bash(1) — Linux manual page

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BASH(1)                  General Commands Manual                  BASH(1)

NAME         top

       bash - GNU Bourne-Again SHell

SYNOPSIS         top

       bash [options] [command_string | file]

COPYRIGHT         top

       Bash is Copyright (C) 1989-2025 by the Free Software Foundation,
       Inc.

DESCRIPTION         top

       Bash is a command language interpreter that executes commands read
       from the standard input, from a string, or from a file.  It is a
       reimplementation and extension of the Bourne shell, the historical
       Unix command language interpreter.  Bash also incorporates useful
       features from the Korn and C shells (ksh and csh).

       POSIX is the name for a family of computing standards based on
       Unix.  Bash is intended to be a conformant implementation of the
       Shell and Utilities portion of the IEEE POSIX specification (IEEE
       Standard 1003.1).  Bash POSIX mode (hereafter referred to as posix
       mode) changes the shell's behavior where its default operation
       differs from the standard to strictly conform to the standard.
       See SEE ALSO below for a reference to a document that details how
       posix mode affects bash's behavior.  Bash can be configured to be
       POSIX-conformant by default.

OPTIONS         top

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

       -c     If the -c option is present, then commands are read from
              the first non-option argument command_string.  If there are
              arguments after the command_string, the first argument is
              assigned to $0 and any remaining arguments are assigned to
              the positional parameters.  The assignment to $0 sets the
              name of the shell, which is used in warning and error
              messages.

       -i     If the -i option is present, the shell is interactive.

       -l     Make bash act as if it had been invoked as a login shell
              (see INVOCATION below).

       -r     If the -r option is present, the shell becomes restricted
              (see RESTRICTED SHELL below).

       -s     If the -s option is present, or if no arguments remain
              after option processing, the shell reads commands from the
              standard input.  This option allows the positional
              parameters to be set when invoking an interactive shell or
              when reading input through a pipe.

       -D     Print a list of all double-quoted strings preceded by $ on
              the standard output.  These are the strings that are
              subject to language translation when the current locale is
              not C or POSIX.  This implies the -n option; no commands
              will be executed.

       [-+]O [shopt_option]
              shopt_option is one of the shell options accepted by the
              shopt builtin (see SHELL BUILTIN COMMANDS below).  If
              shopt_option is present, -O sets the value of that option;
              +O unsets it.  If shopt_option is not supplied, bash prints
              the names and values of the shell options accepted by shopt
              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 a
              shell script filename (see below) and arguments passed to
              that script.  An argument of - is equivalent to --.

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

       --debugger
              Arrange for the debugger profile to be executed before the
              shell starts.  Turns on extended debugging mode (see the
              description of the extdebug option to the shopt builtin
              below).

       --dump-po-strings
              Equivalent to -D, but the output is in the GNU gettext “po”
              (portable object) file format.

       --dump-strings
              Equivalent to -D.

       --help Display a usage message on standard output and exit
              successfully.

       --init-file file
       --rcfile file
              Execute commands from file instead of the 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
              Enable posix mode; change the behavior of bash where the
              default operation differs from the POSIX standard to match
              the standard.

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

       --verbose
              Equivalent to -v.

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

ARGUMENTS         top

       If arguments remain after option processing, and neither the -c
       nor the -s option has been supplied, the first argument is treated
       as the name of a file containing shell commands (a shell script).
       When 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.  Bash first attempts to open the file in the current
       directory, and, if no file is found, searches the directories in
       PATH for the script.

INVOCATION         top

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

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

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

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

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

       When an interactive shell that is not a login shell is started,
       bash reads and executes commands from ~/.bashrc, if that file
       exists.  The --norc option inhibits this behavior.  The --rcfile
       file option causes bash to use 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 does not use the value of the PATH variable to search for the
       filename.

       If bash is invoked with the name sh, it tries to mimic the startup
       behavior of historical versions of sh as closely as possible,
       while conforming to the POSIX standard as well.  When invoked as
       an interactive login shell, or a non-interactive shell with the
       --login option, it first attempts to read and execute commands
       from /etc/profile and ~/.profile, in that order.  The --noprofile
       option inhibits 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 reading the
       startup files.

       When bash is started in posix mode, as with the --posix command
       line option, it follows the POSIX standard for startup files.  In
       this mode, interactive shells expand the ENV variable and read and
       execute commands 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
       historical and rarely-seen remote shell daemon, usually rshd, or
       the secure shell daemon sshd.  If bash determines it is being run
       non-interactively in this fashion, it reads and executes commands
       from ~/.bashrc, if that file exists and is readable.  Bash does
       not read this file if invoked as sh.  The --norc option inhibits
       this behavior, and the --rcfile option makes bash use a different
       file instead of ~/.bashrc, but neither rshd nor sshd generally
       invoke the shell with those options or allow them to be specified.

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

DEFINITIONS         top

       The following definitions are used throughout the rest of this
       document.
       blank  A space or tab.
       whitespace
              A character belonging to the space character class in the
              current locale, or for which isspace(3) returns true.
       word   A sequence of characters considered as a single unit by the
              shell.  Also known as a token.
       name   A word consisting only of alphanumeric characters and
              underscores, and beginning with an alphabetic character or
              an underscore.  Also referred to as an identifier.
       metacharacter
              A character that, when unquoted, separates words.  One of
              the following:
              |  & ; ( ) < > space tab newline
       control operator
              A token that performs a control function.  It is one of the
              following symbols:
              || & && ; ;; ;& ;;& ( ) | |& <newline>

RESERVED WORDS         top

       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         top

       This section describes the syntax of the various forms of shell
       commands.

   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 remaining words are
       passed as arguments to the invoked command.

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

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

              [time [-p]] [ ! ] command1 [ [||&] command2 ... ]

       The standard output of command1 is connected via a pipe to the
       standard input of command2.  This connection is performed before
       any redirections specified by the command1(see REDIRECTION below).
       If |& is the pipeline operator, command1'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 standard output
       is performed after any redirections specified by command1.

       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.  If a pipeline is executed
       synchronously, the shell waits for all commands in the pipeline to
       terminate before returning a value.

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

       When the shell is in posix mode, time may appear by itself as the
       only word in a simple command.  In this case, the shell displays
       the total user and system time consumed by the shell and its
       children.  The TIMEFORMAT variable specifies the format of the
       time information.

       Each command in a multi-command pipeline, where pipes are created,
       is executed in a subshell, which is a separate process.  See
       COMMAND EXECUTION ENVIRONMENT for a description of subshells and a
       subshell environment.  If the lastpipe option is enabled using the
       shopt builtin (see the description of shopt below), and job
       control is not active, the last element of a pipeline may be run
       by the shell process.

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

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

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

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

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

              command1 && command2

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

       An OR list has the form

              command1 || command2

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

   Compound Commands
       A compound command is one of the following.  In most cases a list
       in a command's description 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 (see COMMAND EXECUTION
              ENVIRONMENT below for a description of a subshell
              environment).  Variable assignments and builtin commands
              that affect the shell's environment do not remain in effect
              after the command completes.  The return status is the exit
              status of list.

       { list; }
              list is executed in the current shell environment.  list
              must be terminated with a newline or semicolon.  This is
              known as a group command.  The return status is the exit
              status of list.

              Note that unlike the metacharacters ( and ), { and } are
              reserved words and must occur where a reserved word is
              permitted to be recognized.  Since they do not cause a word
              break, they must be separated from list by whitespace or
              another shell metacharacter.

       ((expression))
              The arithmetic 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.  The expression
              undergoes the same expansions as if it were within double
              quotes, but unescaped double quote characters in expression
              are not treated specially and are removed.  Since this can
              potentially result in empty strings, this command treats
              those as expressions that evaluate to 0.

       [[ expression ]]
              Evaluate the conditional expression expression and return a
              status of zero (true) or non-zero (false).  Expressions are
              composed of the primaries described below under CONDITIONAL
              EXPRESSIONS.  The words between the [[ and ]] do not
              undergo word splitting and pathname expansion.  The shell
              performs tilde expansion, parameter and variable expansion,
              arithmetic expansion, command substitution, process
              substitution, and quote removal on those words.
              Conditional operators such as -f must be unquoted to be
              recognized as primaries.

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

              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 under Pattern
              Matching, as if the extglob shell option were enabled.  The
              = operator is equivalent to ==.  If the nocasematch shell
              option is enabled, the match is performed without regard to
              the case of alphabetic characters.  The return value is 0
              if the string matches (==) or does not match (!=) the
              pattern, and 1 otherwise.  If any part of the pattern is
              quoted, the quoted portion is matched as a string: every
              character in the quoted portion matches itself, instead of
              having any special pattern matching meaning.

              An additional binary operator, =~, is available, with the
              same precedence as == and !=.  When it is used, the string
              to the right of the operator is considered a POSIX extended
              regular expression and matched accordingly (using the POSIX
              regcomp and regexec interfaces usually described in
              regex(3)).  The return value is 0 if the string matches the
              pattern, and 1 otherwise.  If the regular expression is
              syntactically incorrect, the conditional expression's
              return value is 2.  If the nocasematch shell option is
              enabled, the match is performed without regard to the case
              of alphabetic characters.

              If any part of the pattern is quoted, the quoted portion is
              matched literally, as above.  If the pattern is stored in a
              shell variable, quoting the variable expansion forces the
              entire pattern to be matched literally.  Treat bracket
              expressions in regular expressions carefully, since normal
              quoting and pattern characters lose their meanings between
              brackets.

              The match succeeds if the pattern matches any part of the
              string.  Anchor the pattern 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 subexpression.  Bash
              sets BASH_REMATCH in the global scope; declaring it as a
              local variable will lead to unexpected results.

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

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

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

       for name [ [ in word ... ] ; ] do list ; done
              First, expand The list of words following in, generating a
              list of items.  Then, the variable name is set to each
              element of this list in turn, and list is executed each
              time.  If the in word is omitted, the for command executes
              list once for each 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, evaluate the arithmetic expression expr1 according
              to the rules described below under ARITHMETIC EVALUATION.
              Then, repeatedly evaluate the arithmetic expression expr2
              until it evaluates to zero.  Each time expr2 evaluates to a
              non-zero value, execute list and evaluate the arithmetic
              expression expr3.  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
              non-zero if any of the expressions is invalid.

              Use the break and continue builtins (see SHELL BUILTIN
              COMMANDS below) to control loop execution.

       select name [ in word ] ; do list ; done
              First, expand the list of words following in, generating a
              list of items, and print the set of expanded words the
              standard error, each preceded by a number.  If the in word
              is omitted, print the positional parameters (see PARAMETERS
              below).  select then displays the PS3 prompt and reads a
              line from the standard input.  If the line consists of a
              number corresponding to one of the displayed words, then
              select sets the value of name to that word.  If the line is
              empty, select displays the words and prompt again.  If EOF
              is read, select completes and returns 1.  Any other value
              sets name to null.  The line read is saved in the variable
              REPLY.  The list is executed after each selection until a
              break command is executed.  The exit status of select is
              the exit status of the last command executed in list, or
              zero if no commands were executed.

       case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
              A case command first expands word, and tries to match it
              against each pattern in turn, proceeding from first to
              last, using the matching rules described under Pattern
              Matching below.  A pattern list is a set of one or more
              patterns separated by , and the ) operator terminates the
              pattern list.  The word is expanded using tilde expansion,
              parameter and variable expansion, arithmetic expansion,
              command substitution, process substitution and quote
              removal.  Each pattern examined is expanded using tilde
              expansion, parameter and variable expansion, arithmetic
              expansion, command substitution, process substitution, and
              quote removal.  If the nocasematch shell option is enabled,
              the match is performed without regard to the case of
              alphabetic characters.  A clause is a pattern list and an
              associated list.

              When a match is found, case executes the corresponding
              list.  If the ;; operator terminates the case clause, the
              case command completes after the first match.  Using ;& in
              place of ;; causes execution to continue with the list
              associated with the next pattern list.  Using ;;& in place
              of ;; causes the shell to test the next pattern list in the
              statement, if any, and execute any associated list if the
              match succeeds, continuing the case statement execution as
              if the pattern list had not matched.  The exit status is
              zero if no pattern matches.

              Otherwise, it is the exit status of the last command
              executed in the last list executed.

       if list; then list; [ elif list; then list; ] ... [ else list; ]
       fi
              The if list is executed.  If its exit status is zero, the
              then list is executed.  Otherwise, each elif list is
              executed in turn, and if its exit status is zero, the
              corresponding then list is executed and the command
              completes.  Otherwise, the else list is executed, if
              present.  The exit status is the exit status of the last
              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 syntax for a coprocess is:

              coproc [NAME] command [redirections]

       This creates a coprocess named NAME.  command may be either a
       simple command or a compound command (see above).  NAME is a shell
       variable name.  If NAME is not supplied, the default name is
       COPROC.

       The recommended form to use for a coprocess is

              coproc NAME { command [redirections]; }

       This form is preferred because simple commands result in the
       coprocess always being named COPROC, and it is simpler to use and
       more complete than the other compound commands.

       If command is a compound command, NAME is optional. The word
       following coproc determines whether that word is interpreted as a
       variable name: it is interpreted as NAME if it is not a reserved
       word that introduces a compound command.  If command is a simple
       command, NAME is not allowed; this is to avoid confusion between
       NAME and 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 assigned to NAME[0].  The standard input of command
       is connected via a pipe to a file descriptor in the executing
       shell, and that file descriptor is assigned to NAME[1].  This pipe
       is established before any redirections specified by the command
       (see REDIRECTION 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
       subshells.

       The process ID of the shell spawned to execute the coprocess is
       available as the value of the variable NAME_PID.  The wait builtin
       may be used to wait for the coprocess to terminate.

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

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

       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.  If the function reserved
              word is used, but the parentheses are not supplied, the
              braces are recommended.  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         top

       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 beginning with # introduces
       a comment.  A word begins at the beginning of a line, after
       unquoted whitespace, or after an operator.  The comment causes
       that word and all remaining characters on that line to be ignored.
       An interactive shell without the interactive_comments option
       enabled does not allow comments.  The interactive_comments option
       is enabled by default in interactive shells.

QUOTING         top

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

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

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

       There are four quoting mechanisms: the escape character, single
       quotes, double quotes, and dollar-single quotes.

       A non-quoted backslash (\) is the escape character.  It preserves
       the literal value of the next character that follows, removing any
       special meaning it has, with the exception of <newline>.  If a
       \<newline> pair appears, and the backslash is not itself quoted,
       the \<newline> is treated as a line continuation (that is, it is
       removed from the input stream and effectively ignored).

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

       Enclosing characters in double quotes preserves the literal value
       of all characters within the quotes, with the exception of $, `,
       \, and, when history expansion is enabled, !.  When the shell is
       in posix mode, the ! has no special meaning within double quotes,
       even when history expansion is enabled.  The characters $ and `
       retain their special meaning within double quotes.  The backslash
       retains its special meaning only when followed by one of the
       following characters: $, `, ", \, or <newline>.  Backslashes
       preceding characters without a special meaning are left
       unmodified.

       A double quote may be quoted within double quotes by preceding it
       with a backslash.  If enabled, history expansion will be performed
       unless an !  appearing in double quotes is escaped using a
       backslash.  The backslash preceding the !  is not removed.

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

       Character sequences of the form $'string' are treated as a special
       variant of single quotes.  The sequence expands to string, with
       backslash-escaped characters in string 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.

   Translating Strings
       A double-quoted string preceded by a dollar sign ($"string")
       causes the string to be translated according to the current
       locale.  The gettext infrastructure performs the lookup and
       translation, using the LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN
       shell variables.  If the current locale is C or POSIX, if there
       are no translations available, or if the string is not translated,
       the dollar sign is ignored, and the string is treated as double-
       quoted as described above.  This is a form of double quoting, so
       the string remains double-quoted by default, whether or not it is
       translated and replaced.  If the noexpand_translation option is
       enabled using the shopt builtin, translated strings are single-
       quoted instead of double-quoted.  See the description of shopt
       below under SHELL BUILTIN COMMANDS.

PARAMETERS         top

       A parameter is an entity that stores values.  It can be a name, a
       number, or one of the special characters listed below under
       Special Parameters.  A variable is a parameter denoted by a name.
       A variable has a value and zero or more attributes.  Attributes
       are assigned using the declare builtin command (see declare below
       in SHELL BUILTIN COMMANDS).  The export and readonly builtins
       assign specific attributes.

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

       A variable is assigned to using 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 and pathname expansion are not
       performed.  Assignment statements may also appear as arguments to
       the alias, declare, typeset, export, readonly, and local builtin
       commands (declaration commands).  When in posix mode, these
       builtins may appear in a command after one or more instances of
       the command builtin and retain these assignment statement
       properties.

       In the context where an assignment statement is assigning a value
       to a shell variable or array index, the “+=” operator appends to
       or adds to the variable's previous value.  This includes arguments
       to declaration commands such as declare that accept assignment
       statements.  When “+=” is applied to a variable for which the
       integer attribute has been set, the variable's current value and
       value are each evaluated as arithmetic expressions, and the sum of
       the results is assigned as the variable's value.  The current
       value is usually an integer constant, but may be an expression.
       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 arrays) or added as additional key-value pairs in an
       associative array.  When applied to a string-valued variable,
       value is expanded and appended to the variable's value.

       A variable can be assigned the nameref attribute using the -n
       option to the declare or local builtin commands (see the
       descriptions of declare and local below) to create a nameref, or a
       reference to another variable.  This allows variables to be
       manipulated indirectly.  Whenever the nameref variable is
       referenced, 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 local 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 is
       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 executed with
       the name of a nameref variable as an argument, the variable
       referenced by the nameref variable is unset.

       When the shell starts, it reads its environment and creates a
       shell variable from each environment variable that has a valid
       name, as described below (see ENVIRONMENT).

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

       When a positional parameter consisting of more than a single digit
       is expanded, it must be enclosed in braces (see EXPANSION below).
       Without braces, a digit following $ can only refer to one of the
       first nine positional parameters ($1-$9) or the special parameter
       $0 (see the next section).

   Special Parameters
       The shell treats several parameters specially.  These parameters
       may only be referenced; assignment to them is not allowed.
       Special parameters are denoted by one of the following characters.

       *      ($*) Expands to the positional parameters, starting from
              one.  When the expansion is not within double quotes, each
              positional parameter expands to a separate word.  In
              contexts where word expansions are performed, those words
              are subject to further word splitting and pathname
              expansion.  When the expansion occurs within double quotes,
              it expands to a single word with the value of each
              parameter separated by the first character of the IFS
              variable.  That is, "$*" 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 separate word; if
              not within double quotes, these words are subject to word
              splitting.  In contexts where word splitting is not
              performed, such as the value portion of an assignment
              statement, this expands to a single word with each
              positional parameter separated by a space.  When the
              expansion occurs within double quotes, and word splitting
              is performed, each parameter expands to a separate word.
              That is, "$@" is equivalent to "$1" "$2" ...  If the
              double-quoted expansion occurs within a word, the expansion
              of the first parameter is joined with the expansion of the
              beginning part of the original word, and the expansion of
              the last parameter is joined with the expansion of the last
              part of the original word.  When there are no positional
              parameters, "$@" and $@ expand to nothing (i.e., they are
              removed).
       #      ($#) Expands to the number of positional parameters in
              decimal.
       ?      ($?) Expands to the exit status of the most recently
              executed command.
       -      ($-) 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 parent 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      ($0) Expands to the name of the shell or shell script.
              This is set at shell initialization.  If bash is invoked
              with a file of commands, $0 is set to the name of that
              file.  If bash is started with the -c option, then $0 is
              set to the first argument after the string to be executed,
              if one is present.  Otherwise, it is set to the filename
              used to invoke bash, as given by argument zero.

   Shell Variables
       The shell sets following variables:

       _      ($_, an underscore) This has a number of meanings depending
              on context.  At shell startup, _ is set to the pathname
              used to invoke the shell or shell script being executed as
              passed in the environment or argument list.  Subsequently,
              it expands to the last argument to the previous simple
              command executed in the foreground, after expansion.  It is
              also set to the full pathname used to invoke each command
              executed and placed in the environment exported to that
              command.  When checking mail, $_ expands to 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, the shell enables each option in the list before
              reading any startup files.  If this variable is exported,
              child shells will enable each option in the list.  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 appear in the alias
              list; however, unsetting array elements currently does not
              remove aliases from the alias list.  If BASH_ALIASES is
              unset, it loses its special properties, even if it is
              subsequently reset.
       BASH_ARGC
              An array variable whose values are the number of parameters
              in each frame of the current bash execution call stack.
              The number of parameters to the current 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 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.
              Assignments to BASH_ARGC have no effect, and it may not be
              unset.
       BASH_ARGV
              An array variable containing all of the parameters in the
              current bash execution call stack.  The final parameter of
              the last subroutine call is at the top of the stack; the
              first parameter of the initial call is at the bottom.  When
              a subroutine is executed, the shell pushes the supplied
              parameters 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.  Assignments to BASH_ARGV
              have no effect, and it may not be unset.
       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).  Assigning a value to
              BASH_ARGV0 sets $0 to the same value.  If BASH_ARGV0 is
              unset, it loses its special properties, even if it is
              subsequently reset.
       BASH_CMDS
              An associative array variable whose members correspond to
              the internal hash table of commands as maintained by the
              hash builtin.  Adding elements to this array makes them
              appear in the hash table; however, unsetting array elements
              currently does not remove command names from the hash
              table.  If BASH_CMDS is unset, it loses its special
              properties, even if it is subsequently reset.
       BASH_COMMAND
              Expands to the command currently being executed or about to
              be executed, unless the shell is executing a command as the
              result of a trap, in which case it is the command 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 function).  Use LINENO to obtain the current
              line number.  Assignments to BASH_LINENO have no effect,
              and it may not be unset.
       BASH_LOADABLES_PATH
              A colon-separated list of directories in which the enable
              command looks for dynamically loadable builtins.
       BASH_MONOSECONDS
              Each time this variable is referenced, it expands to the
              value returned by the system's monotonic clock, if one is
              available.  If there is no monotonic clock, this is
              equivalent to EPOCHSECONDS.  If BASH_MONOSECONDS is unset,
              it loses its special properties, even if it is subsequently
              reset.
       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
              portion of the string matching the nth parenthesized
              subexpression.
       BASH_SOURCE
              An array variable whose members are the source filenames
              where the corresponding shell function names in the
              FUNCNAME array variable are defined.  The shell function
              ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]}
              and called from ${BASH_SOURCE[$i+1]}.  Assignments to
              BASH_SOURCE have no effect, and it may not be unset.
       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_SUBSHELL is
              unset, it loses its special properties, even if it is
              subsequently reset.
       BASH_TRAPSIG
              Set to the signal number corresponding to the trap action
              being executed during its execution.  See the description
              of trap under SHELL BUILTIN COMMANDS below for information
              about signal numbers and trap execution.
       BASH_VERSINFO
              A readonly array variable whose members hold version
              information for this instance of bash.  The values assigned
              to the array members are as follows:
              BASH_VERSINFO[0]
                     The major version number (the release).
              BASH_VERSINFO[1]
                     The minor version number (the version).
              BASH_VERSINFO[2]
                     The patch level.
              BASH_VERSINFO[3]
                     The build version.
              BASH_VERSINFO[4]
                     The release status (e.g., beta).
              BASH_VERSINFO[5]
                     The value of MACHTYPE.
       BASH_VERSION
              Expands to a string describing the version of this instance
              of bash (e.g., 5.2.37(3)-release).
       COMP_CWORD
              An index into ${COMP_WORDS} of the word containing the
              current cursor position.  This variable is available only
              in shell functions invoked by the programmable completion
              facilities (see Programmable Completion below).
       COMP_KEY
              The key (or final key of a key sequence) used to invoke the
              current completion function.  This variable is available
              only in shell functions and external commands invoked by
              the programmable completion facilities (see Programmable
              Completion below).
       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 current command.  If the current cursor
              position is at the end of the current command, the value of
              this variable is equal to ${#COMP_LINE}.  This variable is
              available only in shell functions and external commands
              invoked by the programmable completion facilities (see
              Programmable Completion below).
       COMP_TYPE
              Set to an integer value corresponding to the type of
              attempted completion that caused a completion function to
              be called: TAB, for normal completion, ?, for listing
              completions after successive tabs, !, for listing
              alternatives on partial word completion, @, to list
              completions if the word is not unmodified, or %, for menu
              completion.  This variable is available only in shell
              functions and external 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 Programmable
              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 displayed by the dirs builtin.
              Assigning to members of this array variable may be used to
              modify directories already in the stack, but the pushd and
              popd builtins must be used to add and remove directories.
              Assigning to this variable does 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
              subsequently reset.
       EUID   Expands to the effective user ID of the current user,
              initialized at shell startup.  This variable is readonly.
       FUNCNAME
              An array variable containing the names of all shell
              functions currently in the 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 executing.  Assignments to FUNCNAME have
              no effect.  If FUNCNAME is unset, it loses its special
              properties, even if it is subsequently reset.

              This variable can be used with BASH_LINENO and BASH_SOURCE.
              Each element of 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.  Assignments to HISTCMD have no effect.
              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 mapfile builtin when no variable name is
              supplied.
       OLDPWD The previous working directory as set by the cd command.
       OPTARG The value of the last option argument processed by the
              getopts builtin command (see SHELL BUILTIN COMMANDS below).
       OPTIND The index of the next argument to be processed by the
              getopts builtin command (see SHELL BUILTIN COMMANDS below).
       OSTYPE Automatically set to a string that describes the operating
              system on which bash is executing.  The default is system-
              dependent.
       PIPESTATUS
              An array variable (see Arrays below) containing a list of
              exit status values from the commands in the most-recently-
              executed foreground pipeline, which may consist of only a
              simple command (see SHELL GRAMMAR above).  Bash sets
              PIPESTATUS after executing multi-element pipelines, timed
              and negated pipelines, simple commands, subshells created
              with the ( operator, the [[ and (( compound commands, and
              after error conditions that result in the shell aborting
              command execution.
       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.
              Seeding the random number generator with the same constant
              value produces the same sequence of values.  If RANDOM is
              unset, it loses its special properties, even if it is
              subsequently reset.
       READLINE_ARGUMENT
              Any numeric argument given to a readline command that was
              defined using “bind -x” (see SHELL BUILTIN COMMANDS below)
              when it was invoked.
       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 between 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, it expands to the
              number of seconds since shell invocation.  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 are always determined by
              querying the system clock at one-second resolution.  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, the shell enables each option in the list
              before reading any startup files.  If this variable is
              exported, child shells will enable each option in the list.
              This variable is read-only.
       SHLVL  Incremented by one each time an instance of bash is
              started.
       SRANDOM
              Each time it is referenced, this variable expands to a
              32-bit pseudo-random number.  The random number generator
              is not linear on systems that support /dev/urandom or
              arc4random(3), so each returned number has no relationship
              to the numbers preceding it.  The random number generator
              cannot be seeded, so assignments 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 shell uses the following variables. 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 COMPATIBILITY 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 default for the current
              version.  If BASH_COMPAT is set to a value that is not one
              of the valid compatibility levels, the shell prints an
              error message and sets the compatibility level to the
              default for the current version.  A subset of the valid
              values correspond to the compatibility levels described
              below under SHELL COMPATIBILITY MODE.  For example, 4.2 and
              42 are valid values that correspond 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 expanded value is interpreted as a filename
              containing commands to initialize the shell before it reads
              and executes commands from the script.  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 writes the trace output generated when
              “set -x” is enabled to that file descriptor, instead of the
              standard error.  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 directories where the shell looks for
              directories specified as arguments to the cd command.  A
              sample value is “.:~:/usr”.
       CHILD_MAX
              Set the number of exited child status values for the shell
              to remember.  Bash will not allow this value to be
              decreased below a POSIX-mandated minimum, and there is a
              maximum value (currently 8192) that this may not exceed.
              The minimum value is 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 enabled or
              in an interactive shell upon receipt of a SIGWINCH.
       COMPREPLY
              An array variable from which bash reads the possible
              completions generated by a shell function invoked by the
              programmable completion facility (see Programmable
              Completion below).  Each array element contains one
              possible completion.
       EMACS  If bash finds this variable in the environment when the
              shell starts with value “t”, it assumes that the shell is
              running in an Emacs shell buffer and disables line editing.
       ENV    Expanded and executed similarly to BASH_ENV (see INVOCATION
              above) when an interactive shell is invoked in posix mode.
       EXECIGNORE
              A colon-separated list of shell patterns (see Pattern
              Matching) defining the set 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 executable files.  The
              pattern matching honors the setting of the extglob shell
              option.
       FCEDIT The default editor for the fc builtin command.
       FIGNORE
              A colon-separated list of suffixes to ignore when
              performing filename completion (see READLINE below).  A
              filename whose suffix matches one of the entries in 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 cause the current command to abort.
       GLOBIGNORE
              A colon-separated list of patterns defining the set of file
              names to be ignored by pathname expansion.  If a file name
              matched by a pathname expansion pattern also matches one of
              the patterns in GLOBIGNORE, it is removed from the list of
              matches.  The pattern matching honors the setting of the
              extglob shell option.
       GLOBSORT
              Controls how the results of pathname expansion are sorted.
              The value of this variable specifies the sort criteria and
              sort order for the results of pathname expansion.  If this
              variable is unset or set to the null string, pathname
              expansion uses the historical behavior of sorting by name,
              in ascending lexicographic order as determined by the
              LC_COLLATE shell variable.

              If set, a valid value begins with an optional +, which is
              ignored, or -, which reverses the sort order from ascending
              to descending, followed by a sort specifier.  The valid
              sort specifiers are name, numeric, size, mtime, atime,
              ctime, and blocks, which sort the files on name, names in
              numeric rather than lexicographic order, file size,
              modification time, access time, inode change time, and
              number of blocks, respectively.  If any of the non-name
              keys compare as equal (e.g., if two files are the same
              size), sorting uses the name as a secondary sort key.

              For example, a value of -mtime sorts the results in
              descending order by modification time (newest first).

              The numeric specifier treats names consisting solely of
              digits as numbers and sorts them using their numeric value
              (so “2” sorts before “10”, for example).  When using
              numeric, names containing non-digits sort after all the
              all-digit names and are sorted by name using the
              traditional behavior.

              A sort specifier of nosort disables sorting completely;
              bash returns the results in the order they are read from
              the file system, ignoring any leading -.

              If the sort specifier is missing, it defaults to name, so a
              value of + is equivalent to the null string, and a value of
              - sorts by name in descending order.  Any invalid value
              restores the historical sorting behavior.
       HISTCONTROL
              A colon-separated list of values controlling how commands
              are saved on the history list.  If the list of values
              includes ignorespace, lines which begin with a space
              character are not saved in the history list.  A value of
              ignoredups causes lines matching the previous history entry
              not to 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, bash saves all lines
              read by the shell parser on the history list, subject to
              the value of HISTIGNORE.  If the first line of a multi-line
              compound command was saved, the second and subsequent lines
              are not tested, and are added to the history regardless of
              the value of HISTCONTROL.  If the first line was not saved,
              the second and subsequent lines of the command are not
              saved either.
       HISTFILE
              The name of the file in which command history is saved (see
              HISTORY below).  Bash assigns a default value of
              ~/.bash_history.  If HISTFILE is unset or null, the shell
              does not save the command history when it 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 the number
              of history entries that total no more than that number of
              lines by removing the oldest entries.  If the history list
              contains multi-line entries, the history file may contain
              more lines than this maximum to avoid leaving partial
              history entries.  The history file is also truncated to
              this size after writing it when a shell exits or by the
              history builtin.  If the value is 0, the history file is
              truncated to zero size.  Non-numeric values and numeric
              values less than zero inhibit truncation.  The shell sets
              the default value to the value of HISTSIZE after reading
              any startup files.
       HISTIGNORE
              A colon-separated list of patterns used to decide which
              command lines should be saved on the history list.  If a
              command line matches one of the patterns in the value of
              HISTIGNORE, it is not saved on the history list.  Each
              pattern is anchored at the beginning of the line and must
              match the complete line (bash does not  implicitly append a
              “*”).  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.  A backslash escapes the
              “&”; the backslash is removed before attempting a match.
              If the first line of a multi-line compound command was
              saved, the second and subsequent lines are not tested, and
              are added to the history regardless of the value of
              HISTIGNORE.  If the first line was not saved, the second
              and subsequent lines of the command are not saved either.
              The pattern matching honors the setting of the extglob
              shell option.
              HISTIGNORE subsumes some of the function of HISTCONTROL.  A
              pattern of “&” is identical to “ignoredups”, and a pattern
              of “[ ]*” is identical to “ignorespace”.  Combining these
              two patterns, separating them with a colon, provides the
              functionality of “ignoreboth”.
       HISTSIZE
              The number of commands to remember in the command history
              (see HISTORY below).  If the value is 0, commands are not
              saved in the history list.  Numeric values less than zero
              result in every command being saved on the history list
              (there is no limit).  The shell sets the default value to
              500 after reading any startup files.
       HISTTIMEFORMAT
              If this variable is set and not null, its value is used as
              a format string for strftime(3) to print the time stamp
              associated with each history entry displayed by the history
              builtin.  If this variable is set, the shell writes time
              stamps to the history file so they may be preserved across
              shell sessions.  This uses the history comment character to
              distinguish timestamps from other history lines.
       HOME   The home directory of the current user; the default
              argument for the cd builtin command.  The value of this
              variable is also used when performing tilde expansion.
       HOSTFILE
              Contains the name of a file in the same format as
              /etc/hosts that should be read when the shell needs to
              complete a hostname.  The list of possible hostname
              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, bash clears the
              hostname list.
       IFS    The Internal Field Separator that is used for word
              splitting after expansion and to split lines into words
              with the read builtin command.  Word splitting is described
              below under EXPANSION.  The default value is
              “<space><tab><newline>”.
       IGNOREEOF
              Controls the action of an interactive shell on receipt of
              an EOF character as the sole input.  If set, the value is
              the number of consecutive EOF characters which must be
              typed as the first characters on an input line before bash
              exits.  If the variable is set but does not have a numeric
              value, or the value is null, the default value is 10.  If
              it is unset, EOF signifies the end of input to the shell.
       INPUTRC
              The filename for the readline startup file, overriding the
              default of ~/.inputrc (see READLINE below).
       INSIDE_EMACS
              If this variable appears in the environment when the shell
              starts, bash assumes that it is running inside an Emacs
              shell buffer and may disable line editing, depending on the
              value of TERM.
       LANG   Used to determine the locale category for any category not
              specifically selected with a variable starting with LC_.
       LC_ALL This variable overrides the value of LANG and any other LC_
              variable specifying a locale category.
       LC_COLLATE
              This variable determines the collation order used when
              sorting the results of pathname expansion, and determines
              the behavior of range expressions, equivalence classes, and
              collating sequences within pathname expansion and pattern
              matching.
       LC_CTYPE
              This variable determines the interpretation of characters
              and the behavior of 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 formatting.
       LINES  Used by the select compound command to determine the column
              length for printing selection lists.  Automatically set if
              the checkwinsize option is enabled or in an interactive
              shell upon receipt of a SIGWINCH.
       MAIL   If the value is set to a file or directory name and the
              MAILPATH variable is not set, bash informs the user of the
              arrival of mail in the specified file or Maildir-format
              directory.
       MAILCHECK
              Specifies how often (in seconds) bash checks for mail.  The
              default is 60 seconds.  When it is time to check for mail,
              the shell does so before displaying the primary prompt.  If
              this variable is unset, or set to a value that is not a
              number greater than or equal to zero, the shell disables
              mail checking.
       MAILPATH
              A colon-separated list of filenames to be checked for mail.
              The message to be printed when mail arrives in a particular
              file may be specified by separating the filename from the
              message with a “?”.  When used in the text of the message,
              $_ expands to the name of the current mailfile.  For
              example:
              MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
              Bash can be configured to supply a default value for this
              variable (there is no value by default), but the location
              of the user mail files that it uses is system dependent
              (e.g., /var/mail/$USER).
       OPTERR If set to the value 1, bash displays error messages
              generated by the getopts builtin command (see SHELL BUILTIN
              COMMANDS below).  OPTERR is initialized to 1 each time the
              shell is invoked or a shell script is executed.
       PATH   The search path for commands.  It is a colon-separated list
              of directories in which the shell looks for commands (see
              COMMAND EXECUTION below).  A zero-length (null) directory
              name in the value of PATH indicates the current directory.
              A null directory name may appear as two adjacent colons, or
              as an initial or trailing colon.  The default path is
              system-dependent, and is set by the administrator who
              installs bash.  A common value is
                   /usr/local/bin:/usr/local/sbin:
                   /usr/bin:/usr/sbin:/bin:/sbin
       POSIXLY_CORRECT
              If this variable is in the environment when bash starts,
              the shell enters posix mode before reading the startup
              files, as if the --posix invocation option had been
              supplied.  If it is set while the shell is running, bash
              enables posix mode, as if the command “set -o posix” had
              been executed.  When the shell enters posix mode, it sets
              this variable if it was not already set.
       PROMPT_COMMAND
              If this variable is set, and is an array, the value of each
              set element is executed 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 character 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 timing information for pipelines
              prefixed with the time reserved word should be displayed.
              The % character introduces an escape sequence that is
              expanded to a time value or other information.  The escape
              sequences and their meanings are as follows; the brackets
              denote optional portions.
              %%     A literal %.
              %[p][l]R
                     The elapsed time in seconds.
              %[p][l]U
                     The number of CPU seconds spent in user mode.
              %[p][l]S
                     The number of CPU seconds spent in system mode.
              %P     The CPU percentage, computed as (%U + %S) / %R.

              The optional p is a digit specifying the precision, the
              number of fractional digits after a decimal point.  A value
              of 0 causes no decimal point or fraction to be output.
              time prints at most six digits after the decimal point;
              values of p greater than 6 are changed to 6.  If p is not
              specified, time prints three digits after the decimal
              point.

              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, bash does not display any timing information.  A
              trailing newline is added when the format string is
              displayed.
       TMOUT  If set to a value greater than zero, the read builtin uses
              the value as its default timeout.  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, simple
              commands consisting of only a single word, 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 or containing the
              word, this selects the most recently accessed job.  The
              name of a stopped job, in this context, is the command line
              used to start it, as displayed by jobs.  If set to the
              value exact, the word must match the name of a stopped job
              exactly; if set to substring, the word 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 (e.g., prefix), the word must be a prefix of a
              stopped job's name; this provides functionality analogous
              to the %string job identifier.
       histchars
              The two or three characters which control history
              expansion, quick substitution, and tokenization (see
              HISTORY EXPANSION below).  The first character is the
              history expansion character, the character which begins a
              history expansion, normally “!”.  The second character is
              the quick substitution character, normally “^”.  When it
              appears as the first character on the line, history
              substitution repeats the previous command, replacing one
              string with another.  The optional third character is the
              history comment character,  normally “#”, which indicates
              that the remainder of the line is a comment when it appears
              as the first character of a word.  The history comment
              character disables history substitution 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 explicitly declares an array.  There is no maximum
       limit on the size of an array, nor any requirement that members be
       indexed or assigned contiguously.  Indexed arrays are referenced
       using arithmetic expressions that must expand to an integer (see
       ARITHMETIC EVALUATION below) and are zero-based; associative
       arrays are referenced using arbitrary strings.  Unless otherwise
       noted, indexed array indices must be non-negative integers.

       The shell performs parameter and variable expansion, arithmetic
       expansion, command substitution, and quote removal on indexed
       array subscripts.  Since this can potentially result in empty
       strings, subscript indexing treats those as expressions that
       evaluate to 0.

       The shell performs tilde expansion, parameter and variable
       expansion, arithmetic expansion, command substitution, and quote
       removal on associative array subscripts.  Empty strings cannot be
       used as associative array keys.

       Bash automatically creates an indexed array 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 greater than or equal to zero.  To explicitly
       declare an indexed array, use
              declare -a name
       (see SHELL BUILTIN COMMANDS below).
              declare -a name[subscript]
       is also accepted; the subscript is ignored.

       Associative arrays are created using
              declare -A name
       .

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

       Arrays are assigned using compound assignments of the form
       name=(value1 ... valuen), where each value may be of the form
       [subscript]=string.  Indexed array assignments do not require
       anything but string.  Each value in the list is expanded using the
       shell expansions described below under EXPANSION, but values that
       are valid variable assignments including the brackets and
       subscript do not undergo brace expansion and word splitting, as
       with individual variable assignments.

       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 either 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 assigning to an indexed array, if name is subscripted by a
       negative number, that number is interpreted as relative to one
       greater than the maximum index of name, so negative indices count
       back from the end of the array, and an index of -1 references the
       last element.

       The “+=” operator appends to an array variable when assigning
       using the compound assignment syntax; see PARAMETERS above.

       An array element is 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,
       unless noted in the description of a builtin or word expansion.
       These subscripts differ only when the word appears within double
       quotes.  If the word is double-quoted, ${name[*]} expands to a
       single word with the value of each array member separated by the
       first character of the IFS special variable, and ${name[@]}
       expands each element of name to a separate word.  When there are
       no array members, ${name[@]} expands to nothing.  If the double-
       quoted expansion occurs within a word, the expansion of the first
       parameter is joined with the beginning part of the expansion of
       the original word, and the expansion of the last parameter is
       joined with the last part of the expansion 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
       negative 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 valid; bash creates an array
       if necessary.

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

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

       The unset builtin is used to destroy arrays.  unset
       name[subscript] unsets the array 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, removes the entire array.
       unset name[subscript] behaves differently depending on whether
       name is an indexed or associative array when subscript is * or @.
       If name is an associative array, this unsets the element with
       subscript * or @.  If name is an indexed array, unset removes all
       of the elements but does not remove the array itself.

       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, (e.g., unset a[4]), the argument is
       subject to pathname expansion.  Quote the argument if pathname
       expansion is not desired (e.g., unset 'a[4]').

       The declare, local, and readonly builtins each accept a -a option
       to specify an indexed array and a -A option to specify an
       associative array.  If both options are supplied, -A takes
       precedence.  The read builtin accepts a -a option to assign a list
       of words read from the standard input to an array.  The set and
       declare builtins display array values in a way that allows them to
       be reused as assignments.  Other builtins accept array name
       arguments as well (e.g., mapfile); see the descriptions of
       individual builtins below for details.  The shell provides a
       number of builtin array variables.

EXPANSION         top

       Expansion is performed on the command line after it has been split
       into words.  The shell performs these expansions: brace expansion,
       tilde expansion, parameter and variable expansion, command
       substitution, arithmetic expansion, word splitting, pathname
       expansion, and quote removal.

       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; pathname expansion; and quote removal.

       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.

       Quote removal is always performed last.  It removes quote
       characters present in the original word, not ones resulting from
       one of the other expansions, unless they have been quoted
       themselves.

       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 to generate arbitrary strings
       sharing a common prefix and suffix, either of which can be empty.
       This mechanism is similar to pathname expansion, but the filenames
       generated need not exist.  Patterns to be brace expanded are
       formed from 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 resulting string, expanding
       left to right.

       Brace expansions may be nested.  The results of each expanded
       string are not sorted; brace expansion preserves left to right
       order.  For example, a{d,c,b}e expands into “ade ace abe”.

       A sequence expression takes the form x..y[..incr], where x and y
       are either integers or single letters, and incr, an optional
       increment, is an integer.  When integers are supplied, the
       expression expands to each number between x and y, inclusive.  If
       either x or y begins with a zero, each generated term will contain
       the same number of digits, zero-padding where necessary.  When
       letters are supplied, the expression expands to each character
       lexicographically between x and y, inclusive, using the C locale.
       Note that both x and y must be of the same type (integer or
       letter).  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 Q , may be quoted with a backslash to prevent its being
       considered part of a brace expression.  To avoid conflicts with
       parameter expansion, the string “${” is not considered eligible
       for brace expansion, and inhibits brace expansion 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
       consequence of brace expansion.  For example, a word entered to sh
       as “file{1,2}” appears identically in the output.  Bash outputs
       that word as “file1 file2” after brace expansion.  Start bash with
       the +B option or disable brace expansion with the +B option to the
       set command (see SHELL BUILTIN COMMANDS below) for strict sh
       compatibility.

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

       If the tilde-prefix is a “~+”, the value of the shell variable PWD
       replaces the tilde-prefix.  If the tilde-prefix is a “~-”, the
       shell substitutes the value of the shell variable OLDPWD, if it is
       set.  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 characters following the tilde in the tilde-
       prefix as an argument.  If the characters following the tilde in
       the tilde-prefix consist of a number without a leading “+” or “-”,
       tilde expansion assumes “+”.

       The results of tilde expansion are treated as if they were quoted,
       so the replacement is not subject to word splitting and pathname
       expansion.

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

       Bash checks each variable assignment for unquoted tilde-prefixes
       immediately following a : or the first =, and performs tilde
       expansion in these cases.  Consequently, one may use filenames
       with tildes in assignments to PATH, MAILPATH, and CDPATH, and the
       shell assigns the expanded value.

       Bash also performs tilde expansion on words satisfying the
       conditions of variable assignments (as described above under
       PARAMETERS) when they appear as arguments to simple 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.

       The basic form of parameter expansion is

       ${parameter}

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

       If the first character of parameter is an exclamation point (!),
       and parameter is not a nameref, it introduces a level of
       indirection.  Bash uses the value formed by expanding the rest of
       parameter as the new parameter; this new parameter is then
       expanded and that value is used in the rest of the expansion,
       rather than the expansion of the original parameter.  This is
       known as indirect expansion.  The value is subject to tilde
       expansion, parameter expansion, command substitution, and
       arithmetic expansion.  If parameter is a nameref, this expands to
       the name of the parameter referenced by parameter instead of
       performing the complete indirect expansion, for compatibility.
       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 tests 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, and the
              expansion is the final value of parameter.  Positional
              parameters and special parameters may not be assigned in
              this way.

       ${parameter:?word}
              Display Error if Null or Unset.  If parameter is null or
              unset, the shell writes the expansion of word (or a message
              to that effect if word is not present) to the standard
              error and, if it is not interactive, exits with a non-zero
              status.  An interactive shell does not exit, but does not
              execute the command associated with the expansion.
              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.  The value of parameter is not used.

       ${parameter:offset}
       ${parameter:offset:length}
              Substring Expansion.  Expands to up to length characters of
              the value of parameter starting at the character specified
              by offset.  If parameter is @ or *, an indexed array
              subscripted by @ or *, or an associative array name, the
              results differ as described below.  If :length is omitted
              (the first form above), this expands to the substring of
              the value of parameter starting at the character specified
              by offset and extending to the end of the value.  If offset
              is omitted, it is treated as 0.  If length is omitted, but
              the colon after offset is present, it is treated as 0.
              length and offset are arithmetic expressions (see
              ARITHMETIC EVALUATION below).

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

              If parameter is @ or *, the result is length positional
              parameters beginning at offset.  A negative offset is taken
              relative to one greater than the greatest positional
              parameter, so an offset of -1 evaluates to the last
              positional parameter (or 0 if there are no positional
              parameters).  It is an expansion error if length evaluates
              to a number less than zero.

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

              Substring expansion applied to an associative array
              produces undefined results.

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

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

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

       ${#parameter}
              Parameter length.  Substitutes the length in characters of
              the expanded value of parameter.  If parameter is * or @,
              the value substituted is the number of positional
              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
              interpreted as relative to one greater than the maximum
              index of parameter, so negative indices count back from the
              end of the array, and an index of -1 references the last
              element.

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

       ${parameter/pattern/string}
       ${parameter//pattern/string}
       ${parameter/#pattern/string}
       ${parameter/%pattern/string}
              Pattern substitution.  The pattern is expanded to produce a
              pattern and matched against the expanded value of parameter
              as described under Pattern Matching below.  The longest
              match of pattern in the expanded value is replaced with
              string.  string undergoes tilde expansion, parameter and
              variable expansion, arithmetic expansion, command and
              process substitution, and quote removal.

              In the first form above, only the first match is replaced.
              If there are two slashes separating parameter and pattern
              (the second form above), all matches of pattern are
              replaced with string.  If pattern is preceded by # (the
              third form above), it must match at the beginning of the
              expanded value of parameter.  If pattern is preceded by %
              (the fourth form above), it must match at the end of the
              expanded value of parameter.

              If the expansion of string is null, matches of pattern are
              deleted and the / following pattern may be omitted.

              If the patsub_replacement shell option is enabled using
              shopt, any unquoted instances of & in string are replaced
              with the matching portion of pattern.

              Quoting any part of string inhibits replacement in the
              expansion of the quoted portion, including replacement
              strings stored in shell variables.  Backslash escapes & in
              string; the backslash is removed in order to permit a
              literal & in the replacement string.  Backslash can also be
              used to escape a backslash; \\ results in a literal
              backslash in the replacement.  Users should take care if
              string is double-quoted to avoid unwanted interactions
              between the backslash and double-quoting, since backslash
              has special meaning within double quotes.  Pattern
              substitution performs the check for unquoted & after
              expanding string; shell programmers should quote any
              occurrences of & they want to be taken literally in the
              replacement and ensure any instances of & they want to be
              replaced are unquoted.

              Like the pattern removal operators, double quotes
              surrounding the replacement string quote the expanded
              characters, while double quotes enclosing the entire
              parameter substitution do not, since the expansion is
              performed in a context that doesn't take any enclosing
              double quotes into account.

              If the nocasematch shell option is enabled, the match is
              performed without regard to the case of alphabetic
              characters.

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

       ${parameter^pattern}
       ${parameter^^pattern}
       ${parameter,pattern}
       ${parameter,,pattern}
              Case modification.  This expansion modifies the case of
              alphabetic characters in parameter.  First, the pattern is
              expanded to produce a pattern as described below under
              Pattern Matching.  Bash then examines characters in the
              expanded value of parameter against pattern as described
              below.  If a character matches the pattern, its case is
              converted.  The pattern should not attempt to match more
              than one character.

              Using “^” converts lowercase letters matching pattern to
              uppercase; “,” converts matching uppercase letters to
              lowercase.  The ^ and , variants examine the first
              character in the expanded value and convert its case if it
              matches pattern; the ^^ and ,, variants examine all
              characters in the expanded value and convert each one that
              matches pattern.  If pattern is omitted, it is treated like
              a ?, which matches every character.

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

       ${parameter@operator}
              Parameter transformation.  The expansion is either a
              transformation of the value of parameter or information
              about parameter itself, depending on the value of 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 declare command that, if
                     evaluated, recreates parameter with its attributes
                     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 sequence of
                     quoted key-value pairs (see Arrays above).  The keys
                     and values are quoted in a format that can be reused
                     as input.
              a      The expansion is a string consisting of flag values
                     representing parameter's attributes.
              k      Like the K transformation, but expands the keys and
                     values of indexed and associative arrays to separate
                     words after word splitting.

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

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

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

              $(command)
       or (deprecated)
              `command`.

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

       With the old-style backquote form of substitution, backslash
       retains its literal 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.

       There is an alternate form of command substitution:

              ${c command;}

       which executes command in the current execution environment and
       captures its output, again with trailing newlines removed.

       The character c following the open brace must be a space, tab,
       newline, or |, and the close brace must be in a position where a
       reserved word may appear (i.e., preceded by a command terminator
       such as semicolon).  Bash allows the close brace to be joined to
       the remaining characters in the word without being followed by a
       shell metacharacter as a reserved word would usually require.

       Any side effects of command take effect immediately in the current
       execution environment and persist in the current environment after
       the command completes (e.g., the exit builtin exits the shell).

       This type of command substitution superficially resembles
       executing an unnamed shell function: local variables are created
       as when a shell function is executing, and the return builtin
       forces command to complete; however, the rest of the execution
       environment, including the positional parameters, is shared with
       the caller.

       If the first character following the open brace is a |, the
       construct expands to the value of the REPLY shell variable after
       command executes, without removing any trailing newlines, and the
       standard output of command remains the same as in the calling
       shell.  Bash creates REPLY as an initially-unset local variable
       when command executes, and restores REPLY to the value it had
       before the command substitution after command completes, as with
       any local variable.

       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, bash does not
       perform word splitting and pathname expansion on the results.

   Arithmetic Expansion
       Arithmetic expansion evaluates an arithmetic expression and
       substitutes the result.  The format for arithmetic expansion is:

              $((expression))

       The expression undergoes the same expansions as if it were within
       double quotes, but unescaped double quote characters in expression
       are not treated specially and are removed.  All tokens in the
       expression undergo parameter and variable expansion, command
       substitution, and quote removal.  The result is treated as the
       arithmetic expression to be evaluated.  Since the way Bash handles
       double quotes can potentially result in empty strings, arithmetic
       expansion treats those as expressions that evaluate to 0.
       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 to standard error indicating failure, does not
       perform the substitution, and does not execute the command
       associated with the expansion.

   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 asynchronously, and its input or
       output appears as a filename.  This filename is passed as an
       argument to the current command as the result of the expansion.

       If the >(list) form is used, writing to the file provides input
       for list.  If the <(list) form is used, reading the file obtains
       the output of list.  No space may appear between the < or > and
       the left parenthesis, otherwise the construct would be interpreted
       as a redirection.

       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 arithmetic expansion that did not occur within
       double quotes for word splitting.  Words that were not expanded
       are not split.

       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.

       An IFS whitespace character is whitespace as defined above (see
       Definitions) that appears in the value of IFS.  Space, tab, and
       newline are always considered IFS whitespace, even if they don't
       appear in the locale's space category.

       If IFS is unset, field splitting acts as if its value were
       <space><tab><newline>, and treats these characters as IFS
       whitespace.  If the value of IFS is null, no word splitting
       occurs, but implicit null arguments (see below) are still removed.

       Word splitting begins by removing sequences of IFS whitespace
       characters from the beginning and end of the results of the
       previous expansions, then splits the remaining words.

       If the value of IFS consists solely of IFS whitespace, any
       sequence of IFS whitespace characters delimits a field, so a field
       consists of characters that are not unquoted IFS whitespace, and
       null fields result only from quoting.

       If IFS contains a non-whitespace character, then any character in
       the value of IFS that is not IFS whitespace, along with any
       adjacent IFS whitespace characters, delimits a field.  This means
       that adjacent non-IFS-whitespace delimiters produce a null field.
       A sequence of IFS whitespace characters also delimits a field.

       Explicit null arguments ("" or '') are retained and passed to
       commands as empty strings.  Unquoted implicit null arguments,
       resulting from the expansion of parameters that have no values,
       are removed.  Expanding a parameter with no value within double
       quotes produces a null field, which is retained and passed to a
       command as an empty string.

       When a quoted null argument appears as part of a word whose
       expansion is non-null, word splitting removes the null argument
       portion, leaving the non-null expansion.  That is, the word “-d''”
       becomes “-d” after word splitting and null argument removal.

   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 a sorted list of filenames
       matching the pattern (see Pattern Matching below) subject to the
       value of the GLOBSORT shell variable.

       If no matching filenames are found, and the shell option nullglob
       is not enabled, the word is left unchanged.  If the nullglob
       option is set, and no matches are found, the word is removed.  If
       the failglob shell option is set, and no matches are found, bash
       prints an error message and does not execute the command.  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.  In
       order to match the filenames . and .., the pattern must begin with
       “.” (for example, “.?”), even if dotglob is set.  If the
       globskipdots shell option is enabled, the filenames . and .. never
       match, even if the pattern begins with a “.”.  When not matching
       pathnames, 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, globskipdots,
       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
       “.” 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 GLOBIGNORE pattern matching honors the setting of the
       extglob shell option.

       The value of the GLOBSORT shell variable controls how the results
       of pathname expansion are sorted, as described above under Shell
       Variables.

       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 match all files and zero or
                     more directories and subdirectories.  If followed by
                     a /, two adjacent *s match only directories and
                     subdirectories.
              ?      Matches any single character.
              [...]  Matches any one of the characters enclosed between
                     the brackets.  This is known as a bracket expression
                     and matches a single character.  A pair of
                     characters separated by a hyphen denotes a range
                     expression; any character that falls between those
                     two characters, inclusive, using the current
                     locale's collating sequence and character set,
                     matches.  If the first character following the [ is
                     a !  or a ^ then any character not within the range
                     matches.  To match a -, include it as the first or
                     last character in the set.  To match a ], include it
                     as the first character in the set.

                     The sorting order of characters in range
                     expressions, and the characters included in the
                     range, are determined by the current locale and the
                     values of the LC_COLLATE or LC_ALL shell variables,
                     if set.  To obtain the traditional interpretation of
                     range expressions, where [a-d] is equivalent to
                     [abcd], set the value of the LC_COLLATE or LC_ALL
                     shell variables to C, or enable the globasciiranges
                     shell option.

                     Within a bracket expression, 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 a bracket expression, 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 a bracket expression, the syntax [.symbol.]
                     matches the collating symbol symbol.

       If the extglob shell option is enabled using the shopt builtin,
       the shell recognizes several extended pattern matching operators.
       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.

       The extglob option changes the behavior of the parser, since the
       parentheses are normally treated as operators with syntactic
       meaning.  To ensure that extended matching patterns are parsed
       correctly, make sure that extglob is enabled before parsing
       constructs containing the patterns, including shell functions and
       command substitutions.

       When matching filenames, the dotglob shell option determines the
       set of filenames that are tested: when dotglob is enabled, the set
       of filenames includes all files beginning with “.”, but . and ..
       must be matched by a pattern or sub-pattern that begins with a
       dot; when it is disabled, the set does not include any filenames
       beginning with “.” unless the pattern or sub-pattern begins with a
       “.”.  If the globskipdots shell option is enabled, the filenames .
       and .. never appear in the set.  As above, “.” only has a special
       meaning when matching filenames.

       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         top

       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.  When used with
       the exec builtin, redirections modify file handles in the current
       shell execution environment.  The following redirection operators
       may precede or appear anywhere within a simple command or may
       follow a command.  Redirections are processed in the order they
       appear, from left to right.

       Each redirection that may be preceded by a file descriptor number
       may instead be preceded by a word of the form {varname}.  In this
       case, for each redirection operator except >&- and <&-, the shell
       allocates a file descriptor greater than or equal to 10 and
       assigns it to varname.  If {varname} precedes >&- or <&-, the
       value of varname defines the file descriptor to close.  If
       {varname} is supplied, the redirection persists beyond the scope
       of the command, which allows the shell programmer to manage the
       file descriptor's lifetime manually without using the exec
       builtin.  The varredir_close shell option manages this behavior.

       In the following descriptions, if the file descriptor number is
       omitted, and the first character of the redirection operator is
       “<”, the redirection refers to the standard input (file descriptor
       0).  If the first character of the redirection operator is “>”,
       the 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,
       pathname expansion, and word splitting.  If it expands to more
       than one word, bash reports an error.

       The order of redirections is significant.  For example, the
       command

              ls > dirlist 2>&1

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

              ls 2>&1 > dirlist

       directs only the standard output to file dirlist, because the
       standard error was directed to the standard output before the
       standard output was redirected to dirlist.

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

              /dev/fd/fd
                     If fd is a valid integer, duplicate file descriptor
                     fd.
              /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
       Redirecting input opens the file whose name results from the
       expansion of word 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
       Redirecting output opens the file whose name results from the
       expansion of word for writing on file descriptor n, or the
       standard output (file descriptor 1) if n is not specified.  If the
       file does not exist it is created; if it does exist it is
       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 command has been enabled, the redirection fails if the
       file whose name results from the expansion of word exists and is a
       regular file.  If the redirection operator is >|, or the
       redirection operator is > and the noclobber option to the set
       builtin is not enabled, bash attempts the redirection even if the
       file named by word exists.

   Appending Redirected Output
       Redirecting output in this fashion opens the file whose name
       results from the expansion of word for appending on file
       descriptor n, or the standard output (file descriptor 1) if n is
       not specified.  If the file does not exist it is created.

       The general format for appending output is:

              [n]>>word

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

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

              &>word
       and
              >&word

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

              >word 2>&1

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

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

       The format for appending standard output and standard error is:

              &>>word

       This is semantically equivalent to

              >>word 2>&1

       (see Duplicating File Descriptors below).

   Here Documents
       This type of redirection instructs the shell to read input from
       the current source until it reads a line containing only delimiter
       (with no trailing blanks).  All of the lines read up to that point
       then become 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

       The shell does not perform parameter and variable expansion,
       command substitution, arithmetic expansion, or pathname expansion
       on word.

       If any part of word is quoted, the delimiter is the result of
       quote removal on word, and the lines in the here-document are not
       expanded.  If word is unquoted, the delimiter is word itself, and
       the here-document text is treated similarly to a double-quoted
       string: all lines of the here-document are subjected to parameter
       expansion, command substitution, and arithmetic expansion, the
       character sequence \<newline> is treated literally, and \ must be
       used to quote the characters \, $, and `; however, double quote
       characters have no special meaning.

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

       If the delimiter is not quoted, the \<newline> sequence is treated
       as a line continuation: the two lines are joined and the
       backslash-newline is removed.  This happens while reading the
       here-document, before the check for the ending delimiter, so
       joined lines can form the end delimiter.

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

              [n]<<<word

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

   Duplicating File Descriptors
       The redirection operator

              [n]<&word

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

       The operator

              [n]>&word

       is used similarly to duplicate output file descriptors.  If n is
       not specified, this uses the standard output (file descriptor 1).
       It is a redirection error if the digits in word do not specify a
       file descriptor open for output.  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 -, this redirects
       the standard output and standard error 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 descriptor 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

       opens the file whose name is the expansion of word 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         top

       Aliases allow a string to be substituted for a word that is in a
       position in the input where it can be the first word of a simple
       command.  Aliases have names and corresponding values that are set
       and unset using the alias and unalias builtin commands (see SHELL
       BUILTIN COMMANDS below).

       If the shell reads an unquoted word in the right position, it
       checks the word to see if it matches an alias name.  If it
       matches, the shell replaces the word with the alias value, and
       reads that value as if it had been read instead of the word.  The
       shell doesn't look at any characters following the word before
       attempting alias substitution.

       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, the shell
       checks the next command word following the alias 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, use a shell function (see FUNCTIONS
       below) instead.

       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 compound command, before
       executing any of the commands on that line or the compound
       command.  Aliases are expanded when a command is read, not when it
       is executed.  Therefore, an alias definition appearing on the same
       line as another command does not take effect until the shell reads
       the next line of input, and an alias definition in a compound
       command does not take effect until the shell parses and executes
       the entire compound command.  The commands following the alias
       definition on that line, or in the rest of a compound command, are
       not affected by the new alias.  This behavior is also an issue
       when functions are executed.  Aliases are expanded when a function
       definition is read, not when the function is executed, because a
       function definition is itself a command.  As a consequence,
       aliases defined in a function are not available until after that
       function is executed.  To be safe, always put alias definitions on
       a separate line, and do not use alias in compound commands.

       For almost every purpose, shell functions are preferable to
       aliases.

FUNCTIONS         top

       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 shell
       executes the list of commands associated with that function name.
       Functions are executed in the context of the calling shell; there
       is no new process created to interpret them (contrast this with
       the execution of a shell script).

       When a function is executed, the arguments to the function become
       the positional parameters during its execution.  The special
       parameter # is updated to reflect the new positional parameters.
       Special 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 are declared with the local
       builtin command (local variables).  Ordinarily, 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).

       In the following description, the current scope is a currently-
       executing function.  Previous scopes consist of that function's
       caller and so on, back to the “global” scope, where the shell is
       not executing any shell function.  A local variable at the current
       scope is a variable declared using the local or declare builtins
       in the function that is currently executing.

       Local variables “shadow” variables with the same name declared at
       previous scopes.  For instance, a local variable declared in a
       function hides variables with the same name declared at previous
       scopes, including global variables: references and assignments
       refer to the local variable, leaving the variables at previous
       scopes unmodified.  When the function returns, the global variable
       is once again visible.

       The shell uses dynamic scoping to control a variable's visibility
       within functions.  With dynamic scoping, visible variables and
       their values are a result of the sequence of 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 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 unsets it; otherwise
       the unset will refer to the variable found in any calling scope as
       described above.  If a variable at the current local scope is
       unset, it remains so (appearing as unset) until it is reset in
       that scope or until the function returns.  Once the function
       returns, any instance of the variable at a previous scope becomes
       visible.  If the unset acts on a variable at a previous scope, any
       instance of a variable with that name that had been shadowed
       becomes visible (see below how the localvar_unset shell option
       changes this behavior).

       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.  If return is supplied a numeric
       argument, that is the function's return status; otherwise the
       function's return status is the exit status of the last command
       executed before the return.  Any command associated with the
       RETURN trap is executed before execution resumes.  When a function
       completes, the values of the positional parameters and the special
       parameter # are restored to the values they had prior to the
       function's execution.

       The -f option to the declare or typeset builtin commands lists
       function names and definitions.  The -F option to declare or
       typeset lists the function names only (and optionally the source
       file and line number, if the extdebug shell option is enabled).
       Functions may be exported so that child shell processes (those
       created when executing a separate shell invocation) automatically
       have them defined with the -f option to the export builtin.  The
       -f option to the unset builtin deletes a function definition.

       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, bash imposes no limit on the
       number of recursive calls.

ARITHMETIC EVALUATION         top

       The shell allows arithmetic expressions to be evaluated, under
       certain circumstances (see the let and declare builtin commands,
       the (( compound command, the arithmetic for command, the [[
       conditional command, and Arithmetic Expansion).

       Evaluation is done in the largest fixed-width integers available,
       with no check for overflow, though division by 0 is trapped and
       flagged as an error.  The operators and their precedence,
       associativity, and values are the same as in the C language.  The
       following list of operators is grouped into levels of equal-
       precedence operators.  The levels are listed in order of
       decreasing precedence.

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

       Shell variables are allowed as operands; parameter expansion is
       performed before the expression is evaluated.  Within an
       expression, shell variables may also be referenced by name without
       using the parameter expansion syntax.  This means you can use "x",
       where x is a shell variable name, in an arithmetic expression, and
       the shell will evaluate its value as an expression and use the
       result.  A shell variable that is null or unset evaluates to 0
       when referenced by name in an expression.

       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 precedence order.  Sub-expressions in
       parentheses are evaluated first and may override the precedence
       rules above.

CONDITIONAL EXPRESSIONS         top

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

       Expressions are formed from the unary or binary primaries listed
       below.  Unary expressions are often used to examine the status of
       a file or shell variable.  Binary operators are used for string,
       numeric, and file attribute comparisons.

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

       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 [[, or when the shell is in posix mode, the < and >
       operators sort lexicographically using the current locale.  When
       the shell is not in posix mode, 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
              accessed.
       -O file
              True if file exists and is owned by the effective user id.
       -S file
              True if file exists and is a socket.
       -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).  If varname is an indexed array variable
              name subscripted by @ or *, this returns true if the array
              has any set elements.  If varname is an associative array
              variable name subscripted by @ or *, this returns true if
              an element with that key is set.
       -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.

       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.

       arg1 OP arg2
              OP is one of -eq, -ne, -lt, -le, -gt, or -ge.  These
              arithmetic binary operators return true if arg1 is equal
              to, not equal to, less than, less than or equal to, greater
              than, or greater than or equal to arg2, respectively.  arg1
              and arg2 may be positive or negative integers.  When used
              with the [[ command, arg1 and arg2 are evaluated as
              arithmetic expressions (see ARITHMETIC EVALUATION above).
              Since the expansions the [[ command performs on arg1 and
              arg2 can potentially result in empty strings, arithmetic
              expression evaluation treats those as expressions that
              evaluate to 0.

SIMPLE COMMAND EXPANSION         top

       When the shell executes a simple command, it performs the
       following expansions, assignments, 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 removal before being
              assigned to the variable.

       If no command name results, the variable assignments affect the
       current shell environment.  In the case of such a command (one
       that consists only of assignment statements and redirections),
       assignment statements are performed before redirections.
       Otherwise, the variables are added to the environment of the
       executed command and do not affect the current shell environment.
       If any of the assignments attempts to assign a value to a readonly
       variable, an error occurs, and the command exits with a non-zero
       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 below.  Otherwise, the command exits.  If
       one of the expansions contained a command substitution, the exit
       status of the command is the exit status of the last command
       substitution performed.  If there were no command substitutions,
       the command exits with a zero status.

COMMAND EXECUTION         top

       After a command has been split into words, if it results in a
       simple command and an optional list of arguments, the shell
       performs the following actions.

       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).  Bash performs a full
       search of the directories in PATH only if the command is not found
       in the hash table.  If the search is unsuccessful, the shell
       searches for a defined shell function named
       command_not_found_handle.  If that function exists, it is invoked
       in a separate execution environment with the original command and
       the original command's arguments as its arguments, and the
       function's exit status becomes the exit status of that subshell.
       If that function is not defined, the shell prints an error message
       and returns an exit status of 127.

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

       If this execution fails because the file is not in executable
       format, and the file is not a directory, it is assumed to be a
       shell script, a file containing shell commands, and the shell
       creates a new instance of itself to execute it.  Bash tries to
       determine whether the file is a text file or a binary, and will
       not execute files it determines to be binaries.  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
       under SHELL BUILTIN COMMANDS are retained by the child.

       If the program is a file beginning with #!, the remainder of the
       first line specifies an interpreter for the program.  The shell
       executes the specified interpreter on operating systems that do
       not handle this executable format themselves.  The arguments to
       the 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         top

       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 parent.

       •      Current traps set by trap.

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

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

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

       •      Options enabled by shopt.

       •      Shell aliases defined with alias.

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

       When a simple command other than a builtin or shell function is to
       be executed, it is invoked in a separate execution environment
       that consists of the following.  Unless 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.

       A subshell is a copy of the shell process.

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

       When the shell is in posix mode, subshells spawned to execute
       command substitutions inherit the value of the -e option from
       their parent shell.  When not in posix mode, bash clears the -e
       option in such subshells.  See the description of the
       inherit_errexit shell option below for how to control this
       behavior when not in posix mode.

       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         top

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

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

       If any parameter assignments, as described above in PARAMETERS,
       appear before a simple command, the variable assignments are part
       of that command's environment for as long as it executes.  These
       assignment statements affect only the environment seen by that
       command.  If these assignments precede a call to a shell function,
       the variables are local to the function and exported to that
       function's children.

       If the -k option is set (see the set builtin command below), then
       all parameter assignments 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 pathname of the command and passed to that command in its
       environment.

EXIT STATUS         top

       The exit status of an executed command is the value returned by
       the waitpid system call or equivalent function.  Exit statuses
       fall between 0 and 255, though, as explained below, the shell may
       use values above 125 specially.  Exit statuses from shell builtins
       and 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 succeeded.  So while an exit status of zero indicates
       success, a non-zero exit status indicates failure.

       When a command terminates on a fatal signal N, bash uses the value
       of 128+N as the exit status.

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

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

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

       The exit status of the last command is available in the special
       parameter $?.

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

SIGNALS         top

       When bash is interactive, in the absence of any traps, it ignores
       SIGTERM (so that kill 0 does not kill an interactive shell), and
       catches and handles SIGINT (so that the wait builtin is
       interruptible).  When bash receives SIGINT, it breaks out of any
       executing loops.  In all cases, bash ignores SIGQUIT.  If job
       control is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

       The trap builtin modifies the shell's signal handling, as
       described below.

       Non-builtin commands bash executes have signal handlers set to the
       values inherited by the shell from its parent, unless trap sets
       them to be ignored, in which case the child process will ignore
       them as well.  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.  The shell sends SIGCONT to stopped jobs to
       ensure that they receive the SIGHUP (see JOB CONTROL below for
       more information about running and stopped jobs).  To prevent the
       shell from sending the signal to a particular job, remove it from
       the jobs table with the disown builtin (see SHELL BUILTIN COMMANDS
       below) or mark it not to receive SIGHUP using disown -h.

       If the huponexit shell option has been set using 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, it will not execute the trap until
       the command completes.  If bash is waiting for an asynchronous
       command via the wait builtin, and it receives a signal for which a
       trap has been set, the wait builtin will return immediately with
       an exit status greater than 128, immediately after which the shell
       executes the trap.

       When job control is not enabled, and bash is waiting for a
       foreground command to complete, the shell receives keyboard-
       generated signals such as SIGINT (usually generated by ^C) that
       users commonly intend to send to that command.  This happens
       because the shell and the command are in the same process group as
       the terminal, and ^C sends SIGINT to all processes in that process
       group.  Since bash does not enable job control by default when the
       shell is not interactive, this scenario is most common in non-
       interactive shells.

       When job control is enabled, and bash is waiting for a foreground
       command to complete, the shell does not receive keyboard-generated
       signals, because it is not in the same process group as the
       terminal.  This scenario is most common in interactive shells,
       where bash attempts to enable job control by default.  See JOB
       CONTROL below for more information about process groups.

       When job control is not enabled, and bash receives SIGINT while
       waiting for a foreground command, it waits until that foreground
       command terminates and then decides what to do about the SIGINT:

       1.     If the command terminates due to the SIGINT, bash concludes
              that the user meant to send the SIGINT to the shell as
              well, and acts on the SIGINT (e.g., by running a SIGINT
              trap, exiting a non-interactive shell, or returning to the
              top level to read a new command).

       2.     If the command does not terminate due to SIGINT, the
              program handled the SIGINT itself and did not treat it as a
              fatal signal.  In that case, bash does not treat SIGINT as
              a fatal signal, either, instead assuming that the SIGINT
              was used as part of the program's normal operation (e.g.,
              emacs uses it to abort editing commands) or deliberately
              discarded.  However, bash will run any trap set on SIGINT,
              as it does with any other trapped signal it receives while
              it is waiting for the foreground command to complete, for
              compatibility.

       When job control is enabled, bash does not receive keyboard-
       generated signals such as SIGINT while it is waiting for a
       foreground command.  An interactive shell does not pay attention
       to the SIGINT, even if the foreground command terminates as a
       result, other than noting its exit status.  If the shell is not
       interactive, and the foreground command terminates due to the
       SIGINT, bash pretends it received the SIGINT itself (scenario 1
       above), for compatibility.

JOB CONTROL         top

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

       The shell associates a job with each pipeline.  It keeps a table
       of currently executing jobs, which the jobs command will display.
       Each job has a job number, which jobs displays between brackets.
       Job numbers start at 1.  When bash starts a job asynchronously (in
       the background), it prints a line that looks like:

              [1] 25647

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

       To facilitate the implementation of the user interface to job
       control, each process has a process group ID, and the operating
       system maintains the notion of a current terminal process group
       ID.  This terminal process group ID is associated with the
       controlling terminal.

       Processes that have the same process group ID are said to be part
       of the same process group.  Members of the foreground process
       group (processes whose process group ID is equal to the current
       terminal process group ID) receive keyboard-generated signals such
       as SIGINT.  Processes in the foreground process group are said to
       be foreground processes.  Background processes are those whose
       process group ID differs from the controlling terminal's; such
       processes are immune to keyboard-generated signals.  Only
       foreground processes are allowed to read from or, if the user so
       specifies with “stty tostop”, write to the controlling terminal.
       The system sends a SIGTTIN (SIGTTOU) signal to background
       processes which attempt to read from (write to when “tostop” is in
       effect) the terminal, 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
       stops that process and returns control to bash.  Typing the
       delayed suspend character (typically ^Y, Control-Y) causes the
       process stop when it attempts to read input from the terminal, and
       returns control to bash.  The user then manipulates the state of
       this job, using the bg command to continue it in the background,
       the fg command to continue it in the foreground, or the kill
       command to kill it.  The suspend character takes effect
       immediately, and has the additional side effect of discarding any
       pending output and typeahead.  To force a background process to
       stop, or stop a process that's not associated with the current
       terminal session, send it the SIGSTOP signal using kill.

       There are a number of ways to refer to a job in the shell.  The %
       character introduces a job specification (jobspec).

       Job number n may be referred to as %n.  A job may also be referred
       to using a prefix of the name used to start it, or using a
       substring that appears in its command line.  For example, %ce
       refers to a job whose command name begins with ce.  Using %?ce, on
       the other hand, refers to any job containing the string ce in its
       command line.  If the prefix or substring matches more than one
       job, bash reports an error.

       The symbols %% and %+ refer to the shell's notion of the current
       job.  A single % (with no accompanying job specification) also
       refers to the current job.  %- refers to the previous job.  When a
       job starts in the background, a job stops while in the foreground,
       or a job is resumed in the background, it becomes the current job.
       The job that was the current job becomes the previous job.  When
       the current job terminates, the previous job becomes the current
       job.  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 marked with a +,
       and the previous job with a -.

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

       The shell learns immediately whenever a job changes state.
       Normally, bash waits until it is about to print a prompt before
       notifying the user about changes in a job's status so as to not
       interrupt any other output, though it will notify of changes in a
       job's status after a foreground command in a list completes,
       before executing the next command in the list.  If the -b option
       to the set builtin command is enabled, bash reports status changes
       immediately.  Bash executes any trap on SIGCHLD for each child
       that terminates.

       When a job terminates and bash notifies the user about it, bash
       removes the job from the table.  It will not appear in jobs
       output, but wait will report its exit status, as long as it's
       supplied the process ID associated with the job as an argument.
       When the table is empty, job numbers start over at 1.

       If a user attempts to exit bash 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 the
       user immediately attempts to exit again, without an intervening
       command, bash does not print another warning, and terminates any
       stopped jobs.

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

PROMPTING         top

       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 examines the value of the array variable PROMPT_COMMAND just
       before printing each primary prompt.  If any elements in
       PROMPT_COMMAND are set and non-null, Bash executes each value, in
       numeric order, just as if it had been typed on the command line.
       Bash displays PS0 after it reads a command but before executing
       it.

       Bash displays PS4 as described above before tracing each command
       when the -x option is enabled.

       Bash allows the prompt strings PS0, PS1, PS2, and PS4, 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 representation.
                     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
                     (e.g., “ttys0”).
              \n     A newline.
              \r     A carriage return.
              \s     The name of the shell: the basename of $0 (the
                     portion following the final slash).
              \t     The current time in 24-hour HH:MM:SS format.
              \T     The current time in 12-hour HH:MM:SS format.
              \@     The current time in 12-hour am/pm format.
              \A     The current time in 24-hour HH:MM format.
              \u     The username of the current user.
              \v     The bash version (e.g., 2.00).
              \V     The bash release, version + patch level (e.g.,
                     2.00.0)
              \w     The value of the PWD shell variable ($PWD), with
                     $HOME abbreviated with a tilde (uses the value of
                     the PROMPT_DIRTRIM variable).
              \W     The basename of $PWD, with $HOME abbreviated with a
                     tilde.
              \!     The history number of this command.
              \#     The command number of this command.
              \$     If the effective UID is 0, a #, otherwise a $.
              \nnn   The character corresponding to the octal number nnn.
              \\     A backslash.
              \[     Begin a sequence of non-printing characters, which
                     could be used to embed a terminal control sequence
                     into the prompt.
              \]     End a sequence of non-printing characters.

       The command number and the history number are usually different:
       the history number of a command is its position in the history
       list, which may include commands restored from the history file
       (see HISTORY below), while the command number is the position in
       the sequence of commands executed during the current shell
       session.  After the string is decoded, it is expanded via
       parameter expansion, command substitution, arithmetic expansion,
       and quote removal, subject to the value of the promptvars shell
       option (see the description of the shopt command under SHELL
       BUILTIN COMMANDS below).  This can have unwanted side effects if
       escaped portions of the string appear within command substitution
       or contain characters special to word expansion.

READLINE         top

       This is the library that handles reading input when using an
       interactive shell, unless the --noediting option is supplied 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
       This section uses Emacs-style editing concepts and uses its
       notation for 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.  The Meta key is often labeled “Alt” or
       “Option”.

       On keyboards without a Meta key, M-x means ESC x, i.e., press and
       release the Escape key, then press and release the x key, in
       sequence.  This makes ESC the meta prefix.  The combination M-C-x
       means ESC Control-x: press and release the Escape key, then press
       and hold the Control key while pressing the x key, then release
       both.

       On some keyboards, the Meta key modifier produces characters with
       the eighth bit (0200) set.  You can use the enable-meta-key
       variable to control whether or not it does this, if the keyboard
       allows it.  On many others, the terminal or terminal emulator
       converts the metafied key to a key sequence beginning with ESC as
       described in the preceding paragraph.

       If your Meta key produces a key sequence with the ESC meta prefix,
       you can make M-key key bindings you specify (see Readline Key
       Bindings below) do the same thing by setting the force-meta-prefix
       variable.

       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) makes
       that command act in a backward direction.  Commands whose behavior
       with arguments deviates from this are noted below.

       The point is the current cursor position, and mark refers to a
       saved cursor position.  The text between the point and mark is
       referred to as the region.  Readline has the concept of an active
       region: when the region is active, readline redisplay highlights
       the region using the value of the active-region-start-color
       variable.  The enable-active-region variable turns this on and
       off.  Several commands set the region to active; those 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 accumulate the deleted
       text 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 shell variable.  If that variable is unset,
       the default is ~/.inputrc.  If that file  does not exist or cannot
       be read, readline looks for /etc/inputrc.  When a program that
       uses the readline library starts up, readline reads the
       initialization file and sets the key bindings and variables found
       there, before reading any user input.

       There are only a few basic constructs allowed in the inputrc file.
       Blank lines are ignored.  Lines beginning with a # are comments.
       Lines beginning with a $ indicate conditional constructs.  Other
       lines denote key bindings and variable settings.

       The default key-bindings in this section may be changed using key
       binding commands in the inputrc file.  Programs that use the
       readline library, including bash, 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.

       Key bindings may contain the following symbolic character names:
       DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT (a destructive
       backspace), SPACE, SPC, 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).  The
       difference between a macro and a command is that a macro is
       enclosed in single or double quotes.

   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 key sequence may be specified in one of two ways: as a
       symbolic key name, possibly with Meta- or Control- prefixes, or as
       a key sequence composed of one or more characters enclosed in
       double quotes.  The key sequence and name are separated by a
       colon.  There can be no whitespace between the name and the colon.

       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 none of the symbolic character names are
       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 available when
       specifying key sequences is
              \C-    A control prefix.
              \M-    Adding the meta prefix or converting the following
                     character to a meta character, as described below
                     under force-meta-prefix.
              \e     An escape character.
              \\     Backslash.
              \"     Literal ", a double quote.
              \'     Literal ', a single quote.

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

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

       Bash will display or modify the current readline key bindings with
       the bind builtin command.  The -o emacs or -o vi options to the
       set builtin (see SHELL BUILTIN COMMANDS below) change the editing
       mode during interactive use.

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

              set variable-name value
       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 readline reads a variable value, empty or null
       values, “on” (case-insensitive), and “1” are equivalent to On.
       All other values are equivalent to Off.

       The bind -V command lists the current readline variable names and
       values (see SHELL BUILTIN COMMANDS below).

       The variables and their default values are:

       active-region-start-color
              A string variable that controls the text color and
              background when displaying the text in the active region
              (see the description of enable-active-region below).  This
              string must not take up any physical character positions on
              the display, so it should consist only of terminal escape
              sequences.  It is output to the terminal before displaying
              the text in the active region.  This variable is reset to
              the default value whenever the terminal type changes.  The
              default value is the string that puts the terminal in
              standout mode, as obtained from the terminal's terminfo
              description.  A sample value might be “\e[01;33m”.
       active-region-end-color
              A string variable that “undoes” the effects of
              active-region-start-color and restores “normal” terminal
              display appearance after displaying text in the active
              region.  This string must not take up any physical
              character positions on the display, so it should consist
              only of terminal escape sequences.  It is output to the
              terminal after displaying the text in the active region.
              This variable is reset to the default value whenever the
              terminal type changes.  The default value is the string
              that restores the terminal from standout mode, as obtained
              from the terminal's terminfo description.  A sample value
              might be “\e[0m”.
       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 that are treated specially by the kernel's
              terminal driver to their readline equivalents.  These
              override the default readline bindings described here.
              Type “stty -a” at a bash prompt to see your current
              terminal settings, including the special control characters
              (usually cchars).
       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 definitions are taken from
              the value of the LS_COLORS environment variable.  If there
              is a color definition in $LS_COLORS for the custom suffix
              “.readline-colored-completion-prefix”, readline uses this
              color for the common prefix instead of its default.
       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 the readline insert-comment command
              inserts.  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 causes 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-insensitive fashion.
       completion-map-case (Off)
              If set to On, and completion-ignore-case is enabled,
              readline treats hyphens (-) and underscores (_) as
              equivalent when performing case-insensitive filename
              matching and completion.
       completion-prefix-display-length(0)
              The maximum length in characters of the common prefix of a
              list of possible completions that is displayed without
              modification.  When set to a value greater than zero,
              readline replaces common prefixes longer than this value
              with an ellipsis when displaying possible completions.  If
              a completion begins with a period, and eadline is
              completing filenames, it uses three underscores instead of
              an ellipsis.
       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 asks whether or not the user
              wishes to view them; otherwise readline simply lists them
              on the terminal.  A zero value means readline should never
              ask; negative values are treated as zero.
       convert-meta (On)
              If set to On, readline converts characters it reads that
              have the eighth bit set to an ASCII key sequence by
              clearing the eighth bit and prefixing it with an escape
              character (converting the character to have the meta
              prefix).  The default is On, but readline sets it to Off if
              the locale contains characters whose encodings may include
              bytes with the eighth bit set.  This variable is dependent
              on the LC_CTYPE locale category, and may change if the
              locale changes.  This variable also affects key bindings;
              see the description of force-meta-prefix below.
       disable-completion (Off)
              If set to On, readline inhibits word completion.
              Completion characters are 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 uses 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 immediately 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.  The \1 and \2 escapes begin and
              end sequences of non-printing characters, which can be used
              to embed a terminal control sequence into the mode string.
       enable-active-region (On)
              When this variable is set to On, readline allows certain
              commands to designate the region as active.  When the
              region is active, readline highlights the text in the
              region using the value of the active-region-start-color
              variable, which defaults to the string that enables the
              terminal's standout mode.  The active region shows the text
              inserted by bracketed-paste and any matching text found by
              incremental and non-incremental history searches.
       enable-bracketed-paste (On)
              When set to On, readline configures the terminal to insert
              each paste into the editing buffer as a single string of
              characters, instead of treating each character as if it had
              been read from the keyboard.  This is called
              bracketed-paste mode; it prevents readline from executing
              any editing commands bound to key sequences appearing in
              the pasted text.
       enable-keypad (Off)
              When set to On, readline tries  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 tries to enable any meta modifier
              key the terminal claims to support.  On many terminals, the
              Meta key is used to send eight-bit characters; this
              variable checks for the terminal capability that indicates
              the terminal can enable and disable a mode that sets the
              eighth bit of a character (0200) if the Meta key is held
              down when the character is typed (a meta character).
       expand-tilde (Off)
              If set to On, readline performs tilde expansion when it
              attempts word completion.
       force-meta-prefix (Off)
              If set to On, readline modifies its behavior when binding
              key sequences containing \M- or Meta- (see Key Bindings
              above) by converting a key sequence of the form \M-C or
              Meta-C to the two-character sequence ESC C (adding the meta
              prefix).  If force-meta-prefix is set to Off (the default),
              readline uses the value of the convert-meta variable to
              determine whether to perform this conversion: if
              convert-meta is On, readline performs the conversion
              described above; if it is Off, readline converts C to a
              meta character by setting the eighth bit (0200).
       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, bash sets the maximum number of
              history entries to the value of the HISTSIZE shell
              variable.  Setting history-size to a non-numeric value will
              set the maximum number of history entries to 500.
       horizontal-scroll-mode (Off)
              Setting this variable 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
              automatically enabled for terminals of height 1.
       input-meta (Off)
              If set to On, readline enables eight-bit input (that is, it
              does not clear the eighth bit in the characters it reads),
              regardless of what the terminal claims it can support.  The
              default is Off, but readline sets it to On if the locale
              contains characters whose encodings may include bytes with
              the eighth bit set.  This variable is dependent on the
              LC_CTYPE locale category, and its value may change if the
              locale changes.  The name meta-flag is a synonym for
              input-meta.
       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 terminate an
              incremental search.
       keymap (emacs)
              Set the current readline keymap.  The set of valid keymap
              names is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
              vi-command, and vi-insert.  vi is equivalent to vi-command;
              emacs is equivalent to emacs-standard.  The default value
              is emacs; the value of editing-mode also affects the
              default keymap.
       keyseq-timeout (500)
              Specifies the duration readline will wait for a character
              when reading an ambiguous key sequence (one that can form a
              complete key sequence using the input read so far, or can
              take additional input to complete a longer key sequence).
              If readline does not receive any input within the timeout,
              it uses the shorter but complete key sequence.  The value
              is specified in milliseconds, so a value of 1000 means that
              readline will wait one second for additional input.  If
              this variable is set to a value less than or equal to zero,
              or to a non-numeric value, readline waits 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, readline displays history lines that have
              been modified with a preceding asterisk (*).
       mark-symlinked-directories (Off)
              If set to On, completed names which are symbolic links to
              directories have a slash appended, subject to the value of
              mark-directories.
       match-hidden-files (On)
              This variable, when set to On, forces readline to match
              files whose names begin with a “.”  (hidden files) when
              performing filename completion.  If set to Off, the user
              must include the leading “.”  in the filename to be
              completed.
       menu-complete-display-prefix (Off)
              If set to On, menu completion displays the common prefix of
              the list of possible completions (which may be empty)
              before cycling through the list.
       output-meta (Off)
              If set to On, readline displays characters with the eighth
              bit set directly rather than as a meta-prefixed escape
              sequence.  The default is Off, but readline sets it to On
              if the locale contains characters whose encodings may
              include bytes with the eighth bit set.  This variable is
              dependent on the LC_CTYPE locale category, and its value
              may change if the locale changes.
       page-completions (On)
              If set to On, readline uses an internal pager resembling
              more(1) to display a screenful of possible completions at a
              time.
       prefer-visible-bell
              See bell-style.
       print-completions-horizontally (Off)
              If set to On, readline displays completions with matches
              sorted horizontally in alphabetical order, rather than down
              the screen.
       revert-all-at-newline (Off)
              If set to On, readline will undo all changes to history
              lines before returning when executing accept-line.  By
              default, history lines may be modified and retain
              individual undo lists across calls to readline.
       search-ignore-case (Off)
              If set to On, readline performs incremental and non-
              incremental history list searches in a case-insensitive
              fashion.
       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 similar to show-all-if-ambiguous.
              If set to On, words which have more than one possible
              completion without any possible partial completion (the
              possible completions don't share a common prefix) cause the
              matches to be listed immediately instead of ringing the
              bell.
       show-mode-in-prompt (Off)
              If set to On, add a string to the beginning of the prompt
              indicating the editing mode: emacs, vi command, or vi
              insertion.  The mode strings are user-settable (e.g.,
              emacs-mode-string).
       skip-completed-text (Off)
              If set to On, this alters the default completion behavior
              when inserting a single match into the line.  It's only
              active when performing completion in the middle of a word.
              If enabled, readline does not insert characters from the
              completion that match characters after point in the word
              being completed, so portions of the word following the
              cursor are not duplicated.
       vi-cmd-mode-string ((cmd))
              If the show-mode-in-prompt variable is enabled, this string
              is displayed immediately before the last line of the
              primary prompt when vi editing mode is active and in
              command mode.  The value is expanded like a key binding, so
              the standard set of meta- and control- prefixes and
              backslash escape sequences is available.  The \1 and \2
              escapes begin and end sequences of non-printing characters,
              which can be used to embed a terminal control sequence into
              the mode string.
       vi-ins-mode-string ((ins))
              If the show-mode-in-prompt variable is enabled, this string
              is displayed immediately before the last line of the
              primary prompt when vi editing mode is active and in
              insertion mode.  The value is expanded like a key binding,
              so the standard set of meta- and control- prefixes and
              backslash escape sequences is available.  The \1 and \2
              escapes begin and end sequences of non-printing characters,
              which can be used to embed a terminal control sequence into
              the mode string.
       visible-stats (Off)
              If set to On, a character denoting a file's type as
              reported by stat(2) is appended to the filename when
              listing possible completions.

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

       $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 required to isolate it.

              mode   The mode= form of the $if directive is used to test
                     whether readline is in emacs or vi mode.  This may
                     be used in conjunction with the set keymap 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 xterm to match both xterm and
                     xterm-256color, for instance.

              version
                     The version test may be used to perform comparisons
                     against specific readline versions.  The version
                     expands to the current readline version.  The set of
                     comparison operators includes =, (and ==), !=, <=,
                     >=, <, and >.  The version number supplied on the
                     right side of the operator consists of a major
                     version number, an optional decimal point, and an
                     optional minor version (e.g., 7.1).  If the minor
                     version is omitted, it defaults to 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 settings.  Each program using
                     the readline library sets the application name, and
                     an initialization file can test for a particular
                     value.  This could be used to bind key sequences to
                     functions useful for a specific program.  For
                     instance, the following command adds a key sequence
                     that quotes the current or previous word in bash:

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

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

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

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

       $include
              This directive takes a single filename as an argument and
              reads commands and key 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 below) for lines containing a specified
       string.  There are two search modes: incremental and non-
       incremental.

       Incremental searches begin before the user has finished typing the
       search string.  As each character of the search string is typed,
       readline displays the next entry from the history matching the
       string typed so far.  An incremental search requires only as many
       characters as needed to find the desired history entry.  When
       using emacs editing mode, type C-r to search backward in the
       history for a particular string.  Typing C-s searches forward
       through the history.  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, ESC and
       C-j terminate an incremental search.  C-g aborts an incremental
       search and restores the original line.  When the search is
       terminated, the history entry containing the search string becomes
       the current line.

       To find other matching entries in the history list, type C-r or
       C-s as appropriate.  This searches 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
       terminates the search and executes that command.  For instance, a
       newline terminates the search and accepts the line, thereby
       executing the command from the history list.  A movement command
       will terminate the search, make the last line found the current
       line, and begin editing.

       Readline remembers the last incremental search string.  If two
       C-rs are typed without any intervening characters defining a new
       search string, readline uses any remembered search string.

       Non-incremental searches read the entire search string before
       starting to search for matching history entries.  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
       position, and mark refers to a cursor position saved by the
       set-mark command.  The text between the point and mark is referred
       to as the region.  Readline has the concept of an active region:
       when the region is active, readline redisplay highlights the
       region using the value of the active-region-start-color variable.
       The enable-active-region readline variable turns this on and off.
       Several commands set the region to active; those are noted below.

   Commands for Moving
       beginning-of-line (C-a)
              Move to the start of the current line.  This may also be
              bound to the Home key on some keyboards.
       end-of-line (C-e)
              Move to the end of the line.  This may also be bound to the
              End key on some keyboards.
       forward-char (C-f)
              Move forward a character.  This may also be bound to the
              right arrow key on some keyboards.
       backward-char (C-b)
              Move back a character.  This may also be bound to the left
              arrow key on some keyboards.
       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 (M-C-f)
              Move forward to the end of the next word.  Words are
              delimited by non-quoted shell metacharacters.
       shell-backward-word (M-C-b)
              Move back to the start of the current or previous word.
              Words are delimited by non-quoted shell metacharacters.
       previous-screen-line
              Attempt to move point to the same physical screen column on
              the previous physical screen line.  This will not have the
              desired effect if the current readline line does not take
              up more than one physical line or if point is not greater
              than the length of the prompt plus the screen width.
       next-screen-line
              Attempt to move point to the same physical screen column on
              the next physical screen line.  This will not have the
              desired effect if the current readline line does not take
              up more than one physical line or if the length of the
              current 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 redraw 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 a numeric
              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 and HISTIGNORE variables.  If
              the line is a modified history line, 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.  This may also be bound to the up arrow
              key on some keyboards.
       next-history (C-n)
              Fetch the next command from the history list, moving
              forward in the list.  This may also be bound to the down
              arrow key on some keyboards.
       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.
       operate-and-get-next (C-o)
              Accept the current line for execution as if a newline had
              been entered, 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.
       fetch-history
              With a numeric argument, fetch that entry from the history
              list and make it the current line.  Without an argument,
              move back to the first entry in the history list.
       reverse-search-history (C-r)
              Search backward starting at the current line and moving
              “up” through the history as necessary.  This is an
              incremental search.  This command sets the region to the
              matched text and activates the region.
       forward-search-history (C-s)
              Search forward starting at the current line and moving
              “down” through the history as necessary.  This is an
              incremental search.  This command sets the region to the
              matched text and activates the region.
       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.  The search string may match anywhere in a
              history line.
       non-incremental-forward-search-history (M-n)
              Search forward through the history using a non-incremental
              search for a string supplied by the user.  The search
              string may match anywhere in a history line.
       history-search-backward
              Search backward through the history for the string of
              characters between the start of the current line and the
              point.  The search string must match at the beginning of a
              history line.  This is a non-incremental search.  This may
              be bound to the Page Up key on some keyboards.
       history-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 must match at the beginning of a
              history line.  This is a non-incremental search.  This may
              be bound to the Page Down key on some keyboards.
       history-substring-search-backward
              Search backward 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.
       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, this
              uses the history expansion facilities to extract the nth
              word, as if the “!n” history expansion had been specified.
       yank-last-arg (M-., M-_)
              Insert the last argument to the previous command (the last
              word of the previous history entry).  With a numeric
              argument, behave exactly like yank-nth-arg.  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 successive calls
              determines the direction to move through the history.  A
              negative argument switches the direction through the
              history (back or forward).  This uses the history expansion
              facilities to extract the last word, as if the “!$” history
              expansion had been specified.
       shell-expand-line (M-C-e)
              Expand the line by performing shell word expansions.  This
              performs alias and history expansion, $'string' and
              $"string" quoting, tilde expansion, parameter and variable
              expansion, arithmetic expansion, command and process
              substitution, word splitting, and quote removal.  An
              explicit argument suppresses command and process
              substitution.  See HISTORY EXPANSION below for a
              description of history expansion.
       history-expand-line (M-^)
              Perform history expansion on the current line.  See HISTORY
              EXPANSION below for a description of history expansion.
       magic-space
              Perform history expansion on the current line and insert a
              space.  See HISTORY EXPANSION below for a description of
              history expansion.
       alias-expand-line
              Perform alias expansion on the current line.  See ALIASES
              above for a description of alias expansion.
       history-and-alias-expand-line
              Perform history and alias expansion on the current line.
       insert-last-argument (M-., M-_)
              A synonym for yank-last-arg.
       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(1).  If this character is read when there are no
              characters on the line, and point is at the beginning of
              the line, readline interprets it as the end of input and
              returns EOF.
       delete-char (C-d)
              Delete the character at point.  If this function is bound
              to the same character as the tty EOF character, as C-d
              commonly is, see above for the effects.  This may also be
              bound to the Delete key on some keyboards.
       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.
       bracketed-paste-begin
              This function is intended to be bound to the “bracketed
              paste” escape sequence sent by some terminals, and such a
              binding is assigned by default.  It allows readline to
              insert the pasted text as a single unit without treating
              each character as if it had been read from the keyboard.
              The pasted characters are inserted as if each one was bound
              to self-insert instead of executing any editing commands.
              Bracketed paste sets the region to the inserted text and
              activates the region.
       transpose-chars (C-t)
              Drag the character before point forward over the character
              at point, moving point forward as well.  If point is at the
              end of the line, then this transposes the two characters
              before point.  Negative arguments have no effect.
       transpose-words (M-t)
              Drag the word before point past the word after point,
              moving point past that word as well.  If point is at the
              end of the line, this transposes the last two words on the
              line.
       shell-transpose-words (M-C-t)
              Drag the word before point past the word after point,
              moving point past that word as well.  If the insertion
              point is at the end of the line, this transposes the last
              two words on the line.  Word boundaries are the same as
              shell-forward-word and shell-backward-word.
       upcase-word (M-u)
              Uppercase the current (or following) word.  With a negative
              argument, uppercase the previous word, but do not move
              point.
       downcase-word (M-l)
              Lowercase the current (or following) word.  With a negative
              argument, lowercase the previous word, but do not move
              point.
       capitalize-word (M-c)
              Capitalize the current (or following) word.  With a
              negative argument, capitalize the previous word, but do not
              move point.
       overwrite-mode
              Toggle overwrite mode.  With an explicit positive numeric
              argument, switches to overwrite mode.  With an explicit
              non-positive numeric argument, switches to insert mode.
              This command affects only emacs mode; vi mode does
              overwrite differently.  Each call to readline() starts in
              insert mode.
              In overwrite mode, characters bound to self-insert replace
              the text at point rather than pushing the text to the
              right.  Characters bound to backward-delete-char replace
              the character before point with a space.  By default, this
              command is unbound, but may be bound to the Insert key on
              some keyboards.

   Killing and Yanking
       kill-line (C-k)
              Kill the text from point to the end of the current line.
              With a negative numeric argument, kill backward from the
              cursor to the beginning of the line.
       backward-kill-line (C-x Rubout)
              Kill backward to the beginning of the current line.  With a
              negative numeric argument, kill forward from the cursor to
              the end of the line.
       unix-line-discard (C-u)
              Kill backward from point to the beginning of the line,
              saving the killed text on the kill-ring.
       kill-whole-line
              Kill all characters on the current line, no matter where
              point is.
       kill-word (M-d)
              Kill from point to the end of the current word, or if
              between words, to the end of the next word.  Word
              boundaries are the same as those used by forward-word.
       backward-kill-word (M-Rubout)
              Kill the word behind point.  Word boundaries are the same
              as those used by backward-word.
       shell-kill-word (M-C-d)
              Kill from point to the end of the current word, or if
              between words, to the end of the next word.  Word
              boundaries are the same as those used by
              shell-forward-word.
       shell-backward-kill-word
              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, saving the killed text on the kill-ring.
       unix-filename-rubout
              Kill the word behind point, using white space and the slash
              character as the word boundaries, saving the killed text 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, so it can
              be yanked immediately.
       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-argument again ends the numeric argument, but is
              otherwise ignored.  As a special case, if this command is
              immediately followed by a character that is neither a digit
              nor minus sign, the argument count for the next command is
              multiplied by four.  The argument count is initially one,
              so executing this function the first time makes the
              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 completion by first checking for any
              programmable completions for the command word (see
              Programmable Completion below), otherwise treating the text
              as a variable (if the text begins with $), username (if the
              text begins with ~), hostname (if the text begins with @),
              or command (including aliases, functions, and builtins) in
              turn.  If none of these produces a match, it falls back to
              filename completion.
       possible-completions (M-?)
              List the possible completions of the text before point.
              When displaying completions, readline sets the number of
              columns used for display to the value of completion-
              display-width, the value of the shell variable COLUMNS, or
              the screen width, in that order.
       insert-completions (M-*)
              Insert all completions of the text before point that would
              have been generated by possible-completions, separated by a
              space.
       menu-complete
              Similar to complete, but replaces the word to be completed
              with a single match from the list of possible completions.
              Repeatedly executing menu-complete steps through the list
              of possible completions, inserting each match in turn.  At
              the end of the list of completions, menu-complete rings the
              bell (subject to the setting of bell-style) and restores
              the original text.  An argument of n moves n positions
              forward in the list of matches; a negative argument moves
              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.
       export-completions
              Perform completion on the word before point as described
              above and write the list of possible completions to
              readline's output stream using the following format,
              writing information on separate lines:

              •      the number of matches N;
              •      the word being completed;
              •      S:E, where S and E are the start and end offsets of
                     the word in the readline line buffer; then
              •      each match, one per line

              If there are no matches, the first line will be “0”, and
              this command does not print any output after the S:E.  If
              there is only a single match, this prints a single line
              containing it.  If there is more than one match, this
              prints the common prefix of the matches, which may be
              empty, on the first line after the S:E, then the matches on
              subsequent lines.  In this case, N will include the first
              line with the common prefix.

              The user or application should be able to accommodate the
              possibility of a blank line.  The intent is that the user
              or application reads N lines after the line containing S:E
              to obtain the match list.  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).  At the
              end of the line, it 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 filename.

       complete-username (M-~)
              Attempt completion on the text before point, treating it as
              a username.

       possible-username-completions (C-x ~)
              List the possible completions of the text before point,
              treating it as a username.

       complete-variable (M-$)
              Attempt completion on the text before point, treating it as
              a shell variable.

       possible-variable-completions (C-x $)
              List the possible completions of the text before point,
              treating it as a shell variable.

       complete-hostname (M-@)
              Attempt completion on the text before point, treating it as
              a hostname.

       possible-hostname-completions (C-x @)
              List the possible completions of the text before point,
              treating it as a hostname.

       complete-command (M-!)
              Attempt completion on the text before point, treating it as
              a command name.  Command completion attempts to match the
              text against aliases, reserved words, shell functions,
              shell builtins, and finally executable filenames, in that
              order.

       possible-command-completions (C-x !)
              List the possible completions of the text before point,
              treating it as a command name.

       dynamic-complete-history (M-TAB)
              Attempt completion on the text before point, comparing the
              text against history list entries for possible completion
              matches.

       dabbrev-expand
              Attempt menu completion on the text before point, comparing
              the text against lines from the history list for possible
              completion matches.

       complete-into-braces (M-{)
              Perform filename completion and insert the list of possible
              completions enclosed within braces so the list is available
              to the shell (see Brace Expansion above).

   Keyboard Macros
       start-kbd-macro (C-x ()
              Begin saving the characters typed into the current keyboard
              macro.
       end-kbd-macro (C-x ))
              Stop saving the characters typed into the current keyboard
              macro and store the 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-<space>)
              Set the mark to the point.  If a numeric argument is
              supplied, set the mark to that position.
       exchange-point-and-mark (C-x C-x)
              Swap the point with the mark.  Set the current cursor
              position to the saved position, then set the mark to the
              old cursor position.
       character-search (C-])
              Read a character and move point to the next occurrence of
              that character.  A negative argument searches for previous
              occurrences.
       character-search-backward (M-C-])
              Read a character and move point to the previous occurrence
              of that character.  A negative argument 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.  CSI sequences
              begin with a Control Sequence Indicator (CSI), usually ESC
              [.  If this sequence is bound to “\e[”, keys producing CSI
              sequences 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, insert the value of the
              readline comment-begin variable 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, insert the value; otherwise delete the
              characters in comment-begin 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.
       spell-correct-word (C-x s)
              Perform spelling correction on the current word, treating
              it as a directory or filename, in the same way as the
              cdspell shell option.  Word boundaries are the same as
              those used by shell-forward-word.
       glob-complete-word (M-g)
              Treat the word before point as a pattern for pathname
              expansion, with an asterisk implicitly appended, then use
              the pattern to generate a list of matching file names for
              possible completions.
       glob-expand-word (C-x *)
              Treat the word before point as a pattern for pathname
              expansion, and insert the list of matching file names,
              replacing the word.  If a numeric argument is supplied,
              append a * before pathname expansion.
       glob-list-expansions (C-x g)
              Display the list of expansions that would have been
              generated by glob-expand-word and redisplay the line.  If a
              numeric argument is supplied, append a * 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 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.
       execute-named-command (M-x)
              Read a bindable readline command name from the input and
              execute the function to which it's bound, as if the key
              sequence to which it was bound appeared in the input.  If
              this function is supplied with a numeric argument, it
              passes that argument to the function it executes.
       display-shell-version (C-x C-v)
              Display version information about the current instance of
              bash.

   Programmable Completion
       When a user attempts word completion for a command or an argument
       to a command for which a completion specification (a compspec) has
       been defined using the complete builtin (see SHELL BUILTIN
       COMMANDS below), readline invokes the programmable completion
       facilities.

       First, bash identifies the command name.  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
       the empty string (completion attempted at the beginning of an
       empty line), bash uses any compspec defined with the -E option to
       complete.  The -I option to complete indicates that the command
       word is the first non-assignment word on the line, or after a
       command delimiter such as ; or |.  This usually indicates command
       name completion.

       If the command word is a full pathname, bash searches for a
       compspec for the full pathname first.  If there is no compspec for
       the full pathname, bash attempts to find a compspec for the
       portion following the final slash.  If those searches do not
       result in a compspec, or if there is no compspec for the command
       word, bash uses any compspec defined with the -D option to
       complete as the default.  If there is no default compspec, bash
       performs alias expansion on the command word as a final resort,
       and attempts to find a compspec for the command word resulting
       from any successful expansion.

       If a compspec is not found, bash performs its default completion
       as described above under Completing.  Otherwise, once a compspec
       has been found, bash uses it to generate the list of matching
       words.

       First, bash performs the actions specified by the compspec.  This
       only returns matches which are prefixes of the word being
       completed.  When the -f or -d option is used for filename or
       directory name completion, bash uses the shell variable FIGNORE to
       filter the matches.

       Next, programmable completion generates matches specified by a
       pathname expansion pattern supplied as an argument to the -G
       option.  The words generated by the pattern need not match the
       word being completed.  Bash uses the FIGNORE variable to filter
       the matches, but does not use the GLOBIGNORE shell variable.

       Next, completion considers the string specified as the argument to
       the -W option.  The string is first split using the characters in
       the IFS special variable as delimiters.  This honors shell quoting
       within the string, in order to provide a mechanism for the words
       to contain shell metacharacters or characters in the value of IFS.
       Each word is then expanded using brace expansion, tilde expansion,
       parameter and variable expansion, command substitution, and
       arithmetic expansion, as described above under EXPANSION.  The
       results are split using the rules described above under Word
       Splitting.  The results of the expansion are prefix-matched
       against the word being completed, and the matching words become
       possible completions.

       After these matches have been generated, bash executes any shell
       function or command specified with the -F and -C options.  When
       the command or function is invoked, bash assigns values to the
       COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables as
       described above under Shell Variables.  If a shell function is
       being invoked, bash also sets the COMP_WORDS and COMP_CWORD
       variables.  When the function or command is invoked, the first
       argument ($1) is the name of the command whose arguments are being
       completed, the second argument ($2) is the word being completed,
       and the third argument ($3) is the word preceding the word being
       completed on the current command line.  There is no filtering of
       the generated completions against the word being completed; the
       function or command has complete freedom in generating the matches
       and they do not need to match a prefix of the word.

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

       Next, any command specified with the -C option is invoked in an
       environment equivalent to command substitution.  It should print a
       list of completions, one per line, to the standard output.
       Backslash will escape a newline, if necessary.  These are added to
       the set of possible completions.

       After generating all of the possible completions, bash applies any
       filter specified with the -X option to the completions in the
       list.  The filter is a pattern as used for pathname expansion; a &
       in the pattern is replaced with the text of the word being
       completed.  A literal & may be escaped with a backslash; the
       backslash is removed before attempting a match.  Any completion
       that matches the pattern is removed from the list.  A leading !
       negates the pattern; in this case bash removes any completion that
       does not match the pattern.  If the nocasematch shell option is
       enabled, bash performs the match without regard to the case of
       alphabetic characters.

       Finally, programmable completion adds any prefix and suffix
       specified with the -P and -S options, respectively, to each
       completion, and returns the result to readline 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, bash attempts directory name completion.

       If the -o plusdirs option was supplied to complete when the
       compspec was defined, bash attempts directory name completion and
       adds any matches to the set of possible completions.

       By default, if a compspec is found, whatever it generates is
       returned to the completion code as the full set of possible
       completions.  The default bash completions and the readline
       default of filename completion are disabled.  If the -o
       bashdefault option was supplied to complete when the compspec was
       defined, and the compspec generates no matches, bash attempts its
       default completions.  If the compspec and, if attempted, the
       default bash completions generate no matches, and the -o default
       option was supplied to complete when the compspec was defined,
       programmable completion performs readline's default completion.

       The options supplied to complete and compopt can control how
       readline treats the completions.  For instance, the -o fullquote
       option tells readline to quote the matches as if they were
       filenames.  See the description of complete below for details.

       When a compspec indicates that it wants directory name completion,
       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 variable.

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

       For instance, assuming that there is a library of compspecs, each
       kept in a file corresponding 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         top

       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 shell saves
       the text of the last HISTSIZE commands (default 500).  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, bash initializes the history list by reading history
       entries from the file named by the HISTFILE variable (default
       ~/.bash_history).  That file is referred to as the history file.
       The history file is truncated, if necessary, to contain no more
       than the number of history entries specified by the value of the
       HISTFILESIZE variable.  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 timestamps
       are optionally displayed depending on the value of the
       HISTTIMEFORMAT variable.  When present, history timestamps delimit
       history entries, making multi-line entries possible.

       When a shell with history enabled exits, bash copies the last
       $HISTSIZE entries from the history list to $HISTFILE.  If the
       histappend shell option is enabled (see the description of shopt
       under SHELL BUILTIN COMMANDS below), bash appends the entries to
       the history file, otherwise it overwrites the history file.  If
       HISTFILE is unset or null, or if the history file is unwritable,
       the history is not saved.  After saving the history, bash
       truncates the history file to contain no more than HISTFILESIZE
       lines as described above.

       If the HISTTIMEFORMAT variable is set, the shell writes the
       timestamp information associated with each history entry to the
       history file, marked with the history comment character, so
       timestamps are preserved across shell sessions.  This uses the
       history comment character to distinguish timestamps from other
       history lines.  As above, when using HISTTIMEFORMAT, the
       timestamps delimit multi-line history entries.

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

       The shell allows control over which commands are saved on the
       history list.  The HISTCONTROL and HISTIGNORE variables are used
       to save only a subset of the commands entered.  If the cmdhist
       shell option is enabled, the shell attempts 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 modifies cmdhist by saving the command with embedded
       newlines instead of semicolons.  See the description of the shopt
       builtin below under SHELL BUILTIN COMMANDS for information on
       setting and unsetting shell options.

HISTORY EXPANSION         top

       The shell supports a history expansion feature that is similar to
       the history expansion in csh.  This section describes what syntax
       features are available.

       History expansion is enabled by default for interactive shells,
       and can be disabled using the +H option to the set builtin command
       (see SHELL BUILTIN COMMANDS below).  Non-interactive shells do not
       perform history expansion by default, but it can be enabled with
       “set -H”.

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

       History expansion is performed immediately after a complete line
       is read, before the shell breaks it into words, and is performed
       on each line individually.  The shell attempts to inform the
       history expansion functions about quoting still in effect from
       previous lines.

       It takes place in two parts.  The first is to determine which
       history list entry to use during substitution.  The second is to
       select portions of that entry to include into the current one.

       The entry selected from the history is the event, and the portions
       of that entry that are acted upon are words.  Various modifiers
       are available to manipulate the selected words.  The entry is
       split into words in the same fashion as when reading input, so
       that several metacharacter-separated words surrounded by quotes
       are considered one word.  The event designator selects the event,
       the optional word designator selects words from the event, and
       various optional modifiers are available to manipulate the
       selected words.

       History expansions are introduced by the appearance of the history
       expansion character, which is ! by default.  History expansions
       may appear anywhere in the input, but do not nest.

       Only backslash (\) and single quotes can quote the history
       expansion character, but the history expansion character is also
       treated as quoted if it immediately precedes the closing double
       quote in a double-quoted string.

       Several characters inhibit history expansion if found immediately
       following the history expansion character, even if it is unquoted:
       space, tab, newline, carriage return, =, and the other shell
       metacharacters defined above.

       There is a special abbreviation for substitution, active when the
       quick substitution character (described above under histchars) is
       the first character on the line.  It selects the previous history
       list entry, using an event designator equivalent to !!, and
       substitutes one string for another in that entry.  It is described
       below under Event Designators.  This is the only history expansion
       that does not begin with the history expansion character.

       Several shell options settable with the shopt builtin will modify
       history expansion behavior (see the description of the shopt
       builtin below).and If the histverify shell option is enabled, and
       readline is being used, history substitutions 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 is reloaded into
       the readline editing buffer for correction.

       The -p option to the history builtin command shows what a history
       expansion will do before using it.  The -s option to the history
       builtin will add commands to the end of the history list without
       actually executing them, so that they are available for subsequent
       recall.

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

   Event Designators
       An event designator is a reference to an entry in the history
       list.  The event designator consists of the portion of the word
       beginning with the history expansion character and ending with the
       word designator if present, or the end of the word.  Unless the
       reference is absolute, events are relative to the current position
       in the history list.

       !      Start a history substitution, except when followed by a
              blank, newline, carriage return, =, or, when the extglob
              shell option is enabled using the shopt builtin, (.
       !n     Refer to history list entry n.
       !-n    Refer to the current entry minus n.
       !!     Refer to the previous entry.  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, this uses the string
              from the most recent search; 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.
       They are optional; if the word designator isn't supplied, the
       history expansion uses the entire event.  A : separates the event
       specification from the word designator.  It may be omitted if the
       word designator begins with a ^, $, *, -, or %.  Words are
       numbered from the beginning of the line, with the first word being
       denoted by 0 (zero).  Words are inserted into the current line
       separated by single spaces.

       0 (zero)
              The zeroth word.  For the shell, this is the command word.
       n      The nth word.
       ^      The first argument: word 1.
       $      The last word.  This is usually the last argument, but will
              expand to the zeroth word if there is only one word in the
              line.
       %      The first word matched by the most recent “?string?”
              search, if the search string begins with a character that
              is part of a word.  By default, searches begin at the end
              of each line and proceed to the beginning, so the first
              word matched is the one closest to the end of the line.
       x-y    A range of words; “-y” abbreviates “0-y”.
       *      All of the words but the zeroth.  This is a synonym for
              “1-$”.  It is not an error to use * if there is just one
              word in the event; it expands to the empty string in that
              case.
       x*     Abbreviates x-$.
       x-     Abbreviates x-$ like x*, but omits the last word.  If x is
              missing, it defaults to 0.

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

   Modifiers
       After the optional word designator, the expansion may include 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 pathname component, leaving only the
              head.
       t      Remove all leading pathname components, leaving the tail.
       r      Remove a trailing suffix of the form .xxx, leaving the
              basename.
       e      Remove all but the trailing suffix.
       p      Print the new command but do not execute it.
       q      Quote the substituted words, escaping further
              substitutions.
       x      Quote the substituted words as with q, but break into words
              at blanks and newlines.  The q and x modifiers are mutually
              exclusive; expansion uses the last one supplied.
       s/old/new/
              Substitute new for the first occurrence of old in the event
              line.  Any character may be used as the delimiter in place
              of /.  The final delimiter is optional if it is the last
              character of the event line.  A single backslash quotes the
              delimiter in old and new.  If & appears in new, it is
              replaced with old.  A single backslash quotes 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
              matching old is deleted.
       &      Repeat the previous substitution.
       g      Cause changes to be applied over the entire event line.
              This is used in conjunction with “:s” (e.g.,
              “:gs/old/new/”) or “:&”.  If used with “:s”, any delimiter
              can be used in place of /, and the final delimiter is
              optional if it is the last character of the event line.  An
              a may be used as a synonym for g.
       G      Apply the following “s” or “&” modifier once to each word
              in the event line.

SHELL BUILTIN COMMANDS         top

       Unless otherwise noted, each builtin command documented in this
       section as accepting options preceded by - accepts -- to signify
       the end of the options.  The :, true, false, and test/[ builtins
       do not accept options and do not treat -- specially.  The exit,
       logout, return, break, continue, let, and shift builtins accept
       and process arguments beginning with - without requiring --.
       Other builtins that accept arguments but are not specified as
       accepting options interpret arguments beginning with - as invalid
       options and require -- to prevent this interpretation.

       : [arguments]
              No effect; the command does nothing beyond expanding
              arguments and performing any specified redirections.  The
              return status is zero.

       . [-p path] filename [arguments]
       source [-p path] filename [arguments]
              The . command (source) reads and execute commands from
              filename in the current shell environment and returns the
              exit status of the last command executed from filename.

              If filename does not contain a slash, . searches for it.
              If the -p option is supplied, . treats path as a colon-
              separated list of directories in which to find filename;
              otherwise, . uses the entries in PATH to find the directory
              containing filename.  filename does not need to be
              executable.  When bash is not in posix mode, it searches
              the current directory if filename is not found in PATH, but
              does not search the current directory if -p is supplied.
              If the sourcepath option to the shopt builtin command is
              turned off, . does not search PATH.

              If any arguments are supplied, they become the positional
              parameters when filename is executed.  Otherwise the
              positional parameters are unchanged.

              If the -T option is enabled, . inherits any trap on DEBUG;
              if it is not, any DEBUG trap string is saved and restored
              around the call to ., and . unsets the DEBUG trap while it
              executes.  If -T is not set, and the sourced file changes
              the DEBUG trap, the new value persists after . completes.
              The return status is the status of the last command
              executed from filename (0 if no commands are executed), and
              non-zero if filename is not found or cannot be read.

       alias [-p] [name[=value] ...]
              With no arguments or with the -p option, alias prints the
              list of aliases in the form alias name=value on standard
              output.  When arguments are supplied, define an alias 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 during command parsing.  For
              each name in the argument list for which no value is
              supplied, print the name and value of the alias name.
              alias returns true unless a name is given (without a
              corresponding =value) 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 uses its notion of the current job.  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] [-lsvSVX]
       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] -p|-P [readline-command]
       bind [-m keymap] keyseq:readline-command
       bind readline-command-line
              Display current readline key and function bindings, bind a
              key sequence to a readline function or macro or to a shell
              command, or set a readline variable.  Each non-option
              argument is a key binding or command as it would appear in
              a readline initialization file such as .inputrc, but each
              binding or command must be passed as a separate argument;
              e.g., '"\C-x\C-r": re-read-init-file'.  In the following
              descriptions, output available to be re-read is formatted
              as commands that would appear in a readline initialization
              file or that would be supplied as individual arguments to a
              bind command.  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 used as an argument to a
                     subsequent bind command or in a readline
                     initialization file.  If arguments remain after
                     option processing, bind treats them as readline
                     command names and restricts output to those names.
              -P     List current readline function names and bindings.
                     If arguments remain after option processing, bind
                     treats them as readline command names and restricts
                     output to those names.
              -s     Display readline key sequences bound to macros and
                     the strings they output in such a way that they can
                     be used as an argument to a subsequent bind command
                     or in a readline initialization file.
              -S     Display readline key sequences bound to macros and
                     the strings they output.
              -v     Display readline variable names and values in such a
                     way that they can be used as an argument to a
                     subsequent bind command or in a readline
                     initialization file.
              -V     List current readline variable names and values.
              -f filename
                     Read key bindings from filename.
              -q function
                     Display key sequences that invoke the named readline
                     function.
              -u function
                     Unbind all key sequences bound to the named readline
                     function.
              -r keyseq
                     Remove any current binding for keyseq.
              -x keyseq[: ]shell-command
                     Cause shell-command to be executed whenever keyseq
                     is entered.  The separator between keyseq and
                     shell-command is either whitespace or a colon
                     optionally followed by whitespace.  If the separator
                     is whitespace, shell-command must be enclosed in
                     double quotes and readline expands any of its
                     special backslash-escapes in shell-command before
                     saving it.  If the separator is a colon, any
                     enclosing double quotes are optional, and readline
                     does not expand the command string before saving it.
                     Since the entire key binding expression must be a
                     single argument, it should be enclosed in single
                     quotes.  When shell-command is executed, the shell
                     sets the READLINE_LINE variable to the contents of
                     the readline line buffer and the READLINE_POINT and
                     READLINE_MARK variables to the current location of
                     the insertion point and the saved insertion point
                     (the mark), respectively.  The shell assigns any
                     numeric argument the user supplied to the
                     READLINE_ARGUMENT variable.  If there was no
                     argument, that variable is not set.  If the executed
                     command changes the value of any of READLINE_LINE,
                     READLINE_POINT, or READLINE_MARK, those new values
                     will be reflected in the editing state.
              -X     List all key sequences bound to shell commands and
                     the associated commands in a format that can be
                     reused as an argument to a subsequent bind command.

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

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

       builtin shell-builtin [arguments]
              Execute the specified shell builtin shell-builtin, passing
              it arguments, and return its exit status.  This is useful
              when defining a function whose name is the same as a shell
              builtin, retaining the functionality of the builtin within
              the function.  The cd builtin is commonly redefined this
              way.  The return status is false if shell-builtin is not a
              shell builtin command.

       caller [expr]
              Returns the context of any active subroutine call (a shell
              function or a script executed with the . or source
              builtins).

              Without expr, caller displays the line number and source
              filename of the current subroutine call.  If a non-negative
              integer is supplied as expr, caller displays the line
              number, subroutine name, and source file corresponding to
              that position in the current execution call stack.  This
              extra information may be used, for example, to print a
              stack trace.  The current frame is frame 0.

              The return value is 0 unless the shell is not executing a
              subroutine call or expr does not correspond to a valid
              position in the call stack.

       cd [-L] [-@] [dir]
       cd -P [-e] [-@] [dir]
              Change the current directory to dir.  if dir is not
              supplied, the value of the HOME shell variable is used as
              dir.  The variable CDPATH exists, and dir does not begin
              with a slash (/), cd uses it as a search path: the shell
              searches each directory name in CDPATH for dir.
              Alternative directory names in CDPATH are separated by a
              colon (:).  A null directory name in CDPATH is the same as
              the current directory, i.e., “.”.

              The -P option causes cd to use the physical directory
              structure by resolving symbolic links while traversing dir
              and before processing instances of .. in dir (see also the
              -P option to the set builtin command).

              The -L option forces cd to follow symbolic links by
              resolving the link after processing instances of .. in dir.
              If .. appears in dir, cd processes it by removing the
              immediately previous pathname component from dir, back to a
              slash or the beginning of dir, and verifying that the
              portion of dir it has processed to that point is still a
              valid directory name after removing the pathname component.
              If it is not a valid directory name, cd returns a non-zero
              status.  If neither -L nor -P is supplied, cd behaves as if
              -L had been supplied.

              If the -e option is supplied with -P, and cd cannot
              successfully determine the current working directory after
              a successful directory change, it returns a non-zero
              status.

              On systems that support it, the -@ option presents the
              extended attributes associated with a file as a directory.

              An argument of - is converted to $OLDPWD before attempting
              the directory change.

              If cd uses a non-empty directory name from CDPATH, or if -
              is the first argument, and the directory change is
              successful, cd writes the absolute pathname of the new
              working directory to the standard output.

              If the directory change is successful, cd sets the value of
              the PWD environment variable to the new directory name, and
              sets the OLDPWD environment variable to the value of the
              current working directory before the change.

              The return value is true if the directory was successfully
              changed; false otherwise.

       command [-pVv] command [arg ...]
              The command builtin runs command with args suppressing the
              normal shell function lookup for command.  Only builtin
              commands or commands found in the PATH named command are
              executed.  If the -p option is supplied, the search for
              command is performed using a default value for PATH that is
              guaranteed to find all of the standard utilities.

              If either the -V or -v option is supplied, command prints a
              description of command.  The -v option displays a single
              word indicating the command or filename used to invoke
              command; the -V option produces a more verbose description.

              If the -V or -v option is supplied, the exit status is zero
              if command was found, and non-zero if not.  If neither
              option is supplied and an error occurred or command cannot
              be found, the exit status is 127.  Otherwise, the exit
              status of the command builtin is the exit status of
              command.

       compgen [-V varname] [option] [word]
              Generate possible completion matches for word according to
              the options, which may be any option accepted by the
              complete builtin with the exceptions of -p, -r, -D, -E, and
              -I, and write the matches to the standard output.

              If the -V option is supplied, compgen stores the generated
              completions into the indexed array variable varname instead
              of writing them to the standard output.

              When using the -F or -C options, the various shell
              variables set by the programmable completion facilities,
              while available, will not have useful values.

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

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

       complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action]
              [-G globpat] [-W wordlist] [-F function] [-C command]
              [-X filterpat] [-P prefix] [-S suffix] name [name ...]
       complete -pr [-DEI] [name ...]
              Specify how arguments to each name should be completed.

              If the -p option is supplied, or if no options or names are
              supplied, print existing completion specifications in a way
              that allows them to be reused as input.  The -r option
              removes a completion specification for each name, or, if no
              names are supplied, all completion specifications.

              The -D option indicates that other supplied options and
              actions should apply to the “default” command completion;
              that is, completion attempted on a command for which no
              completion has previously been defined.  The -E option
              indicates that other supplied options and actions should
              apply to “empty” command completion; that is, completion
              attempted on a blank line.  The -I option indicates that
              other supplied options and actions should apply to
              completion on the initial non-assignment word on the line,
              or after a command delimiter such as ; or |, which is
              usually command name completion.  If multiple options are
              supplied, the -D option takes precedence over -E, and both
              take precedence over -I.  If any of -D, -E, or -I are
              supplied, any other name arguments are ignored; these
              completions only apply to the case specified by the option.

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

              Other options, if specified, have the following meanings.
              The arguments to the -G, -W, and -X options (and, if
              necessary, the -P and -S options) should be quoted to
              protect them from expansion before the complete builtin is
              invoked.

              -o comp-option
                      The comp-option controls several aspects of the
                      compspec's behavior beyond the simple generation of
                      completions.  comp-option may be one of:
                      bashdefault
                              Perform the rest of the default bash
                              completions if the compspec generates no
                              matches.
                      default Use readline's default filename completion
                              if the compspec generates no matches.
                      dirnames
                              Perform directory name completion if the
                              compspec generates no matches.
                      filenames
                              Tell readline that the compspec generates
                              filenames, so it can perform any
                              filename-specific processing (such as
                              adding a slash to directory names, quoting
                              special characters, or suppressing trailing
                              spaces).  This is intended to be used with
                              shell functions.
                      fullquote
                              Tell readline to quote all the completed
                              words even if they are not filenames.
                      noquote Tell readline not to quote the completed
                              words if they are filenames (quoting
                              filenames is the default).
                      nosort  Tell readline not to sort the list of
                              possible completions alphabetically.
                      nospace Tell readline not to append a space (the
                              default) to words completed at the end of
                              the line.
                      plusdirs
                              After generating any matches defined by the
                              compspec, attempt directory name completion
                              and add any matches to the results of the
                              other actions.
              -A action
                      The action may be one of the following to generate
                      a list of possible completions:
                      alias   Alias names.  May also be specified as -a.
                      arrayvar
                              Array variable names.
                      binding Readline key binding names.
                      builtin Names of shell builtin commands.  May also
                              be specified as -b.
                      command Command names.  May also be specified as
                              -c.
                      directory
                              Directory names.  May also be specified as
                              -d.
                      disabled
                              Names of disabled shell builtins.
                      enabled Names of enabled shell builtins.
                      export  Names of exported shell variables.  May
                              also be specified as -e.
                      file    File and directory names, similar to
                              readline's filename completion.  May also
                              be specified as -f.
                      function
                              Names of shell functions.
                      group   Group names.  May also be specified as -g.
                      helptopic
                              Help topics as accepted by the help
                              builtin.
                      hostname
                              Hostnames, as taken from the file specified
                              by the HOSTFILE shell variable.
                      job     Job names, if job control is active.  May
                              also be specified as -j.
                      keyword Shell reserved words.  May also be
                              specified as -k.
                      running Names of running jobs, if job control is
                              active.
                      service Service names.  May also be specified as
                              -s.
                      setopt  Valid arguments for the -o option to the
                              set builtin.
                      shopt   Shell option names as accepted by the shopt
                              builtin.
                      signal  Signal names.
                      stopped Names of stopped jobs, if job control is
                              active.
                      user    User names.  May also be specified as -u.
                      variable
                              Names of all shell variables.  May also be
                              specified as -v.
              -C command
                      command is executed in a subshell environment, and
                      its output is used as the possible completions.
                      Arguments are passed as with the -F option.
              -F function
                      The shell function function is executed in the
                      current shell environment.  When the function is
                      executed, the first argument ($1) is the name of
                      the command whose arguments are being completed,
                      the second argument ($2) is the word being
                      completed, and the third argument ($3) is the word
                      preceding the word being completed on the current
                      command line.  When function finishes, programmable
                      completion retrieves the possible completions from
                      the value of the COMPREPLY array variable.
              -G globpat
                      Expand the pathname expansion pattern globpat to
                      generate the possible completions.
              -P prefix
                      Add prefix to the beginning of each possible
                      completion after all other options have been
                      applied.
              -S suffix
                      Append suffix to each possible completion after all
                      other options have been applied.
              -W wordlist
                      Split the wordlist using the characters in the IFS
                      special variable as delimiters, and expand each
                      resulting word.  Shell quoting is honored within
                      wordlist, in order to provide a mechanism for the
                      words to contain shell metacharacters or characters
                      in the value of IFS.  The possible completions are
                      the members of the resultant list which match a
                      prefix of the word being completed.
              -X filterpat
                      filterpat is a pattern as used for pathname
                      expansion.  It is applied to the list of possible
                      completions generated by the preceding options and
                      arguments, and each completion matching filterpat
                      is removed from the list.  A leading ! in filterpat
                      negates the pattern; in this case, any completion
                      not matching filterpat is removed.

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

       compopt [-o option] [-DEI] [+o option] [name]
              Modify completion options for each name according to the
              options, or for the currently-executing completion if no
              names are supplied.  If no options are supplied, display
              the completion options for each name or the current
              completion.  The possible values of option are those valid
              for the complete builtin described above.

              The -D option indicates that other supplied options should
              apply to the “default” command completion; the -E option
              indicates that other supplied options should apply to
              “empty” command completion; and the -I option indicates
              that other supplied options should apply to completion on
              the initial word on the line.  These are determined in the
              same way as the complete builtin.

              If multiple options are supplied, the -D option takes
              precedence over -E, and both take precedence over -I.

              The return value is true unless an invalid option is
              supplied, an attempt is made to modify the options for a
              name for which no completion specification exists, or an
              output error occurs.

       continue [n]
              continue resumes the next iteration of the enclosing for,
              while, until, or select loop.  If n is specified, bash
              resumes the nth enclosing loop.  n must be ≥ 1.  If n is
              greater than the number of enclosing loops, the shell
              resumes the last enclosing loop (the “top-level” loop).
              The return value is 0 unless n is not greater than or equal
              to 1.

       declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]
       typeset [-aAfFgiIlnrtux] [-p] [name[=value] ...]
              Declare variables and/or give them attributes.  If no names
              are given then display the values of variables or
              functions.  The -p option will display the attributes and
              values of each name.  When -p is used with name arguments,
              additional options, other than -f and -F, are ignored.

              When -p is supplied without name arguments, declare will
              display the attributes and values of all variables having
              the attributes specified by the additional options.  If no
              other options are supplied with -p, declare will display
              the attributes and values of all shell variables.  The -f
              option restricts the display to shell functions.

              The -F option inhibits the display of function definitions;
              only the function name and attributes are printed.  If the
              extdebug shell option is enabled using shopt, the source
              file name and line number where each name is defined are
              displayed as well.  The -F option implies -f.

              The -g option forces variables to be created or modified at
              the global scope, even when declare is executed in a shell
              function.  It is ignored when declare is not executed in a
              shell function.

              The -I option causes local variables to inherit the
              attributes (except the nameref attribute) and value of any
              existing variable with the same name at a surrounding
              scope.  If there is no existing variable, the local
              variable is initially unset.

              The following options can be used to restrict output to
              variables with the specified attribute or to give variables
              attributes:
              -a     Each name is an indexed array variable (see Arrays
                     above).
              -A     Each name is an associative array variable (see
                     Arrays above).
              -f     Each name refers to a shell function.
              -i     The variable is treated as an integer; arithmetic
                     evaluation (see ARITHMETIC EVALUATION above) is
                     performed when the variable is assigned a value.
              -l     When the variable is assigned a value, all upper-
                     case characters are converted to lower-case.  The
                     upper-case attribute is disabled.
              -n     Give each name the nameref attribute, making it a
                     name reference to another variable.  That other
                     variable is defined by the value of name.  All
                     references, assignments, and attribute modifications
                     to name, except those using or changing the -n
                     attribute itself, are performed on the variable
                     referenced by name's value.  The nameref attribute
                     cannot be applied to array variables.
              -r     Make names readonly.  These names cannot then be
                     assigned values by subsequent assignment statements
                     or unset.
              -t     Give each name the trace attribute.  Traced
                     functions inherit the DEBUG and RETURN traps from
                     the calling shell.  The trace attribute has no
                     special meaning for variables.
              -u     When the variable is assigned a value, all lower-
                     case characters are converted to upper-case.  The
                     lower-case attribute is disabled.
              -x     Mark each name for export to subsequent commands via
                     the environment.

              Using “+” instead of “-” turns off the specified attribute
              instead, with the exceptions that +a and +A may not be used
              to destroy array variables and +r will not remove the
              readonly attribute.

              When used in a function, declare and typeset make each name
              local, as with the local command, unless the -g option is
              supplied.  If a variable name is followed by =value, the
              value of the variable is set to value.  When using -a or -A
              and the compound assignment syntax to create array
              variables, additional attributes do not take effect until
              subsequent assignments.

              The return value is 0 unless an invalid option is
              encountered, an attempt is made to define a function using
              “-f foo=bar”, an attempt is made to assign a value to a
              readonly variable, an attempt is made to assign a value to
              an array variable without using the compound assignment
              syntax (see Arrays above), one of the names is not a valid
              shell variable name, an attempt is made to turn off
              readonly status for a readonly variable, an attempt is made
              to turn off array status for an array variable, or an
              attempt is made to display a non-existent function with -f.

       dirs [-clpv] [+n] [-n]
              Without options, display the list of currently remembered
              directories.  The default display is on a single line with
              directory names separated by spaces.  Directories are added
              to the list with the pushd command; the popd command
              removes entries from the list.  The current directory is
              always the first directory in the stack.

              Options, if supplied, have the following meanings:
              -c     Clears the directory stack by deleting all of the
                     entries.
              -l     Produces a listing using full pathnames; the default
                     listing format uses a tilde to denote the home
                     directory.
              -p     Print the directory stack with one entry per line.
              -v     Print the directory stack with one entry per line,
                     prefixing each entry with its index in the stack.
              +n     Displays the nth entry counting from the left of the
                     list shown by dirs when invoked without options,
                     starting with zero.
              -n     Displays the nth entry counting from the right of
                     the list shown by dirs when invoked without options,
                     starting with zero.

              The return value is 0 unless an invalid option is supplied
              or n indexes beyond the end of the directory stack.

       disown [-ar] [-h] [id ...]
              Without options, remove each id from the table of active
              jobs.  Each id may be a job specification jobspec or a
              process ID pid; if id is a pid, disown uses the job
              containing pid as jobspec.

              If the -h option is supplied, disown does not remove the
              jobs corresponding to each id from the jobs table, but
              rather marks them so the shell does not send SIGHUP to the
              job if the shell receives a SIGHUP.

              If no id is supplied, the -a option means to remove or mark
              all jobs; the -r option without an id argument removes or
              marks running jobs.  If no id is supplied, and neither the
              -a nor the -r option is supplied, disown removes or marks
              the current job.

              The return value is 0 unless an id does not specify a valid
              job.

       echo [-neE] [arg ...]
              Output the args, separated by spaces, followed by a
              newline.  The return status is 0 unless a write error
              occurs.  If -n is specified, the trailing newline is not
              printed.

              If the -e option is given, echo interprets the following
              backslash-escaped characters.  The -E option disables
              interpretation of these escape characters, even on systems
              where they are interpreted by default.  The xpg_echo shell
              option determines whether or not echo interprets any
              options and expands these escape characters.  echo does not
              interpret -- to mean the end of options.

              echo interprets the following escape sequences:
              \a     alert (bell)
              \b     backspace
              \c     suppress further output
              \e
              \E     an escape character
              \f     form feed
              \n     new line
              \r     carriage return
              \t     horizontal tab
              \v     vertical tab
              \\     backslash
              \0nnn  The eight-bit character whose value is the octal
                     value nnn (zero to three octal digits).
              \xHH   The eight-bit character whose value is the
                     hexadecimal value HH (one or two hex digits).
              \uHHHH The Unicode (ISO/IEC 10646) character whose value is
                     the hexadecimal value HHHH (one to four hex digits).
              \UHHHHHHHH
                     The Unicode (ISO/IEC 10646) character whose value is
                     the hexadecimal value HHHHHHHH (one to eight hex
                     digits).

              echo writes any unrecognized backslash-escaped characters
              unchanged.

       enable [-a] [-dnps] [-f filename] [name ...]
              Enable and disable builtin shell commands.  Disabling a
              builtin allows an executable file which has the same name
              as a shell builtin to be executed without specifying a full
              pathname, even though the shell normally searches for
              builtins before files.

              If -n is supplied, each name is disabled; otherwise, names
              are enabled.  For example, to use the test binary found
              using PATH instead of the shell builtin version, run
              “enable -n test”.

              If no name arguments are supplied, or if the -p option is
              supplied, print a list of shell builtins.  With no other
              option arguments, the list consists of all enabled shell
              builtins.  If -n is supplied, print only disabled builtins.
              If -a is supplied, the list printed includes all builtins,
              with an indication of whether or not each is enabled.  The
              -s option means to restrict the output to the POSIX special
              builtins.

              The -f option means to load the new builtin command name
              from shared object filename, on systems that support
              dynamic loading.  If filename does not contain a slash,
              Bash will use the value of the BASH_LOADABLES_PATH variable
              as a colon-separated list of directories in which to search
              for filename.  The default for BASH_LOADABLES_PATH is
              system-dependent, and may include “.” to force a search of
              the current directory.  The -d option will delete a builtin
              previously loaded with -f.  If -s is used with -f, the new
              builtin becomes a POSIX special builtin.

              If no options are supplied and a name is not a shell
              builtin, enable will attempt to load name from a shared
              object named name, as if the command were “enable -f name
              name”.

              The return value is 0 unless a name is not a shell builtin
              or there is an error loading a new builtin from a shared
              object.

       eval [arg ...]
              Concatenate the args together into a single command,
              separating them with spaces.  Bash then reads and execute
              this command, and returns its exit status as the return
              status of eval.  If there are no args, or only null
              arguments, eval returns 0.

       exec [-cl] [-a name] [command [arguments]]
              If command is specified, it replaces the shell without
              creating a new process.  command cannot be a shell builtin
              or function.  The arguments become the arguments to
              command.  If the -l option is supplied, the shell places a
              dash at the beginning of the zeroth argument passed to
              command.  This is what login(1) does.  The -c option causes
              command to be executed with an empty environment.  If -a is
              supplied, the shell passes name as the zeroth argument to
              the executed command.

              If command cannot be executed for some reason, a non-
              interactive shell exits, unless the execfail shell option
              is enabled.  In that case, it returns a non-zero status.
              An interactive shell returns a non-zero status if the file
              cannot be executed.  A subshell exits unconditionally if
              exec fails.

              If command is not specified, any redirections take effect
              in the current shell, and the return status is 0.  If there
              is a redirection error, the return status is 1.

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

       export [-fn] [name[=value]] ...
       export -p [-f]
              The supplied names are marked for automatic export to the
              environment of subsequently executed commands.  If the -f
              option is given, the names refer to functions.

              The -n option unexports, or removes the export attribute,
              from each name.  If no names are given, or if only the -p
              option is supplied, export displays a list of names of all
              exported variables on the standard output.  Using -p and -f
              together displays exported functions.  The -p option
              displays output in a form that may be reused as input.

              export allows the value of a variable to be set when it is
              exported or unexported by following the variable name with
              =value.  This sets the value of the variable to value while
              modifying the export attribute.  export returns an exit
              status of 0 unless an invalid option is encountered, one of
              the names is not a valid shell variable name, or -f is
              supplied with a name that is not a function.

       false  Does nothing; returns a non-zero status.

       fc [-e ename] [-lnr] [first] [last]
       fc -s [pat=rep] [cmd]
              The first form selects a range of commands from first to
              last from the history list and displays or edits and re-
              executes them.  First and last may be specified as a string
              (to locate the last command beginning with that string) or
              as a number (an index into the history list, where a
              negative number is used as an offset from the current
              command number).

              When listing, a first or last of 0 is equivalent to -1 and
              -0 is equivalent to the current command (usually the fc
              command); otherwise 0 is equivalent to -1 and -0 is
              invalid.  If last is not specified, it is set to the
              current command for listing (so that “fc -l -10” prints the
              last 10 commands) and to first otherwise.  If first is not
              specified, it is set to the previous command for editing
              and -16 for listing.

              If the -l option is supplied, the commands are listed on
              the standard output.  The -n option suppresses the command
              numbers when listing.  The -r option reverses the order of
              the commands.

              Otherwise, fc invokes the editor named by ename on a file
              containing those commands.  If ename is not supplied, fc
              uses the value of the FCEDIT variable, and the value of
              EDITOR if FCEDIT is not set.  If neither variable is set,
              fc uses vi. When editing is complete, fc reads the file
              containing the edited commands and echoes and executes
              them.

              In the second form, fc re-executes command after replacing
              each instance of pat with rep.  Command is interpreted the
              same as first above.

              A useful alias to use with fc is “r="fc -s"”, so that
              typing “r cc” runs the last command beginning with “cc” and
              typing “r” re-executes the last command.

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

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

       getopts optstring name [arg ...]
              getopts is used by shell scripts and functions to parse
              positional parameters and obtain options and their
              arguments.  optstring contains the option characters to be
              recognized; if a character is followed by a colon, the
              option is expected to have an argument, which should be
              separated from it by white space.  The colon and question
              mark characters may not be used as option characters.

              Each time it is invoked, getopts places the next option in
              the shell variable name, initializing name if it does not
              exist, and the index of the next argument to be processed
              into the variable OPTIND.  OPTIND is initialized to 1 each
              time the shell or a shell script is invoked.  When an
              option requires an argument, getopts places that argument
              into the variable OPTARG.

              The shell does not reset OPTIND automatically; it must be
              manually reset between multiple calls to getopts within the
              same shell invocation to use a new set of parameters.

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

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

              getopts can report errors in two ways.  If the first
              character of optstring is a colon, getopts uses silent
              error reporting.  In normal operation, getopts prints
              diagnostic messages when it encounters invalid options or
              missing option arguments.  If the variable OPTERR is set to
              0, getopts does not display any error messages, even if the
              first character of optstring is not a colon.

              If getopts detects an invalid option, it places ? into name
              and, if not silent, prints an error message and unsets
              OPTARG.  If getopts is silent, it assigns the option
              character found to OPTARG and does not print a diagnostic
              message.

              If a required argument is not found, and getopts is not
              silent, it sets the value of name to a question mark (?),
              unsets OPTARG, and prints a diagnostic message.  If getopts
              is silent, it sets the value of name to a colon (:) and
              sets OPTARG to the option character found.

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

       hash [-lr] [-p filename] [-dt] [name]
              Each time hash is invoked, it remembers the full pathname
              of the command name as determined by searching the
              directories in $PATH.  Any previously-remembered pathname
              associated with name is discarded.  If the -p option is
              supplied, hash uses filename as the full pathname of the
              command.

              The -r option causes the shell to forget all remembered
              locations.  Assigning to the PATH variable also clears all
              hashed filenames.  The -d option causes the shell to forget
              the remembered location of each name.

              If the -t option is supplied, hash prints the full pathname
              corresponding to each name.  If multiple name arguments are
              supplied with -t, hash prints the name before the
              corresponding hashed full pathname.  The -l option displays
              output in a format that may be reused as input.

              If no arguments are given, or if only -l is supplied, hash
              prints information about remembered commands.  The -t, -d,
              and -p options (the options that act on the name arguments)
              are mutually exclusive.  Only one will be active.  If more
              than one is supplied, -t has higher priority than -p, and
              both have higher priority than -d.

              The return status is zero unless a name is not found or an
              invalid option is supplied.

       help [-dms] [pattern]
              Display helpful information about builtin commands.  If
              pattern is specified, help gives detailed help on all
              commands matching pattern as described below; otherwise it
              displays a list of all the builtins and shell compound
              commands.

              Options, if supplied, have the follow meanings:

              -d     Display a short description of each pattern
              -m     Display the description of each pattern in a
                     manpage-like format
              -s     Display only a short usage synopsis for each pattern

              If pattern contains pattern matching characters (see
              Pattern Matching above) it's treated as a shell pattern and
              help prints the description of each help topic matching
              pattern.

              If not, and pattern exactly matches the name of a help
              topic, help prints the description associated with that
              topic.  Otherwise, help performs prefix matching and prints
              the descriptions of all matching help topics.

              The return status is 0 unless no command matches pattern.

       history [n]
       history -c
       history -d offset
       history -d start-end
       history -anrw [filename]
       history -p arg [arg ...]
       history -s arg [arg ...]
              With no options, display the command history list with
              numbers.  Entries prefixed with a * have been modified.  An
              argument of n lists only the last n entries.  If the shell
              variable HISTTIMEFORMAT is set and not null, it is used as
              a format string for strftime(3) to display the time stamp
              associated with each displayed history entry.  If history
              uses HISTTIMEFORMAT, it does not print an intervening space
              between the formatted time stamp and the history entry.

              If filename is supplied, history uses it as the name of the
              history file; if not, it uses the value of HISTFILE.  If
              filename is not supplied and HISTFILE is unset or null, the
              -a, -n, -r, and -w options have no effect.

              Options, if supplied, have the following meanings:
              -c     Clear the history list by deleting all the entries.
                     This can be used with the other options to replace
                     the history list.
              -d offset
                     Delete the history entry at position offset.  If
                     offset is negative, it is interpreted as relative to
                     one greater than the last history position, so
                     negative indices count back from the end of the
                     history, and an index of -1 refers to the current
                     history -d command.
              -d start-end
                     Delete the range of history entries between
                     positions start and end, inclusive.  Positive and
                     negative values for start and end are interpreted as
                     described above.
              -a     Append the “new” history lines to the history file.
                     These are history lines entered since the beginning
                     of the current bash session, but not already
                     appended to the history file.
              -n     Read the history lines not already read from the
                     history file and add them to the current history
                     list.  These are lines appended to the history file
                     since the beginning of the current bash session.
              -r     Read the history file and append its contents to the
                     current history list.
              -w     Write the current history list to the history file,
                     overwriting the history file.
              -p     Perform history substitution on the following args
                     and display the result on the standard output,
                     without storing the results in the history list.
                     Each arg must be quoted to disable normal history
                     expansion.
              -s     Store the args in the history list as a single
                     entry.  The last command in the history list is
                     removed before adding the args.

              If the HISTTIMEFORMAT variable is set, history writes the
              time stamp information associated with each history entry
              to the history file, marked with the history comment
              character as described above.  When the history file is
              read, lines beginning with the history comment character
              followed immediately by a digit are interpreted as
              timestamps for the following history entry.

              The return value is 0 unless an invalid option is
              encountered, an error occurs while reading or writing the
              history file, an invalid offset or range is supplied as an
              argument to -d, or the history expansion supplied as an
              argument to -p fails.

       jobs [-lnprs] [ jobspec ... ]
       jobs -x command [ args ... ]
              The first form lists the active jobs.  The options have the
              following meanings:
              -l     List process IDs in addition to the normal
                     information.
              -n     Display information only about jobs that have
                     changed status since the user was last notified of
                     their status.
              -p     List only the process ID of the job's process group
                     leader.
              -r     Display only running jobs.
              -s     Display only stopped jobs.

              If jobspec is supplied, jobs restricts output to
              information about that job.  The return status is 0 unless
              an invalid option is encountered or an invalid jobspec is
              supplied.

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

       kill [-s sigspec | -n signum | -sigspec] id [ ... ]
       kill -l|-L [sigspec | exit_status]
              Send the signal specified by sigspec or signum to the
              processes named by each id.  Each id may be a job
              specification jobspec or a process ID pid.  sigspec is
              either a case-insensitive signal name such as SIGKILL (with
              or without the SIG prefix) or a signal number; signum is a
              signal number.  If sigspec is not supplied, then kill sends
              SIGTERM.

              The -l option lists the signal names.  If any arguments are
              supplied when -l is given, kill lists the names of the
              signals corresponding to the arguments, and the return
              status is 0.  The exit_status argument to -l is a number
              specifying either a signal number or the exit status of a
              process terminated by a signal; if it is supplied, kill
              prints the name of the signal that caused the process to
              terminate.  kill assumes that process exit statuses are
              greater than 128; anything less than that is a signal
              number.  The -L option is equivalent to -l.

              kill returns true if at least one signal was successfully
              sent, or false if an error occurs or an invalid option is
              encountered.

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

       local [option] [name[=value] ... | - ]
              For each argument, create a local variable named name and
              assign it value.  The option can be any of the options
              accepted by declare.  When local is used within a function,
              it causes the variable name to have a visible scope
              restricted to that function and its children.  It is an
              error to use local when not within a function.

              If name is -, it makes the set of shell options local to
              the function in which local is invoked: any shell options
              changed using the set builtin inside the function after the
              call to local are restored to their original values when
              the function returns.  The restore is performed as if a
              series of set commands were executed to restore the values
              that were in place before the function.

              With no operands, local writes a list of local variables to
              the standard output.

              The return status is 0 unless local is used outside a
              function, an invalid name is supplied, or name is a
              readonly variable.

       logout [n]
              Exit a login shell, returning a status of n to the shell's
              parent.

       mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd]
       [-C callback] [-c quantum] [array]
       readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u
       fd] [-C callback] [-c quantum] [array]
              Read lines from the standard input, or from file descriptor
              fd if the -u option is supplied, into the indexed array
              variable array.  The variable MAPFILE is the default array.
              Options, if supplied, have the following meanings:
              -d     Use the first character of delim to terminate each
                     input line, rather than newline.  If delim is the
                     empty string, mapfile will terminate a line when it
                     reads a NUL character.
              -n     Copy at most count lines.  If count is 0, copy all
                     lines.
              -O     Begin assigning to array at index origin.  The
                     default index is 0.
              -s     Discard the first count lines read.
              -t     Remove a trailing delim (default newline) from each
                     line read.
              -u     Read lines from file descriptor fd instead of the
                     standard input.
              -C     Evaluate callback each time quantum lines are read.
                     The -c option specifies quantum.
              -c     Specify the number of lines read between each call
                     to callback.

              If -C is specified without -c, the default quantum is 5000.
              When callback is evaluated, it is supplied the index of the
              next array element to be assigned and the line to be
              assigned to that element as additional arguments.  callback
              is evaluated after the line is read but before the array
              element is assigned.

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

              mapfile returns zero unless an invalid option or option
              argument is supplied, array is invalid or unassignable, or
              if array is not an indexed array.

       popd [-n] [+n] [-n]
              Remove entries from the directory stack.  The elements are
              numbered from 0 starting at the first directory listed by
              dirs, so popd is equivalent to “popd +0.”  With no
              arguments, popd removes the top directory from the stack,
              and changes to the new top directory.  Arguments, if
              supplied, have the following meanings:
              -n     Suppress the normal change of directory when
                     removing directories from the stack, only manipulate
                     the stack.
              +n     Remove the nth entry counting from the left of the
                     list shown by dirs, starting with zero, from the
                     stack.  For example: “popd +0” removes the first
                     directory, “popd +1” the second.
              -n     Remove the nth entry counting from the right of the
                     list shown by dirs, starting with zero.  For
                     example: “popd -0” removes the last directory, “popd
                     -1” the next to last.

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

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

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

       printf [-v var] format [arguments]
              Write the formatted arguments to the standard output under
              the control of the format.  The -v option assigns the
              output to the variable var rather than printing it to the
              standard output.

              The format is a character string which contains three types
              of objects: plain characters, which are simply copied to
              standard output, character escape sequences, which are
              converted and copied to the standard output, and format
              specifications, each of which causes printing of the next
              successive argument.  In addition to the standard printf(3)
              format characters cCsSndiouxXeEfFgGaA, printf interprets
              the following additional format specifiers:
              %b     causes printf to expand backslash escape sequences
                     in the corresponding argument in the same way as
                     echo -e.
              %q     causes printf to output the corresponding argument
                     in a format that can be reused as shell input.  %q
                     and %Q use the $'' quoting style if any characters
                     in the argument string require it, and backslash
                     quoting otherwise.  If the format string uses the
                     printf alternate form, these two formats quote the
                     argument string using single quotes.
              %Q     like %q, but applies any supplied precision to the
                     argument before quoting it.
              %(datefmt)T
                     causes printf to output the date-time string
                     resulting from using datefmt as a format string for
                     strftime(3).  The corresponding argument is an
                     integer representing the number of seconds since the
                     epoch.  This format specifier recognizes two special
                     argument values: -1 represents the current time, and
                     -2 represents the time the shell was invoked.  If no
                     argument is specified, conversion behaves as if -1
                     had been supplied.  This is an exception to the
                     usual printf behavior.

              The %b, %q, and %T format specifiers all use the field
              width and precision arguments from the format specification
              and write that many bytes from (or use that wide a field
              for) the expanded argument, which usually contains more
              characters than the original.

              The %n format specifier accepts a corresponding argument
              that is treated as a shell variable name.

              The %s and %c format specifiers accept an l (long)
              modifier, which forces them to convert the argument string
              to a wide-character string and apply any supplied field
              width and precision in terms of characters, not bytes.  The
              %S and %C format specifiers are equivalent to %ls and %lc,
              respectively.

              Arguments to non-string format specifiers are treated as C
              constants, except that a leading plus or minus sign is
              allowed, and if the leading character is a single or double
              quote, the value is the numeric value of the following
              character, using the current locale.

              The format is reused as necessary to consume all of the
              arguments.  If the format requires more arguments than are
              supplied, the extra format specifications behave as if a
              zero value or null string, as appropriate, had been
              supplied.  The return value is zero on success, non-zero if
              an invalid option is supplied or a write or assignment
              error occurs.

       pushd [-n] [+n] [-n]
       pushd [-n] [dir]
              Add a directory to the top of the directory stack, or
              rotate the stack, making the new top of the stack the
              current working directory.  With no arguments, pushd
              exchanges the top two elements of the directory stack.
              Arguments, if supplied, have the following meanings:
              -n     Suppress the normal change of directory when
                     rotating or adding directories to the stack, only
                     manipulate the stack.
              +n     Rotate the stack so that the nth directory (counting
                     from the left of the list shown by dirs, starting
                     with zero) is at the top.
              -n     Rotates the stack so that the nth directory
                     (counting from the right of the list shown by dirs,
                     starting with zero) is at the top.
              dir    Adds dir to the directory stack at the top.

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

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

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

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

       read [-Eers] [-a aname] [-d delim] [-i text] [-n nchars] [-N
       nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
              Read one line from the standard input, or from the file
              descriptor fd supplied as an argument to the -u option,
              split it into words as described above under Word
              Splitting, and assign the first word to the first name, the
              second word to the second name, and so on.  If there are
              more words than names, the remaining words and their
              intervening delimiters are assigned to the last name.  If
              there are fewer words read from the input stream than
              names, the remaining names are assigned empty values.  The
              characters in the value of the IFS variable are used to
              split the line into words using the same rules the shell
              uses for expansion (described above under Word Splitting).
              The backslash character (\) removes any special meaning for
              the next character read and is used for line continuation.

              Options, if supplied, have the following meanings:
              -a aname
                     The words are assigned to sequential indices of the
                     array variable aname, starting at 0.  aname is unset
                     before any new values are assigned.  Other name
                     arguments are ignored.
              -d delim
                     The first character of delim terminates the input
                     line, rather than newline.  If delim is the empty
                     string, read will terminate a line when it reads a
                     NUL character.
              -e     If the standard input is coming from a terminal,
                     read uses readline (see READLINE above) to obtain
                     the line.  Readline uses the current (or default, if
                     line editing was not previously active) editing
                     settings, but uses readline's default filename
                     completion.
              -E     If the standard input is coming from a terminal,
                     read uses readline (see READLINE above) to obtain
                     the line.  Readline uses the current (or default, if
                     line editing was not previously active) editing
                     settings, but uses bash's default completion,
                     including programmable completion.
              -i text
                     If readline is being used to read the line, read
                     places text into the editing buffer before editing
                     begins.
              -n nchars
                     read returns after reading nchars characters rather
                     than waiting for a complete line of input, unless it
                     encounters EOF or read times out, but honors a
                     delimiter if it reads fewer than nchars characters
                     before the delimiter.
              -N nchars
                     read returns after reading exactly nchars characters
                     rather than waiting for a complete line of input,
                     unless it encounters EOF or read times out.  Any
                     delimiter characters in the input are not treated
                     specially and do not cause read to return until it
                     has read nchars characters.  The result is not split
                     on the characters in IFS; the intent is that the
                     variable is assigned exactly the characters read
                     (with the exception of backslash; see the -r option
                     below).
              -p prompt
                     Display prompt on standard error, without a trailing
                     newline, before attempting to read any input, but
                     only if input is coming from a terminal.
              -r     Backslash does not act as an escape character.  The
                     backslash is considered to be part of the line.  In
                     particular, a backslash-newline pair may not then be
                     used as a line continuation.
              -s     Silent mode.  If input is coming from a terminal,
                     characters are not echoed.
              -t timeout
                     Cause read to time out and return failure if it does
                     not read a complete line of input (or a specified
                     number of characters) within timeout seconds.
                     timeout may be a decimal number with a fractional
                     portion following the decimal point.  This option is
                     only effective if read is reading input from a
                     terminal, pipe, or other special file; it has no
                     effect when reading from regular files.  If read
                     times out, it saves any partial input read into the
                     specified variable name, and the exit status is
                     greater than 128.  If timeout is 0, read returns
                     immediately, without trying to read any data.  In
                     this case, the exit status is 0 if input is
                     available on the specified file descriptor, or the
                     read will return EOF, non-zero otherwise.
              -u fd  Read input from file descriptor fd instead of the
                     standard input.

              Other than the case where delim is the empty string, read
              ignores any NUL characters in the input.

              If no names are supplied, read assigns the line read,
              without the ending delimiter but otherwise unmodified, to
              the variable REPLY.

              The exit status is zero, unless end-of-file is encountered,
              read times out (in which case the status is greater than
              128), a variable assignment error (such as assigning to a
              readonly variable) occurs, or an invalid file descriptor is
              supplied as the argument to -u.

       readonly [-aAf] [-p] [name[=word] ...]
              The given names are marked readonly; the values of these
              names may not be changed by subsequent assignment or unset.
              If the -f option is supplied, each name refers to a shell
              function.  The -a option restricts the variables to indexed
              arrays; the -A option restricts the variables to
              associative arrays.  If both options are supplied, -A takes
              precedence.  If no name arguments are supplied, or if the
              -p option is supplied, print a list of all readonly names.
              The other options may be used to restrict the output to a
              subset of the set of readonly names.  The -p option
              displays output in a format that may be reused as input.

              readonly allows the value of a variable to be set at the
              same time the readonly attribute is changed by following
              the variable name with =value.  This sets the value of the
              variable is to value while modifying the readonly
              attribute.

              The return status is 0 unless an invalid option is
              encountered, one of the names is not a valid shell variable
              name, or -f is supplied with a name that is not a function.

       return [n]
              Stop executing a shell function or sourced file and return
              the value specified by n to its caller.  If n is omitted,
              the return status is that of the last command executed.  If
              return is executed by a trap handler, the last command used
              to determine the status is the last command executed before
              the trap handler.  If return is executed during a DEBUG
              trap, the last command used to determine the status is the
              last command executed by the trap handler before return was
              invoked.

              When return is used to terminate execution of a script
              being executed by the .  (source) command, it causes the
              shell to stop executing that script and return either n or
              the exit status of the last command executed within the
              script as the exit status of the script.  If n is supplied,
              the return value is its least significant 8 bits.

              Any command associated with the RETURN trap is executed
              before execution resumes after the function or script.

              The return status is non-zero if return is supplied a non-
              numeric argument, or is used outside a function and not
              during execution of a script by . or source.

       set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [arg ...]
       set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [arg ...]
       set -o
       set +o Without options, display the name and value of each shell
              variable in a format that can be reused as input for
              setting or resetting the currently-set variables.  Read-
              only variables cannot be reset.  In posix mode, only shell
              variables are listed.  The output is sorted according to
              the current locale.  When options are specified, they set
              or unset shell attributes.  Any arguments remaining after
              option processing are treated as values for the positional
              parameters and are assigned, in order, to $1, $2, ..., $n.
              Options, if specified, have the following meanings:
              -a      Each variable or function that is created or
                      modified is given the export attribute and marked
                      for export to the environment of subsequent
                      commands.
              -b      Report the status of terminated background jobs
                      immediately, rather than before the next primary
                      prompt or after a foreground command terminates.
                      This is effective only when job control is enabled.
              -e      Exit immediately if a pipeline (which may consist
                      of a single simple command), a list, or a compound
                      command (see SHELL GRAMMAR above), exits with a
                      non-zero status.  The shell does not exit if the
                      command that fails is part of the command list
                      immediately following a while or until reserved
                      word, part of the test following the if or elif
                      reserved words, part of any command executed in a
                      && or || list except the command following the
                      final && or ||, any command in a pipeline but the
                      last (subject to the state of the pipefail shell
                      option), or if the command's return value is being
                      inverted with !.  If a compound command other than
                      a subshell returns a non-zero status because a
                      command failed while -e was being ignored, the
                      shell does not exit.  A trap on ERR, if set, is
                      executed before the shell exits.  This option
                      applies to the shell environment and each subshell
                      environment separately (see COMMAND EXECUTION
                      ENVIRONMENT above), and may cause subshells to exit
                      before executing all the commands in the subshell.

                      If a compound command or shell function executes in
                      a context where -e is being ignored, none of the
                      commands executed within the compound command or
                      function body will be affected by the -e setting,
                      even if -e is set and a command returns a failure
                      status.  If a compound command or shell function
                      sets -e while executing in a context where -e is
                      ignored, that setting will not have any effect
                      until the compound command or the command
                      containing the function call completes.
              -f      Disable pathname expansion.
              -h      Remember the location of commands as they are
                      looked up for execution.  This is enabled by
                      default.
              -k      All arguments in the form of assignment statements
                      are placed in the environment for a command, not
                      just those that precede the command name.
              -m      Monitor mode.  Job control is enabled.  This option
                      is on by default for interactive shells on systems
                      that support it (see JOB CONTROL above).  All
                      processes run in a separate process group.  When a
                      background job completes, the shell prints a line
                      containing its exit status.
              -n      Read commands but do not execute them.  This may be
                      used to check a shell script for syntax errors.
                      This is ignored by interactive shells.
              -o option-name
                      The option-name can be one of the following:
                      allexport
                              Same as -a.
                      braceexpand
                              Same as -B.
                      emacs   Use an emacs-style command line editing
                              interface.  This is enabled by default when
                              the shell is interactive, unless the shell
                              is started with the --noediting option.
                              This also affects the editing interface
                              used for read -e.
                      errexit Same as -e.
                      errtrace
                              Same as -E.
                      functrace
                              Same as -T.
                      hashall Same as -h.
                      histexpand
                              Same as -H.
                      history Enable command history, as described above
                              under HISTORY.  This option is on by
                              default in interactive shells.
                      ignoreeof
                              The effect is as if the shell command
                              “IGNOREEOF=10” had been executed (see Shell
                              Variables above).
                      keyword Same as -k.
                      monitor Same as -m.
                      noclobber
                              Same as -C.
                      noexec  Same as -n.
                      noglob  Same as -f.
                      nolog   Currently ignored.
                      notify  Same as -b.
                      nounset Same as -u.
                      onecmd  Same as -t.
                      physical
                              Same as -P.
                      pipefail
                              If set, the return value of a pipeline is
                              the value of the last (rightmost) command
                              to exit with a non-zero status, or zero if
                              all commands in the pipeline exit
                              successfully.  This option is disabled by
                              default.
                      posix   Enable posix mode; change the behavior of
                              bash where the default operation differs
                              from the POSIX standard to match the
                              standard.  See SEE ALSO below for a
                              reference to a document that details how
                              posix mode affects bash's behavior.
                      privileged
                              Same as -p.
                      verbose Same as -v.
                      vi      Use a vi-style command line editing
                              interface.  This also affects the editing
                              interface used for read -e.
                      xtrace  Same as -x.
                      If -o is supplied with no option-name, set prints
                      the current shell option settings.  If +o is
                      supplied with no option-name, set prints a series
                      of set commands to recreate the current option
                      settings on the standard output.
              -p      Turn on privileged mode.  In this mode, the shell
                      does not read the $ENV and $BASH_ENV files, shell
                      functions are not inherited from the environment,
                      and the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
                      variables, if they appear in the environment, are
                      ignored.  If the shell is started with the
                      effective user (group) id not equal to the real
                      user (group) id, and the -p option is not supplied,
                      these actions are taken and the effective user id
                      is set to the real user id.  If the -p option is
                      supplied at startup, the effective user id is not
                      reset.  Turning this option off causes the
                      effective user and group ids to be set to the real
                      user and group ids.
              -r      Enable restricted shell mode.  This option cannot
                      be unset once it has been set.
              -t      Exit after reading and executing one command.
              -u      Treat unset variables and parameters other than the
                      special parameters “@” and “*”, or array variables
                      subscripted with “@” or “*”, as an error when
                      performing parameter expansion.  If expansion is
                      attempted on an unset variable or parameter, the
                      shell prints an error message, and, if not
                      interactive, exits with a non-zero status.
              -v      Print shell input lines as they are read.
              -x      After expanding each simple command, for command,
                      case command, select command, or arithmetic for
                      command, display the expanded value of PS4,
                      followed by the command and its expanded arguments
                      or associated word list, to the standard error.
              -B      The shell performs brace expansion (see Brace
                      Expansion above).  This is on by default.
              -C      If set, bash does not overwrite an existing file
                      with the >, >&, and <> redirection operators.
                      Using the redirection operator >| instead of > will
                      override this and force the creation of an output
                      file.
              -E      If set, any trap on ERR is inherited by shell
                      functions, command substitutions, and commands
                      executed in a subshell environment.  The ERR trap
                      is normally not inherited in such cases.
              -H      Enable !  style history substitution.  This option
                      is on by default when the shell is interactive.
              -P      If set, the shell does not resolve symbolic links
                      when executing commands such as cd that change the
                      current working directory.  It uses the physical
                      directory structure instead.  By default, bash
                      follows the logical chain of directories when
                      performing commands which change the current
                      directory.
              -T      If set, any traps on DEBUG and RETURN are inherited
                      by shell functions, command substitutions, and
                      commands executed in a subshell environment.  The
                      DEBUG and RETURN traps are normally not inherited
                      in such cases.
              --      If no arguments follow this option, unset the
                      positional parameters.  Otherwise, set the
                      positional parameters to the args, even if some of
                      them begin with a -.
              -       Signal the end of options, and assign all remaining
                      args to the positional parameters.  The -x and -v
                      options are turned off.  If there are no args, the
                      positional parameters remain unchanged.

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

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

       shopt [-pqsu] [-o] [optname ...]
              Toggle the values of settings controlling optional shell
              behavior.  The settings can be either those listed below,
              or, if the -o option is used, those available with the -o
              option to the set builtin command.

              With no options, or with the -p option, display a list of
              all settable options, with an indication of whether or not
              each is set; if any optnames are supplied, the output is
              restricted to those options.  The -p option displays output
              in a form that may be reused as input.

              Other options have the following meanings:
              -s     Enable (set) each optname.
              -u     Disable (unset) each optname.
              -q     Suppresses normal output (quiet mode); the return
                     status indicates whether the optname is set or
                     unset.  If multiple optname arguments are supplied
                     with -q, the return status is zero if all optnames
                     are enabled; non-zero otherwise.
              -o     Restricts the values of optname to be those defined
                     for the -o option to the set builtin.

              If either -s or -u is used with no optname arguments, shopt
              shows only those options which are set or unset,
              respectively.  Unless otherwise noted, the shopt options
              are disabled (unset) by default.

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

              The list of shopt options is:

              array_expand_once
                      If set, the shell suppresses multiple evaluation of
                      associative and indexed array subscripts during
                      arithmetic expression evaluation, while executing
                      builtins that can perform variable assignments, and
                      while executing builtins that perform array
                      dereferencing.
              assoc_expand_once
                      Deprecated; a synonym for array_expand_once.
              autocd  If set, a command name that is the name of a
                      directory is executed as if it were the argument to
                      the cd command.  This option is only used by
                      interactive shells.
              bash_source_fullpath
                      If set, filenames added to the BASH_SOURCE array
                      variable are converted to full pathnames (see Shell
                      Variables above).
              cdable_vars
                      If set, an argument to the cd builtin command that
                      is not a directory is assumed to be the name of a
                      variable whose value is the directory to change to.
              cdspell If set, the cd command attempts to correct minor
                      errors in the spelling of a directory component.
                      Minor errors include transposed characters, a
                      missing character, and one extra character.  If cd
                      corrects the directory name, it prints the
                      corrected filename, and the command proceeds.  This
                      option is only used by interactive shells.
              checkhash
                      If set, bash checks that a command found in the
                      hash table exists before trying to execute it.  If
                      a hashed command no longer exists, bash performs a
                      normal path search.
              checkjobs
                      If set, bash lists the status of any stopped and
                      running jobs before exiting an interactive shell.
                      If any jobs are running, bash defers the exit until
                      a second exit is attempted without an intervening
                      command (see JOB CONTROL above).  The shell always
                      postpones exiting if any jobs are stopped.
              checkwinsize
                      If set, bash checks the window size after each
                      external (non-builtin) command and, if necessary,
                      updates the values of LINES and COLUMNS, using the
                      file descriptor associated with the standard error
                      if it is a terminal.  This option is enabled by
                      default.
              cmdhist If set, bash attempts to save all lines of a
                      multiple-line command in the same history entry.
                      This allows easy re-editing of multi-line commands.
                      This option is enabled by default, but only has an
                      effect if command history is enabled, as described
                      above under HISTORY.
              compat31
              compat32
              compat40
              compat41
              compat42
              compat43
              compat44
                      These control aspects of the shell's compatibility
                      mode (see SHELL COMPATIBILITY MODE below).
              complete_fullquote
                      If set, bash quotes all shell metacharacters in
                      filenames and directory names when performing
                      completion.  If not set, bash removes
                      metacharacters such as the dollar sign from the set
                      of characters that will be quoted in completed
                      filenames when these metacharacters appear in shell
                      variable references in words to be completed.  This
                      means that dollar signs in variable names that
                      expand to directories will not be quoted; however,
                      any dollar signs appearing in filenames will not be
                      quoted, either.  This is active only when bash is
                      using backslashes to quote completed filenames.
                      This variable is set by default, which is the
                      default bash behavior in versions through 4.2.
              direxpand
                      If set, bash replaces directory names with the
                      results of word expansion when performing filename
                      completion.  This changes the contents of the
                      readline editing buffer.  If not set, bash attempts
                      to preserve what the user typed.
              dirspell
                      If set, bash attempts spelling correction on
                      directory names during word completion if the
                      directory name initially supplied does not exist.
              dotglob If set, bash includes filenames beginning with a
                      “.” in the results of pathname expansion.  The
                      filenames . and .. must always be matched
                      explicitly, even if dotglob is set.
              execfail
                      If set, a non-interactive shell will not exit if it
                      cannot execute the file specified as an argument to
                      the exec builtin.  An interactive shell does not
                      exit if exec fails.
              expand_aliases
                      If set, aliases are expanded as described above
                      under ALIASES.  This option is enabled by default
                      for interactive shells.
              extdebug
                      If set at shell invocation, or in a shell startup
                      file, arrange to execute the debugger profile
                      before the shell starts, identical to the
                      --debugger option.  If set after invocation,
                      behavior intended for use by debuggers is enabled:
                      1.     The -F option to the declare builtin
                             displays the source file name and line
                             number corresponding to each function name
                             supplied as an argument.
                      2.     If the command run by the DEBUG trap returns
                             a non-zero value, the next command is
                             skipped and not executed.
                      3.     If the command run by the DEBUG trap returns
                             a value of 2, and the shell is executing in
                             a subroutine (a shell function or a shell
                             script executed by the . or source
                             builtins), the shell simulates a call to
                             return.
                      4.     BASH_ARGC and BASH_ARGV are updated as
                             described in their descriptions above).
                      5.     Function tracing is enabled: command
                             substitution, shell functions, and subshells
                             invoked with ( command ) inherit the DEBUG
                             and RETURN traps.
                      6.     Error tracing is enabled: command
                             substitution, shell functions, and subshells
                             invoked with ( command ) inherit the ERR
                             trap.
              extglob If set, enable the extended pattern matching
                      features described above under Pathname Expansion.
              extquote
                      If set, $'string' and $"string" quoting is
                      performed within ${parameter} expansions enclosed
                      in double quotes.  This option is enabled by
                      default.
              failglob
                      If set, patterns which fail to match filenames
                      during pathname expansion result in an expansion
                      error.
              force_fignore
                      If set, the suffixes specified by the FIGNORE shell
                      variable cause words to be ignored when performing
                      word completion even if the ignored words are the
                      only possible completions.  See Shell Variables
                      above for a description of FIGNORE.  This option is
                      enabled by default.
              globasciiranges
                      If set, range expressions used in pattern matching
                      bracket expressions (see Pattern Matching above)
                      behave as if in the traditional C locale when
                      performing comparisons.  That is, pattern matching
                      does not take the current locale's collating
                      sequence into account, so b will not collate
                      between A and B, and upper-case and lower-case
                      ASCII characters will collate together.
              globskipdots
                      If set, pathname expansion will never match the
                      filenames . and .., even if the pattern begins with
                      a “.”.  This option is enabled by default.
              globstar
                      If set, the pattern ** used in a pathname expansion
                      context will match all files and zero or more
                      directories and subdirectories.  If the pattern is
                      followed by a /, only directories and
                      subdirectories match.
              gnu_errfmt
                      If set, shell error messages are written in the
                      standard GNU error message format.
              histappend
                      If set, the history list is appended to the file
                      named by the value of the HISTFILE variable when
                      the shell exits, rather than overwriting the file.
              histreedit
                      If set, and readline is being used, the user is
                      given the opportunity to re-edit a failed history
                      substitution.
              histverify
                      If set, and readline is being used, the results of
                      history substitution are not immediately passed to
                      the shell parser.  Instead, the resulting line is
                      loaded into the readline editing buffer, allowing
                      further modification.
              hostcomplete
                      If set, and readline is being used, bash will
                      attempt to perform hostname completion when a word
                      containing a @ is being completed (see Completing
                      under READLINE above).  This is enabled by default.
              huponexit
                      If set, bash will send SIGHUP to all jobs when an
                      interactive login shell exits.
              inherit_errexit
                      If set, command substitution inherits the value of
                      the errexit option, instead of unsetting it in the
                      subshell environment.  This option is enabled when
                      posix mode is enabled.
              interactive_comments
                      In an interactive shell, a word beginning with #
                      causes that word and all remaining characters on
                      that line to be ignored, as in a non-interactive
                      shell (see COMMENTS above).  This option is enabled
                      by default.
              lastpipe
                      If set, and job control is not active, the shell
                      runs the last command of a pipeline not executed in
                      the background in the current shell environment.
              lithist If set, and the cmdhist option is enabled, multi-
                      line commands are saved to the history with
                      embedded newlines rather than using semicolon
                      separators where possible.
              localvar_inherit
                      If set, local variables inherit the value and
                      attributes of a variable of the same name that
                      exists at a previous scope before any new value is
                      assigned.  The nameref attribute is not inherited.
              localvar_unset
                      If set, calling unset on local variables in
                      previous function scopes marks them so subsequent
                      lookups find them unset until that function
                      returns.  This is identical to the behavior of
                      unsetting local variables at the current function
                      scope.
              login_shell
                      The shell sets this option if it is started as a
                      login shell (see INVOCATION above).  The value may
                      not be changed.
              mailwarn
                      If set, and a file that bash is checking for mail
                      has been accessed since the last time it was
                      checked, bash displays the message “The mail in
                      mailfile has been read”.
              no_empty_cmd_completion
                      If set, and readline is being used, bash does not
                      search PATH for possible completions when
                      completion is attempted on an empty line.
              nocaseglob
                      If set, bash matches filenames in a
                      case-insensitive fashion when performing pathname
                      expansion (see Pathname Expansion above).
              nocasematch
                      If set, bash matches patterns in a case-insensitive
                      fashion when performing matching while executing
                      case or [[ conditional commands, when performing
                      pattern substitution word expansions, or when
                      filtering possible completions as part of
                      programmable completion.
              noexpand_translation
                      If set, bash encloses the translated results of
                      $"..." quoting in single quotes instead of double
                      quotes.  If the string is not translated, this has
                      no effect.
              nullglob
                      If set, pathname expansion patterns which match no
                      files (see Pathname Expansion above) expand to
                      nothing and are removed, rather than expanding to
                      themselves.
              patsub_replacement
                      If set, bash expands occurrences of & in the
                      replacement string of pattern substitution to the
                      text matched by the pattern, as described under
                      Parameter Expansion above.  This option is enabled
                      by default.
              progcomp
                      If set, enable the programmable completion
                      facilities (see Programmable Completion above).
                      This option is enabled by default.
              progcomp_alias
                      If set, and programmable completion is enabled,
                      bash treats a command name that doesn't have any
                      completions as a possible alias and attempts alias
                      expansion.  If it has an alias, bash attempts
                      programmable completion using the command word
                      resulting from the expanded alias.
              promptvars
                      If set, prompt strings undergo parameter expansion,
                      command substitution, arithmetic expansion, and
                      quote removal after being expanded as described in
                      PROMPTING above.  This option is enabled by
                      default.
              restricted_shell
                      The shell sets this option if it is started in
                      restricted mode (see RESTRICTED SHELL below).  The
                      value may not be changed.  This is not reset when
                      the startup files are executed, allowing the
                      startup files to discover whether or not a shell is
                      restricted.
              shift_verbose
                      If set, the shift builtin prints an error message
                      when the shift count exceeds the number of
                      positional parameters.
              sourcepath
                      If set, the . (source) builtin uses the value of
                      PATH to find the directory containing the file
                      supplied as an argument when the -p option is not
                      supplied.  This option is enabled by default.
              varredir_close
                      If set, the shell automatically closes file
                      descriptors assigned using the {varname}
                      redirection syntax (see REDIRECTION above) instead
                      of leaving them open when the command completes.
              xpg_echo
                      If set, the echo builtin expands backslash-escape
                      sequences by default.  If the posix shell option is
                      also enabled, echo does not interpret any options.

       suspend [-f]
              Suspend the execution of this shell until it receives a
              SIGCONT signal.  A login shell, or a shell without job
              control enabled, cannot be suspended; the -f option will
              override this and force the suspension.  The return status
              is 0 unless the shell is a login shell or job control is
              not enabled and -f is not supplied.

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

              Expressions may be combined using the following operators,
              listed in decreasing order of precedence.  The evaluation
              depends on the number of arguments; see below.  test uses
              operator precedence when there are five or more arguments.
              ! expr True if expr is false.
              ( expr )
                     Returns the value of expr.  This may be used to
                     override normal operator precedence.
              expr1 -a expr2
                     True if both expr1 and expr2 are true.
              expr1 -o expr2
                     True if either expr1 or expr2 is true.

              test and [ evaluate conditional expressions using a set of
              rules based on the number of arguments.

              0 arguments
                     The expression is false.
              1 argument
                     The expression is true if and only if the argument
                     is not null.
              2 arguments
                     If the first argument is !, the expression is true
                     if and only if the second argument is null.  If the
                     first argument is one of the unary conditional
                     operators listed above under CONDITIONAL
                     EXPRESSIONS, the expression is true if the unary
                     test is true.  If the first argument is not a valid
                     unary conditional operator, the expression is false.
              3 arguments
                     The following conditions are applied in the order
                     listed.  If the second argument is one of the binary
                     conditional operators listed above under CONDITIONAL
                     EXPRESSIONS, the result of the expression is the
                     result of the binary test using the first and third
                     arguments as operands.  The -a and -o operators are
                     considered binary operators when there are three
                     arguments.  If the first argument is !, the value is
                     the negation of the two-argument test using the
                     second and third arguments.  If the first argument
                     is exactly ( and the third argument is exactly ),
                     the result is the one-argument test of the second
                     argument.  Otherwise, the expression is false.
              4 arguments
                     The following conditions are applied in the order
                     listed.  If the first argument is !, the result is
                     the negation of the three-argument expression
                     composed of the remaining arguments.  If the first
                     argument is exactly ( and the fourth argument is
                     exactly ), the result is the two-argument test of
                     the second and third arguments.  Otherwise, the
                     expression is parsed and evaluated according to
                     precedence using the rules listed above.
              5 or more arguments
                     The expression is parsed and evaluated according to
                     precedence using the rules listed above.

              When the shell is in posix mode, or if the expression is
              part of the [[ command, the < and > operators sort using
              the current locale.  If the shell is not in posix mode, the
              test and [ commands sort lexicographically using ASCII
              ordering.

              The historical operator-precedence parsing with 4 or more
              arguments can lead to ambiguities when it encounters
              strings that look like primaries.  The POSIX standard has
              deprecated the -a and -o primaries and enclosing
              expressions within parentheses.  Scripts should no longer
              use them.  It's much more reliable to restrict test
              invocations to a single primary, and to replace uses of -a
              and -o with the shell's && and || list operators.

       times  Print the accumulated user and system times for the shell
              and for processes run from the shell.  The return status is
              0.

       trap [-lpP] [[action] sigspec ...]
              The action is a command that is read and executed when the
              shell receives any of the signals sigspec.  If action is
              absent (and there is a single sigspec) or -, each specified
              sigspec is reset to the value it had when the shell was
              started.  If action is the null string the signal specified
              by each sigspec is ignored by the shell and by the commands
              it invokes.

              If no arguments are supplied, trap displays the actions
              associated with each trapped signal as a set of trap
              commands that can be reused as shell input to restore the
              current signal dispositions.  If -p is given, and action is
              not present, then trap displays the actions associated with
              each sigspec or, if none are supplied, for all trapped
              signals, as a set of trap commands that can be reused as
              shell input to restore the current signal dispositions.
              The -P option behaves similarly, but displays only the
              actions associated with each sigspec argument.  -P requires
              at least one sigspec argument.  The -P or -p options may be
              used in a subshell environment (e.g., command substitution)
              and, as long as they are used before trap is used to change
              a signal's handling, will display the state of its parent's
              traps.

              The -l option prints a list of signal names and their
              corresponding numbers.  Each sigspec is either a signal
              name defined in <signal.h>, or a signal number.  Signal
              names are case insensitive and the SIG prefix is optional.
              If -l is supplied with no sigspec arguments, it prints a
              list of valid signal names.

              If a sigspec is EXIT (0), action is executed on exit from
              the shell.  If a sigspec is DEBUG, action is executed
              before every simple command, for command, case command,
              select command, (( arithmetic command, [[ conditional
              command, arithmetic for command, and before the first
              command executes in a shell function (see SHELL GRAMMAR
              above).  Refer to the description of the extdebug shell
              option (see shopt above) for details of its effect on the
              DEBUG trap.  If a sigspec is RETURN, action is executed
              each time a shell function or a script executed with the .
              or source builtins finishes executing.

              If a sigspec is ERR, action is executed whenever a pipeline
              (which may consist of a single simple command), a list, or
              a compound command returns a non-zero exit status, subject
              to the following conditions.  The ERR trap is not executed
              if the failed command is part of the command list
              immediately following a while or until reserved word, part
              of the test in an if statement, part of a command executed
              in a && or || list except the command following the final
              && or ||, any command in a pipeline but the last (subject
              to the state of the pipefail shell option), or if the
              command's return value is being inverted using !.  These
              are the same conditions obeyed by the errexit (-e) option.

              When the shell is not interactive, signals ignored upon
              entry to the shell cannot be trapped or reset.  Interactive
              shells permit trapping signals ignored on entry.  Trapped
              signals that are not being ignored are reset to their
              original values in a subshell or subshell environment when
              one is created.  The return status is false if any sigspec
              is invalid; otherwise trap returns true.

       true   Does nothing, returns a 0 status.

       type [-aftpP] name [name ...]
              Indicate how each name would be interpreted if used as a
              command name.

              If the -t option is used, type prints a string which is one
              of alias, keyword, function, builtin, or file if name is an
              alias, shell reserved word, function, builtin, or
              executable file, respectively.  If the name is not found,
              type prints nothing and returns a non-zero exit status.

              If the -p option is used, type either returns the pathname
              of the executable file that would be found by searching
              $PATH for name or nothing if “type -t name” would not
              return file.  The -P option forces a PATH search for each
              name, even if “type -t name” would not return file.  If
              name is present in the table of hashed commands, -p and -P
              print the hashed value, which is not necessarily the file
              that appears first in PATH.

              If the -a option is used, type prints all of the places
              that contain a command named name.  This includes aliases,
              reserved words, functions, and builtins, but the path
              search options (-p and -P) can be supplied to restrict the
              output to executable files.  type does not consult the
              table of hashed commands when using -a with -p, and only
              performs a PATH search for name.

              The -f option suppresses shell function lookup, as with the
              command builtin.  type returns true if all of the arguments
              are found, false if any are not found.

       ulimit [-HS] -a
       ulimit [-HS] [-bcdefiklmnpqrstuvxPRT [limit]]
              Provides control over the resources available to the shell
              and to processes it starts, on systems that allow such
              control.

              The -H and -S options specify whether the hard or soft
              limit is set for the given resource.  A hard limit cannot
              be increased by a non-root user once it is set; a soft
              limit may be increased up to the value of the hard limit.
              If neither -H nor -S is specified, ulimit sets both the
              soft and hard limits.

              The value of limit can be a number in the unit specified
              for the resource or one of the special values hard, soft,
              or unlimited, which stand for the current hard limit, the
              current soft limit, and no limit, respectively.  If limit
              is omitted, ulimit prints the current value of the soft
              limit of the resource, unless the -H option is given.  When
              more than one resource is specified, the limit name and
              unit, if appropriate, are printed before the value.  Other
              options are interpreted as follows:
              -a     Report all current limits; no limits are set.
              -b     The maximum socket buffer size.
              -c     The maximum size of core files created.
              -d     The maximum size of a process's data segment.
              -e     The maximum scheduling priority (“nice”).
              -f     The maximum size of files written by the shell and
                     its children.
              -i     The maximum number of pending signals.
              -k     The maximum number of kqueues that may be allocated.
              -l     The maximum size that may be locked into memory.
              -m     The maximum resident set size (many systems do not
                     honor this limit).
              -n     The maximum number of open file descriptors (most
                     systems do not allow this value to be set).
              -p     The pipe size in 512-byte blocks (this may not be
                     set).
              -q     The maximum number of bytes in POSIX message queues.
              -r     The maximum real-time scheduling priority.
              -s     The maximum stack size.
              -t     The maximum amount of cpu time in seconds.
              -u     The maximum number of processes available to a
                     single user.
              -v     The maximum amount of virtual memory available to
                     the shell and, on some systems, to its children.
              -x     The maximum number of file locks.
              -P     The maximum number of pseudoterminals.
              -R     The maximum time a real-time process can run before
                     blocking, in microseconds.
              -T     The maximum number of threads.

              If limit is supplied, and the -a option is not used, limit
              is the new value of the specified resource.  If no option
              is supplied, then -f is assumed.

              Values are in 1024-byte increments, except for -t, which is
              in seconds; -R, which is in microseconds; -p, which is in
              units of 512-byte blocks; -P, -T, -b, -k, -n, and -u, which
              are unscaled values; and, when in posix mode, -c and -f,
              which are in 512-byte increments.  The return status is 0
              unless an invalid option or argument is supplied, or an
              error occurs while setting a new limit.

       umask [-p] [-S] [mode]
              Set the user file-creation mask to mode.  If mode begins
              with a digit, it is interpreted as an octal number;
              otherwise it is interpreted as a symbolic mode mask similar
              to that accepted by chmod(1).  If mode is omitted, umask
              prints the current value of the mask.  The -S option
              without a mode argument prints the mask in a symbolic
              format; the default output is an octal number.  If the -p
              option is supplied, and mode is omitted, the output is in a
              form that may be reused as input.  The return status is
              zero if the mode was successfully changed or if no mode
              argument was supplied, and non-zero otherwise.

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

       unset [-fv] [-n] [name ...]
              For each name, remove the corresponding variable or
              function.  If the -v option is given, each name refers to a
              shell variable, and that variable is removed.  If -f is
              specified, each name refers to a shell function, and the
              function definition is removed.  If the -n option is
              supplied, and name is a variable with the nameref
              attribute, name will be unset rather than the variable it
              references.  -n has no effect if the -f option is supplied.
              Read-only variables and functions may not be unset.  When
              variables or functions are removed, they are also removed
              from the environment passed to subsequent commands.  If no
              options are supplied, each name refers to a variable; if
              there is no variable by that name, a function with that
              name, if any, is unset.  Some shell variables may not be
              unset.  If any of BASH_ALIASES, BASH_ARGV0, BASH_CMDS,
              BASH_COMMAND, BASH_SUBSHELL, BASHPID, COMP_WORDBREAKS,
              DIRSTACK, EPOCHREALTIME, EPOCHSECONDS, FUNCNAME, GROUPS,
              HISTCMD, LINENO, RANDOM, SECONDS, or SRANDOM are unset,
              they lose their special properties, even if they are
              subsequently reset.  The exit status is true unless a name
              is readonly or may not be unset.

       wait [-fn] [-p varname] [id ...]
              Wait for each specified child process id and return the
              termination status of the last id.  Each id may be a
              process ID pid or a job specification jobspec; if a jobspec
              is supplied, wait waits for all processes in the job.

              If no options or ids are supplied, wait waits for all
              running background jobs and the last-executed process
              substitution, if its process id is the same as $!, and the
              return status is zero.

              If the -n option is supplied, wait waits for any one of the
              given ids or, if no ids are supplied, any job or process
              substitution, to complete and returns its exit status.  If
              none of the supplied ids is a child of the shell, or if no
              ids are supplied and the shell has no unwaited-for
              children, the exit status is 127.

              If the -p option is supplied, wait assigns the process or
              job identifier of the job for which the exit status is
              returned to the variable varname named by the option
              argument.  The variable, which cannot be readonly, will be
              unset initially, before any assignment.  This is useful
              only when used with the -n option.

              Supplying the -f option, when job control is enabled,
              forces wait to wait for each id to terminate before
              returning its status, instead of returning when it changes
              status.

              If none of the ids specify one of the shell's active child
              processes, the return status is 127.  If wait is
              interrupted by a signal, any varname will remain unset, and
              the return status will be greater than 128, as described
              under SIGNALS above.  Otherwise, the return status is the
              exit status of the last id.

SHELL COMPATIBILITY MODE         top

       Bash-4.0 introduced the concept of a shell compatibility level,
       specified as a set of options to the shopt builtin (compat31,
       compat32, compat40, compat41, and so on).  There is only one
       current compatibility level — each option is mutually exclusive.
       The compatibility level is intended to allow users to select
       behavior from previous versions that is incompatible with newer
       versions while they migrate scripts to use current features and
       behavior.  It's intended to be a temporary solution.

       This section does not mention behavior that is standard for a
       particular version (e.g., setting compat32 means that quoting the
       right hand side of the regexp matching operator quotes special
       regexp characters in the word, which is default behavior in
       bash-3.2 and subsequent versions).

       If a user enables, say, compat32, it may affect the behavior of
       other compatibility levels up to and including the current
       compatibility level.  The idea is that each compatibility level
       controls behavior that changed in that version of bash, but that
       behavior may have been present in earlier versions.  For instance,
       the change to use locale-based comparisons with the [[ command
       came in bash-4.1, and earlier versions used ASCII-based
       comparisons, so enabling compat32 will enable ASCII-based
       comparisons as well.  That granularity may not be sufficient for
       all uses, and as a result users should employ compatibility levels
       carefully.  Read the documentation for a particular feature to
       find out the current behavior.

       Bash-4.3 introduced a new shell variable: BASH_COMPAT.  The value
       assigned to this variable (a decimal version number like 4.2, or
       an integer corresponding to the compatNN option, like 42)
       determines the compatibility level.

       Starting with bash-4.4, bash began deprecating older compatibility
       levels.  Eventually, the options will be removed in favor of
       BASH_COMPAT.

       Bash-5.0 was the final version for which there was an individual
       shopt option for the previous version.  BASH_COMPAT is the only
       mechanism to control the compatibility level in versions newer
       than bash-5.0.

       The following table describes the behavior changes controlled by
       each compatibility level setting.  The compatNN tag is used as
       shorthand for setting the compatibility level to NN using one of
       the following mechanisms.  For versions prior to bash-5.0, the
       compatibility level may be set using the corresponding compatNN
       shopt option.  For bash-4.3 and later versions, the BASH_COMPAT
       variable is preferred, and it is required for bash-5.1 and later
       versions.

       compat31
              •      Quoting the rhs of the [[ command's regexp matching
                     operator (=~) has no special effect.

       compat32
              •      The < and > operators to the [[ command do not
                     consider the current locale when comparing strings;
                     they use ASCII ordering.

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

       compat41
              •      In posix mode, time may be followed by options and
                     still be recognized as a reserved word (this is
                     POSIX interpretation 267).
              •      In posix mode, the parser requires that an even
                     number of single quotes occur in the word portion of
                     a double-quoted parameter expansion and treats them
                     specially, so that characters within the single
                     quotes are considered quoted (this is POSIX
                     interpretation 221).

       compat42
              •      The replacement string in double-quoted pattern
                     substitution does not undergo quote removal, as it
                     does in versions after bash-4.2.
              •      In posix mode, single quotes are considered special
                     when expanding the word portion of a double-quoted
                     parameter expansion and can be used to quote a
                     closing brace or other special character (this is
                     part of POSIX interpretation 221); in later
                     versions, single quotes are not special within
                     double-quoted word expansions.

       compat43
              •      Word expansion errors are considered non-fatal
                     errors that cause the current command to fail, even
                     in posix mode (the default behavior is to make them
                     fatal errors that cause the shell to exit).
              •      When executing a shell function, the loop state
                     (while/until/etc.)  is not reset, so break or
                     continue in that function will break or continue
                     loops in the calling context.  Bash-4.4 and later
                     reset the loop state to prevent this.

       compat44
              •      The shell sets up the values used by BASH_ARGV and
                     BASH_ARGC so they can expand to the shell's
                     positional parameters even if extended debugging
                     mode is not enabled.
              •      A subshell inherits loops from its parent context,
                     so break or continue will cause the subshell to
                     exit.  Bash-5.0 and later reset the loop state to
                     prevent the exit
              •      Variable assignments preceding builtins like export
                     and readonly that set attributes continue to affect
                     variables with the same name in the calling
                     environment even if the shell is not in posix mode.

       compat50
              •      Bash-5.1 changed the way $RANDOM is generated to
                     introduce slightly more randomness.  If the shell
                     compatibility level is set to 50 or lower, it
                     reverts to the method from bash-5.0 and previous
                     versions, so seeding the random number generator by
                     assigning a value to RANDOM will produce the same
                     sequence as in bash-5.0.
              •      If the command hash table is empty, bash versions
                     prior to bash-5.1 printed an informational message
                     to that effect, even when producing output that can
                     be reused as input.  Bash-5.1 suppresses that
                     message when the -l option is supplied.

       compat51
              •      The unset builtin treats attempts to unset array
                     subscripts @ and * differently depending on whether
                     the array is indexed or associative, and differently
                     than in previous versions.
              •      Arithmetic commands ( ((...)) ) and the expressions
                     in an arithmetic for statement can be expanded more
                     than once.
              •      Expressions used as arguments to arithmetic
                     operators in the [[ conditional command can be
                     expanded more than once.
              •      The expressions in substring parameter brace
                     expansion can be expanded more than once.
              •      The expressions in the $((...)) word expansion can
                     be expanded more than once.
              •      Arithmetic expressions used as indexed array
                     subscripts can be expanded more than once.
              •      test -v, when given an argument of A[@], where A is
                     an existing associative array, will return true if
                     the array has any set elements.  Bash-5.2 will look
                     for and report on a key named @.
              •      The ${parameter[:]=value} word expansion will return
                     value, before any variable-specific transformations
                     have been performed (e.g., converting to lowercase).
                     Bash-5.2 will return the final value assigned to the
                     variable.
              •      Parsing command substitutions will behave as if
                     extended globbing (see the description of the shopt
                     builtin above) is enabled, so that parsing a command
                     substitution containing an extglob pattern (say, as
                     part of a shell function) will not fail.  This
                     assumes the intent is to enable extglob before the
                     command is executed and word expansions are
                     performed.  It will fail at word expansion time if
                     extglob hasn't been enabled by the time the command
                     is executed.

       compat52
              •      The test builtin uses its historical algorithm to
                     parse parenthesized subexpressions when given five
                     or more arguments.
              •      If the -p or -P option is supplied to the bind
                     builtin, bind treats any arguments remaining after
                     option processing as bindable command names, and
                     displays any key sequences bound to those commands,
                     instead of treating the arguments as key sequences
                     to bind.

RESTRICTED SHELL         top

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

       •      Changing directories with cd.

       •      Setting or unsetting the values of SHELL, PATH, HISTFILE,
              ENV, or BASH_ENV.

       •      Specifying command names containing /.

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

       •      Using the -p option to the .  builtin command to specify a
              search path.

       •      Specifying a filename containing a slash as an argument to
              the history builtin command.

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

       •      Importing function definitions from the shell environment
              at startup.

       •      Parsing the values of BASHOPTS and SHELLOPTS from the shell
              environment at startup.

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

       •      Using the exec builtin command to replace the shell with
              another command.

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

       •      Using the enable builtin command to enable disabled shell
              builtins.

       •      Specifying the -p option to the command builtin command.

       •      Turning off restricted mode with set +r or shopt -u
              restricted_shell.

       These restrictions are enforced after any startup files are read.

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

SEE ALSO         top

       Bash Reference Manual, Brian Fox and Chet Ramey
       The Gnu Readline Library, Brian Fox and Chet Ramey
       The Gnu History Library, Brian Fox and Chet Ramey
       Portable Operating System Interface (POSIX) Part 2: Shell and
       Utilities, IEEE —
              http://pubs.opengroup.org/onlinepubs/9799919799/
       http://tiswww.case.edu/~chet/bash/POSIX — a description of posix
       mode
       sh(1), ksh(1), csh(1)
       emacs(1), vi(1)
       readline(3)

FILES         top

       /bin/bash
              The bash executable
       /etc/profile
              The systemwide initialization file, executed for login
              shells
       ~/.bash_profile
              The personal initialization file, executed for login shells
       ~/.bashrc
              The individual per-interactive-shell startup file
       ~/.bash_logout
              The individual login shell cleanup file, executed when a
              login shell exits
       ~/.bash_history
              The default value of HISTFILE, the file in which bash saves
              the command history
       ~/.inputrc
              Individual readline initialization file

AUTHORS         top

       Brian Fox, Free Software Foundation
       bfox@gnu.org

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

BUG REPORTS         top

       If you find a bug in bash, you should report it.  But first, you
       should make sure that it really is a bug, and that it appears in
       the latest version of bash.  The latest version is always
       available from ftp://ftp.gnu.org/pub/gnu/bash/ and
       http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash- 
       master.tar.gz.

       Once you have determined that a bug actually exists, use the
       bashbug command to submit a bug report.  If you have a fix, you
       are encouraged to mail that as well!  You may send suggestions and
       “philosophical” bug reports to bug-bash@gnu.org or post them to
       the Usenet newsgroup gnu.bash.bug.

       ALL bug reports should include:

       The version number of bash
       The hardware and operating system
       The compiler used to compile
       A description of the bug behavior
       A short script or “recipe” which exercises the bug

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

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

BUGS         top

       It's too big and too slow.

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

       Aliases are confusing in some uses.

       Shell builtin commands and functions are not
       stoppable/restartable.

       Compound commands and command lists of the form “a ; b ; c” are
       not handled gracefully when combined with process suspension.
       When a process is stopped, the shell immediately executes the next
       command in the list or breaks out of any existing loops.  It
       suffices to enclose the command in parentheses to force it into a
       subshell, which may be stopped as a unit, or to start the command
       in the background and immediately bring it into the foreground.

       Array variables may not (yet) be exported.

COLOPHON         top

       This page is part of the bash (Bourne again shell) project.
       Information about the project can be found at 
       ⟨http://www.gnu.org/software/bash/⟩.  If you have a bug report for
       this manual page, see ⟨http://www.gnu.org/software/bash/⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨git://git.savannah.gnu.org/bash.git⟩ on 2025-08-11.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2025-07-29.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there is
       a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

GNU Bash 5.3                   2025 April 7                       BASH(1)

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