/usr/bin/bash: Difference between revisions

SYNOPSIS

COPYRIGHT

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

DESCRIPTION

      POSIX specification (IEEE Standard 1003.1).  Bash can be configured to be POSIX-conformant by de‐

OPTIONS

      mand, including -o, can be used as options when the shell is invoked.  In addition,  bash  inter‐

      -c        If  the -c option is present, then commands are read from the first non-option argument

                command_string.  If there are arguments after the command_string, the first argument is

                assigned  to  $0 and any remaining arguments are assigned to the positional parameters.

      -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

      -v       Print shell input lines as they are read.

                is not C or POSIX.  This implies the -n option; no commands will be executed.

      [-+]O [shopt_option]

                BUILTIN COMMANDS below).  If shopt_option is present, -O sets the value of that option;

      --        A  -- signals the end of options and disables further option processing.  Any arguments

      Bash  also interprets a number of multi-character options.  These options must appear on the com‐

      mand line before the single-character options to be recognized.

      --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  ini‐

              tialization  files  ~/.bash_profile, ~/.bash_login, or ~/.profile.  By default, bash reads

              these files when it is invoked as a login shell (see INVOCATION below).

      --norc Do not read and execute the system wide initialization file /etc/bash.bashrc and the  per‐

              sonal initialization file ~/.bashrc if the shell is interactive.  This option is on by de‐

      --posix

              details how posix mode affects bash's behavior.

      --restricted

      --verbose

      --version

      If  arguments  remain after option processing, and neither the -c nor the -s option has been sup‐

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

      gin option.

      The following paragraphs describe how bash executes its startup files. If any of the files exist

      but  cannot  be read, bash reports an error. Tildes are expanded in filenames as described below

      under Tilde Expansion in the EXPANSION section.

      After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that or‐

      When an interactive login shell exits, or a non-interactive login shell executes the exit builtin

      command, bash reads and executes commands from the file ~/.bash_logout, if it exists.

      from /etc/bash.bashrc and ~/.bashrc, if these files exist.  This may be inhibited  by  using  the

      --norc  option.   The --rcfile file option will force bash to read and execute commands from file

      When bash is started non-interactively, to run a shell script, for  example,  it  looks  for  the

      panded value as the name of a file to read and execute.  Bash behaves as if the following command

              if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

      as  an  interactive login shell, or a non-interactive shell with the --login option, it first at‐

      the name sh, bash looks for the variable ENV, expands its value if it is defined,  and  uses  the

      fect.  A  non-interactive shell  invoked  with  the  name sh does not attempt to read any other

      standard for  startup  files. In this mode, interactive shells expand the ENV variable and com‐

      mands are read and executed from the file whose name is the expanded  value.   No other  startup

      as  sh.  The  --norc option may be used to inhibit this behavior, and the --rcfile option may be

      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

              || & && ; ;; ;& ;;& ( ) | |& <newline>

      low),  the  third word of a case or select command (only in is valid), or the third word of a for

      command (only in and do are valid):

      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 ... ]

      tion is enabled.  If pipefail is enabled, the pipeline's return status is the value of  the  last

      If the time reserved word precedes a pipeline, the elapsed as well as user and system time  con‐

      scription of TIMEFORMAT under Shell Variables below.

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

      abled  using  the shopt builtin (see the description of shopt below), the last element of a pipe‐

      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>.

      If a command is terminated by the control operator &, the shell executes the command in the back‐

      AND and OR lists are sequences of one or more pipelines separated by the && and || control opera‐

      A compound command is one of the following. In most cases a list in a command's description may

      be separated from the rest of the command by one or more newlines, and may be followed by a new‐

      (list) list is executed in a subshell environment (see  COMMAND  EXECUTION  ENVIRONMENT  below).

              Variable  assignments  and builtin commands that affect the shell's environment do not re‐

      { list; }

              status of list.  Note that unlike the metacharacters ( and ), { and } are  reserved  words

              The expression is evaluated according to the rules described below under ARITHMETIC EVALU‐

              ATION.  If the value of the expression is non-zero, the return status is 0; otherwise the

      [[ expression ]]

              PRESSIONS.  Word  splitting and pathname expansion are not performed on the words between

              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

      glob  shell  option  were enabled. The = operator is equivalent to ==. If the nocasematch shell

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

      The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 other‐

      wise. Any part of the pattern may be quoted to force the quoted  portion  to  be  matched  as  a

      An additional binary operator, =~, is available, with the same precedence as == and !=. When it

      regular  expression  is  syntactically incorrect, the conditional expression's return value is 2.

      dence:

              expression1 && expression2

              expression1 || expression2

              The  && and || operators do not evaluate expression2 if the value of expression1 is suffi‐

      for name [ [ in [ word ... ] ] ; ] do list ; done

      for (( expr1 ; expr2 ; expr3 )) ; do list ; done

              until it  evaluates to zero.  Each time expr2 evaluates to a non-zero value, list is exe‐

      select name [ in word ] ; do list ; done

              The list of words following in is expanded, generating a list of items.  The  set  of  ex‐

              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 num‐

              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

      case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

              corresponding list is executed.  If the ;; operator is used, no subsequent matches are at‐

              associated list on a successful match, continuing the case statement execution as if the

      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.  Other‐

              command, except that the test is negated: list-2 is executed as long as the last  command

      The format for a coprocess is:

              coproc [NAME] command [redirections]

      A  shell  function is an object that is called like a simple command and executes a compound com‐

      mand with a new set of positional parameters. Shell functions are declared as follows:

      fname () compound-command [redirection]

      function fname [()] compound-command [redirection]

      Enclosing characters in single quotes preserves the literal value of each character  within  the

      quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

      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-

              \'     single quote

              \nnn  the eight-bit character whose value is the octal value nnn (one to three octal dig‐

              \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

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

      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  COM‐

              name=[value]

      If value is not given, the variable is assigned the null string. All values undergo tilde expan‐

      moval (see EXPANSION below).  If the variable has its integer attribute set, then value is evalu‐

      pansion  below).   Word splitting is not performed, with the exception of "$@" as explained below

      under Special Parameters. Pathname expansion is not performed. Assignment statements may also

      (declaration commands). When in posix mode, these builtins may appear in a command after one  or

      A variable can be assigned the nameref attribute using the -n option to the declare or  local

      builtin commands (see the descriptions of declare and local below) to create a nameref, or a ref‐

      erence to another variable. This allows variables to be manipulated indirectly.   Whenever the

      nameref  variable  is referenced, assigned to, unset, or has its attributes modified (other than

      using or changing the nameref attribute itself), the operation is actually performed on the vari‐

      able  specified  by  the nameref variable's value.  A nameref is commonly used within shell func‐

      tions to refer to a variable whose name is passed as an argument to the function. For  instance,

      if a variable name is passed to a shell function as its first argument, running

              declare -n ref=$1

      first argument. References and assignments to ref, and changes to its attributes, are treated as

       references, assignments, and attribute modifications to the variable whose name was passed as $1.

      If the control variable in a for loop has the nameref attribute, the list of words can be a list

      of shell variables, and a name reference will be established for each word in the list, in turn,

      when the loop is executed.  Array variables cannot be  given  the  nameref attribute.  However,

       unset using the -n option to the unset builtin.  Otherwise, if unset is executed with the name of

      closed in braces (see EXPANSION below).

              "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS

              is  performed,  this  expands  each positional parameter to a separate word; if not within

              rated by a space.  When the expansion occurs within double quotes, each parameter expands

              If  bash  is invoked with a file of commands, $0 is set to the name of that file. If bash

      The following variables are set by the shell:

              stack.  The final parameter of the last subroutine call is at the top of  the  stack; the

              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[4]       The release status (e.g., beta1).

              BASH_VERSINFO[5]       The value of MACHTYPE.

              An index into ${COMP_WORDS} of the word containing the  current  cursor  position.  This

              If  the  current cursor  position is at the end of the current command, the value of this

              variable is equal to ${#COMP_LINE}.  This variable is available only  in  shell  functions

              An array variable (see Arrays below) containing a list of exit status values from the pro‐