Linux Bash编程man帮助手册

Linux Bash编程man帮助手册

BASH(1)                            General Commands Manual                           BASH(1)

NAME
       bash - GNU Bourne-Again SHell

SYNOPSIS
       bash [options] [command_string | file]

COPYRIGHT
       Bash is Copyright (C) 1989-2020 by the Free Software Foundation, Inc.

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

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

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

       -c        If  the -c option is present, then commands are read from the first non-op‐
                 tion argument command_string.   If  there  are  arguments  after  the  com‐
                 mand_string,  the  first argument is assigned to $0 and any remaining argu‐
                 ments are assigned to the positional parameters.  The assignment to $0 sets
                 the name of the shell, which is used in warning and error messages.
       -i        If the -i option is present, the shell is interactive.
       -l        Make  bash  act  as if it had been invoked as a login shell (see INVOCATION
                 below).
       -r        If the -r option is present, the shell becomes restricted  (see  RESTRICTED
                 SHELL below).
       -s        If  the  -s  option is present, or if no arguments remain after option pro‐
                 cessing, then commands are read from the standard input.  This  option  al‐
                 lows the positional parameters to be set when invoking an interactive shell
                 or when reading input through a pipe.
       -D        A list of all double-quoted strings preceded by $ is printed on  the  stan‐
                 dard  output.   These are the strings that are subject to language transla‐
                 tion when the current locale is not C or POSIX.  This implies  the  -n  op‐
                 tion; 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 values 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 ar‐
                 gument 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  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
              interactive.

       --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 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  refer‐
              ence to a document that details how posix mode affects bash's behavior.

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

       --rpm-requires
              Produce the list of files that are required for the shell script to run.  This
              implies '-n' and is subject to the same  limitations  as  compile  time  error
              checking  checking;  Command  substitutions,  Conditional expressions and eval
              builtin are not parsed so some dependencies may be missed.

       --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  commands.   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  commands  from  this file, then exits.  Bash's exit status is the exit
       status of the last command 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 direc‐
       tory, and, if no file is found, then the shell searches the directories in  PATH  for
       the script.

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

       An interactive shell is one started without non-option arguments (unless -s is speci‐
       fied)  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 file‐
       names 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/pro‐
       file, if that file exists.  After reading that file, it  looks  for  ~/.bash_profile,
       ~/.bash_login,  and  ~/.profile,  in that order, and reads and executes commands from
       the first one that exists and is readable.  The --noprofile option may be  used  when
       the shell is started to inhibit this behavior.

       When  an interactive login shell exits, or a non-interactive login shell executes the
       exit builtin command, bash reads and executes commands from the files  ~/.bash_logout
       and /etc/bash.bash_logout, if the files exists.

       When  an  interactive shell that is not a login shell is started, bash reads and exe‐
       cutes commands from ~/.bashrc, if that file exists.  This may be inhibited  by  using
       the --norc option.  The --rcfile file option will force bash to read and execute com‐
       mands from file instead of ~/.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 following 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 his‐
       torical 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 execute.  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  fol‐
       lows  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, if that file exists and is readable.   It
       will not do this if invoked as sh.  The --norc option may be used to inhibit this be‐
       havior, and the --rcfile option may be used to force another file  to  be  read,  but
       neither  rshd nor sshd generally invoke the shell with those options or allow them to
       be specified.

       If the shell is started with the effective user (group) id not equal to the real user
       (group) id, and the -p option is not supplied, no startup files are read, shell func‐
       tions are not inherited from the environment, the SHELLOPTS,  BASHOPTS,  CDPATH,  and
       GLOBIGNORE  variables, if they appear in the environment, are ignored, and the effec‐
       tive 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  begin‐
              ning  with  an  alphabetic character or an underscore.  Also referred to as an
              identifier.
       metacharacter
              A character that, when unquoted, separates words.  One of the following:
              |  & ; ( ) < > space tab newline
       control operator
              A token that performs a control function.  It is one of the following symbols:
              || & && ; ;; ;& ;;& ( ) | |& <newline>

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

       !  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-
       separated  words  and  redirections, and terminated by a control operator.  The first
       word specifies the command to be executed, and is passed as argument zero.   The  re‐
       maining 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
       terminated by signal n.

   Pipelines
       A pipeline is a sequence of one or more commands separated by one of the control  op‐
       erators | 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 com‐
       mand2.  This connection is performed before any redirections specified by the command
       (see REDIRECTION below).  If |& is used, command's standard error, in addition to its
       standard output, 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 stan‐
       dard 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  commands  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 return‐
       ing 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 op‐
       tion 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 TIMEFORMAT 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 chil‐
       dren.  The TIMEFORMAT variable may be used to specify the format of the time informa‐
       tion.

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

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

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

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

       If  a command is terminated by the control operator &, the shell executes the command
       in the background in a subshell.  The shell does not wait for the command to  finish,
       and  the  return  status is 0.  These are referred to as asynchronous commands.  Com‐
       mands 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 exe‐
       cuted.

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

              command1 && command2

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

       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 de‐
       scription 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 en‐
              vironment  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 termi‐
              nated with a newline or semicolon.  This is known as a group command.  The re‐
              turn 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 per‐
              mitted to be recognized.  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  ex‐
              pression  expression.  Expressions are composed of the primaries described be‐
              low under CONDITIONAL 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 cur‐
              rent locale.

              When the == and != operators are used, the string to the right of the operator
              is considered a pattern and matched according to the rules described below un‐
              der  Pattern Matching, as if the extglob shell option were enabled.  The = op‐
              erator is equivalent to ==.  If the nocasematch shell option is  enabled,  the
              match  is  performed without regard to the case of alphabetic characters.  The
              return value is 0 if the string matches (==) or does not match (!=)  the  pat‐
              tern,  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 con‐
              sidered a POSIX extended regular expression and matched accordingly (using the
              POSIX  regcomp and regexec interfaces usually described in regex(3)).  The re‐
              turn value is 0 if the string matches the pattern, and 1  otherwise.   If  the
              regular  expression  is  syntactically incorrect, the conditional expression's
              return value is 2.  If the nocasematch shell option 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.

              The pattern will match if it matches any part of the string.  Anchor the  pat‐
              tern  using  the ^ and $ regular expression operators to force it to match the
              entire string.  The array variable BASH_REMATCH records  which  parts  of  the
              string matched the pattern.  The element of BASH_REMATCH with index 0 contains
              the portion of the string matching the entire regular expression.   Substrings
              matched  by  parenthesized  subexpressions  within  the regular expression are
              saved in the remaining BASH_REMATCH indices. The element of BASH_REMATCH  with
              index n is the portion of the string matching the nth parenthesized subexpres‐
              sion.

              Expressions may be combined using the following operators, listed in  decreas‐
              ing order of precedence:

              ( expression )
                     Returns the value of expression.  This may be used to override the nor‐
                     mal 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  expres‐
              sion1  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
              variable  name  is  set to each element of this list in turn, and list is exe‐
              cuted each time.  If the in word is omitted, the  for  command  executes  list
              once  for  each  positional parameter that is set (see PARAMETERS below).  The
              return status is the exit status of the last command that  executes.   If  the
              expansion  of the items following in results in an empty list, no commands are
              executed, and the return status is 0.

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

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

       case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
              A case command first expands word, and tries to match it against each  pattern
              in turn, using the matching rules described under Pattern Matching below.  The
              word is expanded using tilde  expansion,  parameter  and  variable  expansion,
              arithmetic expansion, command substitution, process substitution and quote re‐
              moval.  Each pattern examined is expanded using tilde expansion, parameter and
              variable  expansion,  arithmetic  expansion, command substitution, and process
              substitution.  If the nocasematch shell option is enabled, the match  is  per‐
              formed  without  regard to the case of alphabetic characters.  When a match is
              found, the corresponding list is executed.  If the ;;  operator  is  used,  no
              subsequent  matches  are attempted after the first pattern match.  Using ;& in
              place of ;; causes execution to continue with the  list  associated  with  the
              next  set  of patterns.  Using ;;& in place of ;; causes the shell to test the
              next pattern list in the statement, if any, and execute any associated list on
              a  successful match, continuing the case statement execution as if the pattern
              list had not matched.  The exit status is zero if no pattern matches.   Other‐
              wise, 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 exe‐
              cuted.  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 command 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
       executed asynchronously in a subshell, as if the command had been terminated with the
       & control operator, with a two-way pipe established between the executing  shell  and
       the coprocess.

       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); other‐
       wise,  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  executing shell.  The standard output of command is connected via a
       pipe to a file descriptor in the executing shell, and that  file  descriptor  is  as‐
       signed  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 redirections specified by the command (see REDI‐
       RECTION below).  The file descriptors can be utilized as arguments to shell  commands
       and redirections using standard word expansions.  Other than those created to execute
       command and process substitutions, the file descriptors are  not  available  in  sub‐
       shells.  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 de‐
       clared as follows:

       fname () compound-command [redirection]
       function fname [()] compound-command [redirection]
              This defines a function named fname.  The reserved word function is  optional.
              If  the function reserved word is supplied, the parentheses are optional.  The
              body of the function is the compound command  compound-command  (see  Compound
              Commands  above).  That command is usually a list of commands between { and },
              but may be any command listed under Compound Commands above, with  one  excep‐
              tion:  If the function reserved word is used, but the parentheses are not sup‐
              plied, the braces are required.  compound-command is executed  whenever  fname
              is  specified as the name of a simple command.  When in posix mode, fname must
              be a valid shell name and may not be the name of  one  of  the  POSIX  special
              builtins.   In  default  mode,  a function name can be any unquoted shell word
              that does not contain $.  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  executed  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 be‐
       ginning  with  # causes that word and all remaining characters on that line to be ig‐
       nored.  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 expan‐
       sion.

       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 continuation (that is, it is removed from the input stream and  effectively  ig‐
       nored).

       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  en‐
       abled,  !.  When the shell is in posix mode, the ! has no special meaning within dou‐
       ble quotes, even when history expansion is enabled.  The characters $  and  `  retain
       their  special meaning within double quotes.  The backslash retains its special mean‐
       ing 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 PARAM‐
       ETERS below).

       Words of the form $'string' are treated specially.  The word expands to string,  with
       backslash-escaped characters replaced as specified by the ANSI C standard.  Backslash
       escape sequences, if present, are decoded as follows:
              \a     alert (bell)
              \b     backspace
              \e
              \E     an escape character
              \f     form feed
              \n     new line
              \r     carriage return
              \t     horizontal tab
              \v     vertical tab
              \\     backslash
              \'     single quote
              \"     double quote
              \?     question mark
              \nnn   the eight-bit character whose value is the  octal  value  nnn  (one  to
                     three 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)
              \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.  The gettext infrastructure performs
       the message catalog lookup and translation,  using  the  LC_MESSAGES  and  TEXTDOMAIN
       shell  variables.   If  the current locale is C or POSIX, or if there are no transla‐
       tions available, the dollar sign is ignored.  If the string  is  translated  and  re‐
       placed, 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 param‐
       eter  denoted  by  a  name.  A variable has a value and zero or more attributes.  At‐
       tributes 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  com‐
       mand (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
       $((...))  expansion  is not used (see Arithmetic Expansion below).  Word splitting is
       not performed, with the exception of "$@" as explained below  under  Special  Parame‐
       ters.  Pathname expansion is not performed.  Assignment statements may also appear as
       arguments to the alias, declare, typeset, export, readonly, and  local  builtin  com‐
       mands  (declaration  commands).   When  in posix mode, these builtins may appear in a
       command after one or more instances of the command builtin and retain  these  assign‐
       ment statement properties.

       In the context where an assignment statement is assigning a value to a shell variable
       or array index, the += operator can be used to append to or  add  to  the  variable's
       previous value.  This includes arguments to builtin commands such as declare that ac‐
       cept assignment statements (declaration commands).  When += is applied to a  variable
       for which the integer attribute has been set, value is evaluated as an arithmetic ex‐
       pression and added to the variable's current value, which is also evaluated.  When +=
       is  applied  to  an  array variable using compound assignment (see Arrays below), the
       variable's value is not unset (as it is when using =), and new values are appended to
       the  array  beginning  at one greater than the array's maximum index (for indexed ar‐
       rays) 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 manipu‐
       lated indirectly.  Whenever the nameref variable is referenced, assigned  to,  unset,
       or  has  its  attributes modified (other than using or changing the nameref attribute
       itself), 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, and changes to  its
       attributes,  are  treated  as references, assignments, and attribute modifications to
       the variable whose name was passed as $1.  If the control variable in a for loop  has
       the nameref attribute, the list of words can be a list of shell variables, and a name
       reference will be established for each word in the list, in turn, when  the  loop  is
       executed.   Array  variables cannot be given the nameref attribute.  However, nameref
       variables can reference array variables and subscripted  array  variables.   Namerefs
       can  be  unset using the -n option to the unset builtin.  Otherwise, if unset is exe‐
       cuted 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 pa‐
       rameters may not be assigned to with assignment statements.  The  positional  parame‐
       ters  are  temporarily  replaced when a shell function is executed (see FUNCTIONS be‐
       low).

       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 refer‐
       enced; 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 dou‐
              ble quotes, it expands to a single word with the value of each parameter sepa‐
              rated  by  the  first character of the IFS special variable.  That is, "$*" is
              equivalent to "$1c$2c...", where c is the first character of the value of  the
              IFS  variable.   If  IFS is unset, the parameters are separated by spaces.  If
              IFS is null, the parameters are joined without intervening separators.
       @      Expands to the positional parameters, starting from one.   In  contexts  where
              word splitting is performed, this expands each positional parameter to a sepa‐
              rate word; if not within double quotes, these words are subject to word split‐
              ting.   In  contexts  where word splitting is not performed, this expands to a
              single word with each positional parameter separated by a space.  When the ex‐
              pansion  occurs  within  double  quotes,  each parameter expands to a separate
              word.  That is, "$@" is equivalent to "$1" "$2" ...  If the double-quoted  ex‐
              pansion  occurs  within a word, the expansion of the first parameter is joined
              with the beginning part of the original word, and the expansion  of  the  last
              parameter  is  joined with the last part of the original word.  When there are
              no positional parameters, "$@" and $@ expand to nothing (i.e.,  they  are  re‐
              moved).
       #      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
              CONTROL below).
       0      Expands  to  the name of the shell or shell script.  This is set at shell ini‐
              tialization.  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.  Other‐
              wise,  it  is  set  to  the filename used to invoke bash, as given by argument
              zero.

   Shell Variables
       The following variables are set by the shell:

       _      At shell startup, set to the pathname used to invoke the shell or shell script
              being  executed  as passed in the environment or argument list.  Subsequently,
              expands to the last argument to the previous simple command  executed  in  the
              foreground,  after  expansion.   Also  set to the full pathname used to invoke
              each command executed and placed in the environment exported to that  command.
              When  checking  mail, this parameter holds the name of the mail file currently
              being checked.
       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-initialized.   Assignments to BASHPID have no effect.  If BASHPID is unset,
              it loses its special properties, even if it is subsequently reset.
       BASH_ALIASES
              An associative array variable whose members correspond to the internal list of
              aliases  as maintained by the alias builtin.  Elements added to this array ap‐
              pear in the alias list; however, unsetting array elements currently  does  not
              cause aliases to be removed from the alias list.  If BASH_ALIASES is unset, it
              loses its special properties, even if it is subsequently reset.
       BASH_ARGC
              An array variable whose values are the number of parameters in each  frame  of
              the  current  bash execution call stack.  The number of parameters to the cur‐
              rent subroutine (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  ex‐
              tended debugging mode (see the description of the extdebug option to the shopt
              builtin below).  Setting extdebug after the shell has  started  to  execute  a
              script,  or  referencing this variable when extdebug is not set, may result in
              inconsistent values.
       BASH_ARGV
              An array variable containing all of the parameters in the current bash  execu‐
              tion  call  stack.   The final parameter of the last subroutine call is at the
              top of the stack; the first parameter of the initial call is  at  the  bottom.
              When  a  subroutine  is  executed,  the  parameters  supplied  are pushed onto
              BASH_ARGV.  The shell sets BASH_ARGV only when in extended debugging mode (see
              the  description  of the extdebug option to the shopt builtin below).  Setting
              extdebug after the shell has started to execute a script, or referencing  this
              variable when extdebug is not set, may result in inconsistent values.
       BASH_ARGV0
              When  referenced,  this  variable  expands  to  the name of the shell or shell
              script (identical to $0; see the description of special  parameter  0  above).
              Assignment  to BASH_ARGV0 causes the value assigned to also be assigned to $0.
              If BASH_ARGV0 is unset, it loses its special properties, even if it is  subse‐
              quently reset.
       BASH_CMDS
              An  associative  array  variable whose members correspond to the internal hash
              table of commands as maintained by the hash builtin.  Elements added  to  this
              array  appear  in  the hash table; however, unsetting array elements currently
              does not cause command names to be removed from the hash table.  If  BASH_CMDS
              is unset, it loses its special properties, even if it is subsequently reset.
       BASH_COMMAND
              The command currently being executed or about to be executed, unless the shell
              is executing a command as the result of a trap, in which case it is  the  com‐
              mand  executing  at  the time of the trap.  If BASH_COMMAND is unset, it loses
              its special properties, even if it is subsequently reset.
       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 number in the source file  (${BASH_SOURCE[$i+1]})  where  ${FUNCNAME[$i]}
              was  called  (or ${BASH_LINENO[$i-1]} if referenced within another shell func‐
              tion).  Use LINENO to obtain the current line number.
       BASH_LOADABLES_PATH
              A colon-separated list of directories in which the shell looks for dynamically
              loadable builtins specified by the enable command.
       BASH_REMATCH
              An  array variable whose members are assigned by the =~ binary operator to the
              [[ conditional command.  The element with index 0 is the portion of the string
              matching  the entire regular expression.  The element with index n is the por‐
              tion of the string matching the nth parenthesized subexpression.
       BASH_SOURCE
              An array variable whose members are the source filenames where the correspond‐
              ing  shell  function  names  in  the FUNCNAME array variable are defined.  The
              shell function ${FUNCNAME[$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.   If  BASH_SUB‐
              SHELL  is  unset,  it loses its special properties, even if it is subsequently
              reset.
       BASH_VERSINFO
              A readonly array variable whose members hold version information for this  in‐
              stance 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 posi‐
              tion.  This variable is available only in shell functions invoked by the  pro‐
              grammable completion 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
              Programmable Completion below).
       COMP_POINT
              The index of the current cursor position relative to the beginning of the cur‐
              rent  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 pro‐
              grammable 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
              listing completions after successive tabs, !, for listing alternatives on par‐
              tial 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  commands invoked by the programmable completion facilities (see
              Programmable Completion below).
       COMP_WORDBREAKS
              The set of characters that the readline library treats as word separators when
              performing word completion.  If COMP_WORDBREAKS is unset, it loses its special
              properties, even if it is subsequently reset.
       COMP_WORDS
              An array variable (see Arrays below) consisting of the individual words in the
              current  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 functions invoked by the programmable completion facilities (see Pro‐
              grammable Completion below).
       COPROC An array variable (see Arrays below) created to hold the file descriptors  for
              output from and input to an unnamed coprocess (see Coprocesses above).
       DIRSTACK
              An  array  variable  (see Arrays below) containing the current contents of the
              directory stack.  Directories appear in the stack in the order they  are  dis‐
              played  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 vari‐
              able will not change the current directory.  If DIRSTACK is  unset,  it  loses
              its special properties, even if it is subsequently reset.
       EPOCHREALTIME
              Each  time  this  parameter is referenced, it expands to the number of seconds
              since the Unix Epoch (see time(3)) as a floating point value with micro-second
              granularity.   Assignments  to EPOCHREALTIME are ignored.  If EPOCHREALTIME is
              unset, it loses its special properties, even if it is subsequently reset.
       EPOCHSECONDS
              Each time this parameter is referenced, it expands to the  number  of  seconds
              since  the Unix Epoch (see time(3)).  Assignments to EPOCHSECONDS are ignored.
              If EPOCHSECONDS is unset, it loses its special properties, even if it is  sub‐
              sequently 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
              execution  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 execut‐
              ing.  Assignments to FUNCNAME have no effect.  If FUNCNAME is unset, it  loses
              its special properties, even if it is subsequently reset.

              This  variable  can be used with BASH_LINENO and BASH_SOURCE.  Each element of
              FUNCNAME 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
              member.  Assignments to GROUPS have no effect.  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.  As‐
              signments to HISTCMD are ignored.  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
              representing  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 guaranteed 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 map‐
              file 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, it expands to a random integer between
              0  and 32767.  Assigning a value to RANDOM initializes (seeds) the sequence of
              random numbers.  If RANDOM is unset, it loses its special properties, even  if
              it is subsequently reset.
       READLINE_LINE
              The  contents  of  the readline line buffer, for use with "bind -x" (see SHELL
              BUILTIN COMMANDS below).
       READLINE_MARK
              The position of the mark (saved insertion point) in the readline line  buffer,
              for use with "bind -x" (see SHELL BUILTIN COMMANDS below).  The characters be‐
              tween the insertion point and the mark are often called the region.
       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  in‐
              vocation  is  returned.  If a value is assigned to SECONDS, the value returned
              upon subsequent references is the number of seconds since the assignment  plus
              the value assigned.  The number of seconds at shell invocation and the current
              time is always determined by querying the system clock.  If SECONDS is  unset,
              it loses its special properties, even if it is subsequently 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.
       SRANDOM
              This variable expands to a 32-bit pseudo-random number each time it is  refer‐
              enced.  The  random  number  generator  is  not linear on systems that support
              /dev/urandom or arc4random, so each returned number has no relationship to the
              numbers  preceding  it.   The random number generator cannot be seeded, so as‐
              signments to this variable have no effect.  If SRANDOM is unset, it loses  its
              special properties, even if it is subsequently reset.
       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 SHELL COMPATI‐
              BILITY MODE below for a description of the various  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 de‐
              fault 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 message and
              sets the compatibility level to the default  for  the  current  version.   The
              valid  values  correspond  to  the  compatibility levels described below under
              BSHELLCOMPATIBILITYMODE.  For example, 4.2 and 42 are valid values that corre‐
              spond to the compat42 shopt option and set the compatibility level to 42.  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
              interpreted  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.   Unsetting  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  direc‐
              tories  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 mini‐
              mum value is system-dependent.
       COLUMNS
              Used  by  the  select  compound  command  to determine the terminal width when
              printing selection lists.  Automatically set if the checkwinsize option is en‐
              abled 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 Program‐
              mable 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    Expanded and executed similarly to BASH_ENV (see INVOCATION above) when an in‐
              teractive shell is invoked in posix mode.
       EXECIGNORE
              A colon-separated list of shell patterns (see Pattern Matching)  defining  the
              list  of  filenames  to  be ignored by command search using PATH.  Files whose
              full pathnames match one of these patterns are not considered executable files
              for  the  purposes  of completion and command execution via PATH lookup.  This
              does not affect the behavior of the [, test, and [[ commands.  Full  pathnames
              in the command hash table are not subject to EXECIGNORE.  Use this variable to
              ignore shared library files that have the executable bit set, but are not exe‐
              cutable  files.   The pattern matching honors the setting of the extglob shell
              option.
       FCEDIT The default editor for the fc builtin command.
       FIGNORE
              A colon-separated list of suffixes to ignore when performing filename  comple‐
              tion (see READLINE below).  A filename whose suffix matches one of the entries
              in FIGNORE is excluded from the list of matched filenames.  A sample value  is
              ".o:~".
       FUNCNEST
              If  set  to a numeric value greater than 0, defines a maximum function nesting
              level.  Function invocations that exceed this nesting  level  will  cause  the
              current command to abort.
       GLOBIGNORE
              A  colon-separated  list  of patterns defining the set of file names to be ig‐
              nored by pathname expansion.  If a file name matched by a  pathname  expansion
              pattern also matches one of the patterns in GLOBIGNORE, it is removed from the
              list of matches.
       HISTCONTROL
              A colon-separated list of values controlling how commands  are  saved  on  the
              history  list.   If the list of values includes ignorespace, lines which begin
              with a space character are not saved in the history  list.   A  value  of  ig‐
              noredups  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 subsequent lines of a multi-line com‐
              pound 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 val‐
              ues 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 compound command are not tested, and are  added  to  the
              history  regardless  of  the value of HISTIGNORE.  The pattern matching honors
              the setting of the extglob shell option.
       HISTSIZE
              The number of commands to remember in the command history (see HISTORY below).
              If the value is 0, commands are not saved in the history list.  Numeric values
              less than zero result in every command being saved on the history list  (there
              is  no  limit).   The  shell  sets  the default value to 500 after reading any
              startup files.
       HISTTIMEFORMAT
              If this variable is set and not null, its value is used as a format string for
              strftime(3)  to  print  the time stamp associated with each history entry dis‐
              played by the history builtin.  If this variable is set, time stamps are writ‐
              ten  to the history file so they may be preserved across shell sessions.  This
              uses the history comment character to distinguish timestamps from  other  his‐
              tory 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  host‐
              name  completions  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 readable 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  ex‐
              its.   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 ~/.inpu‐
              trc (see READLINE below).
       INSIDE_EMACS
              If this variable appears in the environment when the shell  starts,  bash  as‐
              sumes  that  it  is  running inside an Emacs shell buffer and may disable line
              editing, depending on the value of TERM.
       LANG   Used to determine the locale category for any category  not  specifically  se‐
              lected with a variable starting with LC_.
       LC_ALL This  variable overrides the value of LANG and any other LC_ variable specify‐
              ing a locale category.
       LC_COLLATE
              This variable determines the collation order used when sorting the results  of
              pathname  expansion, and determines the behavior of range expressions, equiva‐
              lence classes, and collating sequences within pathname expansion  and  pattern
              matching.
       LC_CTYPE
              This  variable determines the interpretation of characters and the behavior of
              character classes within pathname expansion and pattern matching.
       LC_MESSAGES
              This variable determines the locale used to  translate  double-quoted  strings
              preceded by a $.
       LC_NUMERIC
              This variable determines the locale category used for number formatting.
       LC_TIME
              This  variable  determines  the locale category used for data and time format‐
              ting.
       LINES  Used by the select compound command to determine the column length for  print‐
              ing  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 sec‐
              onds.   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 mes‐
              sage, $_ expands to the name of the current mailfile.  Example:
              MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
              Bash can be configured to supply a default value for this variable  (there  is
              no  value by default), but the location of the user mail files that it uses is
              system dependent (e.g., /var/mail/$USER).
       OPTERR If set to the value 1, bash displays error messages generated by  the  getopts
              builtin  command (see SHELL BUILTIN COMMANDS below).  OPTERR is initialized to
              1 each time the shell is invoked or a shell script is executed.
       PATH   The search path for commands.  It is a colon-separated list of directories  in
              which  the  shell  looks  for commands (see COMMAND EXECUTION below).  A zero-
              length (null) directory name in the value of PATH indicates the current direc‐
              tory.   A null directory name may appear as two adjacent colons, or as an ini‐
              tial or trailing colon.  The default path is system-dependent, and is  set  by
              the administrator who installs bash.  A common value is ``/usr/lo‐
              cal/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 op‐
              tion had been supplied.  If it is set while the shell is running, bash enables
              posix  mode, as if the command set -o posix had been executed.  When the shell
              enters posix mode, it sets this variable if it was not already set.
       PROMPT_COMMAND
              If this variable is set, and is an array, the value of each set element is ex‐
              ecuted  as a command prior to issuing each primary prompt.  If this is set but
              not an array variable, its value is used as a command to execute instead.
       PROMPT_DIRTRIM
              If set to a number greater than zero, the value  is  used  as  the  number  of
              trailing  directory  components  to retain when expanding the \w and \W prompt
              string escapes (see PROMPTING below).  Characters removed are replaced with an
              ellipsis.
       PS0    The value of this parameter is expanded (see PROMPTING below) and displayed by
              interactive shells after reading a command and before the command is executed.
       PS1    The value of this parameter is expanded (see PROMPTING below) and used as  the
              primary 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  char‐
              acter of the expanded value of PS4 is replicated multiple times, as necessary,
              to indicate multiple levels of indirection.  The default is ``+ ''.
       SHELL  This variable expands to the full pathname to the shell.  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 tim‐
              ing  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 frac‐
              tion to be output.  At most three places after the decimal point may be speci‐
              fied; 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  in‐
              formation is displayed.  A trailing newline is added when the format string is
              displayed.
       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 seconds if a complete line of input does not arrive.
       TMPDIR If  set,  bash uses its value as the name of a directory in which bash creates
              temporary 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 supplied 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  func‐
              tionality 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, nor‐
              mally `!'.  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  necessarily  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  ar‐
       ray.   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 in‐
       tegers  (including arithmetic 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  ac‐
       cepted; 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  ...  val‐
       uen),  where each value may be of the form [subscript]=string.  Indexed array assign‐
       ments do not require anything but string.  Each value in the list is  expanded  using
       all  the shell expansions described below under EXPANSION.  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 words in a compound assignment may be ei‐
       ther  assignment  statements, for which the subscript is required, or a list of words
       that is interpreted as a sequence of alternating keys and values: name=( key1  value1
       key2   value2   ...).    These   are  treated  identically  to  name=(  [key1]=value1
       [key2]=value2 ...).  The first word in the list determines how  the  remaining  words
       are  interpreted;  all  assignments  in  a list must be of the same type.  When using
       key/value pairs, the keys may not be missing or  empty;  a  final  missing  value  is
       treated like the empty string.

       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 assign‐
       ing  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  ap‐
       pears  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  dou‐
       ble-quoted  expansion  occurs  within a word, the expansion of the first parameter is
       joined with the beginning part of the original word, and the expansion  of  the  last
       parameter  is  joined  with the last part of the original word.  This is analogous to
       the expansion of the special parameters *  and  @  (see  Special  Parameters  above).
       ${#name[subscript]}  expands  to the length of ${name[subscript]}.  If subscript is *
       or @, the expansion is the number of elements in the array.  If the subscript used to
       reference  an element of an indexed array evaluates to a number less than zero, it is
       interpreted as relative to one greater than the maximum index of the array, so  nega‐
       tive  indices count back from the end of the array, and an index of -1 references the
       last element.

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

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

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

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

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

       The declare, local, and readonly builtins each accept a -a option to specify  an  in‐
       dexed  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 dis‐
       play 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 variable 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
       fashion); word splitting; and pathname expansion.

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

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

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

   Brace Expansion
       Brace  expansion  is  a  mechanism by which arbitrary strings may be generated.  This
       mechanism is similar to pathname expansion, but the filenames generated need not  ex‐
       ist.   Patterns  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,  followed  by  an  optional postscript.  The preamble is prefixed to each
       string contained within the braces, and the postscript is then appended to  each  re‐
       sulting 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  inte‐
       gers  or single characters, and incr, an optional increment, is an integer.  When in‐
       tegers are supplied, the expression expands to each number between x  and  y,  inclu‐
       sive.   Supplied  integers 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 gener‐
       ated  terms to contain the same number of digits, zero-padding where necessary.  When
       characters are supplied, the expression 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 pa‐
       rameter expansion, the string ${ is not considered eligible for brace expansion,  and
       inhibits brace expansion until the closing }.

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

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

       Brace expansion introduces a slight incompatibility with historical versions  of  sh.
       sh  does  not treat opening or closing braces specially when they appear as part of a
       word, and preserves them in the output.  Bash removes braces from words as  a  conse‐
       quence  of  brace  expansion.  For example, a word entered to sh as file{1,2} appears
       identically in the output.  The same word is output as file1 file2 after expansion by
       bash.   If  strict compatibility 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 COM‐
       MANDS below).

   Tilde Expansion
       If  a  word begins with an unquoted tilde character (`~'), all of the characters pre‐
       ceding 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  pos‐
       sible  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 re‐
       placed 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-prefix.  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.   Conse‐
       quently,  one may use filenames with tildes in assignments to PATH, MAILPATH, and CD‐
       PATH, and the shell assigns the expanded value.

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

   Parameter Expansion
       The `$' character introduces parameter expansion, command substitution, or arithmetic
       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
       backslash 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 fol‐
              lowed 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 ref‐
              erence (Arrays).

       If the first character of parameter is an exclamation point (!), and parameter is not
       a  nameref,  it introduces a level of indirection.  Bash uses the value formed by ex‐
       panding the rest of parameter as the new parameter; this is then  expanded  and  that
       value is used in the rest of the expansion, rather than the expansion of the original
       parameter.  This is known as indirect expansion.  The value is subject to  tilde  ex‐
       pansion, parameter expansion, command substitution, and arithmetic expansion.  If pa‐
       rameter is a nameref, this expands to the name of the parameter referenced by parame‐
       ter  instead  of  performing the complete 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,
       command 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
              substituted.  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.   Posi‐
              tional 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
              standard 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,
              otherwise the expansion of word is substituted.
       ${parameter:offset}
       ${parameter:offset:length}
              Substring  Expansion.   Expands to up to length characters of the value of pa‐
              rameter 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 ex‐
              tending 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  ex‐
              pansion  is  the characters between offset and that result.  Note that a nega‐
              tive 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 pa‐
              rameter.  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 speci‐
              fied array.  It is an expansion error if length evaluates  to  a  number  less
              than zero.

              Substring  expansion  applied  to  an associative array produces undefined re‐
              sults.

              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 posi‐
              tional 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 ar‐
              ray 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  sub‐
              stituted.   If parameter is * or @, the value substituted is the number of po‐
              sitional parameters.  If parameter is an array name subscripted by * or @, the
              value  substituted is the number of elements in the array.  If parameter is an
              indexed array name subscripted by a negative number,  that  number  is  inter‐
              preted as relative to one greater than the maximum index of parameter, so neg‐
              ative indices count back from the end of the array, and an index of -1  refer‐
              ences the last element.

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

       ${parameter/pattern/string}
              Pattern substitution.  The pattern is expanded to produce a pattern just as in
              pathname expansion, Parameter is expanded and the  longest  match  of  pattern
              against  its  value is replaced with string.  The match is performed using the
              rules described under Pattern Matching below.  If pattern begins with  /,  all
              matches of pattern are replaced with string.  Normally only the first match is
              replaced.  If pattern begins with #, it must match at the beginning of the ex‐
              panded value of parameter.  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 the nocasematch shell
              option is enabled, the match is performed without regard to the case of alpha‐
              betic  characters.   If parameter is @ or *, the substitution operation is ap‐
              plied to each positional parameter in turn, and the expansion is the resultant
              list.   If parameter is an array variable subscripted with @ or *, the substi‐
              tution operation is applied to each member of the array in turn, and  the  ex‐
              pansion 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  path‐
              name  expansion.   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 letters matching pattern to uppercase; the , operator  con‐
              verts  matching uppercase letters to lowercase.  The ^^ and ,, expansions con‐
              vert 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 parame‐
              ter  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 modification operation is
              applied to each member of the array in turn, and the expansion is  the  resul‐
              tant list.

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

              U      The expansion is a string that is the value of parameter with lowercase
                     alphabetic characters converted to uppercase.
              u      The expansion is a string that is the value of parameter with the first
                     character converted to uppercase, if it is alphabetic.
              L      The expansion is a string that is the value of parameter with uppercase
                     alphabetic characters converted to lowercase.
              Q      The expansion is a string that is the value of parameter  quoted  in  a
                     format that can be reused as input.
              E      The expansion is a string that is the value of parameter with backslash
                     escape sequences expanded as with the $'...' quoting mechanism.
              P      The expansion is a string that is the result of expanding the value  of
                     parameter as if it were a prompt string (see PROMPTING below).
              A      The expansion is a string in the form of an assignment statement or de‐
                     clare command that, if evaluated, will recreate parameter with its  at‐
                     tributes and value.
              K      Produces  a  possibly-quoted  version of the value of parameter, except
                     that it prints the values of indexed and associative arrays  as  a  se‐
                     quence of quoted key-value pairs (see Arrays above).
              a      The expansion is a string consisting of flag values representing param‐
                     eter's attributes.

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

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

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

              $(command)
       or
              `command`

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

       When the old-style backquote form of substitution is used, backslash retains its lit‐
       eral meaning except when followed by $, `, or \.  The first backquote not preceded by
       a backslash 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 expan‐
       sion are not performed on the results.

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

              $((expression))

       The  expression is treated as if it were within double quotes, but a double quote in‐
       side the parentheses is not treated specially.  All tokens in the expression  undergo
       parameter  and  variable expansion, command substitution, and quote removal.  The re‐
       sult 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
       EVALUATION.  If expression is invalid, bash prints a message indicating  failure  and
       no substitution occurs.

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

       When available, process substitution is performed simultaneously with  parameter  and
       variable expansion, command substitution, and arithmetic expansion.

   Word Splitting
       The  shell scans the results of parameter expansion, command substitution, and arith‐
       metic 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 se‐
       quences of the whitespace characters space, tab, and newline are ignored at  the  be‐
       ginning  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 and passed to commands as empty
       strings.  Unquoted implicit null arguments, resulting from the expansion  of  parame‐
       ters  that  have  no  values,  are removed.  If a parameter with no value is expanded
       within double quotes, a null argument results and is retained and passed to a command
       as  an empty string.  When a quoted null argument appears as part of a word whose ex‐
       pansion is non-null, the null argument is removed.  That is, the word -d'' becomes -d
       after word splitting and null argument removal.

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

   Pathname Expansion
       After word splitting, unless the -f option has been set, bash scans each word for the
       characters *, ?, and [.  If one of these characters appears, and is not quoted,  then
       the word is regarded as a pattern, and replaced with an alphabetically sorted list of
       filenames matching the pattern (see Pattern Matching below).  If  no  matching  file‐
       names  are  found, and the shell option nullglob is not enabled, the word is left un‐
       changed.  If the nullglob option is set, and no matches are found, the  word  is  re‐
       moved.   If the failglob shell option is set, and no matches are found, an error mes‐
       sage 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.
       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.  The filenames ``.''  and ``..''  must always be  matched  ex‐
       plicitly,  even  if  dotglob  is  set.   In  other cases, the ``.''  character is not
       treated specially.  When matching a pathname, the  slash  character  must  always  be
       matched  explicitly  by a slash in the pattern, but in other matching contexts it can
       be matched by a special pattern character as described below under Pattern  Matching.
       See  the description of shopt below under SHELL BUILTIN COMMANDS for a description of
       the nocaseglob, nullglob, failglob, and dotglob shell options.

       The GLOBIGNORE shell variable may be used to restrict the set of file names  matching
       a  pattern.   If  GLOBIGNORE is set, each matching file name that also matches one of
       the patterns in GLOBIGNORE is removed from the list of matches.   If  the  nocaseglob
       option  is  set, the matching against the patterns in GLOBIGNORE is performed without
       regard to case.  The filenames ``.''  and ``..''  are always ignored when  GLOBIGNORE
       is  set and not null.  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 beginning with a ``.'',
       make ``.*''  one of the patterns in GLOBIGNORE.  The dotglob option is disabled  when
       GLOBIGNORE  is  unset.   The pattern matching honors the setting of the extglob shell
       option.

       Pattern Matching

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

       The special pattern characters have the following meanings:

              *      Matches any string, including the null string.  When the globstar shell
                     option  is  enabled, and * is used in a pathname expansion context, two
                     adjacent *s used as a single pattern will match all files and  zero  or
                     more  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 sepa‐
                     rated  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 charac‐
                     ter following the [ is a !  or a ^ then any character not  enclosed  is
                     matched.   The  sorting order of characters in range expressions is de‐
                     termined by the current locale and the  values  of  the  LC_COLLATE  or
                     LC_ALL  shell variables, if set.  To obtain the traditional interpreta‐
                     tion 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 sym‐
                     bol.

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

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

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

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

REDIRECTION
       Before a command is executed, its input and output may be redirected using a  special
       notation  interpreted  by the shell.  Redirection allows commands' file handles to be
       duplicated, opened, closed, made to refer to different  files,  and  can  change  the
       files  the  command 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 com‐
       mand.  Redirections are processed in the order they appear, from left to right.

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

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

       The  word  following  the  redirection operator in the following descriptions, unless
       otherwise noted, is subjected to brace  expansion,  tilde  expansion,  parameter  and
       variable  expansion, command substitution, arithmetic expansion, quote removal, path‐
       name expansion, and word splitting.  If it expands to more than one  word,  bash  re‐
       ports 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 com‐
       mand

              ls 2>&1 > dirlist

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

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

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

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

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

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

       The general format for redirecting input is:

              [n]<word

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

       The general format for redirecting output is:

              [n]>word

       If  the  redirection  operator  is >, and the noclobber option to the set builtin has
       been enabled, the redirection will fail if the file whose name results from  the  ex‐
       pansion  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 redirection 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 ex‐
       pansion of word to be opened for appending on file descriptor n, or the standard out‐
       put (file descriptor 1) if n is not specified.  If the file does not exist it is cre‐
       ated.

       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 ex‐
       pansion 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
       redirection  operators apply (see Duplicating File Descriptors below) for compatibil‐
       ity reasons.

   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 expan‐
       sion of word.

       The format for appending standard output and standard error is:

              &>>word

       This is semantically equivalent to

              >>word 2>&1

       (see Duplicating File Descriptors below).

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

       The format of here-documents is:

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

       No  parameter  and variable expansion, command substitution, arithmetic expansion, or
       pathname expansion is performed on word.  If any part of word is quoted,  the  delim‐
       iter  is  the result of quote removal on word, and the lines in the here-document are
       not expanded.  If word is unquoted, all lines of the here-document are  subjected  to
       parameter  expansion,  command  substitution, and arithmetic expansion, the character
       sequence \<newline> is 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:

              [n]<<<word

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

   Duplicating File Descriptors
       The redirection operator

              [n]<&word

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

       The operator

              [n]>&word

       is  used  similarly to duplicate output file descriptors.  If n is not specified, the
       standard 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 redi‐
       rected 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 de‐
       scriptor 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
       command.

       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, and all lines that make up a com‐
       pound command, before executing any of the commands on that line or the compound com‐
       mand.   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 defi‐
       nition on that line are not affected by the new alias.  This behavior is also an  is‐
       sue  when functions are executed.  Aliases are expanded when a function definition is
       read, not when the function is executed, because a function definition  is  itself  a
       command.  As a consequence, aliases defined in a function are not available until af‐
       ter that function is executed.  To be safe, always put alias definitions on  a  sepa‐
       rate 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
       commands 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 func‐
       tion is executed, the arguments to the function become the positional parameters dur‐
       ing  its  execution.  The special parameter # is updated to reflect the change.  Spe‐
       cial parameter 0 is unchanged.  The first element of the FUNCNAME variable is set  to
       the name of the function while the function 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 functions 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.  Or‐
       dinarily, variables and their values are shared between the function and its  caller.
       If  a  variable is declared local, the variable's visible scope is restricted to that
       function and its children  (including  the  functions  it  calls).   Local  variables
       "shadow"  variables  with the same name declared at previous scopes.  For instance, a
       local variable declared in a function hides a global variable of the same name:  ref‐
       erences  and assignments refer to the local variable, leaving the global variable un‐
       modified.  When the function returns, the global variable is once again visible.

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

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

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

       The  FUNCNEST  variable,  if set to a numeric value greater than 0, defines a maximum
       function 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
       execution resumes with the next command after the function call.  Any command associ‐
       ated with the RETURN trap is executed before execution resumes.  When a function com‐
       pletes, the values of the positional parameters and the special parameter #  are  re‐
       stored 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 op‐
       tion is enabled).  Functions may be exported so  that  subshells  automatically  have
       them  defined with the -f option to the export builtin.  A function definition may be
       deleted using the -f option to the unset builtin.

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

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

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

       Shell variables are allowed as operands; parameter expansion is performed before  the
       expression  is  evaluated.   Within an expression, shell variables may also be refer‐
       enced 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 ex‐
       pansion 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.

       Integer  constants  follow  the  C language definition, without suffixes or character
       constants.  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 decimal 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, if a non-digit is required, 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
       evaluated 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 compar‐
       isons.  The test and [ commands determine their behavior based on the number of argu‐
       ments; see the descriptions of those commands for any other command-specific actions.

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

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

       When used with [[, the < and > operators sort lexicographically using the current lo‐
       cale.  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 opera‐
              tors return true if arg1 is equal to, not equal to, less than,  less  than  or
              equal  to, greater than, or greater than or equal to arg2, respectively.  Arg1
              and arg2 may be positive or negative integers.  When used with the [[ command,
              Arg1  and Arg2 are evaluated as arithmetic expressions (see ARITHMETIC EVALUA‐
              TION above).

SIMPLE COMMAND EXPANSION
       When a simple command is executed, the shell performs the following  expansions,  as‐
       signments, and redirections, from left to right, in the following order.

       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,
              parameter expansion, command substitution, arithmetic expansion, and quote re‐
              moval before being assigned to the variable.

       If no command name results, the variable assignments affect the current  shell  envi‐
       ronment.   Otherwise, the variables are added to the environment of the executed com‐
       mand and do not affect the current shell environment.  If any of the assignments  at‐
       tempts to assign a value to a readonly variable, an error occurs, and the command ex‐
       its with a non-zero status.

       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  be‐
       low.   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  performed.   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 performed 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 com‐
       mand_not_found_handle.  If that function exists, it is invoked in a  separate  execu‐
       tion  environment  with  the original command and the original command's arguments as
       its arguments, and the function's exit status becomes the exit status  of  that  sub‐
       shell.   If  that  function is not defined, the shell prints an error message and re‐
       turns 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  ar‐
       guments 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  com‐
       mands.   A subshell 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 locations of commands remembered by the parent (see hash below un‐
       der 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 op‐
       erating systems that do not handle this executable format themselves.  The  arguments
       to  the  interpreter  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 arguments, 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
              supplied 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 par‐
              ent

       •      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  argu‐
              ments) 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
       otherwise noted, the values are inherited from the shell.

       •      the shell's open files, plus any  modifications  and  additions  specified  by
              redirections 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
              parent, and traps ignored by the shell are ignored

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

       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  inher‐
       ited  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, automat‐
       ically marking it for export to child processes.  Executed commands inherit the envi‐
       ronment.   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  envi‐
       ronment  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  ini‐
       tial  environment,  whose values may be modified 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
       prefixing it with parameter assignments, as described above in PARAMETERS.  These as‐
       signment statements affect only the environment seen by that command.

       If the -k option is set (see the set builtin command below), then all  parameter  as‐
       signments  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  ex‐
       plained  below,  the  shell  may  use values above 125 specially.  Exit statuses from
       shell builtins and compound 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 suc‐
       ceeded.  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 sta‐
       tus is greater than zero.

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

       Bash itself returns the exit status of the last command executed, unless a syntax er‐
       ror  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 con‐
       trol 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 SIG‐
       CONT  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 dis‐
       own -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 sig‐
       nal 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 pro‐
       cesses and continue (resume) their execution at a later point.  A user typically  em‐
       ploys  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 execut‐
       ing  jobs,  which  may be listed with the jobs command.  When bash starts a job asyn‐
       chronously (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
       system 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 terminal's; such processes are immune to keyboard-generated  sig‐
       nals.   Only  foreground processes are allowed to read from or, if the user so speci‐
       fies with stty tostop, write to the terminal.  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 re‐
       turned 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 fore‐
       ground, 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 % intro‐
       duces 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 sub‐
       string that appears in its command line.  For example, %ce refers to  a  stopped  job
       whose  command  name begins with ce.  If a prefix matches more than one job, bash re‐
       ports 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 re‐
       ports an error.  The symbols %% and %+ refer to the shell's  notion  of  the  current
       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 current 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  un‐
       til it is about to print a prompt before reporting changes in a job's status so as to
       not interrupt any other output.  If the -b option to the set builtin command  is  en‐
       abled,  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
       message, 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  warn‐
       ing, and any stopped jobs are terminated.

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

PROMPTING
       When  executing  interactively, bash displays the primary prompt PS1 when it is ready
       to read a command, and the secondary prompt PS2 when it needs more input to  complete
       a command.  Bash displays PS0 after it reads a command but before executing it.  Bash
       displays PS4 as described above before tracing each command when the -x option is en‐
       abled.   Bash  allows  these prompt strings to be customized by inserting a number of
       backslash-escaped special characters that are decoded as follows:
              \a     an ASCII bell character (07)
              \d     the date in "Weekday Month Date" format (e.g., "Tue May 26")
              \D{format}
                     the format is passed to strftime(3) and the result is inserted into the
                     prompt string; an empty format results in a locale-specific time repre‐
                     sentation.  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
              \[     begin a sequence of non-printing characters, which could be used to em‐
                     bed a terminal control sequence into the prompt
              \]     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).
       This  can  have unwanted side effects if escaped portions of the string appear within
       command substitution or contain characters special to word expansion.

READLINE
       This is the library that handles reading input when using an interactive  shell,  un‐
       less  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 combina‐
       tion 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  negative  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 behav‐
       ior with arguments deviates 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.  If that file  does not  exist  or
       cannot  be read, the ultimate default is /etc/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  com‐
       ments.  Lines beginning with a $ indicate 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, NEW‐
       LINE, 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 sym‐
       bolic 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  es‐
       capes 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 us‐
       ing 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  vari‐
       able may be set in the inputrc file with a statement of the form

              set variable-name value
       or using the bind builtin command (see SHELL BUILTIN COMMANDS below).

       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 terminal's bell.
       bind-tty-special-chars (On)
              If  set  to  On, readline attempts to bind the control characters treated spe‐
              cially by the kernel's terminal driver to their readline equivalents.
       blink-matching-paren (Off)
              If set to On, readline attempts to briefly  move  the  cursor  to  an  opening
              parenthesis when a closing parenthesis is inserted.
       colored-completion-prefix (Off)
              If set to On, when listing completions, readline displays the common prefix of
              the set of possible completions using a different color.   The  color  defini‐
              tions are taken from the value of the LS_COLORS environment variable.
       colored-stats (Off)
              If set to On, readline displays possible completions using different colors to
              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 exe‐
              cuted.  This command is bound to M-# in emacs mode and  to  #  in  vi  command
              mode.
       completion-display-width (-1)
              The  number of screen columns used to display possible matches when performing
              completion.  The value is ignored if it is less than 0  or  greater  than  the
              terminal  screen  width.   A value of 0 will cause matches to be displayed one
              per line.  The default value is -1.
       completion-ignore-case (Off)
              If set to On, readline performs filename matching and completion in a case-in‐
              sensitive fashion.
       completion-map-case (Off)
              If  set  to On, and completion-ignore-case is enabled, readline treats hyphens
              (-) and underscores (_) as equivalent when performing  case-insensitive  file‐
              name matching and completion.
       completion-prefix-display-length (0)
              The  length  in  characters of the common prefix of a list of possible comple‐
              tions that is displayed without modification.  When set  to  a  value  greater
              than  zero, common prefixes longer than this value are replaced with an ellip‐
              sis when displaying possible completions.
       completion-query-items (100)
              This determines when the user is queried about viewing the number of  possible
              completions  generated  by the possible-completions command.  It may be set to
              any integer value greater than or equal to zero.  If the  number  of  possible
              completions  is  greater than or equal to the value of this variable, readline
              will ask whether or not the user wishes to view them; otherwise they are  sim‐
              ply 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 charac‐
              ter  (in  effect,  using  escape  as the meta prefix).  The default is On, but
              readline will set it to Off if the locale contains eight-bit characters.
       disable-completion (Off)
              If set to On, readline will inhibit word  completion.   Completion  characters
              will be inserted into the line as if they had been mapped to self-insert.
       echo-control-characters (On)
              When  set  to On, on operating systems that indicate they support it, readline
              echoes a character corresponding to a signal generated from the keyboard.
       editing-mode (emacs)
              Controls whether readline begins with a set of key bindings similar  to  Emacs
              or vi.  editing-mode can be set to either emacs or vi.
       emacs-mode-string (@)
              If the show-mode-in-prompt variable is enabled, this string is displayed imme‐
              diately before the last line of the primary prompt when emacs editing mode  is
              active.   The  value  is  expanded  like a key binding, so the standard set of
              meta- and control prefixes and backslash escape sequences is  available.   Use
              the  \1  and \2 escapes to begin and end sequences of non-printing characters,
              which can be used to embed a terminal control sequence into the mode string.
       enable-bracketed-paste (On)
              When set to On, readline will configure the terminal in a way that will enable
              it  to insert each paste into the editing buffer as a single string of charac‐
              ters, instead of treating each character as if it had been read from the  key‐
              board.   This  can prevent pasted characters from being interpreted as editing
              commands.
       enable-keypad (Off)
              When set to On, readline will try to enable the application keypad when it  is
              called.  Some systems need this to enable the arrow keys.
       enable-meta-key (On)
              When set to On, readline will try to enable any meta modifier key the terminal
              claims to support when it is called.  On many terminals, the meta key is  used
              to send eight-bit characters.
       expand-tilde (Off)
              If set to On, tilde expansion is performed when readline attempts word comple‐
              tion.
       history-preserve-point (Off)
              If set to On, the history code attempts to place point at the same location on
              each history line retrieved with previous-history or next-history.
       history-size (unset)
              Set  the  maximum number of history entries saved in the history list.  If set
              to zero, any existing history entries are  deleted  and  no  new  entries  are
              saved.  If set to a value less than zero, the number of history entries is not
              limited.  By default, the number of history entries is set to the value of the
              HISTSIZE  shell variable.  If an attempt is made to set history-size to a non-
              numeric value, the maximum number of history entries will be set to 500.
       horizontal-scroll-mode (Off)
              When set to On, makes readline use a single line for  display,  scrolling  the
              input  horizontally  on  a  single screen line when it becomes longer than the
              screen width rather than wrapping to a new line.  This  setting  is  automati‐
              cally enabled for terminals of height 1.
       input-meta (Off)
              If set to On, readline will enable eight-bit input (that is, it will not strip
              the eighth bit from the characters it reads), regardless of what the  terminal
              claims  it  can  support.   The name meta-flag is a synonym for this variable.
              The default is Off, but readline will set it to  On  if  the  locale  contains
              eight-bit characters.
       isearch-terminators (``C-[C-J'')
              The  string  of characters that should terminate an incremental search without
              subsequently 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 am‐
              biguous key sequence (one that can form a complete key sequence using the  in‐
              put  read  so  far,  or can take additional input to complete a longer key se‐
              quence).  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-nu‐
              meric 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 pre‐
              ceding 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 be‐
              gin 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 com‐
              pleted.
       menu-complete-display-prefix (Off)
              If set to On, menu completion displays the common prefix of the list of possi‐
              ble 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 di‐
              rectly rather than as a meta-prefixed escape sequence.  The  default  is  Off,
              but readline will set it to On if the locale contains eight-bit characters.
       page-completions (On)
              If set to On, readline uses an internal more-like pager to display a screenful
              of possible completions at a time.
       print-completions-horizontally (Off)
              If set to On, readline will display completions with matches  sorted  horizon‐
              tally 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 immediately instead of ringing the bell.
       show-all-if-unmodified (Off)
              This alters the default behavior of the completion functions in a fashion sim‐
              ilar to show-all-if-ambiguous.  If set to On, words which have more  than  one
              possible completion without any possible partial completion (the possible com‐
              pletions don't share a common prefix) cause the matches to be  listed  immedi‐
              ately instead of ringing the bell.
       show-mode-in-prompt (Off)
              If set to On, add a string to the beginning of the prompt indicating the edit‐
              ing mode: emacs, vi command, or vi insertion.  The mode strings are  user-set‐
              table (e.g., emacs-mode-string).
       skip-completed-text (Off)
              If  set  to  On,  this alters the default completion behavior when inserting a
              single match into the line.  It's only active when  performing  completion  in
              the  middle  of  a word.  If enabled, readline does not insert characters from
              the completion that match characters after point in the word being  completed,
              so portions of the word following the cursor are not duplicated.
       vi-cmd-mode-string ((cmd))
              If the show-mode-in-prompt variable is enabled, this string is displayed imme‐
              diately before the last line of the primary prompt when vi editing mode is ac‐
              tive  and  in  command mode.  The value is expanded like a key binding, so the
              standard set of meta- and control prefixes and backslash escape  sequences  is
              available.  Use the \1 and \2 escapes to begin and end sequences of non-print‐
              ing characters, which can be used to embed a terminal  control  sequence  into
              the mode string.
       vi-ins-mode-string ((ins))
              If the show-mode-in-prompt variable is enabled, this string is displayed imme‐
              diately before the last line of the primary prompt when vi editing mode is ac‐
              tive  and in insertion mode.  The value is expanded like a key binding, so the
              standard set of meta- and control prefixes and backslash escape  sequences  is
              available.  Use the \1 and \2 escapes to begin and end sequences of non-print‐
              ing characters, which can be used to embed a terminal  control  sequence  into
              the mode string.
       visible-stats (Off)
              If set to On, a character denoting a file's type as reported by stat(2) is ap‐
              pended to the filename when listing possible completions.

   Readline Conditional Constructs
       Readline implements a facility similar in spirit to the conditional compilation  fea‐
       tures  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
              terminal being used, or the application using readline.  The text of the test,
              after any comparison operator,
               extends to the end of the line; unless otherwise noted, no characters are re‐
              quired 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 command, 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,
                     perhaps  to  bind  the  key sequences output by the terminal's function
                     keys.  The word on the right side of the = is tested against  both  the
                     full  name  of the terminal and the portion of the terminal name before
                     the first -.  This allows sun to match both sun and  sun-cmd,  for  in‐
                     stance.

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

              application
                     The application construct is used to include application-specific  set‐
                     tings.   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

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

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

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

       $include
              This  directive  takes a single filename as an argument and reads commands and
              bindings 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 be‐
       low) for lines containing a specified string.  There are two search modes:  incremen‐
       tal 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 variable 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 en‐
       try containing the search string becomes the current line.

       To  find  other  matching entries in the history list, type Control-S or Control-R as
       appropriate.  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
       without  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
       contents 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  posi‐
       tion,  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
              characters (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.
       previous-screen-line
              Attempt to move point to the same physical screen column on the previous phys‐
              ical  screen  line. This will not have the desired effect if the current Read‐
              line line does not take up more than one physical line  or  if  point  is  not
              greater than the length of the prompt plus the screen width.
       next-screen-line
              Attempt  to move point to the same physical screen column on the next physical
              screen line. This will not have the desired effect  if  the  current  Readline
              line does not take up more than one physical line or if the length of the cur‐
              rent Readline line is not greater than the  length  of  the  prompt  plus  the
              screen width.
       clear-display (M-C-l)
              Clear  the screen and, if possible, the terminal's scrollback buffer, then re‐
              draw the current line, leaving the current line at the top of the screen.
       clear-screen (C-l)
              Clear the screen, then redraw the current line, leaving the  current  line  at
              the  top  of  the  screen.  With an argument, 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 his‐
              tory as necessary.  This is an incremental search.
       forward-search-history (C-s)
              Search forward starting at the current line and moving `down' through the his‐
              tory 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-
              incremental 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
              supplied by the user.
       history-search-forward
              Search  forward  through  the history for the string of characters between the
              start of the current line and the point.  This is a non-incremental search.
       history-search-backward
              Search backward through the history for the string of characters  between  the
              start of the current line and the point.  This is a non-incremental search.
       history-substring-search-backward
              Search  backward  through the history for the string of characters between the
              start of the current line and the current cursor position  (the  point).   The
              search string may match anywhere in a history line.  This is a non-incremental
              search.
       history-substring-search-forward
              Search forward through the history for the string of  characters  between  the
              start of the current line and the point.  The search string may match anywhere
              in a history line.  This is a non-incremental search.
       yank-nth-arg (M-C-y)
              Insert the first argument to the previous command (usually the second word  on
              the previous line) at point.  With an argument n, insert the nth word from the
              previous command (the words in the previous command begin  with  word  0).   A
              negative  argument  inserts the nth word from the end of the previous command.
              Once the argument n is computed, the argument is extracted as if the "!n" his‐
              tory expansion had been specified.
       yank-last-arg (M-., M-_)
              Insert  the last argument to the previous command (the last word of the previ‐
              ous  history  entry).   With  a  numeric   argument,   behave   exactly   like
              yank-nth-arg.  Successive 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 suc‐
              cessive calls determines the direction to move through the history.   A  nega‐
              tive  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 descrip‐
              tion 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
              current  line  from the history for editing.  A numeric argument, if supplied,
              specifies the history entry to use instead of the current line.
       edit-and-execute-command (C-x C-e)
              Invoke an editor on the current command line, and execute the result as  shell
              commands.   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 charac‐
              ter 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
              characters 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  trans‐
              poses 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, upper‐
              case the previous word, but do not move point.
       downcase-word (M-l)
              Lowercase the current (or following) word.  With a negative  argument,  lower‐
              case the previous word, but do not move point.
       capitalize-word (M-c)
              Capitalize  the  current (or following) word.  With a negative argument, capi‐
              talize 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 re‐
              place 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 for‐
              ward-word.
       backward-kill-word (M-Rubout)
              Kill the word behind point.  Word boundaries are the same  as  those  used  by
              backward-word.
       shell-kill-word
              Kill  from  point  to the end of the current word, or if between words, to the
              end of the next  word.   Word  boundaries  are  the  same  as  those  used  by
              shell-forward-word.
       shell-backward-kill-word
              Kill  the  word  behind  point.  Word boundaries are the same as those used by
              shell-backward-word.
       unix-word-rubout (C-w)
              Kill the word behind point, using white space as a word boundary.  The  killed
              text is saved on the kill-ring.
       unix-filename-rubout
              Kill  the  word behind point, using white space and the slash character as the
              word boundaries.  The killed text is saved on the kill-ring.
       delete-horizontal-space (M-\)
              Delete all spaces and tabs around point.
       kill-region
              Kill the text in the current region.
       copy-region-as-kill
              Copy the text in the region to the kill buffer.
       copy-backward-word
              Copy the word before point to the kill buffer.  The word  boundaries  are  the
              same as backward-word.
       copy-forward-word
              Copy the word following point to the kill buffer.  The word boundaries are the
              same as forward-word.
       yank (C-y)
              Yank the top of the kill ring into the buffer at point.
       yank-pop (M-y)
              Rotate the kill ring, and yank the new top.   Only  works  following  yank  or
              yank-pop.

   Numeric Arguments
       digit-argument (M-0, M-1, ..., M--)
              Add  this digit to the argument already accumulating, or start a new argument.
              M-- starts a negative argument.
       universal-argument
              This is another way to specify an argument.  If this command  is  followed  by
              one  or more digits, optionally with a leading minus sign, those digits define
              the argument.  If the command is followed by digits, executing universal-argu‐
              ment  again ends the numeric argument, but is otherwise ignored.  As a special
              case, if this command is immediately followed by a character that is neither a
              digit nor minus sign, the argument count for the next command is multiplied by
              four.  The argument count is initially one, so  executing  this  function  the
              first  time  makes  the  argument 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 comple‐
              tion 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 (in‐
              cluding  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  set‐
              ting 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
              completions,  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
              possible-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 file‐
              name.
       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 user‐
              name.
       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 host‐
              name.
       complete-command (M-!)
              Attempt completion on the text before point, treating it as  a  command  name.
              Command completion attempts to match the text against aliases, reserved words,
              shell functions, shell builtins, and finally executable filenames, in that or‐
              der.
       possible-command-completions (C-x !)
              List  the possible completions of the text before point, treating it as a com‐
              mand 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  en‐
              closed  within  braces so the list is available to the shell (see Brace Expan‐
              sion 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
              definition.
       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
              setting of bell-style).
       do-lowercase-version (M-A, M-B, M-x, ...)
              If the metafied character x is uppercase, run the command that is bound to the
              corresponding metafied lowercase character.  The behavior is undefined if x is
              already lowercase.
       prefix-meta (ESC)
              Metafy the next character typed.  ESC f is equivalent to Meta-f.
       undo (C-_, C-x C-u)
              Incremental undo, separately remembered for each line.
       revert-line (M-r)
              Undo  all  changes made to this line.  This is like executing the undo command
              enough times to return the line to its initial state.
       tilde-expand (M-&)
              Perform tilde expansion on the current word.
       set-mark (C-@, M-)
              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  charac‐
              ter.  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
              character.  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 In‐
              dicator (CSI), usually ESC-[.  If this sequence is bound to "\[", keys produc‐
              ing  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,  oth‐
              erwise  the  characters 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  comment-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 executed 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  ar‐
              gument 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
              displayed, and the line is redrawn.  If a numeric argument is supplied, an as‐
              terisk 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
              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-macros
              Print  all  of the readline key sequences bound to macros and the strings they
              output.  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 (com‐
       pletion 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 following 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 de‐
       fault.  If there is no default compspec, bash attempts alias expansion on the command
       word as a final resort, and attempts to find a compspec for the command word from any
       successful expansion.

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

       First,  the  actions specified by the compspec are used.  Only matches which are pre‐
       fixed 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 gener‐
       ated  next.   The  words  generated by the pattern need not match the word being com‐
       pleted.  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 ex‐
       pansion,  as  described above under EXPANSION.  The results are split using the rules
       described above under Word Splitting.  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  de‐
       scribed  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  argu‐
       ment ($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  per‐
       formed; the function or command has complete freedom 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  equiva‐
       lent  to  command substitution.  It should print a list of completions, one per line,
       to the standard 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 expan‐
       sion; 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 completion 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.  If the nocasematch shell option is enabled, the match is  performed
       without regard to the case of alphabetic characters.

       Finally, any prefix and suffix specified with the -P and -S options are added to each
       member 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 comple‐
       tion is attempted.

       If the -o plusdirs option was supplied to complete when the compspec was defined, di‐
       rectory 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 comple‐
       tion 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 bashdefault option was supplied to complete when the  compspec  was  defined,  the
       bash  default completions 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
       completion  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 vari‐
       able.

       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 comple‐
       tion  should  be retried by returning an exit status of 124.  If a shell function re‐
       turns 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 cor‐
       responding  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 variable 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 HISTCONTROL.

       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
       necessary, to contain no more than the number of lines  specified  by  the  value  of
       HISTFILESIZE.   If  HISTFILESIZE  is unset, or set to null, a non-numeric value, or a
       numeric value less than zero, the history file is not truncated.   When  the  history
       file is read, lines beginning with the history comment character followed immediately
       by a digit are interpreted as timestamps for the following history line.  These time‐
       stamps  are  optionally  displayed depending on the value of the HISTTIMEFORMAT vari‐
       able.  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 un‐
       set, or if the history file is unwritable, the history is not saved.   If  the  HIST‐
       TIMEFORMAT  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  com‐
       mand-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
       HISTCONTROL  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  COM‐
       MANDS for information on setting and unsetting shell options.

HISTORY EXPANSION
       The  shell supports a history expansion feature that is similar to the history expan‐
       sion in csh.  This section describes what syntax features are available.   This  fea‐
       ture  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-interac‐
       tive shells do not perform history expansion by default.

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

       History  expansion is performed immediately after a complete line is read, before the
       shell breaks it into words, and is performed on each line individually without taking
       quoting  on  previous lines into account.  It takes place in two parts.  The first is
       to determine which line from the history list to use during substitution.  The second
       is  to select portions of that line for inclusion into the current one.  The line se‐
       lected 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 sev‐
       eral  metacharacter-separated  words  surrounded  by  quotes are considered one word.
       History expansions are introduced by the appearance of the history expansion  charac‐
       ter,  which is ! by default.  Only backslash (\) and single quotes can quote the his‐
       tory expansion character, but the history expansion  character  is  also  treated  as
       quoted if it immediately precedes the closing double quote in a double-quoted string.

       Several  characters inhibit history expansion if found immediately following the his‐
       tory expansion character, even if it is unquoted: space, tab, newline,  carriage  re‐
       turn, 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 be‐
       havior of history expansion.  If the histverify shell option is enabled (see the  de‐
       scription  of the shopt builtin below), and readline is being used, history substitu‐
       tions are not immediately passed to the shell parser.  Instead, the expanded line  is
       reloaded  into  the readline 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  be‐
       fore  using  it.  The -s option to the history builtin may be used to add commands to
       the end of the history list without actually executing them, so that they are  avail‐
       able 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  history comment character to mark history timestamps when writing the his‐
       tory file.

   Event Designators
       An event designator is a reference to a command line entry in the history list.   Un‐
       less  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,  car‐
              riage 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.  If string is missing, the string from the most re‐
              cent search is used; it is an error if there is no previous search string.
       ^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 designa‐
       tor  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 ze‐
              roth word if there is only one word in the line.
       %      The first word matched by the most recent `?string?'  search,  if  the  search
              string begins with a character that is part of a word.
       x-y    A range of words; `-y' abbreviates `0-y'.
       *      All  of  the words but the zeroth.  This is a synonym for `1-$'.  It is not an
              error to use * if there is just one word in the event; the empty string is re‐
              turned in that case.
       x*     Abbreviates x-$.
       x-     Abbreviates  x-$  like  x*,  but omits the last word.  If x is missing, it de‐
              faults to 0.

       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
       following modifiers, each preceded by a `:'.  These modify,  or  edit,  the  word  or
       words selected from the history event.

       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 new‐
              lines.  The q and x modifiers are mutually exclusive; the last one supplied is
              used.
       s/old/new/
              Substitute new for the first occurrence of old in the event line.  Any charac‐
              ter may be used as the delimiter in place of /.  The final  delimiter  is  op‐
              tional  if  it  is the last character of the event line.  The delimiter may be
              quoted in old and new with a single backslash.  If & appears in new, it is re‐
              placed  by  old.   A single backslash will quote the &.  If old is null, it is
              set to the last old substituted, or, if no previous history substitutions took
              place,  the last string in a !?string[?]  search.  If new is null, each match‐
              ing old is deleted.
       &      Repeat the previous substitution.
       g      Cause changes to be applied over the entire event line.  This is used in  con‐
              junction with `:s' (e.g., `:gs/old/new/') or `:&'.  If used with `:s', any de‐
              limiter 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' or `&' 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, return, break, continue, let, and shift builtins accept and process ar‐
       guments  beginning with - without requiring --.  Other builtins that accept arguments
       but are not specified as accepting options interpret arguments beginning  with  -  as
       invalid options and require -- to prevent this interpretation.
       : [arguments]
              No  effect; the command does nothing beyond expanding arguments and performing
              any specified redirections.  The return status is zero.

        .  filename [arguments]
       source filename [arguments]
              Read and execute commands from filename in the current shell  environment  and
              return  the  exit status of the last command executed from filename.  If file‐
              name does not contain a slash, filenames in PATH are used to find  the  direc‐
              tory  containing  filename.   The  file  searched for in PATH need not be exe‐
              cutable.  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 com‐
              mand is turned off, the PATH is not searched.  If any arguments are  supplied,
              they  become  the  positional parameters when filename is executed.  Otherwise
              the positional parameters are unchanged.  If the -T option is enabled,  source
              inherits  any  trap on DEBUG; if it is not, any DEBUG trap string is saved and
              restored around the call to source, and source unsets the DEBUG trap while  it
              executes.   If -T is not set, and the sourced file changes the DEBUG trap, the
              new value is retained when source completes.  The return status is the  status
              of  the last command exited within the script (0 if no commands are executed),
              and false if filename is not found or cannot be read.

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

       bg [jobspec ...]
              Resume each suspended job jobspec in the background, as if it had been started
              with  &.   If jobspec is not present, the shell's notion of the current job is
              used.  bg jobspec returns 0 unless run when job control is disabled  or,  when
              run  with  job  control  enabled,  any  specified jobspec was not found or was
              started without job control.

       bind [-m keymap] [-lpsvPSVX]
       bind [-m keymap] [-q function] [-u function] [-r keyseq]
       bind [-m keymap] -f filename
       bind [-m keymap] -x keyseq:shell-command
       bind [-m keymap] keyseq:function-name
       bind [-m keymap] keyseq:readline-command
              Display current readline key and function bindings, bind a key sequence  to  a
              readline function or macro, or set a readline variable.  Each non-option argu‐
              ment 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 supplied, have the following meanings:
              -m keymap
                     Use keymap as the keymap to be affected  by  the  subsequent  bindings.
                     Acceptable   keymap   names   are  emacs,  emacs-standard,  emacs-meta,
                     emacs-ctlx, vi, vi-move, vi-command, and vi-insert.  vi  is  equivalent
                     to  vi-command  (vi-move  is  also  a  synonym); emacs is equivalent to
                     emacs-standard.
              -l     List the names of all readline functions.
              -p     Display readline function names and bindings in such a  way  that  they
                     can be re-read.
              -P     List current readline function names and bindings.
        

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