines read
       up to that point are then used as the standard input (or file descriptor n if n is  speci‐
       fied) for a command.

       The format of here-documents is:

              [n]<<[-]word
                      here-document
              delimiter

       No  parameter and variable expansion, command substitution, arithmetic expansion, or path‐
       name expansion is performed on word.  If any part of word is quoted, the delimiter is  the
       result  of quote removal on word, and the lines in the here-document are not expanded.  If
       word is unquoted, all lines of the here-document are  subjected  to  parameter  expansion,
       command  substitution,  and arithmetic expansion, the character sequence \<newline> is ig‐
       nored, and \ must be used to quote the characters \, $, and `.

       If the redirection operator is <<-, then all leading tab characters are stripped from  in‐
       put  lines  and  the  line  containing delimiter.  This allows here-documents within shell
       scripts to be indented in a natural fashion.

   Here Strings
       A variant of here documents, the format is:

              [n]<<<word

       The word undergoes tilde expansion, parameter and variable  expansion,  command  substitu‐
       tion,  arithmetic expansion, and quote removal.  Pathname expansion and word splitting are
       not performed.  The result is supplied as a single string, with a newline appended, to the
       command on its standard input (or file descriptor n if n is specified).

   Duplicating File Descriptors
       The redirection operator

              [n]<&word

       is  used  to duplicate input file descriptors.  If word expands to one or more digits, the
       file descriptor denoted by n is made to be a copy of that file descriptor.  If the  digits
       in  word  do not specify a file descriptor open for input, a redirection error occurs.  If
       word evaluates to -, file descriptor n is closed.  If n is not specified, the standard in‐
       put (file descriptor 0) is used.

       The operator

              [n]>&word

       is  used similarly to duplicate output file descriptors.  If n is not specified, the stan‐
       dard output (file descriptor 1) is used.  If the digits in word do not specify a file  de‐
       scriptor  open  for  output, a redirection error occurs.  If word evaluates to -, file de‐
       scriptor n is closed.  As a special case, if n is omitted, and word does not expand to one
       or  more  digits  or -, the standard output and standard error are redirected as described
       previously.

   Moving File Descriptors
       The redirection operator

              [n]<&digit-

       moves the file descriptor digit to file descriptor n, or the standard input (file descrip‐
       tor 0) if n is not specified.  digit is closed after being duplicated to n.

       Similarly, the redirection operator

              [n]>&digit-

       moves  the  file  descriptor  digit to file descriptor n, or the standard output (file de‐
       scriptor 1) if n is not specified.

   Opening File Descriptors for Reading and Writing
       The redirection operator

              [n]<>word

       causes the file whose name is the expansion of word to be  opened  for  both  reading  and
       writing  on file descriptor n, or on file descriptor 0 if n is not specified.  If the file
       does not exist, it is created.

ALIASES
       Aliases allow a string to be substituted for a word when it is used as the first word of a
       simple  command.  The shell maintains a list of aliases that may be set and unset with the
       alias and unalias builtin commands (see SHELL BUILTIN COMMANDS below).  The first word  of
       each  simple command, if unquoted, is checked to see if it has an alias.  If so, that word
       is replaced by the text of the alias.  The characters /, $, `, and = and any of the  shell
       metacharacters  or  quoting  characters listed above may not appear in an alias name.  The
       replacement text may contain any valid shell input, including shell  metacharacters.   The
       first  word of the replacement text is tested for aliases, but a word that is identical to
       an alias being expanded is not expanded a second time.  This means that one may  alias  ls
       to  ls -F, for instance, and bash does not try to recursively expand the replacement text.
       If the last character of the alias value is a blank, then the next command word  following
       the alias is also checked for alias expansion.

       Aliases  are  created and listed with the alias command, and removed with the unalias com‐
       mand.

       There is no mechanism for using arguments in  the  replacement  text.   If  arguments  are
       needed, a shell function should be used (see FUNCTIONS below).

       Aliases  are  not  expanded  when  the shell is not interactive, unless the expand_aliases
       shell option is set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS
       below).

       The  rules  concerning the definition and use of aliases are somewhat confusing.  Bash al‐
       ways reads at least one complete line of input, and all lines that make up a compound com‐
       mand,  before executing any of the commands on that line or the compound command.  Aliases
       are expanded when a command is read, not when it is executed.  Therefore, an alias defini‐
       tion  appearing  on  the  same line as another command does not take effect until the next
       line of input is read.  The commands following the alias definition on that line  are  not
       affected  by  the  new alias.  This behavior is also an issue when functions are executed.
       Aliases are expanded when a function definition is read, not when  the  function  is  exe‐
       cuted,  because  a function definition is itself a command.  As a consequence, aliases de‐
       fined in a function are not available until after that function is executed.  To be  safe,
       always  put  alias  definitions  on a separate line, and do not use alias in compound com‐
       mands.

       For almost every purpose, aliases are superseded by shell functions.

FUNCTIONS
       A shell function, defined as described above under SHELL GRAMMAR, stores a series of  com‐
       mands  for later execution.  When the name of a shell function is used as a simple command
       name, the list of commands associated with that function name is executed.  Functions  are
       executed  in the context of the current shell; no new process is created to interpret them
       (contrast this with the execution of a shell script).  When a function  is  executed,  the
       arguments to the function become the positional parameters during its execution.  The spe‐
       cial parameter # is updated to reflect the change.  Special parameter 0 is unchanged.  The
       first  element of the FUNCNAME variable is set to the name of the function while the func‐
       tion is executing.

       All other aspects of the shell execution environment are identical between a function  and
       its  caller  with these exceptions: the DEBUG and RETURN traps (see the description of the
       trap builtin under SHELL BUILTIN COMMANDS below) are not inherited unless the function has
       been  given  the trace attribute (see the description of the declare builtin below) or the
       -o functrace shell option has been enabled with the set builtin (in which case  all  func‐
       tions inherit the DEBUG and RETURN traps), and the ERR trap is not inherited unless the -o
       errtrace shell option has been enabled.

       Variables local to the function may be declared with the local builtin command.   Ordinar‐
       ily,  variables  and  their  values  are shared between the function and its caller.  If a
       variable is declared local, the variable's visible scope is restricted  to  that  function
       and  its  children (including the functions it calls).  Local variables "shadow" variables
       with the same name declared at previous scopes.  For instance, a local  variable  declared
       in  a  function hides a global variable of the same name: references and assignments refer
       to the local variable, leaving the global variable unmodified.  When the function returns,
       the global variable is once again visible.

       The  shell uses dynamic scoping to control a variable's visibility within functions.  With
       dynamic scoping, visible variables and their values are a result of the sequence of  func‐
       tion  calls  that caused execution to reach the current function.  The value of a variable
       that a function sees depends on its value within its caller, if any, whether  that  caller
       is  the  "global"  scope  or  another shell function.  This is also the value that a local
       variable declaration "shadows", and the value that is restored when the function returns.

       For example, if a variable var is declared as local in function func1, and func1 calls an‐
       other  function  func2, references to var made from within func2 will resolve to the local
       variable var from func1, shadowing any global variable named var.

       The unset builtin also acts using the same dynamic scope: if a variable is  local  to  the
       current  scope,  unset will unset it; otherwise the unset will refer to the variable found
       in any calling scope as described above.  If a variable at the current local scope is  un‐
       set,  it  will  remain  so  until it is reset in that scope or until the function returns.
       Once the function returns, any instance of the variable at a previous  scope  will  become
       visible.   If the unset acts on a variable at a previous scope, any instance of a variable
       with that name that had been shadowed will become visible.

       The FUNCNEST variable, if set to a numeric value greater than 0, defines a  maximum  func‐
       tion  nesting  level.  Function invocations that exceed the limit cause the entire command
       to abort.

       If the builtin command return is executed in a function, the function completes and execu‐
       tion  resumes  with the next command after the function call.  Any command associated with
       the RETURN trap is executed before execution resumes.  When a function completes, the val‐
       ues  of  the  positional parameters and the special parameter # are restored to the values
       they had prior to the function's execution.

       Function names and definitions may be listed with the -f option to the declare or  typeset
       builtin  commands.   The -F option to declare or typeset will list the function names only
       (and optionally the source file and line number, if the extdebug shell option is enabled).
       Functions  may  be  exported so that subshells automatically have them defined with the -f
       option to the export builtin.  A function definition may be deleted using the -f option to
       the unset builtin.

       Functions  may  be recursive.  The FUNCNEST variable may be used to limit the depth of the
       function call stack and restrict the number of function invocations.  By default, no limit
       is imposed on the number of recursive calls.

ARITHMETIC EVALUATION
       The  shell allows arithmetic expressions to be evaluated, under certain circumstances (see
       the let and declare builtin commands, the (( compound command, and Arithmetic  Expansion).
       Evaluation  is done in fixed-width integers with no check for overflow, though division by
       0 is trapped and flagged as an error.  The operators and their precedence,  associativity,
       and  values are the same as in the C language.  The following list of operators is grouped
       into levels of equal-precedence operators.  The levels are listed in order  of  decreasing
       precedence.

       id++ id--
              variable post-increment and post-decrement
       - +    unary minus and plus
       ++id --id
              variable pre-increment and pre-decrement
       ! ~    logical and bitwise negation
       **     exponentiation
       * / %  multiplication, division, remainder
       + -    addition, subtraction
       << >>  left and right bitwise shifts
       <= >= < >
              comparison
       == !=  equality and inequality
       &      bitwise AND
       ^      bitwise exclusive OR
       |      bitwise OR
       &&     logical AND
       ||     logical OR
       expr?expr:expr
              conditional operator
       = *= /= %= += -= <<= >>= &= ^= |=
              assignment
       expr1 , expr2
              comma

       Shell  variables  are allowed as operands; parameter expansion is performed before the ex‐
       pression is evaluated.  Within an expression, shell variables may also  be  referenced  by
       name without using the parameter expansion syntax.  A shell variable that is null or unset
       evaluates to 0 when referenced by name without using the parameter expansion syntax.   The
       value  of  a  variable  is evaluated as an arithmetic expression when it is referenced, or
       when a variable which has been given the integer attribute using declare -i is assigned  a
       value.  A null value evaluates to 0.  A shell variable need not have its integer attribute
       turned on to be used in an expression.

       Constants with a leading 0 are interpreted as octal numbers.  A leading 0x or  0X  denotes
       hexadecimal.  Otherwise, numbers take the form [base#]n, where the optional base is a dec‐
       imal number between 2 and 64 representing the arithmetic base, and n is a number  in  that
       base.   If  base# is omitted, then base 10 is used.  When specifying n, the digits greater
       than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in  that
       order.   If  base is less than or equal to 36, lowercase and uppercase letters may be used
       interchangeably to represent numbers between 10 and 35.

       Operators are evaluated in order of precedence.  Sub-expressions in parentheses are evalu‐
       ated first and may override the precedence rules above.

CONDITIONAL EXPRESSIONS
       Conditional  expressions  are  used  by the [[ compound command and the test and [ builtin
       commands to test file attributes and perform string and arithmetic comparisons.  The  test
       abd [ commands determine their behavior based on the number of arguments; see the descrip‐
       tions of those commands for any other command-specific actions.

       Expressions are formed from the following unary or binary primaries.  Bash handles several
       filenames  specially  when they are used in expressions.  If the operating system on which
       bash is running provides these special files, bash will use them; otherwise it  will  emu‐
       late  them  internally with this behavior: If any file argument to one of the primaries is
       of the form /dev/fd/n, then file descriptor n is checked.  If the file argument to one  of
       the  primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or
       2, respectively, is checked.

       Unless otherwise specified, primaries that operate on files follow symbolic links and  op‐
       erate on the target of the link, rather than the link itself.

       When  used with [[, the < and > operators sort lexicographically using the current locale.
       The test command sorts using ASCII ordering.

       -a file
              True if file exists.
       -b file
              True if file exists and is a block special file.
       -c file
              True if file exists and is a character special file.
       -d file
              True if file exists and is a directory.
       -e file
              True if file exists.
       -f file
              True if file exists and is a regular file.
       -g file
              True if file exists and is set-group-id.
       -h file
              True if file exists and is a symbolic link.
       -k file
              True if file exists and its ``sticky'' bit is set.
       -p file
              True if file exists and is a named pipe (FIFO).
       -r file
              True if file exists and is readable.
       -s file
              True if file exists and has a size greater than zero.
       -t fd  True if file descriptor fd is open and refers to a terminal.
       -u file
              True if file exists and its set-user-id bit is set.
       -w file
              True if file exists and is writable.
       -x file
              True if file exists and is executable.
       -G file
              True if file exists and is owned by the effective group id.
       -L file
              True if file exists and is a symbolic link.
       -N file
              True if file exists and has been modified since it was last read.
       -O file
              True if file exists and is owned by the effective user id.
       -S file
              True if file exists and is a socket.
       file1 -ef file2
              True if file1 and file2 refer to the same device and inode numbers.
       file1 -nt file2
              True if file1 is newer (according to modification date) than file2, or if file1 ex‐
              ists and file2 does not.
       file1 -ot file2
              True if file1 is older than file2, or if file2 exists and file1 does not.
       -o optname
              True if the shell option optname is enabled.  See the list of options under the de‐
              scription of the -o option to the set builtin below.
       -v varname
              True if the shell variable varname is set (has been assigned a value).
       -R varname
              True if the shell variable varname is set and is a name reference.
       -z string
              True if the length of string is zero.
       string
       -n string
              True if the length of string is non-zero.

       string1 == string2
       string1 = string2
              True if the strings are equal.  = should be used with the test  command  for  POSIX
              conformance.   When used with the [[ command, this performs pattern matching as de‐
              scribed above (Compound Commands).

       string1 != string2
              True if the strings are not equal.

       string1 < string2
              True if string1 sorts before string2 lexicographically.

       string1 > string2
              True if string1 sorts after string2 lexicographically.

       arg1 OP arg2
              OP is one of -eq, -ne, -lt, -le, -gt, or -ge.  These  arithmetic  binary  operators
              return  true  if  arg1 is equal to, not equal to, less than, less than or equal to,
              greater than, or greater than or equal to arg2, respectively.  Arg1 and arg2 may be
              positive  or  negative  integers.  When used with the [[ command, Arg1 and Arg2 are
              evaluated as arithmetic expressions  (see ARITHMETIC EVALUATION above).

SIMPLE COMMAND EXPANSION
       When a simple command is executed, the shell performs the  following  expansions,  assign‐
       ments, and redirections, from left to right.

       1.     The  words  that the parser has marked as variable assignments (those preceding the
              command name) and redirections are saved for later processing.

       2.     The words that are not variable assignments or redirections are expanded.   If  any
              words remain after expansion, the first word is taken to be the name of the command
              and the remaining words are the arguments.

       3.     Redirections are performed as described above under REDIRECTION.

       4.     The text after the = in each variable assignment undergoes tilde expansion, parame‐
              ter expansion, command substitution, arithmetic expansion, and quote removal before
              being assigned to the variable.

       If no command name results, the variable assignments affect the current shell environment.
       Otherwise,  the  variables are added to the environment of the executed command and do not
       affect the current shell environment.  If any of the  assignments  attempts  to  assign  a
       value  to a readonly variable, an error occurs, and the command exits with a non-zero sta‐
       tus.

       If no command name results, redirections are performed, but  do  not  affect  the  current
       shell environment.  A redirection error causes the command to exit with a non-zero status.

       If  there  is  a command name left after expansion, execution proceeds as described below.
       Otherwise, the command exits.  If one of the expansions contained a command  substitution,
       the  exit  status  of the command is the exit status of the last command substitution per‐
       formed.  If there were no command substitutions, the command exits with a status of zero.

COMMAND EXECUTION
       After a command has been split into words, if it results in a simple command  and  an  op‐
       tional list of arguments, the following actions are taken.

       If the command name contains no slashes, the shell attempts to locate it.  If there exists
       a shell function by that name, that function is invoked as described above  in  FUNCTIONS.
       If  the  name  does  not  match a function, the shell searches for it in the list of shell
       builtins.  If a match is found, that builtin is invoked.

       If the name is neither a shell function nor a  builtin,  and  contains  no  slashes,  bash
       searches  each  element  of the PATH for a directory containing an executable file by that
       name.  Bash uses a hash table to remember the full pathnames of executable files (see hash
       under  SHELL  BUILTIN  COMMANDS  below).  A full search of the directories in PATH is per‐
       formed only if the command is not found in the hash table.  If the search is unsuccessful,
       the  shell  searches for a defined shell function named command_not_found_handle.  If that
       function exists, it is invoked in a separate execution environment with the original  com‐
       mand and the original command's arguments as its arguments, and the function's exit status
       becomes the exit status of that subshell.  If that function  is  not  defined,  the  shell
       prints an error message and returns an exit status of 127.

       If  the  search  is  successful,  or if the command name contains one or more slashes, the
       shell executes the named program in a separate execution environment.  Argument 0  is  set
       to  the  name  given,  and the remaining arguments to the command are set to the arguments
       given, if any.

       If this execution fails because the file is not in executable format, and the file is  not
       a directory, it is assumed to be a shell script, a file containing shell commands.  A sub‐
       shell is spawned to execute it.  This subshell reinitializes itself, so that the effect is
       as if a new shell had been invoked to handle the script, with the exception that the loca‐
       tions of commands remembered by the parent (see hash below under SHELL  BUILTIN  COMMANDS)
       are retained by the child.

       If  the  program is a file beginning with #!, the remainder of the first line specifies an
       interpreter for the program.  The shell executes the specified  interpreter  on  operating
       systems that do not handle this executable format themselves.  The arguments to the inter‐
       preter consist of a single optional argument following the interpreter name on  the  first
       line  of  the  program, followed by the name of the program, followed by the command argu‐
       ments, if any.

COMMAND EXECUTION ENVIRONMENT
       The shell has an execution environment, which consists of the following:

       •      open files inherited by the shell at invocation, as modified by  redirections  sup‐
              plied to the exec builtin

       •      the  current  working  directory  as set by cd, pushd, or popd, or inherited by the
              shell at invocation

       •      the file creation mode mask as set by umask or inherited from the shell's parent

       •      current traps set by trap

       •      shell parameters that are set by variable assignment or with set or inherited  from
              the shell's parent in the environment

       •      shell  functions  defined  during execution or inherited from the shell's parent in
              the environment

       •      options enabled at invocation (either by default or with command-line arguments) or
              by set

       •      options enabled by shopt

       •      shell aliases defined with alias

       •      various  process  IDs, including those of background jobs, the value of $$, and the
              value of PPID

       When a simple command other than a builtin or shell function is to be executed, it is  in‐
       voked  in  a separate execution environment that consists of the following.  Unless other‐
       wise noted, the values are inherited from the shell.

       •      the shell's open files, plus any modifications and additions specified by  redirec‐
              tions to the command

       •      the current working directory

       •      the file creation mode mask

       •      shell  variables and functions marked for export, along with variables exported for
              the command, passed in the environment

       •      traps caught by the shell are reset to the values inherited from the  shell's  par‐
              ent, and traps ignored by the shell are ignored

       A  command  invoked in this separate environment cannot affect the shell's execution envi‐
       ronment.

       Command substitution, commands grouped with parentheses, and asynchronous commands are in‐
       voked  in a subshell environment that is a duplicate of the shell environment, except that
       traps caught by the shell are reset to the values that the shell inherited from its parent
       at  invocation.  Builtin commands that are invoked as part of a pipeline are also executed
       in a subshell environment.  Changes made to the subshell  environment  cannot  affect  the
       shell's execution environment.

       Subshells spawned to execute command substitutions inherit the value of the -e option from
       the parent shell.  When not in posix mode, bash clears the -e option in such subshells.

       If a command is followed by a & and job control is not active, the default standard  input
       for  the command is the empty file /dev/null.  Otherwise, the invoked command inherits the
       file descriptors of the calling shell as modified by redirections.

ENVIRONMENT
       When a program is invoked it is given an array of strings called the environment.  This is
       a list of name-value pairs, of the form name=value.

       The  shell  provides several ways to manipulate the environment.  On invocation, the shell
       scans its own environment and creates a parameter for each name found, automatically mark‐
       ing it for export to child processes.  Executed commands inherit the environment.  The ex‐
       port and declare -x commands allow parameters and functions to be  added  to  and  deleted
       from the environment.  If the value of a parameter in the environment is modified, the new
       value becomes part of the environment, replacing the old.  The  environment  inherited  by
       any executed command consists of the shell's initial environment, whose values may be mod‐
       ified in the shell, less any pairs removed by the unset command, plus  any  additions  via
       the export and declare -x commands.

       The environment for any simple command or function may be augmented temporarily by prefix‐
       ing it with parameter assignments, as described above  in  PARAMETERS.   These  assignment
       statements affect only the environment seen by that command.

       If  the  -k  option is set (see the set builtin command below), then all parameter assign‐
       ments are placed in the environment for a command, not just those that precede the command
       name.

       When  bash  invokes an external command, the variable _ is set to the full filename of the
       command and passed to that command in its environment.

EXIT STATUS
       The exit status of an executed command is the value returned by the waitpid system call or
       equivalent  function.   Exit  statuses fall between 0 and 255, though, as explained below,
       the shell may use values above 125 specially.  Exit statuses from shell builtins and  com‐
       pound  commands  are  also  limited to this range.  Under certain circumstances, the shell
       will use special values to indicate specific failure modes.

       For the shell's purposes, a command which exits with a zero exit status has succeeded.  An
       exit  status of zero indicates success.  A non-zero exit status indicates failure.  When a
       command terminates on a fatal signal N, bash uses the value of 128+N as the exit status.

       If a command is not found, the child process created to execute it  returns  a  status  of
       127.  If a command is found but is not executable, the return status is 126.

       If a command fails because of an error during expansion or redirection, the exit status is
       greater than zero.

       Shell builtin commands return a status of 0 (true) if successful, and non-zero (false)  if
       an  error  occurs while they execute.  All builtins return an exit status of 2 to indicate
       incorrect usage, generally invalid options or missing arguments.

       Bash itself returns the exit status of the last command executed, unless  a  syntax  error
       occurs,  in  which case it exits with a non-zero value.  See also the exit builtin command
       below.

SIGNALS
       When bash is interactive, in the absence of any traps, it ignores SIGTERM (so that kill  0
       does  not  kill  an interactive shell), and SIGINT is caught and handled (so that the wait
       builtin is interruptible).  In all cases, bash ignores SIGQUIT.  If job control is in  ef‐
       fect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

       Non-builtin  commands  run by bash have signal handlers set to the values inherited by the
       shell from its parent.  When job control is not in effect,  asynchronous  commands  ignore
       SIGINT  and  SIGQUIT in addition to these inherited handlers.  Commands run as a result of
       command substitution ignore the keyboard-generated job control signals  SIGTTIN,  SIGTTOU,
       and SIGTSTP.

       The shell exits by default upon receipt of a SIGHUP.  Before exiting, an interactive shell
       resends the SIGHUP to all jobs, running or stopped.  Stopped jobs are sent SIGCONT to  en‐
       sure that they receive the SIGHUP.  To prevent the shell from sending the signal to a par‐
       ticular job, it should be removed from the jobs table with the disown builtin  (see  SHELL
       BUILTIN COMMANDS below) or marked to not receive SIGHUP using disown -h.

       If  the  huponexit  shell  option has been set with shopt, bash sends a SIGHUP to all jobs
       when an interactive login shell exits.

       If bash is waiting for a command to complete and receives a signal for which  a  trap  has
       been set, the trap will not be executed until the command completes.  When bash is waiting
       for an asynchronous command via the wait builtin, the reception of a signal  for  which  a
       trap  has  been  set will cause the wait builtin to return immediately with an exit status
       greater than 128, immediately after which the trap is executed.

JOB CONTROL
       Job control refers to the ability to selectively stop (suspend) the execution of processes
       and continue (resume) their execution at a later point.  A user typically employs this fa‐
       cility via an interactive interface supplied jointly by the operating system kernel's ter‐
       minal driver and bash.

       The  shell  associates  a job with each pipeline.  It keeps a table of currently executing
       jobs, which may be listed with the jobs command.  When bash starts  a  job  asynchronously
       (in the background), it prints a line that looks like:

              [1] 25647

       indicating  that  this  job is job number 1 and that the process ID of the last process in
       the pipeline associated with this job is 25647.  All of the processes in a single pipeline
       are members of the same job.  Bash uses the job abstraction as the basis for job control.

       To  facilitate the implementation of the user interface to job control, the operating sys‐
       tem maintains the notion of a current terminal process group ID.  Members of this  process
       group (processes whose process group ID is equal to the current terminal process group ID)
       receive keyboard-generated signals such as SIGINT.  These processes are said to be in  the
       foreground.  Background processes are those whose process group ID differs from the termi‐
       nal's; such processes are immune to keyboard-generated signals.  Only foreground processes
       are  allowed to read from or, if the user so specifies with stty tostop, write to the ter‐
       minal.  Background processes which attempt to read from (write to when stty tostop  is  in
       effect)  the terminal are sent a SIGTTIN (SIGTTOU) signal by the kernel's terminal driver,
       which, unless caught, suspends the process.

       If the operating system on which bash is running supports job control, bash  contains  fa‐
       cilities  to  use  it.   Typing  the  suspend  character (typically ^Z, Control-Z) while a
       process is running causes that process to be stopped and returns control to bash.   Typing
       the  delayed  suspend character (typically ^Y, Control-Y) causes the process to be stopped
       when it attempts to read input from the terminal, and control to be returned to bash.  The
       user may then manipulate the state of this job, using the bg command to continue it in the
       background, the fg command to continue it in the foreground, or the kill command  to  kill
       it.   A ^Z takes effect immediately, and has the additional side effect of causing pending
       output and typeahead to be discarded.

       There are a number of ways to refer to a job in the shell.  The character %  introduces  a
       job  specification  (jobspec).   Job number n may be referred to as %n.  A job may also be
       referred to using a prefix of the name used to start it, or using a substring that appears
       in  its  command  line.  For example, %ce refers to a stopped ce job.  If a prefix matches
       more than one job, bash reports an error.  Using %?ce, on the other hand,  refers  to  any
       job  containing the string ce in its command line.  If the substring matches more than one
       job, bash reports an error.  The symbols %% and %+ refer to the shell's notion of the cur‐
       rent  job,  which is the last job stopped while it was in the foreground or started in the
       background.  The previous job may be referenced using %-.  If there is only a single  job,
       %+  and %- can both be used to refer to that job.  In output pertaining to jobs (e.g., the
       output of the jobs command), the current job is always flagged with a +, and the  previous
       job with a -.  A single % (with no accompanying job specification) also refers to the cur‐
       rent job.

       Simply naming a job can be used to bring it into the foreground: %1 is a synonym for  ``fg
       %1'', bringing job 1 from the background into the foreground.  Similarly, ``%1 &'' resumes
       job 1 in the background, equivalent to ``bg %1''.

       The shell learns immediately whenever a job changes state.  Normally, bash waits until  it
       is  about to print a prompt before reporting changes in a job's status so as to not inter‐
       rupt any other output.  If the -b option to the set builtin command is enabled,  bash  re‐
       ports  such  changes immediately.  Any trap on SIGCHLD is executed for each child that ex‐
       its.

       If an attempt to exit bash is made while jobs are stopped (or, if the checkjobs shell  op‐
       tion  has  been enabled using the shopt builtin, running), the shell prints a warning mes‐
       sage, and, if the checkjobs option is enabled, lists the jobs  and  their  statuses.   The
       jobs  command  may  then  be used to inspect their status.  If a second attempt to exit is
       made without an intervening command, the shell does not print  another  warning,  and  any
       stopped jobs are terminated.

       When  the shell is waiting for a job or process using the wait builtin, and job control is
       enabled, wait will return when the job changes state. The -f option  will  force  wait  to
       wait until the job or process terminates before returning.

PROMPTING
       When  executing  interactively,  bash  displays the primary prompt PS1 when it is ready to
       read a command, and the secondary prompt PS2 when it needs more input to complete  a  com‐
       mand.   Bash displays PS0 after it reads a command but before executing it.  Bash displays
       PS4 as described above before tracing each command when the -x option  is  enabled.   Bash
       allows  these  prompt  strings to be customized by inserting a number of backslash-escaped
       special characters that are decoded as follows:
              \a     an ASCII bell character (07)
              \d     the date in "Weekday Month Date" format (e.g., "Tue May 26")
              \D{format}
                     the format is passed to strftime(3) and the  result  is  inserted  into  the
                     prompt string; an empty format results in a locale-specific time representa‐
                     tion.  The braces are required
              \e     an ASCII escape character (033)
              \h     the hostname up to the first `.'
              \H     the hostname
              \j     the number of jobs currently managed by the shell
              \l     the basename of the shell's terminal device name
              \n     newline
              \r     carriage return
              \s     the name of the shell, the basename of $0 (the portion following  the  final
                     slash)
              \t     the current time in 24-hour HH:MM:SS format
              \T     the current time in 12-hour HH:MM:SS format
              \@     the current time in 12-hour am/pm format
              \A     the current time in 24-hour HH:MM format
              \u     the username of the current user
              \v     the version of bash (e.g., 2.00)
              \V     the release of bash, version + patch level (e.g., 2.00.0)
              \w     the current working directory, with $HOME abbreviated with a tilde (uses the
                     value of the PROMPT_DIRTRIM variable)
              \W     the basename of the current working directory, with $HOME abbreviated with a
                     tilde
              \!     the history number of this command
              \#     the command number of this command
              \$     if the effective UID is 0, a #, otherwise a $
              \nnn   the character corresponding to the octal number nnn
              \\     a backslash
              \[     begin  a sequence of non-printing characters, which could be used to embed a
                     terminal control sequence into the prompt
              \]     end a sequence of non-printing characters

       The command number and the history number are usually different: the history number  of  a
       command  is its position in the history list, which may include commands restored from the
       history file (see HISTORY below), while the command number is the position in the sequence
       of commands executed during the current shell session.  After the string is decoded, it is
       expanded via parameter expansion, command substitution, arithmetic  expansion,  and  quote
       removal,  subject  to the value of the promptvars shell option (see the description of the
       shopt command under SHELL BUILTIN COMMANDS below).

READLINE
       This is the library that handles reading input when using an interactive shell, unless the
       --noediting option is given at shell invocation.  Line editing is also used when using the
       -e option to the read builtin.  By default, the line editing commands are similar to those
       of  Emacs.   A vi-style line editing interface is also available.  Line editing can be en‐
       abled at any time using the -o emacs or -o vi  options  to  the  set  builtin  (see  SHELL
       BUILTIN  COMMANDS below).  To turn off line editing after the shell is running, use the +o
       emacs or +o vi options to the set builtin.

   Readline Notation
       In this section, the Emacs-style notation is used to denote keystrokes.  Control keys  are
       denoted  by  C-key, e.g., C-n means Control-N.  Similarly, meta keys are denoted by M-key,
       so M-x means Meta-X.  (On keyboards without a meta key, M-x means ESC x, i.e.,  press  the
       Escape  key  then the x key.  This makes ESC the meta prefix.  The combination M-C-x means
       ESC-Control-x, or press the Escape key then hold the Control  key  while  pressing  the  x
       key.)

       Readline  commands  may  be given numeric arguments, which normally act as a repeat count.
       Sometimes, however, it is the sign of the argument that is significant.  Passing  a  nega‐
       tive  argument  to  a  command that acts in the forward direction (e.g., kill-line) causes
       that command to act in a backward direction.  Commands whose behavior with arguments devi‐
       ates from this are noted below.

       When a command is described as killing text, the text deleted is saved for possible future
       retrieval (yanking).  The killed text is saved in a kill ring.   Consecutive  kills  cause
       the text to be accumulated into one unit, which can be yanked all at once.  Commands which
       do not kill text separate the chunks of text on the kill ring.

   Readline Initialization
       Readline is customized by putting commands in an initialization file (the  inputrc  file).
       The  name  of this file is taken from the value of the INPUTRC variable.  If that variable
       is unset, the default is ~/.inputrc.  When a  program  which  uses  the  readline  library
       starts  up,  the  initialization file is read, and the key bindings and variables are set.
       There are only a few basic constructs allowed in the readline initialization file.   Blank
       lines are ignored.  Lines beginning with a # are comments.  Lines beginning with a $ indi‐
       cate conditional constructs.  Other lines denote key bindings and variable settings.

       The default key-bindings may be changed with an inputrc file.   Other  programs  that  use
       this library may add their own commands and bindings.

       For example, placing

              M-Control-u: universal-argument
       or
              C-Meta-u: universal-argument
       into the inputrc would make M-C-u execute the readline command universal-argument.

       The  following  symbolic  character  names are recognized: RUBOUT, DEL, ESC, LFD, NEWLINE,
       RET, RETURN, SPC, SPACE, and TAB.

       In addition to command names, readline allows keys to be bound to a  string  that  is  in‐
       serted when the key is pressed (a macro).

   Readline Key Bindings
       The  syntax  for  controlling key bindings in the inputrc file is simple.  All that is re‐
       quired is the name of the command or the text of a macro and a key sequence  to  which  it
       should  be  bound.   The name may be specified in one of two ways: as a symbolic key name,
       possibly with Meta- or Control- prefixes, or as a key sequence.

       When using the form keyname:function-name or macro, keyname is the name of a  key  spelled
       out in English.  For example:

              Control-u: universal-argument
              Meta-Rubout: backward-kill-word
              Control-o: "> output"

       In  the  above example, C-u is bound to the function universal-argument, M-DEL is bound to
       the function backward-kill-word, and C-o is bound to run the macro expressed on the  right
       hand side (that is, to insert the text ``> output'' into the line).

       In  the second form, "keyseq":function-name or macro, keyseq differs from keyname above in
       that strings denoting an entire key sequence may be  specified  by  placing  the  sequence
       within  double  quotes.  Some GNU Emacs style key escapes can be used, as in the following
       example, but the symbolic character names are not recognized.

              "\C-u": universal-argument
              "\C-x\C-r": re-read-init-file
              "\e[11~": "Function Key 1"

       In this example, C-u is again bound to the function universal-argument.  C-x C-r is  bound
       to  the function re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text ``Function
       Key 1''.

       The full set of GNU Emacs style escape sequences is
              \C-    control prefix
              \M-    meta prefix
              \e     an escape character
              \\     backslash
              \"     literal "
              \'     literal '

       In addition to the GNU Emacs style escape sequences, a second set of backslash escapes  is
       available:
              \a     alert (bell)
              \b     backspace
              \d     delete
              \f     form feed
              \n     newline
              \r     carriage return
              \t     horizontal tab
              \v     vertical tab
              \nnn   the  eight-bit  character  whose  value is the octal value nnn (one to three
                     digits)
              \xHH   the eight-bit character whose value is the hexadecimal value HH (one or  two
                     hex digits)

       When  entering  the  text  of  a macro, single or double quotes must be used to indicate a
       macro definition.  Unquoted text is assumed to be a function name.  In the macro body, the
       backslash  escapes described above are expanded.  Backslash will quote any other character
       in the macro text, including " and '.

       Bash allows the current readline key bindings to be displayed or modified  with  the  bind
       builtin  command.  The editing mode may be switched during interactive use by using the -o
       option to the set builtin command (see SHELL BUILTIN COMMANDS below).

   Readline Variables
       Readline has variables that can be used to further customize its behavior.  A variable may
       be set in the inputrc file with a statement of the form

              set variable-name value

       Except  where  noted,  readline variables can take the values On or Off (without regard to
       case).  Unrecognized variable names are ignored.  When a variable value is read, empty  or
       null  values, "on" (case-insensitive), and "1" are equivalent to On.  All other values are
       equivalent to Off.  The variables and their default values are:

       bell-style (audible)
              Controls what happens when readline wants to ring the terminal  bell.   If  set  to
              none,  readline  never  rings the bell.  If set to visible, readline uses a visible
              bell if one is available.  If set to audible, readline attempts to ring the  termi‐
              nal's bell.
       bind-tty-special-chars (On)
              If set to On, readline attempts to bind the control characters treated specially by
              the kernel's terminal driver to their readline equivalents.
       blink-matching-paren (Off)
              If set to On, readline attempts to briefly move the cursor to an opening  parenthe‐
              sis when a closing parenthesis is inserted.
       colored-completion-prefix (Off)
              If  set to On, when listing completions, readline displays the common prefix of the
              set of possible completions using a different color.   The  color  definitions  are
              taken from the value of the LS_COLORS environment variable.
       colored-stats (Off)
              If  set to On, readline displays possible completions using different colors to in‐
              dicate their file type.  The color definitions are taken  from  the  value  of  the
              LS_COLORS environment variable.
       comment-begin (``#'')
              The  string  that is inserted when the readline insert-comment command is executed.
              This command is bound to M-# in emacs mode and to # in vi command mode.
       completion-display-width (-1)
              The number of screen columns used to display possible matches when performing  com‐
              pletion.   The  value  is ignored if it is less than 0 or greater than the terminal
              screen width.  A value of 0 will cause matches to be displayed one per  line.   The
              default value is -1.
       completion-ignore-case (Off)
              If set to On, readline performs filename matching and completion in a case-insensi‐
              tive fashion.
       completion-map-case (Off)
              If set to On, and completion-ignore-case is enabled, readline  treats  hyphens  (-)
              and  underscores (_) as equivalent when performing case-insensitive filename match‐
              ing and completion.
       completion-prefix-display-length (0)
              The length in characters of the common prefix of a  list  of  possible  completions
              that  is  displayed  without  modification.  When set to a value greater than zero,
              common prefixes longer than this value are replaced with an ellipsis when  display‐
              ing possible completions.
       completion-query-items (100)
              This  determines when the user is queried about viewing the number of possible com‐
              pletions generated by the possible-completions command.  It may be set to any inte‐
              ger  value greater than or equal to zero.  If the number of possible completions is
              greater than or equal to the value of this variable, the user is asked  whether  or
              not he wishes to view them; otherwise they are simply listed on the terminal.
       convert-meta (On)
              If  set to On, readline will convert characters with the eighth bit set to an ASCII
              key sequence by stripping the eighth bit and prefixing an escape character (in  ef‐
              fect,  using  escape as the meta prefix).  The default is On, but readline will set
              it to Off if the locale contains eight-bit characters.
       disable-completion (Off)
              If set to On, readline will inhibit word completion.  Completion characters will be
              inserted into the line as if they had been mapped to self-insert.
       echo-control-characters (On)
              When set to On, on operating systems that indicate they support it, readline echoes
              a character corresponding to a signal generated from the keyboard.
       editing-mode (emacs)
              Controls whether readline begins with a set of key bindings similar to Emacs or vi.
              editing-mode can be set to either emacs or vi.
       emacs-mode-string (@)
              If  the  show-mode-in-prompt  variable is enabled, this string is displayed immedi‐
              ately before the last line of the primary prompt when emacs editing mode is active.
              The  value is expanded like a key binding, so the standard set of meta- and control
              prefixes and backslash escape sequences is available.  Use the \1 and \2 escapes to
              begin  and  end  sequences of non-printing characters, which can be used to embed a
              terminal control sequence into the mode string.
       enable-bracketed-paste (Off)
              When set to On, readline will configure the terminal in a way that will  enable  it
              to  insert each paste into the editing buffer as a single string of characters, in‐
              stead of treating each character as if it had been read from  the  keyboard.   This
              can prevent pasted characters from being interpreted as editing commands.
       enable-keypad (Off)
              When  set  to  On,  readline  will  try to enable the application keypad when it is
              called.  Some systems need this to enable the arrow keys.
       enable-meta-key (On)
              When set to On, readline will try to enable any  meta  modifier  key  the  terminal
              claims  to  support  when it is called.  On many terminals, the meta key is used to
              send eight-bit characters.
       expand-tilde (Off)
              If set to On, tilde expansion is performed when readline attempts word completion.
       history-preserve-point (Off)
              If set to On, the history code attempts to place point at the same location on each
              history line retrieved with previous-history or next-history.
       history-size (unset)
              Set  the  maximum  number  of history entries saved in the history list.  If set to
              zero, any existing history entries are deleted and no new entries  are  saved.   If
              set  to  a  value less than zero, the number of history entries is not limited.  By
              default, the number of history entries is set to the value of  the  HISTSIZE  shell
              variable.   If  an  attempt is made to set history-size to a non-numeric value, the
              maximum number of history entries will be set to 500.
       horizontal-scroll-mode (Off)
              When set to On, makes readline use a single line for display, scrolling  the  input
              horizontally  on  a single screen line when it becomes longer than the screen width
              rather than wrapping to a new line.
       input-meta (Off)
              If set to On, readline will enable eight-bit input (that is, it will not strip  the
              eighth bit from the characters it reads), regardless of what the terminal claims it
              can support.  The name meta-flag is a synonym for this variable.   The  default  is
              Off, but readline will set it to On if the locale contains eight-bit characters.
       isearch-terminators (``C-[C-J'')
              The string of characters that should terminate an incremental search without subse‐
              quently executing the character as a command.  If this variable has not been  given
              a value, the characters ESC and C-J will terminate an incremental search.
       keymap (emacs)
              Set  the  current  readline  keymap.   The  set  of  valid  keymap  names is emacs,
              emacs-standard, emacs-meta, emacs-ctlx,  vi,  vi-command,  and  vi-insert.   vi  is
              equivalent to vi-command; emacs is equivalent to emacs-standard.  The default value
              is emacs; the value of editing-mode also affects the default keymap.
       keyseq-timeout (500)
              Specifies the duration readline will wait for a character when reading an ambiguous
              key  sequence  (one  that  can form a complete key sequence using the input read so
              far, or can take additional input to complete a longer key sequence).  If no  input
              is  received within the timeout, readline will use the shorter but complete key se‐
              quence.  The value is specified in milliseconds, so a  value  of  1000  means  that
              readline  will  wait one second for additional input.  If this variable is set to a
              value less than or equal to zero, or to a non-numeric value, readline will wait un‐
              til another key is pressed to decide which key sequence to complete.
       mark-directories (On)
              If set to On, completed directory names have a slash appended.
       mark-modified-lines (Off)
              If  set to On, history lines that have been modified are displayed with a preceding
              asterisk (*).
       mark-symlinked-directories (Off)
              If set to On, completed names which are symbolic links to directories have a  slash
              appended (subject to the value of mark-directories).
       match-hidden-files (On)
              This  variable,  when  set  to On, causes readline to match files whose names begin
              with a `.' (hidden files) when performing filename completion.  If set to Off,  the
              leading `.' must be supplied by the user in the filename to be completed.
       menu-complete-display-prefix (Off)
              If  set  to  On, menu completion displays the common prefix of the list of possible
              completions (which may be empty) before cycling through the list.
       output-meta (Off)
              If set to On, readline will display characters with the  eighth  bit  set  directly
              rather  than  as a meta-prefixed escape sequence.  The default is Off, but readline
              will set it to On if the locale contains eight-bit characters.
       page-completions (On)
              If set to On, readline uses an internal more-like pager to display a  screenful  of
              possible completions at a time.
       print-completions-horizontally (Off)
              If set to On, readline will display completions with matches sorted horizontally in
              alphabetical order, rather than down the screen.
       revert-all-at-newline (Off)
              If set to On, readline will undo all changes to history lines before returning when
              accept-line  is executed.  By default, history lines may be modified and retain in‐
              dividual undo lists across calls to readline.
       show-all-if-ambiguous (Off)
              This alters the default behavior of the completion functions.  If set to On,  words
              which have more than one possible completion cause the matches to be listed immedi‐
              ately instead of ringing the bell.
       show-all-if-unmodified (Off)
              This alters the default behavior of the completion functions in a  fashion  similar
              to  show-all-if-ambiguous.   If  set to On, words which have more than one possible
              completion without any possible partial completion (the possible completions  don't
              share  a common prefix) cause the matches to be listed immediately instead of ring‐
              ing the bell.
       show-mode-in-prompt (Off)
              If set to On, add a string to the beginning of the prompt  indicating  the  editing
              mode:  emacs,  vi  command,  or  vi  insertion.  The mode strings are user-settable
              (e.g., emacs-mode-string).
       skip-completed-text (Off)
              If set to On, this alters the default completion behavior when inserting  a  single
              match  into the line.  It's only active when performing completion in the middle of
              a word.  If enabled, readline does not insert characters from the  completion  that
              match  characters  after point in the word being completed, so portions of the word
              following the cursor are not duplicated.
       vi-cmd-mode-string ((cmd))
              If the show-mode-in-prompt variable is enabled, this string  is  displayed  immedi‐
              ately before the last line of the primary prompt when vi editing mode is active and
              in command mode.  The value is expanded like a key binding, so the standard set  of
              meta- and control prefixes and backslash escape sequences is available.  Use the \1
              and \2 escapes to begin and end sequences of non-printing characters, which can  be
              used to embed a terminal control sequence into the mode string.
       vi-ins-mode-string ((ins))
              If  the  show-mode-in-prompt  variable is enabled, this string is displayed immedi‐
              ately before the last line of the primary prompt when vi editing mode is active and
              in  insertion  mode.  The value is expanded like a key binding, so the standard set
              of meta- and control prefixes and backslash escape sequences is available.  Use the
              \1  and \2 escapes to begin and end sequences of non-printing characters, which can
              be used to embed a terminal control sequence into the mode string.
       visible-stats (Off)
              If set to On, a character denoting a file's type as reported by stat(2) is appended
              to the filename when listing possible completions.

   Readline Conditional Constructs
       Readline  implements  a facility similar in spirit to the conditional compilation features
       of the C preprocessor which allows key bindings and variable settings to be  performed  as
       the result of tests.  There are four parser directives used.

       $if    The  $if construct allows bindings to be made based on the editing mode, the termi‐
              nal being used, or the application using readline.  The text of the test, after any
              comparison operator,
               extends to the end of the line; unless otherwise noted, no characters are required
              to isolate it.

              mode   The mode= form of the $if directive is used to test whether readline  is  in
                     emacs  or vi mode.  This may be used in conjunction with the set keymap com‐
                     mand, for instance, to set bindings in  the  emacs-standard  and  emacs-ctlx
                     keymaps only if readline is starting out in emacs mode.

              term   The  term=  form may be used to include terminal-specific key bindings, per‐
                     haps to bind the key sequences output by the terminal's function keys.   The
                     word  on the right side of the = is tested against both the full name of the
                     terminal and the portion of the terminal name before the first -.  This  al‐
                     lows sun to match both sun and sun-cmd, for instance.

              version
                     The  version  test may be used to perform comparisons against specific read‐
                     line versions.  The version expands to the current  readline  version.   The
                     set of comparison operators includes =, (and ==), !=, <=, >=, <, and >.  The
                     version number supplied on the right side of the operator consists of a  ma‐
                     jor version number, an optional decimal point, and an optional minor version
                     (e.g., 7.1). If the minor version is omitted, it is assumed to  be  0.   The
                     operator  may be separated from the string version and from the version num‐
                     ber argument by whitespace.

              application
                     The application construct is used to include application-specific  settings.
                     Each  program  using  the readline library sets the application name, and an
                     initialization file can test for a particular value.  This could be used  to
                     bind  key  sequences  to  functions  useful for a specific program.  For in‐
                     stance, the following command adds a key sequence that quotes the current or
                     previous word in bash:

                     $if Bash
                     # Quote the current or previous word
                     "\C-xq": "\eb\"\ef\""
                     $endif

              variable
                     The variable construct provides simple equality tests for readline variables
                     and values.  The permitted comparison operators are  =,  ==,  and  !=.   The
                     variable  name must be separated from the comparison operator by whitespace;
                     the operator may be separated from the value  on  the  right  hand  side  by
                     whitespace.   Both string and boolean variables may be tested. Boolean vari‐
                     ables must be tested against the values on and off.

       $endif This command, as seen in the previous example, terminates an $if command.

       $else  Commands in this branch of the $if directive are executed if the test fails.

       $include
              This directive takes a single filename as an argument and reads commands and  bind‐
              ings from that file.  For example, the following directive would read /etc/inputrc:

              $include  /etc/inputrc

   Searching
       Readline  provides  commands for searching through the command history (see HISTORY below)
       for lines containing a specified string.  There are two search modes: incremental and non-
       incremental.

       Incremental searches begin before the user has finished typing the search string.  As each
       character of the search string is typed, readline displays the next entry from the history
       matching  the string typed so far.  An incremental search requires only as many characters
       as needed to find the desired history entry.  The characters present in the value  of  the
       isearch-terminators  variable  are used to terminate an incremental search.  If that vari‐
       able has not been assigned a value the Escape and Control-J characters will  terminate  an
       incremental  search.   Control-G will abort an incremental search and restore the original
       line.  When the search is terminated, the history entry containing the search  string  be‐
       comes the current line.

       To  find other matching entries in the history list, type Control-S or Control-R as appro‐
       priate.  This will search backward or forward in the history for the next  entry  matching
       the  search  string typed so far.  Any other key sequence bound to a readline command will
       terminate the search and execute that command.  For instance, a newline will terminate the
       search and accept the line, thereby executing the command from the history list.

       Readline  remembers the last incremental search string.  If two Control-Rs are typed with‐
       out any intervening characters defining a new search string, any remembered search  string
       is used.

       Non-incremental  searches  read  the  entire  search  string before starting to search for
       matching history lines.  The search string may be typed by the user or be part of the con‐
       tents of the current line.

   Readline Command Names
       The  following  is  a  list  of the names of the commands and the default key sequences to
       which they are bound.  Command names without an accompanying key sequence are  unbound  by
       default.   In the following descriptions, point refers to the current cursor position, and
       mark refers to a cursor position saved by the set-mark  command.   The  text  between  the
       point and mark is referred to as the region.

   Commands for Moving
       beginning-of-line (C-a)
              Move to the start of the current line.
       end-of-line (C-e)
              Move to the end of the line.
       forward-char (C-f)
              Move forward a character.
       backward-char (C-b)
              Move back a character.
       forward-word (M-f)
              Move forward to the end of the next word.  Words are composed of alphanumeric char‐
              acters (letters and digits).
       backward-word (M-b)
              Move back to the start of the current or previous word.  Words are composed of  al‐
              phanumeric characters (letters and digits).
       shell-forward-word
              Move  forward to the end of the next word.  Words are delimited by non-quoted shell
              metacharacters.
       shell-backward-word
              Move back to the start of the current or previous word.   Words  are  delimited  by
              non-quoted shell metacharacters.
       previous-screen-line
              Attempt  to  move point to the same physical screen column on the previous physical
              screen line. This will not have the desired effect if  the  current  Readline  line
              does  not  take  up more than one physical line or if point is not greater than the
              length of the prompt plus the screen width.
       next-screen-line
              Attempt to move point to the same physical  screen  column  on  the  next  physical
              screen  line.  This  will  not have the desired effect if the current Readline line
              does not take up more than one physical line or if the length of the current  Read‐
              line line is not greater than the length of the prompt plus the screen width.
       clear-screen (C-l)
              Clear  the screen leaving the current line at the top of the screen.  With an argu‐
              ment, refresh the current line without clearing the screen.
       redraw-current-line
              Refresh the current line.

   Commands for Manipulating the History
       accept-line (Newline, Return)
              Accept the line regardless of where the cursor is.  If this line is non-empty,  add
              it  to the history list according to the state of the HISTCONTROL variable.  If the
              line is a modified history line, then restore the  history  line  to  its  original
              state.
       previous-history (C-p)
              Fetch the previous command from the history list, moving back in the list.
       next-history (C-n)
              Fetch the next command from the history list, moving forward in the list.
       beginning-of-history (M-<)
              Move to the first line in the history.
       end-of-history (M->)
              Move to the end of the input history, i.e., the line currently being entered.
       reverse-search-history (C-r)
              Search backward starting at the current line and moving `up' through the history as
              necessary.  This is an incremental search.
       forward-search-history (C-s)
              Search forward starting at the current line and moving `down' through  the  history
              as necessary.  This is an incremental search.
       non-incremental-reverse-search-history (M-p)
              Search backward through the history starting at the current line using a non-incre‐
              mental search for a string supplied by the user.
       non-incremental-forward-search-history (M-n)
              Search forward through the history using a non-incremental search for a string sup‐
              plied by the user.
       history-search-forward
              Search  forward  through the history for the string of characters between the start
              of the current line and the point.  This is a non-incremental search.
       history-search-backward
              Search backward through the history for the string of characters between the  start
              of the current line and the point.  This is a non-incremental search.
       history-substring-search-backward
              Search  backward through the history for the string of characters between the start
              of the current line and the current cursor position (the point).  The search string
              may match anywhere in a history line.  This is a non-incremental search.
       history-substring-search-forward
              Search  forward  through the history for the string of characters between the start
              of the current line and the point.  The search string may match anywhere in a  his‐
              tory line.  This is a non-incremental search.
       yank-nth-arg (M-C-y)
              Insert  the  first argument to the previous command (usually the second word on the
              previous line) at point.  With an argument n, insert the nth word from the previous
              command (the words in the previous command begin with word 0).  A negative argument
              inserts the nth word from the end of the previous command.  Once the argument n  is
              computed, the argument is extracted as if the "!n" history expansion had been spec‐
              ified.
       yank-last-arg (M-., M-_)
              Insert the last argument to the previous command (the last  word  of  the  previous
              history  entry).   With a numeric argument, behave exactly like yank-nth-arg.  Suc‐
              cessive calls to yank-last-arg move back through the history  list,  inserting  the
              last word (or the word specified by the argument to the first call) of each line in
              turn.  Any numeric argument supplied to these successive calls determines  the  di‐
              rection  to  move  through the history.  A negative argument switches the direction
              through the history (back or forward).  The history expansion facilities  are  used
              to extract the last word, as if the "!$" history expansion had been specified.
       shell-expand-line (M-C-e)
              Expand  the  line  as the shell does.  This performs alias and history expansion as
              well as all of the shell word expansions.  See HISTORY EXPANSION below  for  a  de‐
              scription of history expansion.
       history-expand-line (M-^)
              Perform  history  expansion on the current line.  See HISTORY EXPANSION below for a
              description of history expansion.
       magic-space
              Perform history expansion on the current line and insert a space.  See HISTORY  EX‐
              PANSION below for a description of history expansion.
       alias-expand-line
              Perform  alias  expansion on the current line.  See ALIASES above for a description
              of alias expansion.
       history-and-alias-expand-line
              Perform history and alias expansion on the current line.
       insert-last-argument (M-., M-_)
              A synonym for yank-last-arg.
       operate-and-get-next (C-o)
              Accept the current line for execution and fetch the next line relative to the  cur‐
              rent line from the history for editing.  A numeric argument, if supplied, specifies
              the history entry to use instead of the current line.
       edit-and-execute-command (C-x C-e)
              Invoke an editor on the current command line, and execute the result as shell  com‐
              mands.   Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the editor, in that
              order.

   Commands for Changing Text
       end-of-file (usually C-d)
              The character indicating end-of-file as set, for example,  by  ``stty''.   If  this
              character is read when there are no characters on the line, and point is at the be‐
              ginning of the line, Readline interprets it as the end of input and returns EOF.
       delete-char (C-d)
              Delete the character at point.  If this function is bound to the same character  as
              the tty EOF character, as C-d commonly is, see above for the effects.
       backward-delete-char (Rubout)
              Delete  the  character  behind the cursor.  When given a numeric argument, save the
              deleted text on the kill ring.
       forward-backward-delete-char
              Delete the character under the cursor, unless the cursor is at the end of the line,
              in which case the character behind the cursor is deleted.
       quoted-insert (C-q, C-v)
              Add  the  next character typed to the line verbatim.  This is how to insert charac‐
              ters like C-q, for example.
       tab-insert (C-v TAB)
              Insert a tab character.
       self-insert (a, b, A, 1, !, ...)
              Insert the character typed.
       transpose-chars (C-t)
              Drag the character before point forward over the character at point,  moving  point
              forward  as well.  If point is at the end of the line, then this transposes the two
              characters before point.  Negative arguments have no effect.
       transpose-words (M-t)
              Drag the word before point past the word after point, moving point over  that  word
              as well.  If point is at the end of the line, this transposes the last two words on
              the line.
       upcase-word (M-u)
              Uppercase the current (or following) word.  With a negative argument, uppercase the
              previous word, but do not move point.
       downcase-word (M-l)
              Lowercase the current (or following) word.  With a negative argument, lowercase the
              previous word, but do not move point.
       capitalize-word (M-c)
              Capitalize the current (or following) word.  With a negative  argument,  capitalize
              the previous word, but do not move point.
       overwrite-mode
              Toggle  overwrite  mode.   With  an explicit positive numeric argument, switches to
              overwrite mode.  With an explicit non-positive numeric argument, switches to insert
              mode.   This  command  affects only emacs mode; vi mode does overwrite differently.
              Each call to readline() starts in insert mode.  In overwrite mode, characters bound
              to self-insert replace the text at point rather than pushing the text to the right.
              Characters bound to backward-delete-char replace the character before point with  a
              space.  By default, this command is unbound.

   Killing and Yanking
       kill-line (C-k)
              Kill the text from point to the end of the line.
       backward-kill-line (C-x Rubout)
              Kill backward to the beginning of the line.
       unix-line-discard (C-u)
              Kill backward from point to the beginning of the line.  The killed text is saved on
              the kill-ring.
       kill-whole-line
              Kill all characters on the current line, no matter where point is.
       kill-word (M-d)
              Kill from point to the end of the current word, or if between words, to the end  of
              the next word.  Word boundaries are the same as those used by forward-word.
       backward-kill-word (M-Rubout)
              Kill  the  word  behind point.  Word boundaries are the same as those used by back‐
              ward-word.
       shell-kill-word
              Kill from point to the end of the current word, or if between words, to the end  of
              the next word.  Word boundaries are the same as those used by shell-forward-word.
       shell-backward-kill-word
              Kill  the  word  behind  point.   Word  boundaries  are  the  same as those used by
              shell-backward-word.
       unix-word-rubout (C-w)
              Kill the word behind point, using white space as a word boundary.  The killed  text
              is saved on the kill-ring.
       unix-filename-rubout
              Kill  the  word behind point, using white space and the slash character as the word
              boundaries.  The killed text is saved on the kill-ring.
       delete-horizontal-space (M-\)
              Delete all spaces and tabs around point.
       kill-region
              Kill the text in the current region.
       copy-region-as-kill
              Copy the text in the region to the kill buffer.
       copy-backward-word
              Copy the word before point to the kill buffer.  The word boundaries are the same as
              backward-word.
       copy-forward-word
              Copy the word following point to the kill buffer.  The word boundaries are the same
              as forward-word.
       yank (C-y)
              Yank the top of the kill ring into the buffer at point.
       yank-pop (M-y)
              Rotate the kill ring, and yank the new top.  Only works following yank or yank-pop.

   Numeric Arguments
       digit-argument (M-0, M-1, ..., M--)
              Add this digit to the argument already accumulating, or start a new argument.   M--
              starts a negative argument.
       universal-argument
              This  is another way to specify an argument.  If this command is followed by one or
              more digits, optionally with a leading minus sign, those digits  define  the  argu‐
              ment.   If  the  command  is followed by digits, executing universal-argument again
              ends the numeric argument, but is otherwise ignored.  As a special  case,  if  this
              command  is  immediately  followed by a character that is neither a digit nor minus
              sign, the argument count for the next command is multiplied by four.  The  argument
              count  is  initially one, so executing this function the first time makes the argu‐
              ment count four, a second time makes the argument count sixteen, and so on.

   Completing
       complete (TAB)
              Attempt to perform completion on the text before point.  Bash  attempts  completion
              treating  the text as a variable (if the text begins with $), username (if the text
              begins with ~), hostname (if the text begins with @), or command (including aliases
              and  functions) in turn.  If none of these produces a match, filename completion is
              attempted.
       possible-completions (M-?)
              List the possible completions of the text before point.
       insert-completions (M-*)
              Insert all completions of the text before point that would have been  generated  by
              possible-completions.
       menu-complete
              Similar to complete, but replaces the word to be completed with a single match from
              the list of  possible  completions.   Repeated  execution  of  menu-complete  steps
              through the list of possible completions, inserting each match in turn.  At the end
              of the list of completions, the bell is rung (subject to the setting of bell-style)
              and  the  original text is restored.  An argument of n moves n positions forward in
              the list of matches; a negative argument may be used to move backward  through  the
              list.  This command is intended to be bound to TAB, but is unbound by default.
       menu-complete-backward
              Identical to menu-complete, but moves backward through the list of possible comple‐
              tions, as if menu-complete had been given a negative argument.  This command is un‐
              bound by default.
       delete-char-or-list
              Deletes  the  character under the cursor if not at the beginning or end of the line
              (like delete-char).  If at the end of  the  line,  behaves  identically  to  possi‐
              ble-completions.  This command is unbound by default.
       complete-filename (M-/)
              Attempt filename completion on the text before point.
       possible-filename-completions (C-x /)
              List the possible completions of the text before point, treating it as a filename.
       complete-username (M-~)
              Attempt completion on the text before point, treating it as a username.
       possible-username-completions (C-x ~)
              List the possible completions of the text before point, treating it as a username.
       complete-variable (M-$)
              Attempt completion on the text before point, treating it as a shell variable.
       possible-variable-completions (C-x $)
              List  the  possible  completions  of  the text before point, treating it as a shell
              variable.
       complete-hostname (M-@)
              Attempt completion on the text before point, treating it as a hostname.
       possible-hostname-completions (C-x @)
              List the possible completions of the text before point, treating it as a hostname.
       complete-command (M-!)
              Attempt completion on the text before point, treating it as a command  name.   Com‐
              mand  completion  attempts to match the text against aliases, reserved words, shell
              functions, shell builtins, and finally executable filenames, in that order.
       possible-command-completions (C-x !)
              List the possible completions of the text before point, treating it  as  a  command
              name.
       dynamic-complete-history (M-TAB)
              Attempt  completion on the text before point, comparing the text against lines from
              the history list for possible completion matches.
       dabbrev-expand
              Attempt menu completion on the text before point, comparing the text against  lines
              from the history list for possible completion matches.
       complete-into-braces (M-{)
              Perform  filename  completion  and insert the list of possible completions enclosed
              within braces so the list is available to the shell (see Brace Expansion above).

   Keyboard Macros
       start-kbd-macro (C-x ()
              Begin saving the characters typed into the current keyboard macro.
       end-kbd-macro (C-x ))
              Stop saving the characters typed into the current keyboard macro and store the def‐
              inition.
       call-last-kbd-macro (C-x e)
              Re-execute  the  last keyboard macro defined, by making the characters in the macro
              appear as if typed at the keyboard.
       print-last-kbd-macro ()
              Print the last keyboard macro defined in a format suitable for the inputrc file.

   Miscellaneous
       re-read-init-file (C-x C-r)
              Read in the contents of the inputrc file, and incorporate any bindings or  variable
              assignments found there.
       abort (C-g)
              Abort the current editing command and ring the terminal's bell (subject to the set‐
              ting of bell-style).
       do-lowercase-version (M-A, M-B, M-x, ...)
              If the metafied character x is uppercase, run the command that is bound to the cor‐
              responding metafied lowercase character.  The behavior is undefined if x is already
              lowercase.
       prefix-meta (ESC)
              Metafy the next character typed.  ESC f is equivalent to Meta-f.
       undo (C-_, C-x C-u)
              Incremental undo, separately remembered for each line.
       revert-line (M-r)
              Undo all changes made to this line.  This is like executing the undo command enough
              times to return the line to its initial state.
       tilde-expand (M-&)
              Perform tilde expansion on the current word.
       set-mark (C-@, M-<space>)
              Set  the  mark to the point.  If a numeric argument is supplied, the mark is set to
              that position.
       exchange-point-and-mark (C-x C-x)
              Swap the point with the mark.  The current cursor position is set to the saved  po‐
              sition, and the old cursor position is saved as the mark.
       character-search (C-])
              A character is read and point is moved to the next occurrence of that character.  A
              negative count searches for previous occurrences.
       character-search-backward (M-C-])
              A character is read and point is moved to the previous occurrence of  that  charac‐
              ter.  A negative count searches for subsequent occurrences.
       skip-csi-sequence
              Read  enough  characters  to consume a multi-key sequence such as those defined for
              keys like Home and End.  Such sequences begin with  a  Control  Sequence  Indicator
              (CSI),  usually  ESC-[.  If this sequence is bound to "\[", keys producing such se‐
              quences will have no effect unless explicitly bound to a readline command,  instead
              of inserting stray characters into the editing buffer.  This is unbound by default,
              but usually bound to ESC-[.
       insert-comment (M-#)
              Without a numeric argument, the value of the readline comment-begin variable is in‐
              serted  at  the  beginning of the current line.  If a numeric argument is supplied,
              this command acts as a toggle: if the characters at the beginning of  the  line  do
              not  match the value of comment-begin, the value is inserted, otherwise the charac‐
              ters in comment-begin are deleted from the beginning of the line.  In either  case,
              the  line  is  accepted  as if a newline had been typed.  The default value of com‐
              ment-begin causes this command to make the current line a shell comment.  If a  nu‐
              meric  argument  causes  the comment character to be removed, the line will be exe‐
              cuted by the shell.
       glob-complete-word (M-g)
              The word before point is treated as a pattern for pathname expansion, with  an  as‐
              terisk  implicitly  appended.   This pattern is used to generate a list of matching
              filenames for possible completions.
       glob-expand-word (C-x *)
              The word before point is treated as a pattern for pathname expansion, and the  list
              of  matching  filenames  is inserted, replacing the word.  If a numeric argument is
              supplied, an asterisk is appended before pathname expansion.
       glob-list-expansions (C-x g)
              The list of expansions that would have been generated by glob-expand-word  is  dis‐
              played, and the line is redrawn.  If a numeric argument is supplied, an asterisk is
              appended before pathname expansion.
       dump-functions
              Print all of the functions and their key bindings to the  readline  output  stream.
              If  a  numeric  argument is supplied, the output is formatted in such a way that it
              can be made part of an inputrc file.
       dump-variables
              Print all of the settable readline variables and their values to the readline  out‐
              put  stream.   If a numeric argument is supplied, the output is formatted in such a
              way that it can be made part of an inputrc file.
       dump-macros
              Print all of the readline key sequences bound to macros and the strings  they  out‐
              put.  If a numeric argument is supplied, the output is formatted in such a way that
              it can be made part of an inputrc file.
       display-shell-version (C-x C-v)
              Display version information about the current instance of bash.

   Programmable Completion
       When word completion is attempted for an argument to a  command  for  which  a  completion
       specification  (a compspec) has been defined using the complete builtin (see SHELL BUILTIN
       COMMANDS below), the programmable completion facilities are invoked.

       First, the command name is identified.  If the command word is the empty  string  (comple‐
       tion attempted at the beginning of an empty line), any compspec defined with the -E option
       to complete is used.  If a compspec has been defined for that  command,  the  compspec  is
       used  to generate the list of possible completions for the word.  If the command word is a
       full pathname, a compspec for the full pathname is searched for first.  If no compspec  is
       found for the full pathname, an attempt is made to find a compspec for the portion follow‐
       ing the final slash.  If those searches do not result in a compspec, any compspec  defined
       with  the  -D option to complete is used as the default.  If there is no default compspec,
       bash attempts alias expansion on the command word as a final resort, and attempts to  find
       a compspec for the command word from any successful expansion.

       Once  a  compspec has been found, it is used to generate the list of matching words.  If a
       compspec is not found, the default bash completion as described above under Completing  is
       performed.

       First, the actions specified by the compspec are used.  Only matches which are prefixed by
       the word being completed are returned.  When the -f or -d option is used for  filename  or
       directory name completion, the shell variable FIGNORE is used to filter the matches.

       Any  completions  specified by a pathname expansion pattern to the -G option are generated
       next.  The words generated by the pattern need not match the word  being  completed.   The
       GLOBIGNORE  shell  variable is not used to filter the matches, but the FIGNORE variable is
       used.

       Next, the string specified as the argument to the -W option is considered.  The string  is
       first split using the characters in the IFS special variable as delimiters.  Shell quoting
       is honored.  Each word is then expanded using brace expansion, tilde expansion,  parameter
       and variable expansion, command substitution, and arithmetic expansion, as described above
       under EXPANSION.  The results are split using the rules described above under Word  Split‐
       ting.   The  results of the expansion are prefix-matched against the word being completed,
       and the matching words become the possible completions.

       After these matches have been generated, any shell function or command specified with  the
       -F  and  -C  options  is invoked.  When the command or function is invoked, the COMP_LINE,
       COMP_POINT, COMP_KEY, and COMP_TYPE variables are assigned values as described above under
       Shell  Variables.   If  a  shell  function is being invoked, the COMP_WORDS and COMP_CWORD
       variables are also set.  When the function or command is invoked, 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.  No filtering of the generated completions
       against the word being completed is performed; the function or command has complete  free‐
       dom in generating the matches.

       Any  function  specified  with -F is invoked first.  The function may use any of the shell
       facilities, including the compgen builtin described below, to generate  the  matches.   It
       must put the possible completions in the COMPREPLY array variable, one per array element.

       Next,  any command specified with the -C option is invoked in an environment equivalent to
       command substitution.  It should print a list of completions, one per line, to  the  stan‐
       dard output.  Backslash may be used to escape a newline, if necessary.

       After  all of the possible completions are generated, any filter specified with the -X op‐
       tion is applied to the list.  The filter is a pattern as used for pathname expansion; a  &
       in  the pattern is replaced with the text of the word being completed.  A literal & may be
       escaped with a backslash; the backslash is removed before attempting a match.  Any comple‐
       tion that matches the pattern will be removed from the list.  A leading ! negates the pat‐
       tern; in this case any completion not matching the pattern will be removed.   If  the  no‐
       casematch  shell  option  is enabled, the match is performed without regard to the case of
       alphabetic characters.

       Finally, any prefix and suffix specified with the -P and -S options are added to each mem‐
       ber  of the completion list, and the result is returned to the readline completion code as
       the list of possible completions.

       If the previously-applied actions do not generate any matches, and the -o dirnames  option
       was  supplied  to complete when the compspec was defined, directory name completion is at‐
       tempted.

       If the -o plusdirs option was supplied to complete when the compspec was  defined,  direc‐
       tory  name  completion  is attempted and any matches are added to the results of the other
       actions.

       By default, if a compspec is found, whatever it generates is returned  to  the  completion
       code  as  the  full set of possible completions.  The default bash completions are not at‐
       tempted, and the readline default of filename completion is disabled.  If the  -o  bashde‐
       fault option was supplied to complete when the compspec was defined, the bash default com‐
       pletions are attempted if the compspec generates no matches.  If the -o default option was
       supplied  to complete when the compspec was defined, readline's default completion will be
       performed if the compspec (and, if attempted, the default bash  completions)  generate  no
       matches.

       When a compspec indicates that directory name completion is desired, the programmable com‐
       pletion functions force readline to append a slash to completed names which  are  symbolic
       links  to directories, subject to the value of the mark-directories readline variable, re‐
       gardless of the setting of the mark-symlinked-directories readline variable.

       There is some support for dynamically modifying completions.  This  is  most  useful  when
       used  in  combination with a default completion specified with complete -D.  It's possible
       for shell functions executed as completion handlers to indicate that completion should  be
       retried  by returning an exit status of 124.  If a shell function returns 124, and changes
       the compspec associated with the command on which completion is being attempted  (supplied
       as  the  first  argument  when the function is executed), programmable completion restarts
       from the beginning, with an attempt to find a new compspec for that command.  This  allows
       a set of completions to be built dynamically as completion is attempted, rather than being
       loaded all at once.

       For instance, assuming that there is a library of compspecs, each kept in  a  file  corre‐
       sponding  to the name of the command, the following default completion function would load
       completions dynamically:

       _completion_loader()
       {
            . "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
       }
       complete -D -F _completion_loader -o bashdefault -o default

HISTORY
       When the -o history option to the set builtin is enabled, the shell provides access to the
       command  history,  the list of commands previously typed.  The value of the HISTSIZE vari‐
       able is used as the number of commands to save in a history list.  The text  of  the  last
       HISTSIZE  commands  (default  500) is saved.  The shell stores each command in the history
       list prior to parameter and variable expansion (see EXPANSION above) but after history ex‐
       pansion is performed, subject to the values of the shell variables HISTIGNORE and HISTCON‐
       TROL.

       On startup, the history is initialized from the file named by the variable  HISTFILE  (de‐
       fault  ~/.bash_history).   The file named by the value of HISTFILE is truncated, if neces‐
       sary, to contain no more than the number of lines specified by the value of  HISTFILESIZE.
       If  HISTFILESIZE  is  unset,  or set to null, a non-numeric value, or a numeric value less
       than zero, the history file is not truncated.  When the history file is read, lines begin‐
       ning with the history comment character followed immediately by a digit are interpreted as
       timestamps for the following history line.  These timestamps are optionally displayed  de‐
       pending  on  the  value of the HISTTIMEFORMAT variable.  When a shell with history enabled
       exits, the last $HISTSIZE lines are copied from the history list  to  $HISTFILE.   If  the
       histappend  shell option is enabled (see the description of shopt under SHELL BUILTIN COM‐
       MANDS below), the lines are appended to the history file, otherwise the  history  file  is
       overwritten.   If  HISTFILE is unset, or if the history file is unwritable, the history is
       not saved.  If the HISTTIMEFORMAT variable is set, time stamps are written to the  history
       file,  marked  with  the  history comment character, so they may be preserved across shell
       sessions.  This uses the history comment character to distinguish  timestamps  from  other
       history lines.  After saving the history, the history file is truncated to contain no more
       than HISTFILESIZE lines.  If HISTFILESIZE is unset, or set to null, a  non-numeric  value,
       or a numeric value less than zero, the history file is not truncated.

       The  builtin command fc (see SHELL BUILTIN COMMANDS below) may be used to list or edit and
       re-execute a portion of the history list.  The history builtin may be used to  display  or
       modify the history list and manipulate the history file.  When using command-line editing,
       search commands are available in each editing mode that  provide  access  to  the  history
       list.

       The  shell allows control over which commands are saved on the history list.  The HISTCON‐
       TROL and HISTIGNORE variables may be set to cause the shell to save only a subset  of  the
       commands  entered.   The  cmdhist shell option, if enabled, causes the shell to attempt to
       save each line of a multi-line command in the same history entry, adding semicolons  where
       necessary to preserve syntactic correctness.  The lithist shell option causes the shell to
       save the command with embedded newlines instead of semicolons.  See the description of the
       shopt  builtin below under SHELL BUILTIN COMMANDS for information on setting and unsetting
       shell options.

HISTORY EXPANSION
       The shell supports a history expansion feature that is similar to the history expansion in
       csh.   This section describes what syntax features are available.  This feature is enabled
       by default for interactive shells, and can be disabled using the  +H  option  to  the  set
       builtin command (see SHELL BUILTIN COMMANDS below).  Non-interactive shells do not perform
       history expansion by default.

       History expansions introduce words from the history list into the input stream, making  it
       easy to repeat commands, insert the arguments to a previous command into the current input
       line, or fix errors in previous commands quickly.

       History expansion is performed immediately after a complete line is read, before the shell
       breaks it into words, and is performed on each line individually without taking quoting on
       previous lines into account.  It takes place in two parts.   The  first  is  to  determine
       which line from the history list to use during substitution.  The second is to select por‐
       tions of that line for inclusion into the current one.  The line selected from the history
       is  the event, and the portions of that line that are acted upon are words.  Various modi‐
       fiers are available to manipulate the selected words.  The line is broken  into  words  in
       the same fashion as when reading input, so that several metacharacter-separated words sur‐
       rounded by quotes are considered one word.  History expansions are introduced by  the  ap‐
       pearance  of  the  history expansion character, which is ! by default.  Only backslash (\)
       and single quotes can quote the history expansion character,  but  the  history  expansion
       character is also treated as quoted if it immediately precedes the closing double quote in
       a double-quoted string.

       Several characters inhibit history expansion if found immediately  following  the  history
       expansion  character, even if it is unquoted: space, tab, newline, carriage return, and =.
       If the extglob shell option is enabled, ( will also inhibit expansion.

       Several shell options settable with the shopt builtin may be used to tailor  the  behavior
       of  history  expansion.  If the histverify shell option is enabled (see the description of
       the shopt builtin below), and readline is being used, history substitutions are not  imme‐
       diately passed to the shell parser.  Instead, the expanded line is reloaded into the read‐
       line editing buffer for  further  modification.   If  readline  is  being  used,  and  the
       histreedit  shell  option  is enabled, a failed history substitution will be reloaded into
       the readline editing buffer for correction.  The -p option to the history builtin  command
       may be used to see what a history expansion will do before using it.  The -s option to the
       history builtin may be used to add commands to the end of the history list  without  actu‐
       ally executing them, so that they are available for subsequent recall.

       The shell allows control of the various characters used by the history expansion mechanism
       (see the description of histchars above under Shell Variables).  The shell uses  the  his‐
       tory comment character to mark history timestamps when writing the history file.

   Event Designators
       An  event  designator  is a reference to a command line entry in the history list.  Unless
       the reference is absolute, events are relative to the  current  position  in  the  history
       list.

       !      Start  a  history  substitution, except when followed by a blank, newline, carriage
              return, = or ( (when the extglob shell option is enabled using the shopt builtin).
       !n     Refer to command line n.
       !-n    Refer to the current command minus n.
       !!     Refer to the previous command.  This is a synonym for `!-1'.
       !string
              Refer to the most recent command preceding the current position in the history list
              starting with string.
       !?string[?]
              Refer to the most recent command preceding the current position in the history list
              containing string.  The trailing ? may be omitted if string is followed immediately
              by a newline.
       ^string1^string2^
              Quick  substitution.   Repeat the previous command, replacing string1 with string2.
              Equivalent to ``!!:s/string1/string2/'' (see Modifiers below).
       !#     The entire command line typed so far.

   Word Designators
       Word designators are used to select desired words from the event.  A : separates the event
       specification  from  the word designator.  It may be omitted if the word designator begins
       with a ^, $, *, -, or %.  Words are numbered from the beginning  of  the  line,  with  the
       first  word being denoted by 0 (zero).  Words are inserted into the current line separated
       by single spaces.

       0 (zero)
              The zeroth word.  For the shell, this is the command word.
       n      The nth word.
       ^      The first argument.  That is, word 1.
       $      The last word.  This is usually the last argument, but will expand  to  the  zeroth
              word if there is only one word in the line.
       %      The word matched by the most recent `?string?' search.
       x-y    A range of words; `-y' abbreviates `0-y'.
       *      All  of the words but the zeroth.  This is a synonym for `1-$'.  It is not an error
              to use * if there is just one word in the event; the empty string  is  returned  in
              that case.
       x*     Abbreviates x-$.
       x-     Abbreviates x-$ like x*, but omits the last word.

       If  a  word designator is supplied without an event specification, the previous command is
       used as the event.

   Modifiers
       After the optional word designator, there may appear a sequence of one or more of the fol‐
       lowing modifiers, each preceded by a `:'.

       h      Remove a trailing filename component, leaving only the head.
       t      Remove all leading filename components, leaving the tail.
       r      Remove a trailing suffix of the form .xxx, leaving the basename.
       e      Remove all but the trailing suffix.
       p      Print the new command but do not execute it.
       q      Quote the substituted words, escaping further substitutions.
       x      Quote the substituted words as with q, but break into words at blanks and newlines.
       s/old/new/
              Substitute  new  for  the first occurrence of old in the event line.  Any delimiter
              can be used in place of /.  The final delimiter is optional if it is the last char‐
              acter  of the event line.  The delimiter may be quoted in old and new with a single
              backslash.  If & appears in new, it is replaced by old.  A  single  backslash  will
              quote the &.  If old is null, it is set to the last old substituted, or, if no pre‐
              vious history substitutions took place, the last string in a !?string[?]  search.
       &      Repeat the previous substitution.
       g      Cause changes to be applied over the entire event line.  This is used  in  conjunc‐
              tion  with  `:s'  (e.g., `:gs/old/new/') or `:&'.  If used with `:s', any delimiter
              can be used in place of /, and the final delimiter is optional if it  is  the  last
              character of the event line.  An a may be used as a synonym for g.
       G      Apply the following `s' modifier once to each word in the event line.

SHELL BUILTIN COMMANDS
       Unless  otherwise  noted, each builtin command documented in this section as accepting op‐
       tions 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 executed from filename.  If filename does not
              contain a slash, filenames in PATH are used to find the directory containing  file‐
              name.   The  file searched for in PATH need not be executable.  When bash is not in
              posix mode, the current directory is searched if no file is found in PATH.  If  the
              sourcepath  option  to  the  shopt  builtin  command is turned off, the PATH is not
              searched.  If any arguments are supplied, they  become  the  positional  parameters
              when  filename is executed.  Otherwise the positional parameters are unchanged.  If
              the -T option is enabled, source inherits any trap on DEBUG; if it is not, any  DE‐
              BUG  trap string is saved and restored around the call to source, and source 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 source completes.  The return status
              is the status of the last command exited within the script (0 if  no  commands  are
              executed), and false if filename is not found or cannot be read.

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

       bg [jobspec ...]
              Resume 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
              Display  current readline key and function bindings, bind a key sequence to a read‐
              line function or macro, or set a readline variable.  Each non-option argument is  a
              command  as it would appear in .inputrc, but each binding or command must be passed
              as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'.   Options,  if  sup‐
              plied, have the following meanings:
              -m keymap
                     Use keymap as the keymap to be affected by the subsequent bindings.  Accept‐
                     able keymap names are emacs,  emacs-standard,  emacs-meta,  emacs-ctlx,  vi,
                     vi-move, vi-command, and vi-insert.  vi is equivalent to vi-command (vi-move
                     is also a synonym); emacs is equivalent to emacs-standard.
              -l     List the names of all readline functions.
              -p     Display readline function names and bindings in such a way that they can  be
                     re-read.
              -P     List current readline function names and bindings.
              -s     Display  readline  key sequences bound to macros and the strings they output
                     in such a way that they can be re-read.
              -S     Display readline key sequences bound to macros and the strings they output.
              -v     Display readline variable names and values in such a way that  they  can  be
                     re-read.
              -V     List current readline variable names and values.
              -f filename
                     Read key bindings from filename.
              -q function
                     Query about which keys invoke the named function.
              -u function
                     Unbind all keys bound to the named function.
              -r keyseq
                     Remove any current binding for keyseq.
              -x keyseq:shell-command
                     Cause  shell-command  to  be  executed  whenever  keyseq  is  entered.  When
                     shell-command is executed, the shell sets the READLINE_LINE variable to  the
                     contents  of the readline line buffer and the READLINE_POINT variable to the
                     current location of the insertion point.  If the  executed  command  changes
                     the  value  of READLINE_LINE or READLINE_POINT, those new values will be re‐
                     flected 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 sta‐
              tus.   This  is  useful  when defining a function whose name is the same as a shell
              builtin, retaining the functionality of the builtin within the  function.   The  cd
              builtin   is   commonly  redefined  this  way.   The  return  status  is  false  if
              shell-builtin is not a shell builtin command.

       caller [expr]
              Returns the context of any active subroutine call (a shell function or a script ex‐
              ecuted with the . or source builtins).  Without expr, caller displays the line num‐
              ber 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  ex‐
              tra  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  sub‐
              routine call or expr does not correspond to a valid position in the call stack.

       cd [-L|[-P [-e]] [-@]] [dir]
              Change the current directory to dir.  if dir is not supplied, the value of the HOME
              shell variable is the default.  Any additional arguments following dir are ignored.
              The  variable CDPATH defines the search path for the directory containing dir: each
              directory name in CDPATH is searched for dir.  Alternative directory names  in  CD‐
              PATH  are separated by a colon (:).  A null directory name in CDPATH is the same as
              the current directory, i.e., ``.''.  If dir begins with a slash (/), then CDPATH is
              not  used.   The -P option causes cd to use the physical directory structure by re‐
              solving 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 after processing  instances  of
              ..  in dir.  If .. appears in dir, it is processed by removing the immediately pre‐
              vious 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 success‐
              fully determined after a successful directory change, cd will return an  unsuccess‐
              ful  status.   On  systems that support it, the -@ option presents the extended at‐
              tributes associated with a file as a directory.  An argument of - is  converted  to
              $OLDPWD  before  the  directory change is attempted.  If a non-empty directory name
              from CDPATH is used, or if - is the first argument, and  the  directory  change  is
              successful,  the  absolute  pathname of the new working directory is written to the
              standard output.  The return value  is  true  if  the  directory  was  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 guaran‐
              teed to find all of the standard utilities.  If either the -V or -v option is  sup‐
              plied, a description of command is printed.  The -v option causes a single word in‐
              dicating the command or filename used to invoke command to be displayed; the -V op‐
              tion  produces a more verbose description.  If the -V or -v option is supplied, the
              exit status is 0 if command was found, and 1 if not.  If neither option is supplied
              and  an  error occurred or command cannot be found, the exit status is 127.  Other‐
              wise, 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 avail‐
              able, will not have useful values.

              The matches will be generated in the same way as  if  the  programmable  completion
              code  had  generated  them  directly  from a completion specification with the same
              flags.  If word is specified, only those completions matching  word  will  be  dis‐
              played.

              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 sup‐
              plied, or if no  options  are  supplied,  existing  completion  specifications  are
              printed  in  a way that allows them to be reused as input.  The -r option removes a
              completion specification for each name, or, if no names are supplied,  all  comple‐
              tion  specifications.   The -D option indicates that other supplied options and ac‐
              tions should apply to the ``default'' command completion; that is,  completion  at‐
              tempted  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
              inital  non-assignment  word on the line, or after a command delimiter such as ; or
              |, which is usually command name completion.  If multiple options are supplied, the
              -D  option  takes  precedence over -E, and both take precedence over -I.  If any of
              -D, -E, or -I are supplied, any other name 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 at‐
              tempted is described above under Programmable Completion.

              Other options, if specified, have the following meanings.  The arguments to the -G,
              -W,  and  -X options (and, if necessary, the -P and -S options) should be quoted to
              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  gener‐
                              ates no matches.
                      dirnames
                              Perform  directory  name  completion  if  the compspec generates no
                              matches.
                      filenames
                              Tell readline that the compspec generates filenames, so it can per‐
                              form  any  filename-specific processing (like adding a slash to di‐
                              rectory 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 file‐
                              names (quoting filenames is the default).
                      nosort  Tell readline not to sort the list of possible  completions  alpha‐
                              betically.
                      nospace Tell  readline  not  to  append a space (the default) to words com‐
                              pleted 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 re‐
                              sults of the other actions.
              -A action
                      The action may be one of the following to generate a list of possible  com‐
                      pletions:
                      alias   Alias names.  May also be specified as -a.
                      arrayvar
                              Array variable names.
                      binding Readline key binding names.
                      builtin Names of shell builtin commands.  May also be specified as -b.
                      command Command names.  May also be specified as -c.
                      directory
                              Directory names.  May also be specified as -d.
                      disabled
                              Names of disabled shell builtins.
                      enabled Names of enabled shell builtins.
                      export  Names of exported shell variables.  May also be specified as -e.
                      file    File names.  May also be specified as -f.
                      function
                              Names of shell functions.
                      group   Group names.  May also be specified as -g.
                      helptopic
                              Help topics as accepted by the help builtin.
                      hostname
                              Hostnames,  as  taken from the file specified by the HOSTFILE shell
                              variable.
                      job     Job names, if job control is active.  May also be specified as -j.
                      keyword Shell reserved words.  May also be specified as -k.
                      running Names of running jobs, if job control is active.
                      service Service names.  May also be specified as -s.
                      setopt  Valid arguments for the -o option to the set builtin.
                      shopt   Shell option names as accepted by the shopt builtin.
                      signal  Signal names.
                      stopped Names of stopped jobs, if job control is active.
                      user    User names.  May also be specified as -u.
                      variable
                              Names of all shell variables.  May also be specified as -v.
              -C command
                      command is executed in a subshell environment, and its output  is  used  as
                      the possible completions.
              -F function
                      The  shell  function function is executed in the current shell environment.
                      When the function is executed, the first argument ($1) is the name  of  the
                      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  ar‐
                      ray 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 expanded.  Shell quoting is honored
                      within wordlist, in order to provide a mechanism for the words  to  contain
                      shell  metacharacters or characters in the value of IFS.  The possible com‐
                      pletions 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 ar‐
                      guments,  and  each completion matching filterpat is removed from the list.
                      A leading ! in filterpat negates the pattern; in this case, any  completion
                      not matching filterpat is removed.

              The return value is true unless an invalid option is supplied, an option other than
              -p or -r is supplied without a name argument, an attempt is made to remove  a  com‐
              pletion 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  cur‐
              rently-executing  completion  if  no  names are supplied.  If no options are given,
              display the completion options for each name or the current completion.  The possi‐
              ble values of option are those valid for the complete builtin described above.  The
              -D option indicates that other supplied options should  apply  to  the  ``default''
              command completion; that is, completion attempted on a command for which no comple‐
              tion has previously been defined.  The -E option indicates that other supplied  op‐
              tions  should  apply to ``empty'' command completion; that is, completion attempted
              on a blank line.  The -I option indicates that other supplied options should  apply
              to completion on the inital non-assignment word on the line, or after a command de‐
              limiter such as ; or |, which is usually command name completion.

              The return value is true unless an invalid option is supplied, an attempt  is  made
              to  modify  the options for a name for which no completion specification exists, or
              an 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 [-aAfFgilnrtux] [-p] [name[=value] ...]
       typeset [-aAfFgilnrtux] [-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 defini‐
              tions; only the function name and attributes are printed.  If  the  extdebug  shell
              option is enabled using shopt, the source file name and line number where each name
              is defined are 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  following  op‐
              tions  can  be used to restrict output to variables with the specified attribute or
              to give variables attributes:
              -a     Each name is an indexed array variable (see Arrays above).
              -A     Each name is an associative array variable (see Arrays above).
              -f     Use function names only.
              -i     The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC
                     EVALUATION above) is performed when the variable is assigned a value.
              -l     When  the  variable  is assigned a value, all upper-case characters are con‐
                     verted to lower-case.  The upper-case attribute is disabled.
              -n     Give each name the nameref attribute, making it a name reference to  another
                     variable.   That other variable is defined by the value of name.  All refer‐
                     ences, assignments, and attribute modifications to name, except those  using
                     or  changing  the  -n attribute itself, are performed on the variable refer‐
                     enced 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 subse‐
                     quent 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  con‐
                     verted to upper-case.  The lower-case attribute 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
              attributes  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  com‐
              pound  assignment  syntax (see Arrays above), one of the names is not a valid shell
              variable name, an attempt is made to turn off readonly status for a readonly  vari‐
              able,  an attempt is made to turn off array status for an array variable, or an at‐
              tempt is made to display a non-existent function with -f.

       dirs [-clpv] [+n] [-n]
              Without options, displays the list of currently remembered  directories.   The  de‐
              fault display is on a single line with directory names separated by spaces.  Direc‐
              tories are added to the list with the pushd command; the popd command  removes  en‐
              tries  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 op‐
              tion 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  sup‐
              pressed.   If the -e option is given, interpretation of the following backslash-es‐
              caped 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  ex‐
              pands  these  escape characters by default.  echo does not interpret -- to mean the
              end of options.  echo interprets the following escape sequences:
              \a     alert (bell)
              \b     backspace
              \c     suppress further output
              \e
              \E     an escape character
              \f     form feed
              \n     new line
              \r     carriage return
              \t     horizontal tab
              \v     vertical tab
              \\     backslash
              \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  com‐
              mand 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 com‐
              mand name from shared object filename, on systems  that  support  dynamic  loading.
              The  -d  option  will delete a builtin previously loaded with -f.  If no name argu‐
              ments 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  sup‐
              plied, the list printed includes all builtins, with an indication of whether or not
              each is enabled.  If -s is supplied, the output is restricted to the POSIX  special
              builtins.   The  return value is 0 unless a name is not a shell builtin or there is
              an error loading a new builtin from a shared object.

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

       exec [-cl] [-a name] [command [arguments]]
              If command is specified, it replaces the shell.  No new process  is  created.   The
              arguments become the arguments to command.  If the -l option is supplied, the shell
              places a dash at the beginning of the zeroth argument passed to command.   This  is
              what  login(1) does.  The -c option causes command to be executed with an empty en‐
              vironment.  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 status is 1.

       exit [n]
              Cause the shell to exit with a status of n.  If n is omitted, the  exit  status  is
              that  of  the  last  command executed.  A trap on EXIT is executed before the shell
              terminates.

       export [-fn] [name[=word]] ...
       export -p
              The supplied names are marked for automatic export to  the  environment  of  subse‐
              quently  executed  commands.   If  the -f option is given, the names refer to func‐
              tions.  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 op‐
              tion is encountered, one of the names is not a valid shell variable name, or -f  is
              supplied with a name that is not a function.

       fc [-e ename] [-lnr] [first] [last]
       fc -s [pat=rep] [cmd]
              The first form selects a range of commands from first to last from the history list
              and displays or edits and re-executes them.  First and last may be specified  as  a
              string  (to  locate the last command beginning with that string) or as a number (an
              index into the history list, where a negative number is used as an offset from  the
              current  command  number).  If last is not specified, it is set to the current com‐
              mand for listing (so that ``fc -l -10'' prints the last 10 commands) and  to  first
              otherwise.   If first is not specified, it is set to the previous command for edit‐
              ing and -16 for listing.

              The -n option suppresses the command numbers when listing.  The -r option  reverses
              the  order  of the commands.  If the -l option is given, the commands are listed on
              standard output.  Otherwise, the editor given by ename is invoked on  a  file  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  en‐
              countered or first or last specify history lines out of range.  If the -e option is
              supplied, the return value is the value of the last command executed or failure  if
              an  error  occurs with the temporary file of commands.  If the second form is used,
              the return status is that of the command re-executed, unless cmd does not specify a
              valid history line, in which case fc returns failure.

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

       getopts optstring name [args]
              getopts is used by shell procedures to parse positional parameters.  optstring con‐
              tains the option characters to be recognized; if  a  character  is  followed  by  a
              colon,  the  option is expected to have an argument, which should be separated from
              it by white space.  The colon and question mark characters may not be used  as  op‐
              tion  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 invoked.  When an option requires an argu‐
              ment,  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 given
              in args, getopts parses those instead.

              getopts can report errors in two ways.  If the first character of  optstring  is  a
              colon,  silent  error  reporting is used.  In normal operation, diagnostic messages
              are printed when invalid options or missing option arguments are  encountered.   If
              the  variable  OPTERR is set to 0, no error messages will be displayed, even if the
              first character of optstring is not a colon.

              If an invalid option is seen, getopts places ? into name and, if not silent, prints
              an  error  message  and  unsets OPTARG.  If getopts is silent, the option character
              found is placed in OPTARG and no diagnostic message is printed.

              If a required argument is not found, and getopts is not silent, a question mark (?)
              is  placed  in  name,  OPTARG  is  unset,  and a diagnostic message is printed.  If
              getopts is silent, then a colon (:) is placed in name and OPTARG is set to the  op‐
              tion character found.

              getopts  returns true if an option, specified or unspecified, is found.  It returns
              false if the end of options is encountered or an error occurs.

       hash [-lr] [-p filename] [-dt] [name]
              Each time hash is invoked, the full pathname of the command name is  determined  by
              searching the directories in $PATH and remembered.  Any previously-remembered path‐
              name 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 op‐
              tion causes output to be displayed in a format that may be reused as input.  If  no
              arguments  are  given, or if only -l is supplied, information about remembered com‐
              mands is printed.  The return status is true unless a name is not found or  an  in‐
              valid option is supplied.

       help [-dms] [pattern]
              Display  helpful information about builtin commands.  If pattern is specified, help
              gives detailed help on all commands matching pattern; otherwise help  for  all  the
              builtins and shell control structures is printed.
              -d     Display a short description of each pattern
              -m     Display the description of each pattern in a manpage-like format
              -s     Display only a short usage synopsis for each pattern

              The return status is 0 unless no command matches pattern.

       history [n]
       history -c
       history -d offset
       history -d start-end
       history -anrw [filename]
       history -p arg [arg ...]
       history -s arg [arg ...]
              With  no options, display the command history list with line numbers.  Lines listed
              with a * have been modified.  An argument of n lists only the last n lines.  If the
              shell  variable  HISTTIMEFORMAT  is set and not null, it is used as a format string
              for strftime(3) to display the time stamp associated with  each  displayed  history
              entry.   No  intervening  blank is printed between the formatted time stamp and the
              history line.  If filename is supplied, it is used as the name of the history file;
              if  not,  the  value of HISTFILE is used.  Options, if supplied, have the following
              meanings:
              -c     Clear the history list by deleting all the entries.
              -d offset
                     Delete the history entry at position offset.  If offset is negative,  it  is
                     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 history entries between positions start and end, inclusive.  Pos‐
                     itive and negative values for start and end  are  interpreted  as  described
                     above.
              -a     Append  the  ``new''  history  lines to the history file.  These are history
                     lines entered since the beginning of the current bash session, but  not  al‐
                     ready appended to the history file.
              -n     Read  the history lines not already read from the history file into the cur‐
                     rent history list.  These are lines appended to the history file  since  the
                     beginning of the current bash session.
              -r     Read the contents of the history file and append them to the current history
                     list.
              -w     Write the current history list to the history file, overwriting the  history
                     file's contents.
              -p     Perform history substitution on the following args and display the result on
                     the standard output.  Does not store the results in the history list.   Each
                     arg must be quoted to disable normal history expansion.
              -s     Store  the  args in the history list as a single entry.  The last command in
                     the history list is removed before the args are added.

              If the HISTTIMEFORMAT variable is set, the time stamp information  associated  with
              each  history entry is written to the history file, marked with the history comment
              character.  When the history file is read, lines beginning with the history comment
              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 encoun‐
              tered, an error occurs while reading or writing the history file, an invalid offset
              is supplied as an argument to -d, or the history expansion supplied as an  argument
              to -p fails.

       jobs [-lnprs] [ jobspec ... ]
       jobs -x command [ args ... ]
              The first form lists the active jobs.  The options have the following 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  re‐
              turn  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, re‐
              turning 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  job‐
              spec.   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  corre‐
              sponding  to the arguments are listed, and the return status is 0.  The exit_status
              argument to -l is a number specifying either a signal number or the exit status  of
              a process terminated by a signal.  The -L option is equivalent to -l.  kill returns
              true if at least one signal was successfully sent, or false if an error  occurs  or
              an invalid option is encountered.

       let arg [arg ...]
              Each  arg  is  an  arithmetic expression to be evaluated (see ARITHMETIC EVALUATION
              above).  If the last arg evaluates to 0, let returns 1; 0 is returned otherwise.

       local [option] [name[=value] ... | - ]
              For each argument, a local variable named name is created, and assigned value.  The
              option  can be any of the options accepted by declare.  When local is used within a
              function, it causes the variable name to have a visible scope  restricted  to  that
              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.  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 quan‐
       tum] [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 supplied.  The variable MAPFILE is  the  de‐
              fault 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 eval‐
              uated,  it  is  supplied the index of the next array element to be assigned and the
              line to be assigned to that element as additional arguments.  callback is evaluated
              after the line is read but before the array element is assigned.

              If  not supplied with an explicit origin, mapfile will clear array before assigning
              to it.

              mapfile returns successfully unless an invalid option or option  argument  is  sup‐
              plied, array is invalid or unassignable, or if array is not an indexed array.

       popd [-n] [+n] [-n]
              Removes  entries  from the directory stack.  With no arguments, removes the top di‐
              rectory from the stack, and performs a cd to the new top directory.  Arguments,  if
              supplied, have the following meanings:
              -n     Suppresses the normal change of directory when removing directories from the
                     stack, so that only the stack is manipulated.
              +n     Removes the nth entry counting from the left of  the  list  shown  by  dirs,
                     starting  with  zero.  For example: ``popd +0'' removes the first directory,
                     ``popd +1'' the second.
              -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 popd command is successful, a dirs is performed as well, and the return sta‐
              tus  is  0.   popd returns false if an invalid option is encountered, the directory
              stack is empty, a non-existent directory stack entry is specified, or the directory
              change fails.

       printf [-v var] format [arguments]
              Write  the formatted arguments to the standard output under the control of the for‐
              mat.  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 char‐
              acters, 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 exten‐
              sions:
              %b     causes printf to expand backslash escape sequences in the corresponding  ar‐
                     gument in the same way as echo -e.
              %q     causes  printf  to output the corresponding argument in a format that can be
                     reused as shell input.
              %(datefmt)T
                     causes printf to output the date-time string resulting from using datefmt as
                     a  format  string for strftime(3).  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.

              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 directories and returns 0,  unless  the  directory  stack  is
              empty.  Arguments, if supplied, have the following meanings:
              -n     Suppresses  the  normal change of directory when rotating or adding directo‐
                     ries 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, making it the new current  work‐
                     ing directory as if it had been supplied as the argument to the cd builtin.

              If the pushd command is successful, a dirs is performed as well.  If the first form
              is used, pushd returns 0 unless the cd to dir fails.  With the second  form,  pushd
              returns  0 unless the directory stack is empty, a non-existent directory stack ele‐
              ment is specified, or the directory change to the specified new  current  directory
              fails.

       pwd [-LP]
              Print the absolute pathname of the current working directory.  The pathname printed
              contains no symbolic links if the -P option is supplied or the -o  physical  option
              to  the  set  builtin  command  is enabled.  If the -L option is used, the pathname
              printed may contain symbolic links.  The return status is 0 unless an error  occurs
              while reading the name of the current directory or an invalid option is supplied.

       read  [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t time‐
       out] [-u fd] [name ...]
              One line is read from the standard input, or from the file descriptor  fd  supplied
              as  an  argument  to  the -u option, split into words as described above under Word
              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 assigned to the last name.  If there are fewer
              words read from the input stream than names, the remaining names are assigned empty
              values.  The characters in IFS are used to split the line into words using the same
              rules  the  shell  uses  for expansion (described above under Word Splitting).  The
              backslash character (\) may be used to remove any  special  meaning  for  the  next
              character read and for line continuation.  Options, if supplied, have the following
              meanings:
              -a aname
                     The words are assigned to sequential indices of the  array  variable  aname,
                     starting  at  0.   aname is unset before any new values are assigned.  Other
                     name arguments are ignored.
              -d delim
                     The first character of delim is used to terminate  the  input  line,  rather
                     than newline.  If delim is the empty string, read will terminate a line when
                     it reads a NUL character.
              -e     If the standard input is coming from  a  terminal,  readline  (see  READLINE
                     above)  is  used to obtain the line.  Readline uses the current (or default,
                     if line editing was not previously active) editing settings, but uses  Read‐
                     line'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  com‐
                     plete  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.  De‐
                     limiter 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  as‐
                     signed exactly the characters read (with the exception of backslash; see the
                     -r option below).
              -p prompt
                     Display prompt on standard error, without a  trailing  newline,  before  at‐
                     tempting to read any input.  The prompt is displayed 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 termi‐
                     nal, 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 speci‐
                     fied variable name.  If timeout is 0, read returns immediately, without try‐
                     ing  to  read  any  data.  The exit status is 0 if input is available on the
                     specified file descriptor, 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 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 assign‐
              ing to a readonly variable) occurs, or an invalid file descriptor  is  supplied  as
              the argument to -u.

       readonly [-aAf] [-p] [name[=word] ...]
              The  given  names are marked readonly; the values of these names may not be changed
              by subsequent assignment.  If the -f option is supplied, the functions  correspond‐
              ing  to  the names are so marked.  The -a option restricts the variables to indexed
              arrays; the -A option restricts the variables to associative arrays.  If  both  op‐
              tions 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 format that may be  reused  as  input.
              If  a variable name is followed by =word, the value of the variable is set to word.
              The return status is 0 unless an invalid option is encountered, one of the names is
              not  a valid shell variable name, or -f is supplied with a name that is not a func‐
              tion.

       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 sta‐
              tus is the last command executed by the trap handler before return was invoked.  If
              return is used outside a function, but during  execution  of  a  script  by  the  .
              (source)  command, it causes the shell to stop executing that script and return ei‐
              ther 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 signif‐
              icant 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  execu‐
              tion resumes after the function or script.

       set [--abefhkmnptuvxBCEHPT] [-o option-name] [arg ...]
       set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg ...]
              Without  options, the name and value of each shell variable are displayed in a for‐
              mat that can be reused as input for setting or resetting  the  currently-set  vari‐
              ables.   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  as‐
              signed,  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 environment of subsequent commands.
              -b      Report  the  status  of terminated background jobs immediately, rather than
                      before the next primary prompt.  This is effective only when job control is
                      enabled.
              -e      Exit  immediately  if a pipeline (which may consist of a single simple com‐
                      mand), a list, or a compound command (see SHELL GRAMMAR above), exits  with
                      a  non-zero  status.   The shell does not exit if the command that fails is
                      part of the command list immediately following a while  or  until  keyword,
                      part  of the test following the if or elif reserved words, part of any com‐
                      mand 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  sub‐
                      shell 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  be‐
                      fore  the  shell  exits.   This option applies to the shell environment and
                      each subshell environment separately  (see  COMMAND  EXECUTION  ENVIRONMENT
                      above),  and  may cause subshells to exit before executing all the commands
                      in the subshell.

                      If a compound command or shell function executes in a context where  -e  is
                      being ignored, none of the commands executed within the compound command or
                      function body will be affected by the -e setting, even if -e is set  and  a
                      command  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 envi‐
                      ronment 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 above).  All
                      processes run in a separate process group.   When  a  background  job  com‐
                      pletes, the shell prints a line containing its exit status.
              -n      Read  commands  but do not execute them.  This may be used to check a shell
                      script for syntax errors.  This is ignored by interactive shells.
              -o option-name
                      The option-name can be one of the following:
                      allexport
                              Same as -a.
                      braceexpand
                              Same as -B.
                      emacs   Use an emacs-style command line editing interface.  This is enabled
                              by  default  when  the  shell  is  interactive, unless the shell is
                              started with the --noediting option.  This also affects the editing
                              interface used for read -e.
                      errexit Same as -e.
                      errtrace
                              Same as -E.
                      functrace
                              Same as -T.
                      hashall Same as -h.
                      histexpand
                              Same as -H.
                      history Enable command history, as described above under HISTORY.  This op‐
                              tion is on by default in interactive shells.
                      ignoreeof
                              The effect is as if the shell command ``IGNOREEOF=10'' had been ex‐
                              ecuted (see Shell Variables above).
                      keyword Same as -k.
                      monitor Same as -m.
                      noclobber
                              Same as -C.
                      noexec  Same as -n.
                      noglob  Same as -f.
                      nolog   Currently ignored.
                      notify  Same as -b.
                      nounset Same as -u.
                      onecmd  Same as -t.
                      physical
                              Same as -P.
                      pipefail
                              If  set,  the  return  value of a pipeline is the value of the last
                              (rightmost) command to exit with a non-zero status, or zero if  all
                              commands  in  the  pipeline exit successfully.  This option is dis‐
                              abled by default.
                      posix   Change the behavior of bash where  the  default  operation  differs
                              from  the  POSIX  standard to match the standard (posix mode).  See
                              SEE ALSO below for a reference to a document that details how posix
                              mode 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 option-name, a series of set com‐
                      mands to recreate the current option settings is displayed on the  standard
                      output.
              -p      Turn  on  privileged  mode.  In this mode, the $ENV and $BASH_ENV files are
                      not processed, shell functions are not inherited from the environment,  and
                      the  SHELLOPTS,  BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear
                      in the environment, are ignored.  If the shell is started with  the  effec‐
                      tive  user (group) id not equal to the real user (group) id, and the -p op‐
                      tion 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 ef‐
                      fective user id is not reset.  Turning this option off causes the effective
                      user and group ids to be set to the real user and group ids.
              -t      Exit after reading and executing one command.
              -u      Treat  unset variables and parameters other than the special parameters "@"
                      and "*" as an error when performing parameter expansion.  If  expansion  is
                      attempted on an unset variable or parameter, the shell prints an error mes‐
                      sage, and, if not interactive, exits with a non-zero status.
              -v      Print shell input lines as they are read.
              -x      After expanding each simple command, for command, case command, select com‐
                      mand,  or  arithmetic  for command, display the expanded value of PS4, fol‐
                      lowed by the command and its expanded arguments or associated word list.
              -B      The shell performs brace expansion (see Brace Expansion above).  This is on
                      by default.
              -C      If set, bash does not overwrite an existing file with the >, >&, and <> re‐
                      direction operators.  This may be overridden when creating output files  by
                      using the redirection operator >| instead of >.
              -E      If  set, any trap on ERR is inherited by shell functions, command substitu‐
                      tions, and commands executed in a subshell environment.  The  ERR  trap  is
                      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
                      directories 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 un‐
                      set.  Otherwise, the positional parameters are set to  the  args,  even  if
                      some of them begin with a -.
              -       Signal  the  end of options, cause all remaining args to be assigned to the
                      positional parameters.  The -x and -v options are turned off.  If there are
                      no args, the positional parameters remain unchanged.

              The  options  are  off  by  default  unless otherwise noted.  Using + rather than -
              causes these options to be turned off.  The options can also be specified as  argu‐
              ments  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  repre‐
              sented 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 op‐
              tion, 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 op‐
              tions.  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 other‐
                     wise.
              -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 op‐
              tions which are set or unset, respectively.  Unless otherwise noted, the shopt  op‐
              tions 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  un‐
              less an optname is not a valid shell option.

              The list of shopt options is:

              assoc_expand_once
                      If  set, the shell suppresses multiple evaluation of associative array sub‐
                      scripts during arithmetic expression evaluation, while  executing  builtins
                      that  can  perform  variable assignments, and while executing builtins that
                      perform array dereferencing.
              autocd  If set, a command name that is the name of a directory is executed as if it
                      were  the argument to the cd command.  This option is only used by interac‐
                      tive 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  com‐
                      mand  will be corrected.  The errors checked for are transposed characters,
                      a missing character, and one character too many.  If a correction is found,
                      the  corrected  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  exit‐
                      ing 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  above).  The shell always postpones exiting if any jobs
                      are stopped.
              checkwinsize
                      If set, bash checks the window size after each external (non-builtin)  com‐
                      mand  and, if necessary, updates the values of LINES and COLUMNS.  This op‐
                      tion 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  his‐
                      tory is enabled, as described above under HISTORY.
              compat31
                      If  set,  bash  changes its behavior to that of version 3.1 with respect to
                      quoted arguments to the [[ conditional command's =~  operator  and  locale-
                      specific  string comparison when using the [[ conditional command's < and >
                      operators.  Bash versions prior to bash-4.1 use ASCII  collation  and  str‐
                      cmp(3);  bash-4.1 and later use the current locale's collation sequence and
                      strcoll(3).
              compat32
                      If set, bash changes its behavior to that of version 3.2  with  respect  to
                      locale-specific string comparison when using the [[ conditional command's <
                      and > operators (see previous item) and the effect of interrupting  a  com‐
                      mand list.  Bash versions 3.2 and earlier continue with the next command in
                      the list after one terminates due to an interrupt.
              compat40
                      If set, bash changes its behavior to that of version 4.0  with  respect  to
                      locale-specific string comparison when using the [[ conditional command's <
                      and > operators (see description of compat31) and the effect of  interrupt‐
                      ing  a  command list.  Bash versions 4.0 and later interrupt the list as if
                      the shell received the interrupt; previous versions continue with the  next
                      command in the list.
              compat41
                      If  set, bash, when in posix mode, treats a single quote in a double-quoted
                      parameter expansion as a special character.  The single quotes  must  match
                      (an  even  number) and the characters between the single quotes are consid‐
                      ered quoted.  This is the behavior of posix mode through version 4.1.   The
                      default bash behavior remains as in previous versions.
              compat42
                      If set, bash does not process the replacement string in the pattern substi‐
                      tution word expansion using quote removal.
              compat43
                      If set, bash does not print a warning message if an attempt is made to  use
                      a  quoted  compound  array assignment as an argument to declare, makes word
                      expansion errors non-fatal errors that cause the current  command  to  fail
                      (the  default behavior is to make them fatal errors that cause the shell to
                      exit), and does not reset the loop state when a shell function is  executed
                      (this  allows  break or continue in a shell function to affect loops in the
                      caller's context).
              compat44
                      If set, bash saves the positional parameters to BASH_ARGV and BASH_ARGC be‐
                      fore they are used, regardless of whether or not extended debugging mode is
                      enabled.
              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 ref‐
                      erences 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 filenames.  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 above under ALIASES.  This option
                      is enabled by default for interactive shells.
              extdebug
                      If set at shell invocation, arrange to execute the debugger profile  before
                      the shell starts, identical to the --debugger option.  If set after invoca‐
                      tion, behavior intended for use by debuggers is enabled:
                      1.     The -F option to the declare builtin displays the source  file  name
                             and  line  number corresponding to each function name supplied as an
                             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  descrip‐
                             tions above.
                      5.     Function  tracing is enabled: command substitution, shell functions,
                             and subshells invoked with ( command ) inherit the DEBUG and  RETURN
                             traps.
                      6.     Error tracing is enabled: command substitution, shell functions, and
                             subshells invoked with ( command ) inherit the ERR trap.
              extglob If set, the extended pattern matching features described above under  Path‐
                      name 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 completion even if the  ignored  words  are
                      the only possible completions.  See SHELL VARIABLES above for a description
                      of FIGNORE.  This option is enabled by default.
              globasciiranges
                      If set, range expressions used in pattern matching bracket expressions (see
                      Pattern  Matching above) behave as if in the traditional C locale when per‐
                      forming comparisons.  That is, the current locale's collating  sequence  is
                      not  taken  into account, so b will not collate between A and B, and upper-
                      case and lower-case ASCII characters will collate together.
              globstar
                      If set, the pattern ** used in a pathname expansion context will match  all
                      files  and  zero or more directories and subdirectories.  If the pattern is
                      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 above).  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,  in‐
                      stead  of unsetting it in the subshell environment.  This option is enabled
                      when posix mode is enabled.
              interactive_comments
                      If set, allow a word beginning with # to cause that word and all  remaining
                      characters on that line to be ignored in an interactive shell (see COMMENTS
                      above).  This option is enabled by default.
              lastpipe
                      If set, and job control is not active, the shell runs the last command of a
                      pipeline not executed in the background in the current shell environment.
              lithist If set, and the cmdhist option is enabled, multi-line commands are saved to
                      the history with embedded newlines rather than using  semicolon  separators
                      where possible.
              localvar_inherit
                      If  set,  local variables inherit the value and attributes of a variable of
                      the same name that exists at a previous scope before any new value  is  as‐
                      signed.  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 cur‐
                      rent function scope.
              login_shell
                      The shell sets this option if it is started as a login shell  (see  INVOCA‐
                      TION above).  The value may not be changed.
              mailwarn
                      If  set,  and a file that bash is checking for mail has been accessed since
                      the last time it was checked, the message ``The mail in mailfile  has  been
                      read'' is displayed.
              no_empty_cmd_completion
                      If  set,  and  readline  is being used, bash will not attempt to search the
                      PATH for possible completions when completion  is  attempted  on  an  empty
                      line.
              nocaseglob
                      If  set, bash matches filenames in a case-insensitive fashion when perform‐
                      ing pathname expansion (see Pathname Expansion above).
              nocasematch
                      If set, bash matches patterns in a case-insensitive fashion when performing
                      matching  while  executing case or [[ conditional commands, when performing
                      pattern substitution word expansions, or when  filtering  possible  comple‐
                      tions as part of programmable completion.
              nullglob
                      If  set,  bash allows patterns which match no files (see Pathname Expansion
                      above) to expand to a null string, rather than themselves.
              progcomp
                      If set, the programmable completion facilities (see Programmable Completion
                      above) are enabled.  This option is enabled by default.
              progcomp_alias
                      If  set, and programmable completion is enabled, bash treats a command name
                      that doesn't have any completions as a possible alias  and  attempts  alias
                      expansion.  If it has an alias, bash attempts programmable completion using
                      the command word resulting from the expanded alias.
              promptvars
                      If set, prompt strings undergo parameter expansion,  command  substitution,
                      arithmetic  expansion,  and quote removal after being expanded as described
                      in PROMPTING above.  This option is enabled by default.
              restricted_shell
                      The shell sets this option if it is started in  restricted  mode  (see  RE‐
                      STRICTED  SHELL  below).   The value may not be changed.  This is not reset
                      when the startup files are executed, allowing the startup files to discover
                      whether or not a shell is restricted.
              shift_verbose
                      If  set, the shift builtin prints an error message when the shift count ex‐
                      ceeds the number of positional parameters.
              sourcepath
                      If set, the source (.) builtin uses the value of PATH to find the directory
                      containing the file supplied as an argument.  This option is enabled by de‐
                      fault.
              xpg_echo
                      If set, the echo builtin expands backslash-escape sequences by default.

       suspend [-f]
              Suspend the execution of this shell until it receives a SIGCONT  signal.   A  login
              shell cannot be suspended; the -f option can be used to override this and force the
              suspension.  The return status is 0 unless the shell is a login shell and -f is not
              supplied, or if job control is not enabled.

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

              Expressions may be combined using the following operators, listed in decreasing or‐
              der 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 prece‐
                     dence 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  num‐
              ber 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 op‐
                     erators  listed  above under CONDITIONAL EXPRESSIONS, the expression is true
                     if the unary test is true.  If the first argument is not a valid unary  con‐
                     ditional operator, the expression is false.
              3 arguments
                     The following conditions are applied in the order listed.  If the second ar‐
                     gument is one of the binary conditional operators listed above under  CONDI‐
                     TIONAL 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  opera‐
                     tors are considered binary operators when there are three arguments.  If the
                     first argument is !, the value is the negation of the two-argument test  us‐
                     ing  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
                     If the first argument is !, the result is the negation of the three-argument
                     expression composed of the remaining arguments.  Otherwise,  the  expression
                     is  parsed  and  evaluated  according  to  precedence using the rules listed
                     above.
              5 or more arguments
                     The expression is parsed and evaluated according  to  precedence  using  the
                     rules listed above.

              When  used with test or [, the < and > operators sort lexicographically using ASCII
              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 single sigspec)  or  -,  each  specified
              signal  is reset to its original disposition (the value it had upon entrance to the
              shell).  If arg is the null string the signal specified by each sigspec is  ignored
              by the shell and by the commands it invokes.  If arg is not present and -p has been
              supplied, then the trap commands associated with each sigspec are displayed.  If no
              arguments are supplied or if only -p is given, trap prints the list of commands as‐
              sociated with each signal.  The -l option causes the shell to print a list of  sig‐
              nal  names  and  their corresponding numbers.  Each sigspec is either a signal name
              defined in <signal.h>, or a signal number.  Signal names are case  insensitive  and
              the SIG prefix is optional.

              If  a sigspec is EXIT (0) the command arg is executed on exit from the shell.  If a
              sigspec is DEBUG, the command arg is executed before every simple command, for com‐
              mand,  case  command,  select command, every arithmetic for command, and before the
              first command executes in a shell function (see SHELL GRAMMAR above).  Refer to the
              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 execut‐
              ing.

              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 ex‐
              ecuted  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 exe‐
              cuted  in a && or || list except the command following the final && or ||, any com‐
              mand in a pipeline but the last, or if the command's return value is being inverted
              using !.  These are the same conditions obeyed by the errexit (-e) option.

              Signals  ignored  upon entry to the shell cannot be trapped or reset.  Trapped sig‐
              nals 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, key‐
              word, 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
              specified as a command name, or nothing if ``type -t name'' would not return  file.
              The  -P  option  forces a PATH search for each name, even if ``type -t name'' would
              not return file.  If a command is hashed, -p and -P print the hashed  value,  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 executable named name.  This includes
              aliases and functions, if and only if the -p option is not also used.  The table of
              hashed commands is not consulted when using -a.  The  -f  option  suppresses  shell
              function  lookup, as with the command builtin.  type returns true if all of the ar‐
              guments are found, false if any are not found.

       ulimit [-HSabcdefiklmnpqrstuvxPT [limit]]
              Provides control over the resources available to the shell and to processes started
              by  it, on systems that allow such control.  The -H and -S options specify that the
              hard or soft limit is set for the given resource.  A hard limit cannot be increased
              by a non-root user once it is set; a soft limit may be increased up to the value of
              the hard limit.  If neither -H nor -S is specified, both the soft and  hard  limits
              are set.  The value of limit can be a number in the unit specified for the resource
              or one of the special values hard, soft, or unlimited, which stand for the  current
              hard  limit, the current soft limit, and no limit, respectively.  If limit is omit‐
              ted, the current value of the soft limit of the resource is printed, unless the  -H
              option is given.  When more than one resource is specified, the limit name and unit
              are printed before the value.  Other options are interpreted as follows:
              -a     All current limits are reported
              -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
              -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; -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 error occurs while setting
              a new limit.

       umask [-p] [-S] [mode]
              The user file-creation mask is set to mode.  If mode begins with a digit, it is in‐
              terpreted  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 omit‐
              ted,  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 variable, and that variable is  removed.   Read-
              only  variables  may not be unset.  If -f is specified, each name refers to a shell
              function, and the function definition is removed.  If the -n  option  is  supplied,
              and  name  is a variable with the nameref attribute, name will be unset rather than
              the variable it references.  -n has no effect if the -f option is supplied.  If  no
              options  are  supplied,  each name refers to a variable; if there is no variable by
              that name, any function with that name is unset.  Each unset variable  or  function
              is  removed  from  the  environment  passed  to  subsequent  commands.   If  any of
              COMP_WORDBREAKS, RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME,  GROUPS,  or  DIRSTACK
              are unset, they lose their special properties, even if they are subsequently reset.
              The exit status is true unless a name is readonly.

       wait [-fn] [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,  all  currently  active
              child processes are waited for, and the return status is zero.  If the -n option is
              supplied, wait waits for any job to terminate and returns its exit status.  If  the
              -f  option is supplied, and job control is enabled, wait forces id to terminate be‐
              fore returning its status, instead of returning when  it  changes  status.   If  id
              specifies  a non-existent process or job, the return status is 127.  Otherwise, the
              return status is the exit status of the last process or job waited for.

RESTRICTED SHELL
       If bash is started with the name rbash, or the -r option is supplied  at  invocation,  the
       shell  becomes  restricted.  A restricted shell is used to set up an environment more con‐
       trolled than the standard shell.  It behaves identically to bash with the  exception  that
       the following are disallowed or not performed:

       •      changing directories with cd

       •      setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV

       •      specifying command names containing /

       •      specifying a filename containing a / as an argument to the .  builtin command

       •      specifying  a  filename  containing  a slash as an argument to the -p option to the
              hash builtin command

       •      importing function definitions from the shell environment at startup

       •      parsing the value of SHELLOPTS from the shell environment at startup

       •      redirecting output using the >, >|, <>, >&, &>, and >> redirection operators

       •      using the exec builtin command to replace the shell with another command

       •      adding or deleting builtin commands with the  -f  and  -d  options  to  the  enable
              builtin command

       •      using the enable builtin command to enable disabled shell builtins

       •      specifying the -p option to the command builtin command

       •      turning off restricted mode with set +r or set +o restricted.

       These restrictions are enforced after any startup files are read.

       When  a  command  that  is  found  to be a shell script is executed (see COMMAND EXECUTION
       above), rbash turns off any restrictions in the shell spawned to execute the script.

SEE ALSO
       Bash Reference Manual, Brian Fox and Chet Ramey
       The Gnu Readline Library, Brian Fox and Chet Ramey
       The Gnu History Library, Brian Fox and Chet Ramey
       Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE --
              http://pubs.opengroup.org/onlinepubs/9699919799/
       http://tiswww.case.edu/~chet/bash/POSIX -- a description of posix mode
       sh(1), ksh(1), csh(1)
       emacs(1), vi(1)
       readline(3)

FILES
       /bin/bash
              The bash executable
       /etc/profile
              The systemwide initialization file, executed for login shells
       /etc/bash.bashrc
              The systemwide per-interactive-shell startup file
       /etc/bash.bash.logout
              The systemwide login shell cleanup file, executed when a login shell exits
       ~/.bash_profile
              The personal initialization file, executed for login shells
       ~/.bashrc
              The individual per-interactive-shell startup file
       ~/.bash_logout
              The individual login shell cleanup file, executed when a login shell exits
       ~/.inputrc
              Individual readline initialization file

AUTHORS
       Brian Fox, Free Software Foundation
       bfox@gnu.org

       Chet Ramey, Case Western Reserve University
       chet.ramey@case.edu

BUG REPORTS
       If you find a bug in bash, you should report it.  But first, you should make sure that  it
       really is a bug, and that it appears in the latest version of bash.  The latest version is
       always available from ftp://ftp.gnu.org/pub/gnu/bash/.

       Once you have determined that a bug actually exists, use the bashbug command to  submit  a
       bug  report.  If you have a fix, you are encouraged to mail that as well!  Suggestions and
       `philosophical' bug reports may be mailed to bug-bash@gnu.org  or  posted  to  the  Usenet
       newsgroup gnu.bash.bug.

       ALL bug reports should include:

       The version number of bash
       The hardware and operating system
       The compiler used to compile
       A description of the bug behaviour
       A short script or `recipe' which exercises the bug

       bashbug inserts the first three items automatically into the template it provides for fil‐
       ing a bug report.

       Comments  and  bug  reports  concerning  this  manual   page   should   be   directed   to
       chet.ramey@case.edu.

BUGS
       It's too big and too slow.

       There  are some subtle differences between bash and traditional versions of sh, mostly be‐
       cause of the POSIX specification.

       Aliases are confusing in some uses.

       Shell builtin commands and functions are not stoppable/restartable.

       Compound commands and command sequences of the form `a ; b ; c' are not handled gracefully
       when  process  suspension  is attempted.  When a process is stopped, the shell immediately
       executes the next command in the sequence.  It suffices to place the sequence of  commands
       between parentheses to force it into a subshell, which may be stopped as a unit.

       Array variables may not (yet) be exported.

       There may be only one active coprocess at a time.

GNU Bash 5.0                             2018 December 7                                  BASH(1)

 

 

Súgó kimenet

bash --help
GNU bash, 5.0.3(1)-release-(x86_64-pc-linux-gnu) verzió
Használat:      bash [GNU hosszú kapcsoló] [kapcsoló] ...
        bash [GNU hosszú kapcsoló] [kapcsoló] parancsfájl ...
GNU hosszú kapcsolók:
        --debug
        --debugger
        --dump-po-strings
        --dump-strings
        --help
        --init-file
        --login
        --noediting
        --noprofile
        --norc
        --posix
        --pretty-print
        --rcfile
        --restricted
        --verbose
        --version
Parancsértelmező-kapcsolók:
        -ilrsD vagy -c parancs vagy -O shopt_option             (csak hívás)
        -abefhkmnptuvxBCHP vagy -o kapcsoló
A „bash -c "help set"” további információt ad a parancsértelmező-beállításokról.
A „bash -c help” további információt ad a beépített parancsokról.
A „bashbug” paranccsal jelenthet hibákat.

a bash honlapja: <http://www.gnu.org/software/bash>
Általános segítség a GNU szoftverek használatához: <http://www.gnu.org/gethelp/>

 

Kapcsolódó tartalom

 

Címkék