bash手册

BASH(1) General Commands Manual       BASH(1)






NAME
       bash - GNU Bourne-Again SHell


SYNOPSIS
       bash [options] [file]


COPYRIGHT
       Bash is Copyright (C) 1989-2011 by the Free Software Foundation, Inc.


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


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


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


       -c string If the -c option is present, then commands are read from string.  If there are arguments  after  the
string, they are assigned to the positional parameters, starting with $0.
       -i If the -i option is present, the shell is interactive.
       -l Make bash act as if it had been invoked as a login shell (see INVOCATION below).
       -r If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below).
       -s If  the  -s  option  is present, or if no arguments remain after option processing, then commands are
read from the standard input. This option allows the positional parameters to be set when  invoking
an interactive shell.
       -D A  list  of all double-quoted strings preceded by $ is printed on the standard output. These are the
strings that are subject to language translation when the current locale is not  C  or POSIX.  This
implies the -n option; no commands will be executed.
       [-+]O [shopt_option]
shopt_option  is  one of the shell options accepted by the shopt builtin (see SHELL BUILTIN COMMANDS
below).  If shopt_option is present, -O sets the value of that option; +O unsets it.  If shopt_option
is not supplied, the names and values of the shell options accepted by shopt are printed on the stan‐
dard output.  If the invocation option is +O, the output is displayed in a format that may be reused
as input.
       -- A  -- signals the end of options and disables further option processing.  Any arguments after the --
are treated as filenames and arguments.  An argument of - is equivalent to --.


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


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


       --login
     Equivalent to -l.


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


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


       --norc Do  not  read  and execute the personal initialization file ~/.bashrc if the shell is interactive.  This
     option is on by default if the shell is invoked as sh.


       --posix
     Change the behavior of bash where the default operation differs from the POSIX  standard to  match  the
     standard (posix mode).


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


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


       --verbose
     Equivalent to  -v.


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


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


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


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


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


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


       When   a  login  shell  exits,   bash reads and  executes  commands from  the  files  ~/.bash_logout  and
       /etc/bash.bash_logout, if the files exists.


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


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


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


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


       Bash attempts to determine when it is being run with its standard input connected to a network  connection,  as
       when executed by the remote shell daemon, usually rshd, or the secure shell daemon sshd. If bash determines it
       is being run in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and  is read‐
       able.   It  will not do this if invoked as sh.  The --norc option may be used to inhibit this behavior, and the
       --rcfile option may be used to force another file to be read, but rshd does not generally invoke the shell with
       those options or allow them to be specified.


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


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


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


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


SHELL GRAMMAR
   Simple Commands
       A simple command is a sequence of optional variable assignments followed by blank-separated words and  redirec‐
       tions,  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]] [ ! ] command [ [|⎪|&] command2 ... ]


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


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


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


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


       Each command in a pipeline is executed as a separate process (i.e., in a subshell).


   Lists
       A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally
       terminated by one of ;, &, or <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.   Commands  separated
       by  a ; are executed sequentially; the shell waits for each command to terminate in turn.  The return status is
       the exit status of the last command executed.


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


     command1 && command2


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


       An OR list has the form


     command1 || command2


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


   Compound Commands
       A compound command is one of the following:


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


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


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


     When used with [[, the < and > operators sort lexicographically using the 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.  If the shell option nocase‐
     match is enabled, the match is performed without regard to  the  case  of alphabetic  characters.   The
     return  value  is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise.  Any
     part of the pattern may be quoted to force it to be matched as a string.


     An additional binary operator, =~, is available, with the same precedence as ==  and  !=.  When it  is
     used,  the  string to the right of the operator is considered an extended regular expression and matched
     accordingly (as in regex(3)).  The return value is 0 if the string matches the pattern, and 1 otherwise.
     If  the  regular expression is syntactically incorrect, the conditional expression's return value is 2.
     If the shell option nocasematch is enabled, the match is performed without regard to the case of alpha‐
     betic  characters.   Any part of the pattern may be quoted to force it to be matched as a string.  Sub‐
     strings matched by parenthesized subexpressions within the regular expression are saved in  the array
     variable BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching
     the entire regular expression.  The element of BASH_REMATCH with index n is the portion  of  the string
     matching the nth parenthesized subexpression.


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


     ( expression )
    Returns  the  value  of expression.  This may be used to override the normal precedence of opera‐
    tors.
     ! 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 deter‐
     mine the return value of the entire conditional expression.


       for name [ [ in [ word ... ] ] ; ] do list ; done
     The  list of  words  following in is expanded, generating a list of items.  The variable name is set to
     each element of this list in turn, and list is 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,  the arithmetic expression expr1 is evaluated according to the rules described below under ARITH‐
     METIC EVALUATION. The arithmetic expression expr2 is then evaluated repeatedly until  it evaluates  to
     zero.   Each  time  expr2 evaluates to a non-zero value, list is executed and the arithmetic expression
     expr3 is evaluated.  If any expression is omitted, it behaves as if it evaluates to 1.  The return value
     is  the  exit status of the last command in list that is executed, or false if any of the expressions is
     invalid.


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


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


       if list; then list; [ elif list; then list; ] ... [ else list; ] fi
     The  if  list is executed.  If its exit status is zero, the then list is executed.  Otherwise, each elif
     list is executed in turn, and if its exit status is zero, the corresponding then list  is executed  and
     the  command  completes. Otherwise, the else list is executed, if present.  The exit status is the exit
     status of the last 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  sta‐
     tus.  The exit status of the while and until commands is the exit status of the last command executed in
     list-2, or zero if none was executed.


   Coprocesses
       A coprocess is a shell command preceded by the coproc reserved word.  A coprocess is executed asynchronously in
       a  subshell, as if the command had been terminated with the & control operator, with a two-way pipe established
       between the executing shell and the coprocess.


       The format for a coprocess is:


     coproc [NAME] command [redirections]


       This creates a coprocess named NAME.  If NAME is not supplied, the default name is COPROC.  NAME must  not  be
       supplied if command is a simple command (see above); otherwise, it is interpreted as the first word of the sim‐
       ple command.  When the coproc 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 descrip‐
       tor 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 expan‐
       sions.  The process ID of the shell spawned to execute the coprocess is available as the value of the  variable
       NAME_PID.  The wait builtin command may be used to wait for the coprocess to terminate.


       The return status of a coprocess is the exit status of command.


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


       name () compound-command [redirection]
       function name [()] compound-command [redirection]
     This defines a function named name.  The reserved word function is optional.  If the  function  reserved
     word  is supplied, the parentheses are optional. The body of the function is the compound command com‐
     pound-command (see Compound Commands above).  That command is usually a list of commands between {  and
     },  but  may be any command listed under Compound Commands above. compound-command is executed whenever
     name is specified as the name of a simple command.  Any redirections (see REDIRECTION  below)  specified
     when  a  function is defined are performed when the function is executed. The exit status of a function
     definition is zero unless a syntax error occurs or a  readonly  function with  the  same name  already
     exists.  When executed, the exit status of a function is the exit status of the last command executed in
     the body. (See FUNCTIONS below.)


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


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


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


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


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


       A  non-quoted backslash (\) is the escape character.  It preserves the literal value of the next character that
       follows, with the exception of <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  sin‐
       gle quote may not occur between single quotes, even when preceded by a backslash.


       Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the
       exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their  special
       meaning within double quotes.  The backslash retains its special meaning only when followed by one of the fol‐
       lowing characters: $, `, ", \, or <newline>.  A double quote may be quoted within double quotes by preceding it
       with  a backslash.  If enabled, history expansion will be performed unless an ! appearing in double quotes is
       escaped using a backslash.  The backslash preceding the !  is not removed.


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


       Words of the form $'string' are treated specially.  The word expands to string, with backslash-escaped  charac‐
       ters replaced as specified by the ANSI C standard.  Backslash escape sequences, if present, are decoded as fol‐
       lows:
     \a     alert (bell)
     \b     backspace
     \e
     \E     an escape character
     \f     form feed
     \n     new line
     \r     carriage return
     \t     horizontal tab
     \v     vertical tab
     \\     backslash
     \'     single quote
     \"     double quote
     \nnn   the eight-bit character whose value is the octal value nnn (one to three digits)
     \xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
     \uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four  hex
    digits)
     \UHHHHHHHH
    the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight
    hex digits)
     \cx    a control-x character


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


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


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


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


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


     name=[value]


       If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter
       and  variable  expansion,  command substitution, arithmetic expansion, and quote removal (see EXPANSION below).
       If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if  the
       $((...)) expansion  is not  used (see Arithmetic Expansion below).  Word splitting is not performed, with the
       exception of "$@" as explained below under Special Parameters.  Pathname expansion is not  performed.   Assign‐
       ment  statements may  also  appear  as arguments  to the alias, declare, typeset, export, readonly, and local
       builtin commands.


       In the context where an assignment statement is assigning a value to a shell variable or array  index,  the  +=
       operator can  be  used to append to or add to the variable's previous value.  When += is applied to a variable
       for which the integer attribute has been set, value is evaluated as an arithmetic expression and added  to  the
       variable's  current  value,  which  is  also evaluated. When += is applied to an array variable using compound
       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.


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


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


   Special Parameters
       The  shell treats several parameters specially. These parameters may only be referenced; assignment to them is
       not allowed.
       *      Expands to the positional parameters, starting from  one.  When the  expansion occurs within double
     quotes, it expands to a single word with the value of each parameter separated by the first character of
     the IFS special variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of
     the  value  of  the  IFS variable.  If IFS is unset, the parameters are separated by spaces.  If IFS is
     null, the parameters are joined without intervening separators.
       @      Expands to the positional parameters, starting from  one.  When the  expansion occurs within double
     quotes, each parameter expands to a separate word.  That is, "$@" is equivalent to "$1" "$2" ... If the
     double-quoted expansion occurs within a word, the expansion of the first parameter is  joined  with  the
     beginning part  of  the original word, and the expansion of the last parameter is joined with the last
     part of the original word.  When there are no positional parameters,  "$@"  and  $@  expand  to  nothing
     (i.e., they are removed).
       #      Expands to the number of positional parameters in decimal.
       ?      Expands to the exit status of the most recently executed foreground pipeline.
       -      Expands  to  the current option flags as specified upon invocation, by the set builtin command, or those
     set by the shell itself (such as the -i option).
       $      Expands to the process ID of the shell.  In a () subshell, it expands to the process ID of  the  current
     shell, not the subshell.
       !      Expands to the process ID of the most recently executed background (asynchronous) command.
       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. Other‐
     wise, it is set to the file name used to invoke bash, as given by argument zero.
       _      At shell startup, set to the absolute pathname used to invoke the shell or shell script  being  executed
     as passed in the environment or argument list.  Subsequently, expands to the last argument to the previ‐
     ous command, after expansion.  Also set to the full pathname used to invoke each command executed  and
     placed  in  the environment exported to that command.  When checking mail, this parameter holds the name
     of the mail file currently being checked.


   Shell Variables
       The following variables are set by the shell:


       BASH   Expands to the full file name used to invoke this instance of bash.
       BASHOPTS
     A colon-separated list of enabled shell options. Each word in the list is a valid argument for  the  -s
     option  to  the  shopt  builtin  command (see  SHELL BUILTIN COMMANDS below).  The options appearing in
     BASHOPTS are those reported as on by shopt.  If this variable is in the environment when bash starts up,
     each  shell option in the list will be enabled before reading any startup files. This variable is read-
     only.
       BASHPID
     Expands to the process ID of the current bash process.  This  differs  from  $$  under  certain  circum‐
     stances, such as subshells that do not require bash to be re-initialized.
       BASH_ALIASES
     An  associative array variable whose members correspond to the internal list of aliases as maintained by
     the alias builtin.  Elements added to this array appear in the  alias  list;  unsetting  array  elements
     cause aliases to be removed from the alias list.
       BASH_ARGC
     An  array variable whose values are the number of parameters in each frame of the current bash execution
     call stack.  The number of parameters to the current 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)
       BASH_ARGV
     An  array variable containing all of the parameters in the current bash execution call stack.  The final
     parameter of the last subroutine call is at the top of the stack; the first  parameter  of  the  initial
     call  is at  the bottom.   When a  subroutine is  executed,  the parameters supplied are pushed onto
     BASH_ARGV.  The shell sets BASH_ARGV only when in extended debugging mode (see the  description  of  the
     extdebug option to the shopt builtin below)
       BASH_CMDS
     An  associative  array variable whose members correspond to the internal hash table of commands as main‐
     tained by the hash builtin.  Elements added to this array appear in the hash table; unsetting array ele‐
     ments cause commands to be removed from the hash table.
       BASH_COMMAND
     The command currently being executed or about to be executed, unless the shell is executing a command as
     the result of a trap, in which case it is the command executing at the time of the trap.
       BASH_EXECUTION_STRING
     The command argument to the -c invocation option.
       BASH_LINENO
     An array variable whose members are the line numbers in source files where each corresponding member  of
     FUNCNAME was  invoked.  ${BASH_LINENO[$i]} is the line 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.
       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  ele‐
     ment with index n is the portion of the string matching the nth parenthesized subexpression.  This vari‐
     able is read-only.
       BASH_SOURCE
     An array variable whose members are the source filenames where the corresponding shell function names in
     the  FUNCNAME  array  variable  are  defined.  The shell function ${FUNCNAME[$i]} is defined in the file
     ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}.
       BASH_SUBSHELL
     Incremented by one each time a subshell or subshell environment is spawned.  The initial value is 0.
       BASH_VERSINFO
     A readonly array variable whose members hold version information for this instance of bash.  The values
     assigned to the array members are as follows:
     BASH_VERSINFO[0]      The major version number (the release).
     BASH_VERSINFO[1]      The minor version number (the version).
     BASH_VERSINFO[2]      The patch level.
     BASH_VERSINFO[3]      The build version.
     BASH_VERSINFO[4]      The release status (e.g., beta1).
     BASH_VERSINFO[5]      The value of MACHTYPE.
       BASH_VERSION
     Expands to a string describing the version of this instance of bash.
       COMP_CWORD
     An index into ${COMP_WORDS} of the word containing the current cursor position.  This variable is avail‐
     able only in shell functions invoked by the programmable completion facilities (see Programmable Comple‐
     tion below).
       COMP_KEY
     The key (or final key of a key sequence) used to invoke the current completion function.
       COMP_LINE
     The  current  command  line.   This  variable is available only in shell functions and external commands
     invoked by the programmable completion facilities (see Programmable Completion below).
       COMP_POINT
     The index of the current cursor position relative to the beginning of the current command.  If the  cur‐
     rent  cursor  position  is  at  the  end of the current command, the value of this variable is equal to
     ${#COMP_LINE}.  This variable is available only in shell functions and external commands invoked by  the
     programmable completion facilities (see Programmable Completion below).
       COMP_TYPE
     Set to an integer value corresponding to the type of completion attempted that caused a completion func‐
     tion 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 comple‐
     tion.  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.   Directo‐
     ries  appear  in the stack in the order they are displayed by the dirs builtin.  Assigning to members of
     this array variable may be used to modify directories already in the  stack,  but the  pushd  and  popd
     builtins must  be  used to add and remove directories.  Assignment to this variable will not change the
     current directory.  If DIRSTACK is unset, it loses its special properties, even if  it  is  subsequently
     reset.
       EUID   Expands  to  the effective user ID of the current user, initialized at shell startup.  This variable is
     readonly.
       FUNCNAME
     An array variable containing the names of all shell functions currently in  the  execution  call stack.
     The element with index 0 is the name of any currently-executing shell function.  The bottom-most element
     (the one with the highest index) is "main".  This variable exists only when a shell function is  execut‐
     ing.  Assignments to FUNCNAME have no effect and return an error status. If FUNCNAME is unset, it loses
     its special properties, even if it is subsequently reset.


     This variable can be used with BASH_LINENO and BASH_SOURCE.  Each element of 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  dis‐
     plays 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 and return an error status. If GROUPS is unset, it loses its special  properties,
     even if it is subsequently reset.
       HISTCMD
     The history number, or index in the history list, of the current command. If HISTCMD is unset, it loses
     its special properties, even if it is subsequently reset.
       HOSTNAME
     Automatically set to the name of the current host.
       HOSTTYPE
     Automatically set to a string that uniquely describes the type of machine on which  bash is  executing.
     The default is system-dependent.
       LINENO Each  time this parameter is referenced, the shell substitutes a decimal number 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 vari‐
     able 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  COM‐
     MANDS below).
       OPTIND The  index  of  the next argument to be processed by the getopts builtin command (see SHELL BUILTIN COM‐
     MANDS below).
       OSTYPE Automatically set to a string that describes the operating system on  which  bash  is  executing.   The
     default is system-dependent.
       PIPESTATUS
     An  array variable (see Arrays below) containing a list of exit status values from the processes in the
     most-recently-executed foreground pipeline (which may contain only a single command).
       PPID   The process ID of the shell's parent.  This variable is readonly.
       PWD    The current working directory as set by the cd command.
       RANDOM Each time this parameter is referenced, a random integer between 0 and 32767 is generated.  The sequence
     of  random  numbers may be initialized by assigning a value to RANDOM.  If RANDOM is unset, it loses its
     special properties, even if it is subsequently reset.
       READLINE_LINE
     The contents of the readline line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS below).
       READLINE_POINT
     The position of the insertion point in the readline line buffer, for  use with  "bind  -x"  (see SHELL
     BUILTIN COMMANDS below).
       REPLY  Set to the line of input read by the read builtin command when no arguments are supplied.
       SECONDS
     Each  time this parameter is referenced, the number of seconds since shell invocation is returned.  If a
     value is assigned to SECONDS, the value returned upon subsequent references is  the  number  of  seconds
     since  the  assignment  plus  the value assigned. 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 SHEL‐
     LOPTS are those reported as on by set -o. If this variable is in the environment when bash  starts  up,
     each  shell option in the list will be enabled before reading any startup files. This variable is read-
     only.
       SHLVL  Incremented by one each time an instance of bash is started.
       UID    Expands to the user ID of the current user, initialized at shell startup. This variable is readonly.


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


       BASH_ENV
     If  this parameter is set when bash is executing a shell script, its value is interpreted as a filename
     containing commands to initialize the shell, as in ~/.bashrc.  The value of  BASH_ENV  is subjected  to
     parameter expansion,  command substitution, and arithmetic expansion before being interpreted as a file
     name.  PATH is not used to search for the resultant file name.
       BASH_XTRACEFD
     If set to an integer corresponding to a valid file descriptor, bash will write the trace output gener‐
     ated  when  set -x is enabled to that file descriptor.  The file descriptor is closed when BASH_XTRACEFD
     is unset or assigned a new value. Unsetting BASH_XTRACEFD or assigning it the empty string  causes  the
     trace output to be sent to the standard error.  Note that setting BASH_XTRACEFD to 2 (the standard error
     file descriptor) and then unsetting it will result in the standard error being closed.
       CDPATH The search path for the cd command.  This is a colon-separated list of directories in  which  the shell
     looks for destination directories specified by the cd command.  A sample value is ".:~:/usr".
       COLUMNS
     Used  by the  select  compound  command to determine the terminal width when printing selection lists.
     Automatically set 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).
       EMACS  If bash finds this variable in the environment when the shell starts with value "t", it assumes that the
     shell is running in an Emacs shell buffer and disables line editing.
       ENV    Similar to BASH_ENV; used when the shell is invoked in POSIX mode.
       FCEDIT The default editor for the fc builtin command.
       FIGNORE
     A colon-separated list of suffixes to ignore when performing filename completion (see  READLINE  below).
     A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched file‐
     names.  A sample value is ".o:~".
       FUNCNEST
     If set to a numeric value greater than 0, defines a maximum function nesting  level.   Function  invoca‐
     tions that exceed this nesting level will cause the current command to abort.
       GLOBIGNORE
     A colon-separated  list of patterns defining the set of filenames to be ignored by pathname expansion.
     If a filename matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it
     is removed from the list of matches.
       HISTCONTROL
     A colon-separated list of values controlling how commands are saved on the history list. If the list of
     values includes ignorespace, lines which begin with a space character are not saved in the history list.
     A value of  ignoredups causes lines matching the previous history entry to not be saved.  A value of
     ignoreboth is shorthand for ignorespace and ignoredups.  A value of erasedups causes all previous lines
     matching the current line to be removed from the history list before that line is saved. Any value not
     in the above list is ignored.  If HISTCONTROL is unset, or does not include a  valid  value,  all lines
     read  by the shell parser are saved on the history list, subject to the value of HISTIGNORE.  The second
     and subsequent lines of a multi-line compound command are not tested,  and  are  added  to  the  history
     regardless of the value of HISTCONTROL.
       HISTFILE
     The  name of  the  file in  which  command history is saved (see HISTORY below).  The default value is
     ~/.bash_history. If unset, the command history is not saved when an interactive shell exits.
       HISTFILESIZE
     The maximum number of lines contained in the history file.  When this variable is assigned a value,  the
     history  file  is truncated, if necessary, by removing the oldest entries, to contain no more than that
     number of lines. The default value is 500.  The history file is also truncated to this size after writ‐
     ing it when an interactive shell exits.
       HISTIGNORE
     A colon-separated  list of  patterns used to decide which command lines should be saved on the history
     list.  Each pattern is anchored at the beginning of the line  and must  match  the  complete  line  (no
     implicit `*' is appended).  Each pattern is tested against the line after the checks specified by HIST‐
     CONTROL are applied.  In addition to the normal shell pattern matching characters, `&' matches the  pre‐
     vious  history line.  `&' may be escaped using a backslash; the backslash is removed before attempting a
     match.  The second and subsequent lines of a multi-line compound command are not tested, and  are added
     to the history regardless of the value of HISTIGNORE.
       HISTSIZE
     The  number  of  commands to remember in the command history (see HISTORY below).  The default value is
     500.
       HISTTIMEFORMAT
     If this variable is set and not null, its value is used as a format string for strftime(3) to print  the
     time  stamp  associated  with  each history entry displayed by the history builtin.  If this variable is
     set, time stamps are written to the 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 run‐
     ning; 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 HOST‐
     FILE is unset, the hostname list is cleared.
       IFS    The  Internal  Field  Separator  that is used for word splitting after expansion and to split lines into
     words with the read builtin command.  The default value is ``<space><tab><newline>''.
       IGNOREEOF
     Controls the action of an interactive shell on receipt of an EOF character as the sole input.   If  set,
     the  value is the number of consecutive EOF characters which must be typed as the first characters on an
     input line before bash exits.  If the variable exists but does not have  a  numeric  value,  or  has  no
     value, the default value is 10.  If it does not exist, EOF signifies the end of input to the shell.
       INPUTRC
     The filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE below).
       LANG   Used  to determine the locale category for any category not specifically selected with a variable start‐
     ing 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 path‐
     name 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.
       LINES  Used  by the select compound command to determine the column length for printing selection lists. Auto‐
     matically set upon receipt of a SIGWINCH.
       MAIL   If this parameter is set to a file or directory name and the MAILPATH variable is not set, bash  informs
     the user of the arrival of mail in the specified file or Maildir-format directory.
       MAILCHECK
     Specifies how  often (in seconds) bash checks for mail. The default is 60 seconds.  When it is time to
     check for mail, the shell does so before displaying the primary prompt.  If this variable is  unset,  or
     set to a value that is not a number greater than or equal to zero, the shell disables mail checking.
       MAILPATH
     A colon-separated  list of  file  names to  be checked for mail.  The message to be printed when mail
     arrives in a particular file may be specified by separating the file name from the message with  a  `?'.
     When used in the text of the message, $_ expands to the name of the current mailfile.  Example:
     MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
     Bash supplies a default value for this variable, but the location of the user mail files that it uses is
     system dependent (e.g., /var/mail/$USER).
       OPTERR If set to the value 1, bash displays error messages generated by the getopts builtin command (see SHELL
     BUILTIN COMMANDS below). OPTERR is initialized to 1 each time the shell is invoked or a shell script is
     executed.
       PATH   The search path for commands.  It is a colon-separated list of directories in which the shell looks  for
     commands (see COMMAND EXECUTION below). A zero-length (null) directory name in the value of PATH indi‐
     cates 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/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
       POSIXLY_CORRECT
     If this variable is in the environment when bash starts, the shell enters posix mode before reading  the
     startup  files,  as if the --posix invocation option had been supplied.  If it is set while the shell is
     running, bash enables posix mode, as if the command set -o posix had been executed.
       PROMPT_COMMAND
     If set, the value is executed as a command prior to issuing each primary prompt.
       PROMPT_DIRTRIM
     If set to a number greater than zero, the value is used as the number of trailing directory  components
     to  retain when expanding the \w and \W prompt string escapes (see PROMPTING below).  Characters removed
     are replaced with an ellipsis.
       PS1    The value of this parameter is expanded (see PROMPTING below) and used as the  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 PS4 is replicated multiple times, as neces‐
     sary, to indicate multiple levels of indirection. The default is ``+ ''.
       SHELL  The full pathname to the shell is kept in this environment variable.  If it is not set  when  the shell
     starts, bash assigns to it the full pathname of the current user's login shell.
       TIMEFORMAT
     The  value  of this parameter is used as a format string specifying how the timing information for pipe‐
     lines prefixed with the time reserved word should be displayed.  The % character introduces  an escape
     sequence that is expanded to a time value or other information.  The escape sequences and their meanings
     are as follows; the braces denote optional portions.
     %% A literal %.
     %[p][l]R The elapsed time in seconds.
     %[p][l]U The number of CPU seconds spent in user mode.
     %[p][l]S The number of CPU seconds spent in system mode.
     %P The CPU percentage, computed as (%U + %S) / %R.


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


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


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


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


       An indexed array is  created  automatically  if any  variable  is  assigned  to using the  syntax  name[sub‐
       script]=value. The  subscript is treated as an arithmetic expression that must evaluate to a number.  If sub‐
       script evaluates to a number less than zero, it is used as an offset from one greater than the array's  maximum
       index  (so  a subcript of -1 refers to the last element of the array).  To explicitly declare an indexed array,
       use declare -a name (see SHELL BUILTIN COMMANDS below). declare -a name[subscript] is also accepted; the  sub‐
       script is ignored.


       Associative arrays are created using declare -A name.


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


       Arrays are assigned to using compound assignments of the form name=(value1 ... valuen), where each value is  of
       the form [subscript]=string.  Indexed array assignments do not require the bracket and subscript.  When assign‐
       ing to indexed arrays, if the optional brackets and subscript are supplied, that index is assigned  to; other‐
       wise  the  index of  the  element  assigned is the last index assigned to by the statement plus one.  Indexing
       starts at zero.


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


       This syntax is also accepted by the declare builtin.  Individual array elements may be assigned to  using  the
       name[subscript]=value syntax introduced above.


       Any element of an array may be referenced using ${name[subscript]}.  The braces are required to avoid conflicts
       with pathname expansion. If subscript is @ or *, the word expands to all members of  name.   These  subscripts
       differ  only when the word appears within double quotes. If the word is double-quoted, ${name[*]} expands to a
       single word with the value of each array member separated by the first character of the IFS  special  variable,
       and  ${name[@]} expands each element of name to a separate word.  When there are no array members, ${name[@]}
       expands to nothing.  If the double-quoted expansion occurs within a word, the expansion of the first  parameter
       is  joined with the beginning part of the original word, and the expansion of the last parameter is joined with
       the last part of the original word.  This is analogous to the expansion of the special parameters * and @  (see
       Special Parameters above).  ${#name[subscript]} expands to the length of ${name[subscript]}.  If subscript is *
       or @, the expansion is the number of elements in the array.  Referencing an array variable without a  subscript
       is equivalent to referencing the array with a subscript of 0.


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


       The unset builtin is used to destroy arrays.  unset name[subscript] destroys the array element  at  index  sub‐
       script. Care  must  be taken to avoid unwanted side effects caused by pathname expansion.  unset name, where
       name is an array, or unset name[subscript], where subscript is * or @, removes the entire array.


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


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


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


       On systems that can support it, there is an additional expansion available: process substitution.


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


       ${parameter}
     The value of parameter is substituted.  The braces are required when parameter is a positional parameter
     with more than one digit, or when parameter is followed by a character which is not to be interpreted as
     part of its name.


       If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced.
       Bash  uses  the value of the variable formed from the rest of parameter as the name of the variable; this vari‐
       able is then expanded and that value is used in the rest of the substitution, rather than the value of  parame‐
       ter itself.  This is known as indirect expansion.  The exceptions to this are the expansions of ${!prefix*} and
       ${!name[@]} described below.  The exclamation point must immediately follow the left brace in order  to intro‐
       duce 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, bash tests for a parameter  that  is
       unset or null.  Omitting the colon results in a test only for a parameter that is unset.


       ${parameter:-word}
     Use  Default  Values.   If parameter is unset or null, the expansion of word is substituted.  Otherwise,
     the value of parameter is substituted.
       ${parameter:=word}
     Assign Default Values.  If parameter is unset or null, the expansion of word is assigned to  parameter.
     The  value  of  parameter is then substituted.  Positional parameters and special parameters may not be
     assigned to in this way.
       ${parameter:?word}
     Display Error if Null or Unset.  If parameter is null or unset, the expansion of word (or a  message  to
     that  effect if word is not present) is written to the standard error and the shell, if it is not inter‐
     active, exits.  Otherwise, the value of parameter is substituted.
       ${parameter:+word}
     Use Alternate Value.  If parameter is null or unset, nothing is substituted, otherwise the expansion  of
     word is substituted.
       ${parameter:offset}
       ${parameter:offset:length}
     Substring Expansion.   Expands to up to length characters of parameter starting at the character speci‐
     fied by offset.  If length is omitted, expands to the substring of parameter starting at the  character
     specified by  offset.   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 from the end of the value
     of parameter.  If length evaluates to a number less than zero, and parameter is not @ and not an indexed
     or associative array, it is interpreted as an offset from the end of the value of parameter rather  than
     a number of characters, and the expansion is the characters between the two offsets.  If parameter is @,
     the result is length positional parameters beginning at offset.  If parameter is an indexed  array  name
     subscripted  by  @  or  *, the result is the length members of the array beginning with ${parameter[off‐
     set]}.  A negative offset is taken relative to one greater than  the  maximum  index  of the  specified
     array.   Substring  expansion  applied  to an associative array produces undefined results.  Note that a
     negative offset must be separated from the colon by at least one space to avoid being confused with  the
     :- expansion.  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 pre‐
     fixed to the list.


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


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


       ${#parameter}
     Parameter length. The length in characters of the value of parameter is substituted.  If parameter is *
     or @, the value substituted is the number of positional parameters.  If parameter is an array name  sub‐
     scripted by * or @, the value substituted is the number of elements in the array.


       ${parameter#word}
       ${parameter##word}
     Remove  matching prefix pattern.  The word is expanded to produce a pattern just as in pathname expan‐
     sion.  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 expan‐
     sion.  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}
     Pattern  substitution.   The  pattern  is expanded  to produce a pattern just as in pathname expansion.
     Parameter is expanded and the longest match of pattern against its value is replaced  with  string.   If
     pattern  begins  with /, all matches of pattern are replaced with string. Normally only the first match
     is replaced.  If pattern begins with #, it must match at the beginning of the expanded value of  parame‐
     ter.   If pattern begins with %, it must match at the end of the expanded value of parameter.  If string
     is null, matches of pattern are deleted and the / following pattern may be omitted.  If parameter is  @
     or  *,  the substitution operation is applied to each positional parameter in turn, and the expansion is
     the resultant list.  If parameter is an array variable subscripted with @ or *, the substitution opera‐
     tion is applied to each member of the array in turn, and the expansion is the resultant list.


       ${parameter^pattern}
       ${parameter^^pattern}
       ${parameter,pattern}
       ${parameter,,pattern}
     Case modification.  This expansion modifies the case of alphabetic characters in parameter.  The pattern
     is expanded to produce a pattern just as in pathname expansion.  The ^ operator converts lowercase  let‐
     ters  matching  pattern  to  uppercase; the , operator converts matching uppercase letters to lowercase.
     The ^^ and ,, expansions convert each matched character in the expanded value; the ^  and ,  expansions
     match  and convert only the first character in the expanded value.  If pattern is omitted, it is treated
     like a ?, which matches every character. If parameter is @ or *, the  case  modification operation  is
     applied  to each positional parameter in turn, and the expansion is the resultant list.  If parameter is
     an array variable subscripted with @ or *, the case modification operation is applied to each member  of
     the array in turn, and the expansion is the resultant list.


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


     $(command)
       or
     `command`


       Bash  performs the expansion by executing command and replacing the command substitution with the standard out‐
       put 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).


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


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


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


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


     $((expression))


       The expression is treated as if it were within double quotes, but a double quote inside the parentheses is  not
       treated specially.  All tokens in the expression undergo parameter expansion, string expansion, command substi‐
       tution, and quote removal.  Arithmetic expansions may be nested.


       The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION.  If expression  is
       invalid, bash prints a message indicating failure and no substitution occurs.


   Process Substitution
       Process substitution  is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming
       open files.  It takes the form of <(list) or >(list).  The process list is run with its input  or  output  con‐
       nected  to  a FIFO or some file in /dev/fd.  The name of this file is passed as an argument to the current com‐
       mand as the result of the expansion.  If the >(list) form is used, writing to the file will provide  input  for
       list.  If the <(list) form is used, the file passed as an argument should be read to obtain the output of list.


       When available, process substitution is performed simultaneously with parameter and 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.


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


       Explicit null  arguments ("" or '') are retained.  Unquoted implicit null arguments, resulting from the expan‐
       sion of parameters that have no values, are removed.  If a parameter with no value is  expanded within double
       quotes, a null argument results and is retained.


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


   Pathname Expansion
       After  word  splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [.
       If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically
       sorted  list  of file  names  matching the pattern.  If no matching file names are found, and the shell option
       nullglob is not enabled, the word is left unchanged.  If the nullglob option is set, and no matches are found,
       the  word  is  removed. If  the  failglob  shell option is set, and no matches are found, an error message is
       printed and the command is not executed. If the shell option nocaseglob is enabled,  the  match is  performed
       without regard to the case of alphabetic characters.  When a pattern is used for pathname expansion, the char‐
       acter ``.''  at the start of a name or immediately following a slash must be  matched  explicitly,  unless  the
       shell  option dotglob is set.  When matching a pathname, the slash character must always be matched explicitly.
       In other cases, the ``.''  character is not treated specially.  See the description of shopt below under SHELL
       BUILTIN COMMANDS for a description of the nocaseglob, nullglob, failglob, and dotglob shell options.


       The  GLOBIGNORE shell variable may be used to restrict the set of 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.  The  file names ``.'' 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  file
       names  beginning with  a  ``.''  will match.  To get the old behavior of ignoring file names beginning with a
       ``.'', make ``.*''  one of the patterns in GLOBIGNORE.  The dotglob  option  is disabled  when GLOBIGNORE  is
       unset.


       Pattern Matching


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


       The special pattern characters have the following meanings:


     *     Matches any string, including the null string.  When the globstar shell option is enabled, and  *
    is used in a pathname expansion context, two adjacent *s used as a single pattern will match all
    files and zero or more directories and subdirectories.  If followed by a /, two adjacent *s  will
    match only directories and subdirectories.
     ?     Matches any single character.
     [...]  Matches any one of the enclosed characters.  A pair of characters separated by a hyphen denotes a
    range expression; any character that sorts between those two  characters, inclusive,  using  the
    current  locale's collating sequence and character set, is matched.  If the first character fol‐
    lowing the [ is a !  or a ^ then any character not enclosed is matched.   The  sorting  order  of
    characters in range expressions is determined by the current locale and the value of the LC_COL‐
    LATE shell variable, if set.  A - may be matched by including it as the first or  last  character
    in the set.  A ] may be matched by including it as the first character in the set.


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


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


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


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


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


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


REDIRECTION
       Before  a  command  is executed, its input and output may be redirected using a special notation interpreted by
       the shell.  Redirection may also be used to open and close files for the current shell  execution  environment.
       The following redirection operators may precede or appear anywhere within a simple command or may follow a com‐
       mand.  Redirections are processed in the order they appear, from left to right.


       Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form
       {varname}.   In this  case,  for  each redirection operator except >&- and <&-, the shell will allocate a file
       descriptor greater than 10 and assign it to varname.  If >&- or <&- is preceded by {varname}, the value of var‐
       name defines the file descriptor to close.


       In  the following descriptions, if the file descriptor number is omitted, and the first character of the redi‐
       rection operator is <, the redirection refers to the standard input (file descriptor 0). If the first  charac‐
       ter 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 expansion, command  substitution, arithmetic  expansion, quote
       removal, pathname expansion, and word splitting. If it expands to more than one word, bash reports an error.


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


     ls > dirlist 2>&1


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


     ls 2>&1 > dirlist


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


       Bash handles several filenames specially when they are used in redirections, as described in the following  ta‐
       ble:


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


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


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


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


       The general format for redirecting input is:


     [n]<word


   Redirecting Output
       Redirection of output causes the file whose name results from the expansion of word to be opened for writing on
       file descriptor n, or the standard output (file 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 has been enabled, the redirection
       will fail if the file whose name results from the expansion of word exists and is a regular file.  If the redi‐
       rection operator is >|, or the redirection operator is > and the noclobber option to the set builtin command is
       not enabled, the redirection is attempted even if the file named by word exists.


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


       The general format for appending output is:


     [n]>>word


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


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


     &>word
       and
     >&word


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


     >word 2>&1


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


       The format for appending standard output and standard error is:


     &>>word


       This is semantically equivalent to


     >>word 2>&1


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


       The format of here-documents is:


     <<[-]word
     here-document
     delimiter


       No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word.
       If  any characters  in word are quoted, the delimiter is the result of quote removal on word, and the lines in
       the here-document are not expanded.  If word is unquoted, all lines  of the  here-document  are subjected  to
       parameter  expansion,  command  substitution,  and  arithmetic  expansion.   In the latter case, the character
       sequence \<newline> is ignored, and \ must be used to quote the characters \, $, and `.


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


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


     <<<word


       The word is expanded and supplied to the command on its standard input.


   Duplicating File Descriptors
       The redirection operator


     [n]<&word


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


       The operator


     [n]>&word


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


   Moving File Descriptors
       The redirection operator


     [n]<&digit-


       moves  the  file descriptor  digit to file descriptor n, or the standard input (file 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


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


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


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


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


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


       The  rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one
       complete line of input before executing any of the commands on that line.  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 next line of input is read.  The commands following the alias definition on that
       line  are  not affected by the new alias.  This behavior is also an issue when functions are executed.  Aliases
       are expanded when a function definition is read, not when the function is executed, because a function  defini‐
       tion  is itself a  compound command.  As a consequence, aliases defined in a function are not available until
       after that function is executed. To be safe, always put alias definitions on a separate line, and do  not  use
       alias in compound commands.


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


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


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


       Variables local to the function may be declared with the local  builtin command.   Ordinarily, variables  and
       their values are shared between the function and its caller.


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


       If the builtin command return is executed in a function, the function completes and execution resumes with  the
       next command after the function call.  Any command 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.


       Function names and  definitions  may be listed with the -f option to the declare or typeset builtin commands.
       The -F option to declare or typeset will list the function names only (and optionally the source file and  line
       number, if  the extdebug shell option is enabled).  Functions may be exported so that subshells automatically
       have them defined with the -f option to the export builtin.  A function definition may be deleted using the  -f
       option to the unset builtin.  Note that shell functions and variables with the same name may result in multiple
       identically-named entries in the environment passed to the shell's children.  Care should  be  taken  in cases
       where this may cause a problem.


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


ARITHMETIC EVALUATION
       The  shell  allows arithmetic expressions to be evaluated, under certain circumstances (see the let and declare
       builtin commands and Arithmetic Expansion).  Evaluation is done in fixed-width integers with no check for over‐
       flow, though division by 0 is trapped and flagged as an error.  The operators and their precedence, associativ‐
       ity, 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 syn‐
       tax.  A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter
       expansion syntax.  The value of a variable is evaluated as an arithmetic expression when it is  referenced,  or
       when  a variable which has been given the integer attribute using declare -i is assigned a value.  A null value
       evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.


       Constants with a leading 0 are interpreted as octal numbers.  A leading 0x or 0X denotes hexadecimal. Other‐
       wise, 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.   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 num‐
       bers between 10 and 35.


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


CONDITIONAL EXPRESSIONS
       Conditional expressions are used by the [[ compound command and the test and [ builtin commands to  test  file
       attributes  and perform string and arithmetic comparisons.  Expressions are formed from the following unary or
       binary primaries.  If any file argument to one of the primaries is of the form /dev/fd/n, then file  descriptor
       n  is checked.  If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr,
       file descriptor 0, 1, or 2, respectively, is checked.


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


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


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


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


       string1 < string2
     True if string1 sorts before string2 lexicographically.


       string1 > string2
     True if string1 sorts after string2 lexicographically.


       arg1 OP arg2
     OP is one of -eq, -ne, -lt, -le, -gt, or -ge.  These arithmetic binary operators return true if arg1  is
     equal  to,  not  equal  to,  less than, less than or equal to, greater than, or greater than or equal to
     arg2, respectively.  Arg1 and arg2 may be positive or negative integers.


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


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


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


       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. 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 re‐
       direction 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 status of zero.


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


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


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


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


       If this execution fails because the file is not in executable format, and the file is not a  directory, it  is
       assumed to  be a  shell script, a file containing shell commands.  A subshell is spawned to execute it.  This
       subshell reinitializes itself, so that the effect is as if a new shell had been invoked to handle  the  script,
       with  the exception that the locations of commands remembered by the parent (see hash below 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 inter‐
       preter  name  on the  first  line of the program, followed by the name of the program, followed by the command
       arguments, if any.


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


       ·      open files inherited by the shell at invocation, as  modified  by redirections  supplied to  the  exec
     builtin


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


       ·      the file creation mode mask as set by umask or inherited from the shell's 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.


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


     [1] 25647


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


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


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


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


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


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


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


PROMPTING
       When  executing interactively, bash displays the primary prompt PS1 when it is ready to read a command, and the
       secondary prompt PS2 when it needs more input to complete a command.  Bash allows these prompt  strings to  be
       customized by inserting a number of backslash-escaped special characters that are decoded as follows:
     \a     an ASCII bell character (07)
     \d     the date in "Weekday Month Date" format (e.g., "Tue May 26")
     \D{format}
    the  format  is passed to strftime(3) and the result is inserted into the prompt string; an empty
    format results in a locale-specific time 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
     \n     newline
     \r     carriage return
     \s     the name of the shell, the basename of $0 (the portion following the final slash)
     \t     the current time in 24-hour HH:MM:SS format
     \T     the current time in 12-hour HH:MM:SS format
     \@     the current time in 12-hour am/pm format
     \A     the current time in 24-hour HH:MM format
     \u     the username of the current user
     \v     the version of bash (e.g., 2.00)
     \V     the release of bash, version + patch level (e.g., 2.00.0)
     \w     the current working directory, with $HOME abbreviated  with  a  tilde  (uses  the value  of  the
    PROMPT_DIRTRIM variable)
     \W     the basename of the current working directory, with $HOME abbreviated with a tilde
     \!     the history number of this command
     \#     the command number of this command
     \$     if the effective UID is 0, a #, otherwise a $
     \nnn   the character corresponding to the octal number nnn
     \\     a backslash
     \[     begin  a  sequence of  non-printing  characters, which could be used to embed a terminal control
    sequence into the prompt
     \]     end a sequence of non-printing characters


       The command number and the history number are usually different: the history number of a command is  its posi‐
       tion  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 expan‐
       sion, and quote removal, subject to the value of the promptvars shell option (see the description of the shopt
       command under SHELL BUILTIN COMMANDS below).


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


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


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


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


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


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


       For example, placing


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


     set variable-name value


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


       bell-style (audible)
     Controls what happens when readline wants to ring the terminal bell.  If set  to none,  readline never
     rings  the  bell. If set to visible, readline uses a visible bell if one is available.  If set to audi‐
     ble, readline attempts to ring the terminal's bell.
       bind-tty-special-chars (On)
     If set to On, readline attempts to bind the control characters treated specially by the kernel's termi‐
     nal driver to their readline equivalents.
       comment-begin (``#'')
     The string that is inserted when the readline insert-comment command is executed. This command is bound
     to M-# in emacs mode and to # in vi command mode.
       completion-ignore-case (Off)
     If set to On, readline performs filename matching and completion in a case-insensitive fashion.
       completion-prefix-display-length (0)
     The length in characters of the common prefix of a list of possible completions that is displayed with‐
     out  modification.   When set  to a value greater than zero, common prefixes longer than this value are
     replaced with an ellipsis when displaying possible completions.
       completion-query-items (100)
     This determines when the user is queried about viewing the number of possible completions generated  by
     the possible-completions command. It may be set to any integer value greater than or equal to zero.  If
     the number of possible completions is greater than or equal to the value of this variable, the  user  is
     asked whether or not he wishes to view them; otherwise they are simply listed on the terminal.
       convert-meta (On)
     If  set  to  On, readline  will convert characters with the eighth bit set to an ASCII key sequence by
     stripping the eighth bit and prefixing an escape character (in effect, using escape as the meta prefix).
       disable-completion (Off)
     If set to On, readline will inhibit word completion.  Completion characters will be  inserted  into  the
     line as if they had been mapped to self-insert.
       editing-mode (emacs)
     Controls whether readline begins with a set of key bindings similar to Emacs or vi.  editing-mode can be
     set to either emacs or vi.
       echo-control-characters (On)
     When set to On, on operating systems that indicate they support it, readline echoes a  character corre‐
     sponding to a signal generated from the keyboard.
       enable-keypad (Off)
     When set to On, readline will try to enable the application keypad when it is called.  Some systems need
     this to enable the arrow keys.
       enable-meta-key (On)
     When set to On, readline will try to enable any meta modifier key the terminal claims to support when it
     is called.  On many terminals, the meta key is used to send eight-bit characters.
       expand-tilde (Off)
     If set to On, tilde expansion is performed when readline attempts word completion.
       history-preserve-point (Off)
     If  set  to  On, the  history  code  attempts  to place point at the same location on each history line
     retrieved with previous-history or next-history.
       history-size (0)
     Set the maximum number of history entries saved in the history list.  If set  to zero,  the  number  of
     entries in the history list is not limited.
       horizontal-scroll-mode (Off)
     When set to On, makes readline use a single line for display, scrolling the input horizontally on a sin‐
     gle screen line when it becomes longer than the screen width rather than wrapping to a new line.
       input-meta (Off)
     If set to On, readline will enable eight-bit input (that is, it will not strip the  high bit  from  the
     characters  it  reads),  regardless of what the terminal claims it can support.  The name meta-flag is a
     synonym for this variable.
       isearch-terminators (``C-[C-J'')
     The string of characters that should terminate an incremental search without subsequently executing  the
     character as  a command.  If this variable has not been given a value, the characters ESC and C-J will
     terminate an incremental search.
       keymap (emacs)
     Set the current readline keymap. The set of valid keymap names is  emacs,  emacs-standard,  emacs-meta,
     emacs-ctlx,  vi, vi-command,  and  vi-insert.   vi  is equivalent to vi-command; emacs is equivalent to
     emacs-standard.  The default value is emacs; the value of editing-mode also affects the default keymap.
       mark-directories (On)
     If set to On, completed directory names have a slash appended.
       mark-modified-lines (Off)
     If set to On, history lines that have been modified are displayed with a preceding asterisk (*).
       mark-symlinked-directories (Off)
     If set to On, completed names which are symbolic links to directories have a slash appended (subject  to
     the value of mark-directories).
       match-hidden-files (On)
     This  variable,  when  set  to  On,  causes readline to match files whose names begin with a `.' (hidden
     files) when performing filename completion.  If set to Off, the leading `.' must be supplied by the user
     in the filename to be completed.
       menu-complete-display-prefix (Off)
     If  set to On, menu completion displays the common prefix of the list of possible completions (which may
     be empty) before cycling through the list.
       output-meta (Off)
     If set to On, readline will display characters with the eighth bit set directly rather than as  a meta-
     prefixed escape sequence.
       page-completions (On)
     If  set  to On, readline uses an internal more-like pager to display a screenful of possible completions
     at a time.
       print-completions-horizontally (Off)
     If set to On, readline will display completions with matches sorted horizontally in alphabetical order,
     rather than down the screen.
       revert-all-at-newline (Off)
     If  set to On, readline will undo all changes to history lines before returning when accept-line is exe‐
     cuted.  By default, history lines may be modified and retain individual undo lists across calls to read‐
     line.
       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-ambigu‐
     ous.  If set to On, words which have more than one possible completion without any possible partial com‐
     pletion (the possible completions don't share a common prefix) cause the matches to  be  listed  immedi‐
     ately instead of ringing the bell.
       skip-completed-text (Off)
     If  set  to On, this alters the default completion behavior when inserting a single match into the line.
     It's only active when performing completion in the middle of a word.   If enabled,  readline  does  not
     insert  characters from the completion that match characters after point in the word being completed, so
     portions of the word following the cursor are not duplicated.
       visible-stats (Off)
     If set to On, a character denoting a file's type as reported by stat(2) is appended to the filename when
     listing possible completions.


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


       $if    The  $if construct allows bindings to be made based on the editing mode, the terminal being used, or the
     application using readline.  The text of the test extends to the end of  the  line;  no  characters  are
     required to isolate it.


     mode   The  mode= form  of  the $if directive is used to test whether readline is in emacs or vi mode.
    This may be used in conjunction with the set keymap 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  the both full name of the terminal and the portion of the terminal name before the first
    -. This allows sun to match both sun and sun-cmd, for instance.


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


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


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


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


       $include
     This directive takes a single filename as an argument and reads commands and 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.  The  charac‐
       ters  present in the value of the isearch-terminators variable are used to terminate an incremental search.  If
       that variable has not been assigned a value the Escape and Control-J characters will terminate  an  incremental
       search. Control-G  will abort an incremental search and restore the original line.  When the search is termi‐
       nated, the history entry containing the search string becomes the current line.


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


       Readline remembers the last incremental search string.  If two Control-Rs are  typed  without  any  intervening
       characters defining a new search string, any remembered search string is used.


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


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


   Commands for Moving
       beginning-of-line (C-a)
     Move to the start of the current line.
       end-of-line (C-e)
     Move to the end of the line.
       forward-char (C-f)
     Move forward a character.
       backward-char (C-b)
     Move back a character.
       forward-word (M-f)
     Move  forward  to the end of the next word.  Words are composed of alphanumeric characters (letters and
     digits).
       backward-word (M-b)
     Move back to the start of the current or previous word.  Words are composed of  alphanumeric  characters
     (letters and digits).
       shell-forward-word
     Move forward to the end of the next word. Words are delimited by non-quoted shell metacharacters.
       shell-backward-word
     Move  back  to  the  start  of  the  current  or previous word.  Words are delimited by non-quoted shell
     metacharacters.
       clear-screen (C-l)
     Clear the screen leaving the current line at the top of the screen.  With an argument, refresh the  cur‐
     rent line without clearing the screen.
       redraw-current-line
     Refresh the current line.


   Commands for Manipulating the History
       accept-line (Newline, Return)
     Accept  the  line regardless  of where the cursor is.  If this line is non-empty, add it to the history
     list according to the state of the HISTCONTROL variable. If the line is a modified history  line,  then
     restore the history line to its original state.
       previous-history (C-p)
     Fetch the previous command from the history list, moving back in the list.
       next-history (C-n)
     Fetch the next command from the history list, moving forward in the list.
       beginning-of-history (M-<)
     Move to the first line in the history.
       end-of-history (M->)
     Move to the end of the input history, i.e., the line currently being entered.
       reverse-search-history (C-r)
     Search  backward starting at the current line and moving `up' through the history as necessary.  This is
     an incremental search.
       forward-search-history (C-s)
     Search forward starting at the current line and moving `down' through the history as necessary.  This is
     an incremental search.
       non-incremental-reverse-search-history (M-p)
     Search  backward through the history starting at the current line using a non-incremental search for a
     string supplied by the user.
       non-incremental-forward-search-history (M-n)
     Search forward through the history using a non-incremental search for a string supplied by the user.
       history-search-forward
     Search forward through the history for the string of characters between the start of  the current  line
     and the point.  This is a non-incremental search.
       history-search-backward
     Search  backward through the history for the string of characters between the start of the current line
     and the point.  This is a non-incremental search.
       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  com‐
     mand.   Once  the argument n is computed, the argument is extracted 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  for‐
     ward).   The  history expansion facilities are used to extract the last argument, as if the "!$" history
     expansion had been specified.
       shell-expand-line (M-C-e)
     Expand the line as the shell does.  This performs alias and history expansion as well  as  all  of  the
     shell word expansions.  See HISTORY EXPANSION below for a description of history expansion.
       history-expand-line (M-^)
     Perform history expansion on the current line.  See HISTORY EXPANSION below for a description of history
     expansion.
       magic-space
     Perform history expansion on the current line and insert a space. See HISTORY  EXPANSION below for  a
     description of history expansion.
       alias-expand-line
     Perform alias expansion on the current line.  See ALIASES above for a description of alias expansion.
       history-and-alias-expand-line
     Perform history and alias expansion on the current line.
       insert-last-argument (M-., M-_)
     A synonym for yank-last-arg.
       operate-and-get-next (C-o)
     Accept the current line for execution and fetch the next line relative to the current line from the his‐
     tory for editing. Any argument is ignored.
       edit-and-execute-command (C-xC-e)
     Invoke an editor on the current command line, and execute the result as shell commands. Bash  attempts
     to invoke $VISUAL, $EDITOR, and emacs as the editor, in that order.


   Commands for Changing Text
       delete-char (C-d)
     Delete the character at point.  If point is at the beginning of the line, there are no characters in the
     line, and the last character typed was not bound to delete-char, then return EOF.
       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 exam‐
     ple.
       tab-insert (C-v TAB)
     Insert a tab character.
       self-insert (a, b, A, 1, !, ...)
     Insert the character typed.
       transpose-chars (C-t)
     Drag  the character before point forward over the character at point, moving point forward as well.  If
     point is at the end of the line, then this transposes the two characters before point.   Negative argu‐
     ments have no effect.
       transpose-words (M-t)
     Drag  the word before point past the word after point, moving point over that word as well.  If point is
     at the end of the line, this transposes the last two words on the line.
       upcase-word (M-u)
     Uppercase the current (or following) word.  With a negative argument, uppercase the previous  word,  but
     do not move point.
       downcase-word (M-l)
     Lowercase the  current (or following) word.  With a negative argument, lowercase the previous word, but
     do not move point.
       capitalize-word (M-c)
     Capitalize the current (or following) word.  With a negative argument, capitalize the previous word, but
     do not move point.
       overwrite-mode
     Toggle overwrite mode.  With an explicit positive numeric argument, switches to overwrite mode.  With an
     explicit non-positive numeric argument, switches to insert mode. This command affects only emacs mode;
     vi  mode does overwrite differently.  Each call to readline() starts in insert mode.  In overwrite mode,
     characters bound to self-insert replace the text at point rather than pushing the text  to  the right.
     Characters  bound to backward-delete-char replace the character before point with a space.  By default,
     this command is unbound.


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


   Numeric Arguments
       digit-argument (M-0, M-1, ..., M--)
     Add this digit to the argument already accumulating, or start a new argument.   M--  starts  a  negative
     argument.
       universal-argument
     This  is another way to specify an argument.  If this command is followed by one or more digits, option‐
     ally 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 or minus sign,  the
     argument count for the next command is multiplied by four.  The argument count is initially one, so exe‐
     cuting 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 treating the text as a
     variable (if the text begins with $), username (if the text begins with ~), hostname (if the text begins
     with @), or command (including aliases and functions) in turn.  If none of these produces a match, file‐
     name completion is attempted.
       possible-completions (M-?)
     List the possible completions of the text before point.
       insert-completions (M-*)
     Insert all completions of the text before point that would have been generated by possible-completions.
       menu-complete
     Similar to complete, but replaces the word to be completed with a single match from the list of possible
     completions.   Repeated  execution  of  menu-complete  steps  through  the list of possible completions,
     inserting each match in turn.  At the end of the list of completions, the bell is rung (subject  to  the
     setting of bell-style) and the original text is restored. An argument of n moves n positions forward in
     the list of matches; a negative argument may be used to move backward through the list.  This command is
     intended to be bound to TAB, but is unbound by default.
       menu-complete-backward
     Identical to menu-complete, but moves backward through the list of possible completions, as if menu-com‐
     plete had been given a negative argument. This command is unbound by default.
       delete-char-or-list
     Deletes the character under the cursor if not at the beginning or end of the  line  (like delete-char).
     If  at  the  end of  the line, behaves identically to possible-completions.  This command is unbound by
     default.
       complete-filename (M-/)
     Attempt filename completion on the text before point.
       possible-filename-completions (C-x /)
     List the possible completions of the text before point, treating it as a 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 exe‐
     cutable filenames, in that order.
       possible-command-completions (C-x !)
     List the possible completions of the text before point, treating it as a command name.
       dynamic-complete-history (M-TAB)
     Attempt completion on the text before point, comparing the text against lines from the history list  for
     possible completion matches.
       dabbrev-expand
     Attempt menu completion on the text before point, comparing the text against lines from the history list
     for possible completion matches.
       complete-into-braces (M-{)
     Perform filename completion and insert the list of possible completions enclosed within  braces  so  the
     list is available to the shell (see Brace Expansion above).


   Keyboard Macros
       start-kbd-macro (C-x ()
     Begin saving the characters typed into the current keyboard macro.
       end-kbd-macro (C-x ))
     Stop saving the characters typed into the current keyboard macro and store the 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.


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


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


       First, the command name is identified.  If the command word is the empty string (completion  attempted  at  the
       beginning  of  an  empty line), any compspec defined with the -E option to complete is used.  If a compspec has
       been defined for that command, the compspec is used to generate the list of possible completions for the word.
       If the command word is a full pathname, a compspec for the full pathname is searched for first. If no compspec
       is found for the full pathname, an attempt is made to find a compspec  for  the portion following  the final
       slash. If  those searches do not result in a compspec, any compspec defined with the -D option to complete is
       used as the default.


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


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


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


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


       After these matches have been generated, any shell function or command specified with the -F and -C options  is
       invoked. When the command or function is invoked, the COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables
       are assigned values as described above under Shell Variables.  If  a  shell  function  is  being invoked,  the
       COMP_WORDS  and COMP_CWORD variables are also set.  When the function or command is invoked, the first argument
       is the name of the command whose arguments are being completed, the second argument  is the  word  being  com‐
       pleted, and the third argument is the word preceding the word being completed on the current command line.  No
       filtering of the generated completions against the word being completed is performed; the function  or  command
       has complete freedom in generating the matches.


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


       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 may be used to escape a
       newline, if necessary.


       After  all of the possible completions are generated, any filter specified with the -X option is applied to the
       list.  The filter is a pattern as used for pathname expansion; a & in the pattern is replaced with the text  of
       the  word  being completed.   A literal  &  may  be escaped with a backslash; the backslash is removed before
       attempting a match.  Any completion that matches the pattern will be  removed  from  the list. A  leading  !
       negates the pattern; in this case any completion not matching the pattern will be removed.


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


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


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


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


       When  a compspec  indicates  that  directory name completion is desired, the programmable completion functions
       force readline to append a slash to completed names which are symbolic links to directories,  subject  to  the
       value  of  the  mark-directories readline variable, regardless of the setting of the mark-symlinked-directories
       readline 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 han‐
       dlers 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 (sup‐
       plied as the first argument when the function is executed), programmable completion restarts  from  the begin‐
       ning,  with  an attempt to find a new compspec for that command.  This allows a set of completions to be built
       dynamically as completion is attempted, rather than being loaded all at once.


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




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


       On  startup, the history is initialized from the file named by the variable HISTFILE (default ~/.bash_history).
       The file named by the value of HISTFILE is truncated, if necessary, to contain no more than the number of lines
       specified  by  the value of HISTFILESIZE.  When the history file is read, lines beginning with the history com‐
       ment character followed immediately by a digit are interpreted as timestamps for the  preceding history line.
       These  timestamps  are  optionally  displayed  depending on the value of the HISTTIMEFORMAT variable.  When an
       interactive shell exits, the last $HISTSIZE lines are copied from  the  history list  to  $HISTFILE.   If  the
       histappend shell option is enabled (see the description of shopt under SHELL BUILTIN COMMANDS below), the lines
       are appended to the history file, otherwise the history file is overwritten.  If HISTFILE is unset, or  if  the
       history file  is unwritable, the history is not saved. If the HISTTIMEFORMAT variable is set, time stamps are
       written to the history file, marked with the history comment character, so they may be preserved across shell
       sessions.   This uses the history comment character to distinguish timestamps from other history lines. After
       saving the history, the history file is truncated to contain no more than HISTFILESIZE lines.  If  HISTFILESIZE
       is not set, no truncation is performed.


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


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


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


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


       History expansion is performed immediately after a complete line is read,  before  the  shell  breaks  it  into
       words. It takes place in two parts.  The first is to determine which line from the history list to use during
       substitution.  The second is to select portions of that line for inclusion into the  current  one.   The  line
       selected from  the history is the event, and the portions of that line that are acted upon are words.  Various
       modifiers are available to manipulate the selected words.  The line is broken into words in the same fashion as
       when reading input, so that several metacharacter-separated words surrounded by quotes are considered one word.
       History expansions are introduced by the appearance of the history expansion character, which is ! by  default.
       Only backslash (\) and single quotes can quote the history expansion character.


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


       Several shell options settable with the shopt builtin may be used to tailor the behavior of history expansion.
       If the histverify shell option is enabled (see the description of the shopt builtin  below),  and  readline  is
       being  used,  history substitutions are not immediately passed to the shell parser.  Instead, the expanded line
       is reloaded into the readline editing buffer for further modification.  If readline  is being  used,  and  the
       histreedit  shell  option  is enabled, a failed history substitution will be reloaded into the readline editing
       buffer for correction.  The -p option to the history builtin command may be used to see what a  history expan‐
       sion  will  do before using it. The -s option to the history builtin may be used to add commands to the end of
       the history list without 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 time‐
       stamps when writing the history file.


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


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


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


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


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


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


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


SHELL BUILTIN COMMANDS
       Unless  otherwise  noted,  each builtin command documented in this section as accepting options preceded by -
       accepts -- to signify the end of the options.  The :, true, false, and test builtins do not accept options  and
       do not treat -- specially.  The exit, logout, break, continue, let, and shift builtins accept and process argu‐
       ments 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 inter‐
       pretation.
       : [arguments]
     No effect; the command does nothing beyond expanding arguments and  performing  any  specified  redirec‐
     tions.  A zero exit code is returned.


.  filename [arguments]
       source filename [arguments]
     Read  and execute commands from filename in the current shell environment and return the exit status of
     the last command executed from filename. If filename does not contain a slash, file names in  PATH  are
     used  to find the directory containing filename. The file searched for in PATH need not be executable.
     When bash is not in posix mode, the current directory is searched if no file is found in PATH.   If  the
     sourcepath  option  to  the shopt builtin command is turned off, the PATH is not searched.  If any argu‐
     ments are supplied, they become the positional parameters when  filename is  executed. Otherwise  the
     positional  parameters are unchanged.  The return status is the status of the last command exited within
     the script (0 if no commands are executed), and false if filename is not found or cannot be read.


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


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


       bind [-m keymap] [-lpsvPSV]
       bind [-m keymap] [-q function] [-u function] [-r keyseq]
       bind [-m keymap] -f filename
       bind [-m keymap] -x keyseq:shell-command
       bind [-m keymap] keyseq:function-name
       bind readline-command
     Display current readline key and function bindings, bind a key sequence to a readline function or macro,
     or set a readline variable.  Each non-option argument is a command as it would appear in .inputrc,  but
     each  binding  or command must be passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'.
     Options, if 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; emacs is equivalent to emacs-standard.
     -l     List the names of all readline functions.
     -p     Display readline function names and bindings in such a way that they can be re-read.
     -P     List current readline function names and bindings.
     -s     Display readline key sequences bound to macros and the strings they output in  such  a  way  that
    they can be re-read.
     -S     Display readline key sequences bound to macros and the strings they output.
     -v     Display readline variable names and values in such a way that they can be re-read.
     -V     List current readline variable names and values.
     -f filename
    Read key bindings from filename.
     -q function
    Query about which keys invoke the named function.
     -u function
    Unbind all keys bound to the named function.
     -r keyseq
    Remove any current binding for keyseq.
     -x keyseq:shell-command
    Cause  shell-command  to be executed whenever keyseq is entered.  When shell-command is executed,
    the shell sets the READLINE_LINE variable to the contents of the readline line  buffer  and  the
    READLINE_POINT  variable to the current location of the insertion point.  If the executed command
    changes the value of READLINE_LINE or READLINE_POINT, those new values will be reflected  in  the
    editing state.


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


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


       builtin shell-builtin [arguments]
     Execute  the  specified shell builtin, passing it arguments, and return its exit status. This is useful
     when defining a 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 sub‐
     routine call.  If a non-negative integer is supplied as expr, caller displays the line  number,  subrou‐
     tine  name,  and source file corresponding to that position in the current execution call stack.  This
     extra information may be used, for example, to print a stack trace.  The current frame is frame 0.   The
     return  value  is 0 unless the shell is not executing a subroutine call or expr does not correspond to a
     valid position in the call stack.


       cd [-L|[-P [-e]]] [dir]
     Change the current directory to dir.  The variable HOME is the default dir.  The variable CDPATH defines
     the  search  path for the directory containing dir.  Alternative directory names in CDPATH are separated
     by a colon (:).  A null directory name in CDPATH is the same as the current directory, i.e., ``.''.   If
     dir  begins  with a slash (/), then CDPATH is not used. The -P option says to use the physical directory
     structure instead of following symbolic links (see also the -P option to the set builtin command);  the
     -L  option  forces symbolic links to be followed. If the -e option is supplied with -P, and the current
     working directory cannot be successfully determined after a successful directory change, cd will return
     an  unsuccessful status. An argument of - is equivalent to $OLDPWD.  If a non-empty directory name from
     CDPATH is used, or if - is the first argument, and the directory change is  successful, the  absolute
     pathname of  the new working directory is written to the standard output.  The return value is true if
     the directory was successfully changed; false otherwise.


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


       compgen [option] [word]
     Generate possible completion matches for word according to the options, which may be any option accepted
     by the complete builtin with the exception of -p and -r, and write the matches to the  standard  output.
     When  using the -F or -C options, the various shell variables set by the programmable completion facili‐
     ties, 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 comple‐
     tions matching word will be displayed.


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


       complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G globpat] [-W wordlist] [-F function]  [-C  com‐
       mand]
     [-X filterpat] [-P prefix] [-S suffix] name [name ...]
       complete -pr [-DE] [name ...]
     Specify  how arguments to each name should be completed. If the -p option is supplied, or if no options
     are supplied, existing completion specifications are printed in a way that allows them to be  reused  as
     input.   The  -r option removes a completion specification for each name, or, if no names are supplied,
     all completion specifications.  The -D option indicates that the remaining options  and  actions should
     apply  to the  ``default''  command completion; that is, completion attempted on a command for which no
     completion has previously been defined.  The -E option indicates that the remaining options and  actions
     should apply to ``empty'' command completion; that is, completion attempted on a blank line.


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


     Other options, if specified, have the following meanings. The arguments to the -G, -W, and  -X  options
     (and,  if necessary,  the -P and -S options) should be quoted to protect them from expansion before the
     complete builtin is invoked.
     -o comp-option
     The comp-option controls several aspects of the compspec's behavior beyond the simple generation
     of completions.  comp-option may be one of:
     bashdefault
     Perform the rest of the default bash completions if the compspec generates no matches.
     default Use readline's default filename completion if the compspec 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-spe‐
     cific processing (like adding a slash to directory names, quoting special characters, or
     suppressing trailing spaces).  Intended to be used with shell functions.
     nospace Tell  readline  not to append a space (the default) to words completed at the end of the
     line.
     plusdirs
     After any matches defined by the compspec are generated, directory  name completion  is
     attempted and any matches are added to the results of the other actions.
     -A action
     The action may be one of the following to generate a list of possible completions:
     alias   Alias names.  May also be specified as -a.
     arrayvar
     Array variable names.
     binding Readline key binding names.
     builtin Names of shell builtin commands. May also be specified as -b.
     command Command names.  May also be specified as -c.
     directory
     Directory names. May also be specified as -d.
     disabled
     Names of disabled shell builtins.
     enabled Names of enabled shell builtins.
     export  Names of exported shell variables.  May also be specified as -e.
     file    File names.  May also be specified as -f.
     function
     Names of shell functions.
     group   Group names.  May also be specified as -g.
     helptopic
     Help topics as accepted by the help builtin.
     hostname
     Hostnames, as taken from the file specified by the HOSTFILE shell variable.
     job     Job names, if job control is active.  May also be specified as -j.
     keyword Shell reserved words.  May also be specified as -k.
     running Names of running jobs, if job control is active.
     service Service names.  May also be specified as -s.
     setopt  Valid arguments for the -o option to the set builtin.
     shopt   Shell option names as accepted by the shopt builtin.
     signal  Signal names.
     stopped Names of stopped jobs, if job control is active.
     user    User names.  May also be specified as -u.
     variable
     Names of all shell variables.  May also be specified as -v.
     -C command
     command  is  executed  in a subshell environment, and its output is used as the possible comple‐
     tions.
     -F function
     The shell function function is executed in the current shell environment. When it finishes, the
     possible completions are retrieved from the value of the COMPREPLY array variable.
     -G globpat
     The pathname expansion pattern globpat is expanded to generate the possible completions.
     -P prefix
     prefix  is  added at the beginning of each possible completion after all other options have been
     applied.
     -S suffix
     suffix is appended to each possible completion after all other options have been applied.
     -W wordlist
     The wordlist is split using the characters in the IFS special variable as delimiters,  and  each
     resultant word  is  expanded.   The  possible completions are the members of the resultant list
     which match the word being completed.
     -X filterpat
     filterpat is a pattern as used for pathname expansion.  It is applied to the  list  of  possible
     completions  generated by the preceding options and arguments, and each completion matching fil‐
     terpat is removed from the list. A leading ! in filterpat negates the pattern;  in  this case,
     any completion not matching filterpat is removed.


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


       compopt [-o option] [-DE] [+o option] [name]
     Modify completion options for each name according to the options, or for the currently-executing comple‐
     tion if no names are supplied.  If no options are given, display the completion options for each name or
     the  current  completion.  The  possible values of  option  are  those valid for the complete builtin
     described above. The -D option indicates that the remaining options should  apply  to  the  ``default''
     command  completion;  that  is, completion attempted on a command for which no completion has previously
     been defined.  The -E option indicates that the remaining options should apply to ``empty'' command com‐
     pletion; that is, completion attempted on a blank line.


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


       continue [n]
     Resume the next iteration of the enclosing for, while, until, or select loop.  If n is specified, resume
     at the nth enclosing loop.  n must be ≥ 1.  If n is greater than the number of enclosing loops, the last
     enclosing loop (the ``top-level'' loop) is resumed.  The return value is 0 unless n is not greater  than
     or equal to 1.


       declare [-aAfFgilrtux] [-p] [name[=value] ...]
       typeset [-aAfFgilrtux] [-p] [name[=value] ...]
     Declare  variables  and/or give them attributes. If no names are given then display the values of vari‐
     ables.  The -p option will display the attributes and values of each name.  When -p is  used  with  name
     arguments,  additional options are ignored.  When -p is supplied without name arguments, it will display
     the attributes and values of all variables having the attributes specified by  the  additional  options.
     If  no  other  options are supplied with -p, declare will display the attributes and values of all shell
     variables.  The -f option will restrict the display to shell functions.  The -F option inhibits the dis‐
     play  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 the function  is  defined  are
     displayed as well.  The -F option implies -f.  The -g option forces variables to be created or modified
     at the global scope, even when declare is executed in a shell function.  It  is  ignored in  all other
     cases.   The  following options can be used to restrict output to variables with the specified attribute
     or to give variables attributes:
     -a     Each name is an indexed array variable (see Arrays above).
     -A     Each name is an associative array variable (see Arrays above).
     -f     Use function names only.
     -i     The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION above) is
    performed when the variable is assigned a value.
     -l     When  the variable  is  assigned a value, all upper-case characters are converted to lower-case.
    The upper-case attribute is disabled.
     -r     Make names readonly.  These names cannot then be assigned values by subsequent assignment state‐
    ments 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 names for export to subsequent commands via the environment.


     Using `+' instead of `-' turns off the attribute instead, with the exceptions that +a may not be used to
     destroy an array variable and +r will not remove the readonly attribute. When used in a function, makes
     each name local, as with the local command, unless the -g option is supplied, If a variable name is fol‐
     lowed by =value, the value of the variable is set to value.  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 [+n] [-n] [-clpv]
     Without options, displays the list of currently remembered directories.  The default  display  is on  a
     single  line with directory names separated by spaces.  Directories are added to the list with the pushd
     command; the popd command removes entries from the list.
     +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.
     -c     Clears the directory stack by deleting all of the entries.
     -l     Produces a longer listing; 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.


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


       disown [-ar] [-h] [jobspec ...]
     Without options, each jobspec is removed from the table of active jobs.  If jobspec is not present,  and
     neither  -a  nor -r  is supplied,  the shell's notion of the current job is used.  If the -h option is
     given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to  the  job
     if  the shell receives a SIGHUP. If no jobspec is present, and neither the -a nor the -r option is sup‐
     plied, the current job is used.  If no jobspec is supplied, the -a option means to remove or  mark  all
     jobs; the -r option without a jobspec argument restricts operation to running jobs.  The return value is
     0 unless a jobspec does not specify a valid job.


       echo [-neE] [arg ...]
     Output the args, separated by spaces, followed by a newline.  The return status is always 0.  If -n  is
     specified, the trailing newline is suppressed.  If the -e option is given, interpretation of the follow‐
     ing backslash-escaped characters is enabled.  The -E option disables the interpretation of these escape
     characters,  even on  systems  where they are interpreted by default.  The xpg_echo shell option may be
     used to dynamically determine whether or not echo expands these escape characters by default.  echo does
     not interpret -- to mean the end of options.  echo interprets the following escape sequences:
     \a     alert (bell)
     \b     backspace
     \c     suppress further output
     \e
     \E     an escape character
     \f     form feed
     \n     new line
     \r     carriage return
     \t     horizontal tab
     \v     vertical tab
     \\     backslash
     \0nnn  the eight-bit character whose value is the octal value nnn (zero to three octal digits)
     \xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
     \uHHHH the  Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex
    digits)
     \UHHHHHHHH
    the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight
    hex digits)


       enable [-a] [-dnps] [-f filename] [name ...]
     Enable and disable builtin shell commands.  Disabling a builtin allows a disk command which has the same
     name as a shell builtin to be executed without specifying a full pathname, even though  the  shell  nor‐
     mally  searches  for  builtins  before  disk commands.  If -n is used, each name is disabled; otherwise,
     names are enabled.  For example, to use the test binary found via the PATH instead of the shell  builtin
     version, run  ``enable -n test''.  The -f option means to load the new builtin command name from shared
     object filename, on systems that support dynamic loading. The -d option will delete  a  builtin previ‐
     ously  loaded with -f.  If no name arguments are given, or if the -p option is supplied, a list of shell
     builtins is printed.  With no other option arguments, the list consists of all enabled  shell  builtins.
     If -n is supplied, only disabled builtins are printed.  If -a is supplied, the list printed includes all
     builtins, with an indication of whether or not each is enabled. If  -s is  supplied,  the  output  is
     restricted to the POSIX special builtins. The return value is 0 unless a name is not a shell builtin or
     there is an error loading a new builtin from a shared object.


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


       exec [-cl] [-a name] [command [arguments]]
     If command is specified, it replaces the shell.  No new process is created.  The arguments  become  the
     arguments to  command. If  the -l option is supplied, the shell places a dash at the beginning of the
     zeroth argument passed to command.  This is what login(1) does.  The -c option causes command to be exe‐
     cuted 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  shell  option  execfail is enabled, in which case it returns failure.  An interactive shell returns
     failure if the file cannot be executed.  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 com‐
     mand executed.  A trap on EXIT is executed before the shell terminates.


       export [-fn] [name[=word]] ...
       export -p
     The supplied names are marked for automatic export to the environment of subsequently executed commands.
     If  the -f option is given, the names refer to functions. If no names are given, or if the -p option is
     supplied, a list of all names that are exported in this shell is printed.  The  -n  option  causes  the
     export property to be removed from each name.  If a variable name is followed by =word, the value of the
     variable is set to word. export returns an exit status of 0 unless an invalid  option  is  encountered,
     one  of  the names is not a valid shell variable name, or -f is supplied with a name that is not a func‐
     tion.


       fc [-e ename] [-lnr] [first] [last]
       fc -s [pat=rep] [cmd]
     Fix Command.  In the first form, a range of commands from first to last is  selected  from  the  history
     list.   First  and  last may  be specified as a string (to locate the last command beginning with that
     string) or as a number (an index into the history list, where a negative number is  used as  an offset
     from the current command number). If last is not specified it is set to the current command for listing
     (so that ``fc -l -10'' prints the last 10 commands) and to first otherwise.  If first is not  specified
     it is set to the previous command for editing and -16 for listing.


     The -n option suppresses the command numbers when listing.  The -r option reverses the order of the com‐
     mands.  If the -l option is given, the commands are listed on standard output.   Otherwise,  the editor
     given  by ename is invoked on a file containing those commands.  If ename is not given, the value of the
     FCEDIT variable is used, and the value of EDITOR if FCEDIT is not set.  If neither variable is  set,  vi
     is used. When editing is complete, the edited commands are echoed and executed.


     In  the  second  form,  command  is re-executed after each instance of pat is replaced by rep.  A useful
     alias to use with this is ``r="fc -s"'', so that typing ``r cc'' runs the last  command  beginning  with
     ``cc'' and typing ``r'' re-executes the last command.


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


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


       getopts optstring name [args]
     getopts is used by shell procedures to parse positional parameters.  optstring contains the option char‐
     acters  to be recognized; if a character is followed by a colon, the option is expected to have an argu‐
     ment, 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  pro‐
     cessed  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 if a new set of parameters is to be used.


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


     getopts  normally parses the  positional  parameters, but if more arguments are given in args, getopts
     parses those instead.


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


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


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


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


       hash [-lr] [-p filename] [-dt] [name]
     Each time hash is invoked, the full pathname of the command name is determined by searching the directo‐
     ries  in $PATH  and  remembered. Any previously-remembered pathname is discarded.  If the -p option is
     supplied, no path search is performed, and filename is used as the full file name of the command.   The
     -r option causes the shell to forget all remembered locations.  The -d option causes the shell to forget
     the remembered location of each name.  If the -t option is supplied, the full  pathname  to  which  each
     name  corresponds is  printed. If  multiple  name arguments are supplied with -t, the name is printed
     before the hashed full pathname. The -l option causes output to be displayed in a format that  may  be
     reused  as  input.   If  no arguments are given, or if only -l is supplied, information about remembered
     commands is printed.  The return status is true unless a name is not found or an invalid option is  sup‐
     plied.


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


     The return status is 0 unless no command matches pattern.


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


     If  the HISTTIMEFORMAT variable is set, the time stamp information associated with each history entry is
     written to the history file, marked with the history comment character.  When the history file is read,
     lines  beginning with  the history comment character followed immediately by a digit are interpreted as
     timestamps for the previous history line. The return value is 0 unless an  invalid  option  is  encoun‐
     tered,  an  error occurs while reading or writing the history file, an invalid offset is supplied as an
     argument to -d, or the history expansion supplied as an argument to -p fails.


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


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


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


       kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
       kill -l [sigspec | exit_status]
     Send  the signal named by sigspec or signum to the processes named by pid or jobspec.  sigspec is either
     a case-insensitive signal name such as SIGKILL (with or without the SIG  prefix) or  a  signal  number;
     signum is a signal number.  If sigspec is not present, then SIGTERM is assumed.  An argument of -l lists
     the signal names. If any arguments are supplied when -l is given, the names of the signals  correspond‐
     ing to the arguments are listed, and the return status is 0.  The exit_status argument to -l is a number
     specifying either a signal number or the exit status of a process terminated by a signal. kill  returns
     true  if at least one signal was successfully sent, or false if an error occurs or an invalid option is
     encountered.


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


       local [option] [name[=value] ...]
     For each argument, a local variable named name is created, and assigned value.  The option can be any of
     the options accepted by declare. When local is used within a function, it causes the variable  name  to
     have  a  visible scope  restricted to that function and its children.  With no operands, local writes a
     list of local variables to the standard output.  It is an error to use local when not within a function.
     The  return status is 0 unless local is used outside a function, an invalid name is supplied, or name is
     a readonly variable.


       logout Exit a login shell.


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


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


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


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


       popd [-n] [+n] [-n]
     Removes entries from the directory stack. With no arguments, removes the top directory from the stack,
     and performs a cd to the new top directory.  Arguments, if supplied, have the following meanings:
     -n     Suppresses the normal change of directory when removing directories from the stack, so that only
    the stack is manipulated.
     +n     Removes the nth entry counting from the left of the list shown by dirs, starting with zero.   For
    example: ``popd +0'' removes the first directory, ``popd +1'' the second.
     -n     Removes the nth entry counting from the right of the list shown by dirs, starting with zero.  For
    example: ``popd -0'' removes the last directory, ``popd -1'' the next to last.


     If the popd command is successful, a dirs is performed as well,  and  the return status is  0.  popd
     returns  false  if an invalid option is encountered, the directory stack is empty, a non-existent direc‐
     tory stack entry is specified, or the directory change fails.


       printf [-v var] format [arguments]
     Write the formatted arguments to the standard output under the control of the  format.   The  -v option
     causes the output to be assigned to the variable var rather than being printed to the standard output.


     The format is a character string which contains three types of objects: plain characters, which are sim‐
     ply copied to standard output, character escape sequences, which are converted and copied to  the stan‐
     dard  output,  and format specifications, each of which causes printing of the next successive argument.
     In addition to the standard printf(1) format specifications, printf interprets the following extensions:
     %b     causes printf to expand backslash escape sequences in the corresponding argument (except that  \c
    terminates output,  backslashes  in  \', \", and \? are not removed, and octal escapes beginning
    with \0 may contain up to four digits).
     %q     causes printf to output the corresponding argument in a format that can be reused as shell input.
     %(datefmt)T
    causes printf to output the date-time string resulting from using datefmt as a format string  for
    strftime(3).   The corresponding argument is an integer representing the number of seconds since
    the epoch. Two special argument values may be used: -1 represents the current time, and -2  rep‐
    resents the time the shell was invoked.


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


     The  format  is  reused as necessary to consume all of the arguments.  If the format requires more argu‐
     ments than are supplied, the extra format specifications behave as if a zero value or  null  string,  as
     appropriate, had been supplied.  The return value is zero on success, non-zero on failure.


       pushd [-n] [+n] [-n]
       pushd [-n] [dir]
     Adds  a  directory  to  the  top of the directory stack, or rotates the stack, making the new top of the
     stack the current working directory.  With no arguments, exchanges the top two directories  and  returns
     0, unless the directory stack is empty.  Arguments, if supplied, have the following meanings:
     -n     Suppresses the normal change of directory when adding directories to the stack, so that only the
    stack is manipulated.
     +n     Rotates the stack so that the nth directory (counting from the left of the list  shown  by dirs,
    starting with zero) is at the top.
     -n     Rotates  the  stack so that the nth directory (counting from the right of the list shown by dirs,
    starting with zero) is at the top.
     dir    Adds dir to the directory stack at the top, making it the new current working directory.


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


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


       read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
     One line is read from the standard input, or from the file descriptor fd supplied as an argument to  the
     -u  option, and the first word is assigned to the first name, the second word to the second name, and so
     on, with leftover words and their intervening separators assigned to the last name.  If there are fewer
     words read from the input stream than names, the remaining names are assigned empty values.  The charac‐
     ters in IFS are used to split the line into words.  The backslash character (\) may be  used  to remove
     any  special  meaning for the next character read and for line continuation.  Options, if supplied, have
     the following meanings:
     -a aname
    The words are assigned to sequential indices of the array variable aname, starting at  0. aname
    is unset before any new values are assigned.  Other name arguments are ignored.
     -d delim
    The first character of delim is used to terminate the input line, rather than newline.
     -e     If the standard input is coming from a terminal, readline (see READLINE above) is used to obtain
    the line. Readline uses the current (or default, if line editing was  not  previously  active)
    editing settings.
     -i text
    If readline is being used to read the line, text is placed into the editing buffer before editing
    begins.
     -n nchars
    read returns after reading nchars characters rather than waiting for a complete  line  of input,
    but honor a delimiter if fewer than nchars characters are read before the delimiter.
     -N nchars
    read  returns  after reading exactly nchars characters rather than waiting for a complete line of
    input, unless EOF is encountered or read times out.   Delimiter  characters  encountered  in  the
    input are not treated specially and do not cause read to return until nchars characters are read.
     -p prompt
    Display  prompt  on  standard  error,  without  a trailing newline, before attempting to read any
    input.  The prompt is displayed only if input is 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 be used as a line continuation.
     -s     Silent mode.  If input is coming from a terminal, characters are not echoed.
     -t timeout
    Cause  read to time out and return failure if a complete line of input is not read within timeout
    seconds.  timeout may be a decimal number with a fractional portion following the decimal point.
    This  option  is  only effective if read is reading input from a terminal, pipe, or other special
    file; it has no effect when reading from regular files.  If timeout is 0, read returns success if
    input  is available  on  the  specified  file descriptor, failure otherwise.  The exit status is
    greater than 128 if the timeout is exceeded.
     -u fd  Read input from file descriptor fd.


     If no names are supplied, the line read is assigned to the variable REPLY.  The  return  code  is zero,
     unless  end-of-file  is encountered, read times out (in which case the return code is greater than 128),
     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  assign‐
     ment.   If  the  -f  option is supplied, the functions corresponding to the names are so marked. The -a
     option restricts the variables to indexed arrays; the -A option restricts the variables  to  associative
     arrays. If  both options are supplied, -A takes precedence.  If no name arguments are given, or if the
     -p option is supplied, a list of all readonly names is printed. The  other  options  may  be  used  to
     restrict the  output  to a subset of the set of readonly names. The -p option causes output to be dis‐
     played in a format that may be reused as input.  If a variable name is followed by =word, the  value  of
     the variable is set to word.  The return status is 0 unless an invalid option is encountered, one of the
     names is not a valid shell variable name, or -f is supplied with a name that is not a function.


       return [n]
     Causes a function to exit with the return value specified by n.  If n is omitted, the return  status  is
     that  of the last command executed in the function body. If used outside a function, but during execu‐
     tion of a script by the . (source) command, it causes the shell to  stop  executing  that  script  and
     return  either n or the exit status of the last command executed within the script as the exit status of
     the script.  If used outside a function and not during execution of a script by ., the return status  is
     false.  Any command associated with the RETURN trap is executed before execution resumes after the func‐
     tion or script.


       set [--abefhkmnptuvxBCEHPT] [-o option-name] [arg ...]
       set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg ...]
     Without options, the name and value of each shell variable are displayed in a 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      Automatically mark variables and functions which are modified or created for export to the envi‐
     ronment of subsequent commands.
     -b      Report the status of terminated background jobs immediately, rather than before the next primary
     prompt.  This is effective only when job control is enabled.
     -e      Exit  immediately if a pipeline (which may consist of a single simple command),  a subshell com‐
     mand enclosed in parentheses, or one of the commands executed as part of a command list enclosed
     by  braces  (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  key‐
     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, or if the command's return value is being inverted with !.  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.
     -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). Background processes run in a separate process
     group and a line containing their exit status is printed upon their completion.
     -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   Change  the behavior of bash where the default operation differs from the POSIX standard
     to match the standard (posix mode).
     privileged
     Same as -p.
     verbose Same as -v.
     vi      Use a vi-style command line editing interface.  This also affects the editing  interface
     used for read -e.
     xtrace  Same as -x.
     If  -o is supplied with no option-name, the values of the current options are printed.  If +o is
     supplied with no option-name, a series of set commands to recreate the current  option  settings
     is displayed on the standard output.
     -p      Turn  on privileged  mode.  In this mode, the $ENV and $BASH_ENV files are not processed, shell
     functions are not inherited from the environment, and the SHELLOPTS, BASHOPTS, CDPATH, and  GLO‐
     BIGNORE 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.
     -t      Exit after reading and executing one command.
     -u      Treat  unset  variables and parameters other than the special parameters "@" and "*" as an error
     when performing parameter expansion.  If expansion is attempted on an unset variable or  parame‐
     ter, 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 argu‐
     ments or associated word list.
     -B      The shell performs brace expansion (see Brace Expansion above).  This is on by default.
     -C      If  set, bash does not overwrite an existing file with the >, >&, and <> redirection operators.
     This may be overridden when creating output files by using the redirection operator  >|  instead
     of >.
     -E      If  set, any  trap  on ERR is inherited by shell functions, command 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  interac‐
     tive.
     -P      If  set, the shell does not follow symbolic links when executing commands such as cd that change
     the current working directory.  It uses the physical directory structure instead.  By  default,
     bash  follows the logical chain of directories when performing commands which change the current
     directory.
     -T      If set, any traps on DEBUG and RETURN are inherited by shell functions,  command substitutions,
     and  commands  executed  in a subshell environment.  The DEBUG and RETURN traps are normally not
     inherited in such cases.
     --      If no arguments follow this option, then the positional parameters are  unset.   Otherwise,  the
     positional parameters are set to the args, even if some of them begin with a -.
     -      Signal the end of options, cause all remaining args to be assigned to the positional parameters.
     The -x and -v options are turned off.  If there are no args, the positional  parameters remain
     unchanged.


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


       shift [n]
     The positional parameters from n+1 ... are renamed to $1 ....  Parameters represented by the numbers  $#
     down  to $#-n+1 are  unset.   n must be a non-negative number less than or equal to $#. If n is 0, no
     parameters are changed.  If n is not given, it is assumed to be 1.  If n is greater than $#,  the posi‐
     tional  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 variables controlling optional shell behavior.  With no options,  or with  the  -p
     option,  a  list of all settable options is displayed, with an indication of whether or not each is set.
     The -p option causes output to be displayed in a form that may be reused as input.  Other options  have
     the following meanings:
     -s     Enable (set) each optname.
     -u     Disable (unset) each optname.
     -q     Suppresses normal output (quiet mode); the return status indicates whether the optname is set or
    unset.  If multiple optname arguments are given with -q, the return status is zero if  all  opt‐
    names 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, the display is limited to those options which are
     set or unset, respectively.  Unless otherwise noted, the shopt options are disabled (unset) by default.


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


     The list of shopt options is:


     autocd  If set, a command name that is the name of a directory is executed as if it were the argument to
     the cd command.  This option is only used by interactive shells.
     cdable_vars
     If set, an argument to the cd builtin command that is not a directory is assumed to be the  name
     of a variable whose value is the directory to change to.
     cdspell If set, minor errors in the spelling of a directory component in a cd command will be corrected.
     The errors checked for are transposed characters, a missing character,  and  one character  too
     many.   If  a correction is found, the corrected file name is printed, and the command proceeds.
     This option is only used by interactive shells.
     checkhash
     If set, bash checks that a command found in the hash table exists before trying to  execute  it.
     If a hashed command no longer exists, a normal path search is performed.
     checkjobs
     If  set, bash  lists  the  status of any stopped and running jobs before exiting an interactive
     shell.  If any jobs are running, this causes the exit to be deferred  until  a  second  exit  is
     attempted without  an  intervening command (see JOB CONTROL above).  The shell always postpones
     exiting if any jobs are stopped.
     checkwinsize
     If set, bash checks the window size after each command and, if necessary, updates the values  of
     LINES and COLUMNS.
     cmdhist If  set, bash  attempts to save all lines of a multiple-line command in the same history entry.
     This allows easy re-editing of multi-line commands.
     compat31
     If set, bash changes its behavior to that of version 3.1 with respect to quoted arguments to the
     [[ conditional command's =~ operator.
     compat32
     If  set, bash changes its behavior to that of version 3.2 with respect to locale-specific string
     comparison when using the [[ conditional command's < and > operators.  Bash  versions  prior  to
     bash-4.1 use  ASCII collation and strcmp(3); bash-4.1 and later use the current locale's colla‐
     tion sequence and strcoll(3).
     compat40
     If set, bash changes its behavior to that of version 4.0 with respect to locale-specific string
     comparison when using the [[ conditional command's < and > operators (see previous item) and the
     effect of interrupting a command list.
     compat41
     If set, bash, when in posix mode, treats a single quote in a double-quoted  parameter  expansion
     as  a  special  character.   The single quotes must match (an even number) and the characters
     between the single quotes are considered quoted. This is the behavior  of  posix mode  through
     version 4.1.  The default bash behavior remains as in previous versions.
     direxpand
     If  set, bash replaces directory names with the results of word expansion when performing file‐
     name completion. This changes the contents of the readline editing buffer.  If  not  set,  bash
     attempts to preserve what the user typed.
     dirspell
     If  set, bash  attempts spelling  correction  on directory names during word completion if the
     directory name initially supplied does not exist.
     dotglob If set, bash includes filenames beginning with a `.' in the results of pathname expansion.
     execfail
     If set, a non-interactive shell will not exit if it cannot execute  the  file  specified as  an
     argument to the exec builtin command.  An interactive shell does not exit if exec fails.
     expand_aliases
     If  set, aliases are  expanded as  described  above under ALIASES.  This option is enabled by
     default for interactive shells.
     extdebug
     If set, behavior intended for use by debuggers is enabled:
     1.     The -F option to the declare builtin displays the source file name and line number corre‐
    sponding to each function name supplied as an argument.
     2.     If the  command  run  by the  DEBUG  trap returns a non-zero value, the next command is
    skipped and not executed.
     3.     If the command run by the DEBUG trap returns a value of 2, and the shell is executing  in
    a subroutine (a shell function or a shell script executed by the . or source builtins), a
    call to return is simulated.
     4.     BASH_ARGC and BASH_ARGV are updated as described in their 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, the extended pattern matching features described above  under  Pathname Expansion  are
     enabled.
     extquote
     If  set, $'string' and $"string" quoting is performed within ${parameter} expansions enclosed in
     double quotes.  This option is enabled by default.
     failglob
     If set, patterns which fail to match filenames during pathname expansion result in an  expansion
     error.
     force_fignore
     If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when per‐
     forming 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.
     globstar
     If  set, the  pattern  ** used in a pathname expansion context will match all files and zero or
     more directories and subdirectories.  If the pattern is followed by a /, only  directories  and
     subdirectories match.
     gnu_errfmt
     If set, shell error messages are written in the standard GNU error message format.
     histappend
     If  set, the  history  list is appended to the file named by the value of the HISTFILE variable
     when the shell exits, rather than overwriting the file.
     histreedit
     If set, and readline is being used, a user is given the opportunity to re-edit a failed  history
     substitution.
     histverify
     If  set, and  readline  is  being used, the results of history substitution are not immediately
     passed to the shell parser.  Instead, the resulting line is loaded  into the  readline  editing
     buffer, allowing further modification.
     hostcomplete
     If set, and readline is being used, bash will attempt to perform hostname completion when a word
     containing a @ is being completed (see Completing under READLINE above). This  is  enabled  by
     default.
     huponexit
     If set, bash will send SIGHUP to all jobs when an interactive login shell exits.
     interactive_comments
     If  set, allow  a word beginning with # to cause that word and all remaining characters on that
     line to be ignored in an interactive shell (see COMMENTS above). This  option  is  enabled  by
     default.
     lastpipe
     If  set, and  job control is not active, the shell runs the last command of a pipeline not exe‐
     cuted in the background in the current shell environment.
     lithist If set, and the cmdhist option is enabled, multi-line commands are saved to  the history  with
     embedded newlines rather than using semicolon separators where possible.
     login_shell
     The  shell sets this option if it is started as a login shell (see 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, the message ``The mail in mailfile has been read'' is displayed.
     no_empty_cmd_completion
     If  set, and readline is being used, bash will not attempt to search the PATH for possible com‐
     pletions 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 exe‐
     cuting case or [[ conditional commands.
     nullglob
     If set, bash allows patterns which match no files (see Pathname Expansion above) to expand to  a
     null string, rather than themselves.
     progcomp
     If  set, the programmable completion facilities (see Programmable Completion above) are enabled.
     This option is enabled by default.
     promptvars
     If set, prompt strings undergo parameter expansion, command substitution, arithmetic  expansion,
     and  quote removal after being expanded as described in PROMPTING above. This option is enabled
     by default.
     restricted_shell
     The shell sets this option if it is started in restricted mode  (see  RESTRICTED SHELL  below).
     The  value  may not be changed.  This is not reset when the startup files are executed, allowing
     the startup files to discover whether or not a shell is restricted.
     shift_verbose
     If set, the shift builtin prints an error message when the shift count  exceeds  the  number  of
     positional parameters.
     sourcepath
     If  set, the source (.) builtin uses the value of PATH to find the directory containing the file
     supplied as an argument. This option is enabled by default.
     xpg_echo
     If set, the echo builtin expands backslash-escape sequences by default.


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


       test expr
       [ expr ]
     Return a status of 0 or 1 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 argu‐
     ment of -- as signifying the end of options.


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


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


     0 arguments
    The expression is false.
     1 argument
    The expression is true if and only if the argument is not null.
     2 arguments
    If the  first  argument is !, the expression is true if and only if the second argument is null.
    If the first argument is one of the unary conditional operators listed  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
    If the first argument is !, the result is the negation of the three-argument expression  composed
    of the  remaining arguments.   Otherwise,  the  expression is parsed and evaluated according to
    precedence using the rules listed above.
     5 or more arguments
    The expression is parsed and evaluated according to precedence using the rules listed above.


     When used with test or [, the < and > operators sort lexicographically using ASCII ordering.


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


       trap [-lp] [[arg] sigspec ...]
     The  command arg is to be read and executed when the shell receives signal(s) sigspec.  If arg is absent
     (and there is a single sigspec) or -, each specified signal is reset to its  original  disposition  (the
     value  it had  upon  entrance  to  the  shell). If arg is the null string the signal specified by each
     sigspec is ignored by the shell and by the commands it invokes.  If arg is not present and -p  has  been
     supplied, then  the trap commands associated with each sigspec are displayed.  If no arguments are sup‐
     plied or if only -p is given, trap prints the list of commands associated with  each  signal.   The  -l
     option  causes  the shell to print a list of signal names and their corresponding numbers.  Each sigspec
     is either a signal name defined in <signal.h>, or a signal number.  Signal names are  case  insensitive
     and the SIG prefix is optional.


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


     If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit  status,
     subject  to the following conditions.  The ERR trap is not executed if the failed command is part of the
     command list immediately following a while or until keyword, part of the test in an if  statement,  part
     of  a  command  executed in  a && or || list, or if the command's return value is being inverted via !.
     These are the same conditions obeyed by the errexit option.


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


       type [-aftpP] name [name ...]
     With no options, indicate how each name would be interpreted if used as  a  command  name.   If  the  -t
     option  is used, type prints a string which is one of alias, keyword, function, builtin, or file if name
     is an alias, shell reserved word, function, builtin, or disk file, respectively. If  the name  is  not
     found, then nothing is printed, and an exit status of false is returned. If the -p option is used, type
     either returns the name of the disk file that would be executed if name  were  specified as  a  command
     name, or nothing if ``type -t name'' would not return file.  The -P option forces a PATH search for each
     name, even if ``type -t name'' would not return file.  If a command is  hashed,  -p  and -P  print  the
     hashed  value,  not  necessarily the  file  that appears first in PATH. If the -a option is used, type
     prints all of the places that contain an executable named name.  This includes aliases and functions, if
     and  only if  the -p option is not also used.  The table of hashed commands is not consulted when using
     -a.  The -f option suppresses shell function lookup, as with the command builtin. type returns true  if
     all of the arguments are found, false if any are not found.


       ulimit [-HSTabcdefilmnpqrstuvx [limit]]
     Provides control over  the resources available to the shell and to processes started by it, on systems
     that allow such control. The -H and -S options specify that the hard or soft limit is set for the given
     resource.  A  hard  limit  cannot  be  increased by a non-root user once it is set; a soft limit may be
     increased up to the value of the hard limit.  If neither -H nor -S is specified, both the soft and  hard
     limits are set.  The value of limit can be a number in the unit specified for the resource or one of the
     special values hard, soft, or unlimited, which stand for the current hard limit, the current soft limit,
     and no limit, respectively.  If limit is omitted, the current value of the soft limit of the resource is
     printed, unless the -H option is given.  When more than one resource is specified, the  limit  name  and
     unit are printed before the value.  Other options are interpreted as follows:
     -a     All current limits are reported
     -b     The maximum socket buffer size
     -c     The maximum size of core files created
     -d     The maximum size of a process's data segment
     -e     The maximum scheduling priority ("nice")
     -f     The maximum size of files written by the shell and its children
     -i     The maximum number of pending signals
     -l     The maximum size that may be locked into memory
     -m     The maximum resident set size (many systems do not honor this limit)
     -n     The maximum number of open file descriptors (most systems do not allow this value to be set)
     -p     The pipe size in 512-byte blocks (this may not be set)
     -q     The maximum number of bytes in POSIX message queues
     -r     The maximum real-time scheduling priority
     -s     The maximum stack size
     -t     The maximum amount of cpu time in seconds
     -u     The maximum number of processes available to a single user
     -v     The maximum amount of virtual memory available to the shell and, on some systems, to its children
     -x     The maximum number of file locks
     -T     The maximum number of threads


     If limit is given, it is the new value of the specified resource (the -a option is display only). If no
     option is given, then -f is assumed.  Values are in 1024-byte increments, except for  -t, which is  in
     seconds, -p,  which  is in units of 512-byte blocks, and -T, -b, -n, and -u, which are unscaled values.
     The return status is 0 unless an invalid option or argument is supplied, or an error occurs  while  set‐
     ting a new limit.


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


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


       unset [-fv] [name ...]
     For  each name, remove the corresponding variable or function. If no options are supplied, or the -v
     option is given, each name refers to a shell variable.  Read-only variables may not be unset.  If -f  is
     specified,  each name  refers  to a shell function, and the function definition is removed.  Each unset
     variable or function is removed  from  the  environment  passed  to  subsequent  commands.   If  any  of
     COMP_WORDBREAKS, RANDOM, SECONDS,  LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are unset, they lose
     their special properties, even if they are subsequently reset.  The exit status is true unless a name is
     readonly.


       wait [n ...]
     Wait  for each specified process and return its termination status.  Each n may be a process ID or a job
     specification; if a job spec is given, all processes in that job's pipeline are waited for.  If n is not
     given,  all currently active child processes are waited for, and the return status is zero.  If n speci‐
     fies a non-existent process or job, the return status is 127.  Otherwise, the return status is the  exit
     status of the last process or job waited for.


RESTRICTED SHELL
       If  bash is  started  with  the name  rbash,  or  the -r option is supplied at invocation, the shell becomes
       restricted.  A restricted shell is used to set up an environment more 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, ENV, or BASH_ENV


       ·      specifying command names containing /


       ·      specifying a file name containing a / as an argument to the .  builtin command


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


       ·      importing function definitions from the shell environment at startup


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


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


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


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


       ·      using the enable builtin command to enable disabled shell builtins


       ·      specifying the -p option to the command builtin command


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


       These restrictions are enforced after any startup files are read.


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


SEE ALSO
       Bash Reference Manual, Brian Fox and Chet Ramey
       The Gnu Readline Library, Brian Fox and Chet Ramey
       The Gnu History Library, Brian Fox and Chet Ramey
       Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE
       sh(1), ksh(1), csh(1)
       emacs(1), vi(1)
       readline(3)


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


AUTHORS
       Brian Fox, Free Software Foundation
       [email protected]


       Chet Ramey, Case Western Reserve University
       [email protected]


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


       Once you have determined that a bug actually exists, use the bashbug command (from the source package) to  sub‐
       mit a bug report.  If you have a fix, you are encouraged to mail that as well!  Suggestions and `philosophical'
       bug reports may be mailed to [email protected] or posted to the Usenet newsgroup gnu.bash.bug.


       ALL bug reports should include:


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


       Comments and bug reports concerning this manual page should be directed to [email protected].


BUGS
       It's too big and too slow.


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


       Aliases are confusing in some uses.


       Shell builtin commands and functions are not stoppable/restartable.


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


       Array variables may not (yet) be exported.


       There may be only one active coprocess at a time.






GNU Bash-4.2   2010 December 28       BASH(1)