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
"$1c$2c…", where c
is the first character of the value of the
IFS is unset, the parameters are separated by spaces.
IFS is null, the parameters are joined without intervening
($@) Expands to the positional parameters, starting from one.
In contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting.
In contexts where word splitting is not performed,
this expands to a single word
with each positional parameter separated by a space.
expansion occurs within double quotes, and word splitting is performed,
each parameter expands to a
separate word. That is,
"$@" is equivalent to
"$1" "$2" ….
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters,
expand to nothing (i.e., they are removed).
($#) 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
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
bg builtin (see Job Control Builtins).
($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
(see Shell Scripts),
$0 is set to the name of that file.
If Bash is started with the -c option (see Invoking Bash),
$0 is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
($_, 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 simple command executed in the foreground, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file.