(bash.info) Special Parameters

Info Catalog

(bash.info) Positional Parameters

(bash.info) Shell Parameters

 
 3.4.2 Special Parameters
 ------------------------
 
 The shell treats several parameters specially.  These parameters may
 only be referenced; assignment to them is not allowed.
 
 `*'
      ($*) Expands to the positional parameters, starting from one.
      When the expansion is not within double quotes, each positional
      parameter expands to a separate word.  In contexts where it is
      performed, those words are subject to further word splitting and
      pathname expansion.  When the expansion occurs within double
      quotes, it expands to a single word with the value of each
      parameter separated by the first character of the `IFS' special
      variable.  That is, `"$*"' is equivalent to `"$1C$2C..."', where C
      is the first character of the value of the `IFS' variable.  If
      `IFS' is unset, the parameters are separated by spaces.  If `IFS'
      is null, the parameters are joined without intervening separators.
 
 `@'
      ($@) Expands to the positional parameters, starting from one.
      When the expansion occurs within double quotes, each parameter
      expands to a separate word.  That is, `"$@"' is equivalent to
      `"$1" "$2" ...'.  If the double-quoted expansion occurs within a
      word, the expansion of the first parameter is joined with the
      beginning part of the original word, and the expansion of the last
      parameter is joined with the last part of the original word.  When
      there are no positional parameters, `"$@"' 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.
 
 `-'
      ($-, a hyphen.)  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 invoking shell, not the
      subshell.
 
 `!'
      ($!) Expands to the process ID of the job most recently placed
      into the background, whether executed as an asynchronous command
      or using the `bg' builtin ( Job Control Builtins).
 
 `0'
      ($0) Expands to the name of the shell or shell script.  This is
      set at shell initialization.  If Bash is invoked with a file of
      commands ( Shell Scripts), `$0' is set to the name of that
      file.  If Bash is started with the `-c' option ( Invoking
      Bash), then `$0' is set to the first argument after the string
      to be executed, if one is present.  Otherwise, it is set to the
      filename used to invoke Bash, as given by argument zero.
 
 `_'
      ($_, an underscore.)  At shell startup, set to the absolute
      pathname used to invoke the shell or shell script being executed
      as passed in the environment or argument list.  Subsequently,
      expands to the last argument to the previous command, after
      expansion.  Also set to the full pathname used to invoke each
      command executed and placed in the environment exported to that
      command.  When checking mail, this parameter holds the name of the
      mail file.
 

Info Catalog

(bash.info) Positional Parameters

(bash.info) Shell Parameters