bash <root
BASH(1)                              General Commands Manual                              BASH(1)

NAME
       bash - GNU Bourne-Again SHell

SYNOPSIS
       bash [options] [command_string | file]

COPYRIGHT
       Bash is Copyright (C) 1989-2013 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 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,  they
                 are assigned to the positional parameters, starting with $0.
       -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.
       -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
                 arguments 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
              option 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
       positional  parameters  are  set to the remaining arguments.  Bash reads and executes com‐
       mands from this file, then exits.  Bash's exit status is the exit status of the last  com‐
       mand  executed  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 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 $- includes 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 a login shell exits, bash reads and executes commands from the  file  ~/.bash_logout,
       if it exists.

       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
       secure 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
       behavior,  and  the --rcfile option may be used to force another file to be read, but nei‐
       ther rshd nor sshd generally invoke the shell with those options or allow them to be spec‐
       ified.

       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
       control operator
              A token that performs a control function.  It is one of the following symbols:
              || & && ; ;; ( ) | |& 

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).

   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 .

       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.  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 of 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.

       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
              below).  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
       shell option nocasematch 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 an 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  incor‐
       rect,  the conditional expression's return value is 2.  If the shell option nocasematch is
       enabled, the match is performed without regard to the case of alphabetic characters.   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 regular 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 element of
       BASH_REMATCH with index n is the portion of the string matching the nth parenthesized sub‐
       expression.

       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
              described 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  evalu‐
              ated.   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 same matching rules as for pathname expansion (see Pathname  Expan‐
              sion  below).   The  word is expanded using tilde expansion, parameter and variable
              expansion, arithmetic substitution, command substitution, process substitution  and
              quote  removal.  Each pattern examined is expanded using tilde expansion, parameter
              and variable expansion, arithmetic substitution, command substitution, and  process
              substitution.   If  the shell option nocasematch 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 execu‐
              tion 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
       interpreted  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
       input  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.  The
       file  descriptors  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
       returns 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.  compound-command is executed when‐
              ever 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 function 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 exe‐
              cuted in the body.  (See FUNCTIONS below.)

COMMENTS
       In a non-interactive shell, or an interactive  shell  in  which  the  interactive_comments
       option  to  the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word begin‐
       ning with # causes that word and all remaining characters on that line to be ignored.   An
       interactive 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
       reserved 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
       below), the history expansion character, usually !, must  be  quoted  to  prevent  history
       expansion.

       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 .  If a \ pair
       appears, and the backslash is not itself quoted, the \ is treated as a line  con‐
       tinuation (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, !.  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  .   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
              \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)
              \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
       denoted by a name.  A variable has a value and zero or more  attributes.   Attributes  are
       assigned 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.  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.  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 variable using  compound  assign‐
       ment  (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
       local 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
       indirectly.  Whenever the nameref variable is referenced or assigned to, the operation  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 are treated as references and
       assignments 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 established for each word in the list, in turn, when  the  loop  is
       executed.   Array  variables cannot be given the -n 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
       invoked,  and  may be reassigned using the set builtin command.  Positional parameters may
       not be assigned to with assignment statements.  The positional parameters are  temporarily
       replaced 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$2c...",  where  c
              is  the  first  character  of  the value of the IFS variable.  If IFS is unset, the
              parameters are separated by spaces.  If IFS is  null,  the  parameters  are  joined
              without intervening separators.
       @      Expands to the positional parameters, starting from one.  When the expansion occurs
              within double quotes, each parameter expands to a separate word.  That is, "$@"  is
              equivalent  to  "$1" "$2" ...  If the double-quoted expansion occurs within a word,
              the expansion of the first parameter is joined with the beginning part of the orig‐
              inal  word, and the expansion of the last parameter is joined with the last part of
              the original word.  When there are no positional parameters, "$@" 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 command, after expansion.  Also set to
              the full pathname used to invoke each command executed and placed in  the  environ‐
              ment  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.
       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; unsetting array elements cause aliases to be removed from the alias
              list.
       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)
       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)
       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; unsetting array elements cause commands to be removed from the
              hash table.
       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_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
              begins 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
              facilities (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.
       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 and return an error status.  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 and return an error status.  If GROUPS
              is unset, it loses its special 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
              reset.
       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
              assigned.  If SECONDS is unset, it loses its special properties, even if it is sub‐
              sequently 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
              error 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.
       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 filenames to be ignored by
              pathname expansion.  If a filename 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
              unset, 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
              before  attempting  a  match.  The second and subsequent lines of a multi-line com‐
              pound command are not tested, and are added to the history regardless of the  value
              of HISTIGNORE.
       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
              ``''.
       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).
       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.
       LINES  Used by the select compound command to determine the  column  length  for  printing
              selection  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, $_
              expands to the name of the current mailfile.  Example:
              MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
              Bash supplies a default value for this variable, 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.
       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.
       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  PS4
              is replicated multiple times, as necessary, to indicate multiple levels of indirec‐
              tion.  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
              issuing  the primary prompt.  Bash terminates after waiting for that number of sec‐
              onds 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
              allowed; 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 sup‐
              plied needs to match a substring of the name of a stopped job.  The substring value
              provides 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
       require 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
       assigned 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
       assigned 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
       required to avoid conflicts with pathname expansion.  If subscript is @  or  *,  the  word
       expands 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.
       Referencing  an  array variable without a subscript is equivalent to referencing the array
       with a subscript of 0.  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 references the last element.

       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
       element at index subscript.  Negative subscripts to  indexed  arrays  are  interpreted  as
       described  above.   Care  must  be taken to avoid unwanted side effects caused by pathname
       expansion.  unset name, where name is an array, or unset name[subscript], where  subscript
       is * or @, removes the entire array.

       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
       expansion,  arithmetic  expansion, and command substitution (done in a left-to-right fash‐
       ion); 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.

       Only brace expansion, word splitting, and pathname expansion  can  change  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[@]}"  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
       series 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
       default 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
       apply 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.

       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
       login 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
       assumed.

       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
       assigns the expanded value.

   Parameter Expansion
       The `$' character introduces parameter  expansion,  command  substitution,  or  arithmetic
       expansion.   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
       following 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 (!), it introduces a level  of
       variable indirection.  Bash uses the value of the variable formed from the rest of parame‐
       ter as the name of the variable; this variable is then expanded and that value is used  in
       the rest of the substitution, rather than the value of parameter itself.  This is known as
       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
              assigned  to  parameter.   The  value of parameter is then substituted.  Positional
              parameters 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
              described  below.   If  length is omitted, expands to the substring of the value of
              parameter 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
              parameter, 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
              indices (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.  If the pattern matches the beginning of the value of param‐
              eter, 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%word}
       ${parameter%%word}
              Remove matching suffix pattern.  The word is expanded to produce a pattern just  as
              in  pathname  expansion.  If the pattern matches a trailing 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 match‐
              ing 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.  If pattern begins with /, all matches  of  pat‐
              tern are replaced with string.  Normally only the first match is replaced.  If pat‐
              tern begins with #, it must match at the beginning of the expanded value of parame‐
              ter.   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  /  following
              pattern  may  be  omitted.   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 subscripted 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
              attempt  to  match more than one character.  The ^ operator converts lowercase let‐
              ters matching pattern to uppercase; the , operator converts matching uppercase let‐
              ters  to lowercase.  The ^^ and ,, expansions convert each matched character in the
              expanded 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 applied 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.

   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 and replacing the command substitution
       with the standard output of the command, with any  trailing  newlines  deleted.   Embedded
       newlines are not deleted, but they may be removed during word splitting.  The command sub‐
       stitution $(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 is supported on systems that  support  named  pipes  (FIFOs)  or  the
       /dev/fd  method  of  naming  open  files.   It  takes the form of <(list) or >(list).  The
       process list is run with its input or output connected to a FIFO or some file in  /dev/fd.
       The name of this file 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.

       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 , the default, then sequences of , ,
       and    at  the  beginning  and  end of the results of the previous expansions are
       ignored, 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 and tab 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  characters,
       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.   Unquoted  implicit  null  arguments,
       resulting from the expansion of parameters that have no values, are removed.  If a parame‐
       ter with no value is expanded within  double  quotes,  a  null  argument  results  and  is
       retained.

       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
       option  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
       immediately following a slash must be matched explicitly, unless the shell option  dotglob
       is  set.  When matching a pathname, the slash character must always be matched explicitly.
       In other cases, the ``.''  character is not treated specially.   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 filenames matching a pat‐
       tern.   If GLOBIGNORE is set, each matching filename that also matches one of the patterns
       in GLOBIGNORE is removed from the list of matches.  The filenames ``.''  and  ``..''   are
       always ignored when GLOBIGNORE is set and not null.  However, 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 behavior of ignoring filenames begin‐
       ning with a ``.'', make ``.*''  one of the patterns in GLOBIGNORE.  The dotglob option  is
       disabled when GLOBIGNORE is unset.

       Pattern Matching

       Any  character  that  appears  in  a  pattern,  other  than the special pattern characters
       described below, matches itself.  The NUL character may not occur in a pattern.   A  back‐
       slash  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
                     option  is enabled, and * is used in a pathname expansion context, two adja‐
                     cent *s used as a single pattern will match  all  files  and  zero  or  more
                     directories  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
                     sequence  and  character  set, is matched.  If the first character following
                     the [ is a !  or a ^ then any character not enclosed is matched.  The  sort‐
                     ing  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  obtain  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
                     defined 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

   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.

       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
       expansion, 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:

              /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

       If  the  redirection  operator  is >, and the noclobber option to the set builtin has been
       enabled, 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
       operator is > and the noclobber option to the set builtin command is not enabled, the  re‐
       direction 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 for a command.

       The format of here-documents is:

              <<[-]word
                      here-document
              delimiter

       No parameter and variable expansion, command substitution, arithmetic expansion, or  path‐
       name  expansion is performed on word.  If any characters in word are 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  \ is ignored, and \ must be used to quote the characters \, $, and `.

       If  the  redirection  operator  is  <<-, then all leading tab characters are stripped from
       input 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:

              <<&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
       descriptor open for output, a redirection error occurs.  If  word  evaluates  to  -,  file
       descriptor  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
       descriptor 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
       always  reads  at least one complete line of input before executing any of the commands on
       that line.  Aliases are expanded when a command is read, not when it is executed.   There‐
       fore,  an  alias  definition  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 func‐
       tions are executed.  Aliases are expanded when a function definition is read, not when the
       function  is  executed,  because a function definition is itself a compound command.  As a
       consequence, aliases defined 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 commands.

       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.

       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.  Note that shell functions and variables with the same name may  result
       in  multiple  identically-named entries in the environment passed to the shell's children.
       Care should be taken in cases where this may cause a problem.

       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 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
       ++id --id
              variable pre-increment and pre-decrement
       - +    unary minus and plus
       ! ~    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
       expression 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.   Expres‐
       sions  are  formed  from the following unary or binary primaries.  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
       operate 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
              exists 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
              description 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
              described 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.

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
       optional 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 with the original command and the original command's argu‐
       ments as its arguments, and the function's exit status becomes  the  exit  status  of  the
       shell.   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
       invoked 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
       invoked 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
       export  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.

       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
       effect, 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
       ensure that they receive the SIGHUP.  To prevent the shell from sending the  signal  to  a
       particular  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
       facility  via  an  interactive interface supplied jointly by the operating system kernel's
       terminal 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
       facilities 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
       reports such changes immediately.  Any trap on SIGCHLD is executed  for  each  child  that
       exits.

       If  an  attempt  to  exit  bash is made while jobs are stopped (or, if the checkjobs shell
       option 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.

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 allows these prompt strings to be customized by inserting a  number  of  back‐
       slash-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
       enabled  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
       inserted 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
       required  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.
       colored-stats (Off)
              If  set  to  On,  readline  displays possible completions using different colors to
              indicate 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-ignore-case (Off)
              If set to On, readline performs filename matching and completion in a case-insensi‐
              tive fashion.
       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
              effect, using escape as the meta prefix).
       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.
       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.
       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.
       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 (0)
              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 not limited.
       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
              high  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.
       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
              sequence.   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
              until 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.
       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
              individual 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 character to the beginning of the prompt indicating the editing
              mode: emacs (@), vi command (:) or vi insertion (+).
       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.
       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 extends to
              the end of the line; 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 the both full name of the
                     terminal and the portion of the terminal name  before  the  first  -.   This
                     allows sun to match both sun and sun-cmd, for instance.

              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
                     instance,  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

       $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
       becomes 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
              alphanumeric 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.
       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.
       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
              direction  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
              description 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
              EXPANSION 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.  Any argument is ignored.
       edit-and-execute-command (C-xC-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
              beginning 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 (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 shell-forward-word.
       shell-backward-kill-word (M-Rubout)
              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  or  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
              unbound 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-uppercase-version (M-a, M-b, M-x, ...)
              If the metafied character x is lowercase, run the command that is bound to the cor‐
              responding uppercase character.
       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-)
              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
              position, 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
              sequences 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
              inserted  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
              numeric  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
              asterisk  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.

       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
       option 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 com‐
       pletion  that  matches the pattern will be removed from the list.  A leading ! negates the
       pattern; in this case any completion not matching the pattern will be removed.

       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
       attempted.

       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
       attempted, 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,
       regardless 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
       expansion  is performed, subject to the values of the shell variables HISTIGNORE and HIST‐
       CONTROL.

       On startup, the history is initialized from  the  file  named  by  the  variable  HISTFILE
       (default  ~/.bash_history).  The file named by the value of HISTFILE is truncated, if nec‐
       essary, to contain no more than the number of lines specified by the  value  of  HISTFILE‐
       SIZE.   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
       beginning  with  the  history comment character followed immediately by a digit are inter‐
       preted as timestamps for the preceding history line.  These timestamps are optionally dis‐
       played  depending  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
       COMMANDS 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.  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  portions  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 modifiers 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 surrounded by
       quotes  are  considered  one word.  History expansions are introduced by the appearance of
       the history expansion character, which is ! by default.  Only  backslash  (\)  and  single
       quotes can quote the history expansion character.

       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
       options  preceded  by - accepts -- to signify the end of the options.  The :, true, false,
       and test builtins do not accept options and do not treat -- specially.  The exit,  logout,
       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.  A zero exit code is returned.

        .  filename [arguments]
       source filename [arguments]
              Read and execute commands from filename in the current shell environment and return
              the exit status of the last command executed from filename.  If filename  does  not
              contain  a slash, 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.  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 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;  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
                     reflected in the editing state.
              -X     List  all  key sequences bound to shell commands and the associated commands
                     in a format that can be reused as input.

              The return value is 0 unless an unrecognized option is given or an error occurred.

       break [n]
              Exit from within a for, while, until, or select loop.  If n is specified,  break  n
              levels.   n  must  be ≥ 1.  If n is greater than the number of enclosing loops, all
              enclosing loops are exited.  The return value is 0 unless n is not greater than  or
              equal to 1.

       builtin shell-builtin [arguments]
              Execute the specified shell builtin, passing it arguments, and return its exit 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
              executed with the . or source builtins).  Without expr, caller  displays  the  line
              number and source filename of the current subroutine call.  If a non-negative inte‐
              ger is supplied as expr, caller displays the  line  number,  subroutine  name,  and
              source  file  corresponding  to  that position in the current execution call stack.
              This extra information may be used, for example, to print a stack trace.  The  cur‐
              rent  frame  is frame 0.  The return value is 0 unless the shell is not executing a
              subroutine call or expr does not correspond to a valid position in the call stack.

       cd [-L|[-P [-e]] [-@]] [dir]
              Change the current directory to dir.  if dir is not supplied, the value of the HOME
              shell variable is the default.  Any additional arguments following dir are ignored.
              The variable CDPATH defines the search path for the directory containing dir:  each
              directory  name  in  CDPATH  is  searched  for dir.  Alternative directory names in
              CDPATH are separated by a colon (:).  A null directory name in CDPATH is  the  same
              as the current directory, i.e., ``.''.  If dir begins with a slash (/), then CDPATH
              is not used. The -P option causes cd to use the  physical  directory  structure  by
              resolving symbolic links while traversing dir and before processing instances of ..
              in dir (see also the -P option to the set builtin command); the  -L  option  forces
              symbolic  links  to be 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
              attributes associated with a file as a directory.  An argument of - is converted to
              $OLDPWD before the directory change is attempted.  If a  non-empty  directory  name
              from  CDPATH  is  used,  or if - is the first argument, and the directory change is
              successful, the absolute pathname of the new working directory is  written  to  the
              standard  output.   The  return  value  is  true  if the directory was 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
              indicating  the  command or filename used to invoke command to be displayed; the -V
              option produces a more verbose description.  If the -V or -v  option  is  supplied,
              the exit status is 0 if command was found, and 1 if not.  If neither option is sup‐
              plied and an error occurred or command cannot be found, the  exit  status  is  127.
              Otherwise, the exit status of the command builtin is the exit status of command.

       compgen [option] [word]
              Generate  possible  completion matches for word according to the options, which may
              be any option accepted by the complete builtin with the exception of -p and -r, and
              write  the  matches  to  the standard output.  When using the -F or -C options, the
              various shell variables set by the programmable completion facilities, while 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] [-DE] [-A action] [-G globpat] [-W wordlist] [-F
       function] [-C command]
              [-X filterpat] [-P prefix] [-S suffix] name [name ...]
       complete -pr [-DE] [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  the  remaining  options  and
              actions  should  apply  to  the ``default'' command completion; that is, completion
              attempted on a command for which no completion has previously been defined.  The -E
              option  indicates  that the remaining options and actions should apply to ``empty''
              command completion; that is, completion attempted on a blank line.

              The process of applying these completion specifications  when  word  completion  is
              attempted is described above under Programmable Completion.

              Other options, if specified, have the following meanings.  The arguments to the -G,
              -W, and -X options (and, if necessary, the -P and -S options) should be  quoted  to
              protect them from expansion before the complete builtin is invoked.
              -o comp-option
                      The  comp-option controls several aspects of the compspec's behavior beyond
                      the simple generation of completions.  comp-option may be one of:
                      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
                              directory  names, quoting special characters, or suppressing trail‐
                              ing 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).
                      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
                              results 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
                      array variable.
              -G globpat
                      The pathname expansion pattern globpat is expanded to generate the possible
                      completions.
              -P prefix
                      prefix is added at the beginning of  each  possible  completion  after  all
                      other options have been applied.
              -S suffix
                      suffix is appended to each possible completion after all other options have
                      been applied.
              -W wordlist
                      The wordlist is split using the characters in the IFS special  variable  as
                      delimiters,  and each resultant word is expanded.  The possible completions
                      are the members of the resultant list which match the word being completed.
              -X filterpat
                      filterpat is a pattern as used for pathname expansion.  It  is  applied  to
                      the  list  of  possible  completions generated by the preceding options and
                      arguments, and each completion matching filterpat is removed from the list.
                      A  leading ! in filterpat negates the pattern; in this case, any completion
                      not matching filterpat is removed.

              The return value is true unless an invalid option is supplied, an option other than
              -p  or  -r is supplied without a name argument, an attempt is made to remove a com‐
              pletion specification for a name for which no specification  exists,  or  an  error
              occurs adding a completion specification.

       compopt [-o option] [-DE] [+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 the remaining options should apply to the ``default'' com‐
              mand completion; that is, completion attempted on a command for which no completion
              has  previously  been  defined.  The -E option indicates that the remaining options
              should apply to ``empty'' command completion; that is, completion  attempted  on  a
              blank line.

              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 the func‐
              tion  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  executed in a shell function.  It is ignored in all other cases.  The following
              options can be used to restrict output to variables with the specified attribute or
              to give variables attributes:
              -a     Each name is an indexed array variable (see Arrays above).
              -A     Each name is an associative array variable (see Arrays above).
              -f     Use function names only.
              -i     The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC
                     EVALUATION above) is performed when the variable is assigned a value.
              -l     When the variable is assigned a value, all upper-case  characters  are  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  and assignments to name, except for changing the -n attribute itself,
                     are performed on the variable referenced by name's value.  The -n  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 may not be used to destroy an array variable 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  compound  assign‐
              ment  syntax  (see  Arrays  above),  one of the names is not a valid shell variable
              name, an attempt is made to turn off readonly status for a  readonly  variable,  an
              attempt  is  made  to turn off array status for an array variable, or an attempt is
              made to display a non-existent function with -f.

       dirs [-clpv] [+n] [-n]
              Without options, displays  the  list  of  currently  remembered  directories.   The
              default  display  is  on  a  single  line with directory names separated by spaces.
              Directories are added to the list with the pushd command; the popd command  removes
              entries from the list.
              -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 ...]
              Without options, remove each jobspec from the table of active jobs.  If jobspec  is
              not  present,  and neither the -a nor the -r option is supplied, the current job is
              used.  If the -h option is given, each jobspec is not removed from the  table,  but
              is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP.  If
              no jobspec is supplied, the -a option means to remove or  mark  all  jobs;  the  -r
              option  without a jobspec argument restricts operation to running jobs.  The return
              value is 0 unless a jobspec does not specify a valid job.

       echo [-neE] [arg ...]
              Output the args, separated by spaces, followed by a newline.  The return status  is
              0  unless  a  write error occurs.  If -n is specified, the trailing newline is sup‐
              pressed.  If the -e option is given, interpretation  of  the  following  backslash-
              escaped  characters is enabled.  The -E option disables the interpretation of these
              escape characters, even on systems where they  are  interpreted  by  default.   The
              xpg_echo  shell  option  may  be  used to dynamically determine whether or not echo
              expands these 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
              environment.   If  -a  is supplied, the shell passes name as the zeroth argument to
              the executed command.  If command cannot be executed for some reason, a  non-inter‐
              active  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  exe‐
              cuted.   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
              option  is  encountered, one of the names is not a valid shell variable name, or -f
              is supplied with a name that is not a function.

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

              The  -n option suppresses the command numbers when listing.  The -r option reverses
              the order of the commands.  If the -l option is given, the commands are  listed  on
              standard  output.   Otherwise,  the editor given by ename is invoked on a file 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 intepreted the same as first above.  A useful alias to use with
              this is ``r="fc -s"'', so that typing ``r cc'' runs the last command beginning with
              ``cc'' and typing ``r'' re-executes the last command.

              If  the  first  form  is  used,  the  return value is 0 unless an invalid option is
              encountered or first or last specify history lines out of range.  If the -e  option
              is  supplied, the return value is the value of the last command executed or failure
              if an error occurs with the temporary file of commands.   If  the  second  form  is
              used,  the  return  status  is that of the command re-executed, unless cmd does not
              specify a valid history line, in which case fc returns failure.

       fg [jobspec]
              Resume jobspec in the foreground, and make it the current job.  If jobspec  is  not
              present,  the  shell's notion of the current job is used.  The return value is that
              of the command placed into the foreground, or failure if run when  job  control  is
              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
              option characters.  Each time it is invoked, getopts places the next option in  the
              shell  variable  name, initializing name if it does not exist, and the index of the
              next argument to be processed into the variable OPTIND.  OPTIND is initialized to 1
              each time the shell or a shell script is 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
              option character found.

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

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

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

              The return status is 0 unless no command matches pattern.

       history [n]
       history -c
       history -d offset
       history -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.
              -a     Append  the ``new'' history lines (history lines entered since the beginning
                     of the current bash session) 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
              previous  history  line.  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
              return status is 0 unless an invalid option is encountered or an invalid jobspec is
              supplied.

              If  the  -x  option is supplied, jobs replaces any jobspec found in command or args
              with the corresponding process group ID, and  executes  command  passing  it  args,
              returning its exit status.

       kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
       kill -l [sigspec | exit_status]
              Send  the  signal  named by sigspec or signum to the processes named by pid or 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.  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.  With no operands, local writes a list of local vari‐
              ables to the standard output.  It is an error to use local when not within a  func‐
              tion.   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 [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
       readarray [-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
              default array.  Options, if supplied, have the following meanings:
              -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 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
              directory 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
                     argument  (except  that  \c terminates output, backslashes in \', \", and \?
                     are not removed, and octal escapes beginning with \0 may contain up to  four
                     digits).
              %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,
              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 adding directories to the
                     stack, so that only the stack is manipulated.
              +n     Rotates the stack so that the nth directory (counting from the left  of  the
                     list shown by dirs, starting with zero) is at the top.
              -n     Rotates  the stack so that the nth directory (counting from the right of the
                     list shown by dirs, starting with zero) is at the top.
              dir    Adds dir to the directory stack at the top, 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, and the first word is assigned to the first name,
              the second word to the second name, and so on, with leftover words and their inter‐
              vening  separators  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.
              -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.
              -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 honor a delimiter if fewer than nchars characters
                     are read before the delimiter.
              -N nchars
                     read returns after reading exactly nchars characters rather than waiting for
                     a  complete  line  of  input,  unless  EOF is encountered or read times out.
                     Delimiter characters encountered in the input are not treated specially  and
                     do not cause read to return until nchars characters are read.
              -p prompt
                     Display  prompt  on  standard  error,  without  a  trailing  newline, before
                     attempting to read any input.  The prompt is displayed only if input is com‐
                     ing 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  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
              return  code  is  zero, unless end-of-file is encountered, read times out (in which
              case the return code is greater than 128), a variable  assignment  error  (such  as
              assigning to a readonly variable) occurs, or an invalid file descriptor is supplied
              as the argument to -u.

       readonly [-aAf] [-p] [name[=word] ...]
              The given names are marked readonly; the values of these names may not  be  changed
              by  subsequent assignment.  If the -f option is supplied, the functions 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
              options are supplied, -A takes precedence.  If no name arguments are given,  or  if
              the  -p  option  is  supplied,  a list of all readonly names is printed.  The other
              options may be used to restrict the output to a  subset  of  the  set  of  readonly
              names.   The -p option causes output to be displayed in a 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 function.

       return [n]
              Causes  a  function  to  stop  executing and return the value specified by n to its
              caller.  If n is omitted, the return status is that of the last command executed in
              the function body.  If return is used outside a function, but during execution of a
              script by the .  (source) command, it causes  the  shell  to  stop  executing  that
              script  and  return either n or the exit status of the last command executed within
              the script as the exit status of the script.  If n is supplied, the return value is
              its  least significant 8 bits.  The return status is non-zero if return is supplied
              a non-numeric argument, or is used outside a function and not during execution of a
              script  by  .  or  source.  Any command associated with the RETURN trap is executed
              before execution resumes after the function or script.

       set [--abefhkmnptuvxBCEHPT] [-o option-name] [arg ...]
       set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg ...]
              Without options, 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
              assigned, in order, to $1, $2, ...  $n.  Options, if specified, have the  following
              meanings:
              -a      Automatically  mark  variables  and functions which are modified or created
                      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
                      before the shell exits.  This option applies to the shell  environment  and
                      each  subshell  environment  separately  (see COMMAND EXECUTION ENVIRONMENT
                      above), and may cause subshells to exit before executing all  the  commands
                      in the subshell.

                      If  a  compound command or shell function executes in a context where -e is
                      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
                              option is on by default in interactive shells.
                      ignoreeof
                              The  effect  is  as  if the shell command ``IGNOREEOF=10'' had been
                              executed (see Shell Variables 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
                      option  is  not supplied, these actions are taken and the effective user id
                      is set to the real user id.  If the -p option is supplied at  startup,  the
                      effective  user id is not reset.  Turning this option off causes the effec‐
                      tive 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
                      unset.   Otherwise,  the positional parameters are set to the args, even if
                      some of them begin with a -.
              -       Signal the end of options, cause all remaining args to be assigned  to  the
                      positional parameters.  The -x and -v options are turned off.  If there are
                      no args, the positional parameters remain unchanged.

              The options are off by default unless otherwise  noted.   Using  +  rather  than  -
              causes  these options to be turned off.  The options can also be specified as 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
              option,  a list of all settable options is displayed, with an indication of whether
              or not each is set.  The -p option causes output to be displayed in a form that may
              be reused as input.  Other options have the following meanings:
              -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
              options which are set or unset, respectively.  Unless otherwise  noted,  the  shopt
              options are disabled (unset) by default.

              The  return  status  when listing options is zero if all optnames are enabled, non-
              zero otherwise.  When setting or unsetting  options,  the  return  status  is  zero
              unless an optname is not a valid shell option.

              The list of shopt options is:

              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 command and, if necessary,
                      updates the values of LINES and COLUMNS.
              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.
              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).
              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.
              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.
              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, 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), a call  to  return  is
                             simulated.
                      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.
              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.
              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.
              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.
              promptvars
                      If  set,  prompt strings undergo parameter expansion, command substitution,
                      arithmetic expansion, and quote removal after being expanded  as  described
                      in PROMPTING above.  This option is enabled by default.
              restricted_shell
                      The  shell  sets  this  option  if  it  is  started in restricted mode (see
                      RESTRICTED SHELL below).  The value may not be changed.  This is not  reset
                      when the startup files are executed, allowing the startup files to discover
                      whether or not a shell is restricted.
              shift_verbose
                      If set, the shift builtin prints an error  message  when  the  shift  count
                      exceeds the number of positional parameters.
              sourcepath
                      If set, the source (.) builtin uses the value of PATH to find the directory
                      containing the file supplied as an argument.  This  option  is  enabled  by
                      default.
              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
              order of precedence.  The evaluation depends on the number of arguments; see below.
              Operator precedence is used when there are five or more arguments.
              ! expr True if expr is false.
              ( expr )
                     Returns the value of expr.  This may be used to override the  normal  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
                     operators 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
                     argument  is one of the binary conditional operators listed above under CON‐
                     DITIONAL EXPRESSIONS, the result of the expression  is  the  result  of  the
                     binary  test using the first and third arguments as operands.  The -a and -o
                     operators are considered binary operators when there  are  three  arguments.
                     If  the  first  argument is !, the value is the negation of the two-argument
                     test using the second and third arguments.  If the first argument is exactly
                     (  and  the third argument is exactly ), the result is the one-argument test
                     of the second argument.  Otherwise, the expression is false.
              4 arguments
                     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
              associated  with  each  signal.   The -l option causes the shell to print a list of
              signal names and their corresponding numbers.  Each sigspec is either a signal name
              defined  in  , 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 a pipeline (which may
              consist of a single simple command), a  list,  or  a  compound  command  returns  a
              non-zero  exit  status,  subject  to the following conditions.  The ERR trap is not
              executed if the failed command is part of the command list immediately following  a
              while or until keyword, part of the test in an if statement, part of a command 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
              arguments are found, false if any are not found.

       ulimit [-HSTabcdefilmnpqrstuvx [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
              -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
              -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; and -T, -b, -n, and -u, which are  unscaled  values.   The  return
              status  is  0  unless an invalid option or argument is supplied, or an error occurs
              while setting a new limit.

       umask [-p] [-S] [mode]
              The user file-creation mask is set to mode.  If mode begins with  a  digit,  it  is
              interpreted as an octal number; otherwise it is interpreted as a symbolic mode mask
              similar to that accepted by chmod(1).  If mode is omitted, the current value of the
              mask is printed.  The -S option causes the mask to be printed in symbolic form; the
              default output is an octal number.  If the -p option is supplied, and mode is 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 [-n] [n ...]
              Wait for each specified child process and return its termination  status.   Each  n
              may  be  a process ID or a job specification; if a job spec is given, all processes
              in that job's pipeline are waited for.  If n is not  given,  all  currently  active
              child processes are waited for, and the return status is zero.  If the -n option is
              supplied, wait waits for any job to terminate and returns its exit  status.   If  n
              specifies  a non-existent process or job, the return status is 127.  Otherwise, the
              return status is the exit status of the last process or job waited for.

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
       because 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 4.3                             2014 February 2                                  BASH(1)

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