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3.4.2
Special Parameters
The shell treats several parameters specially. These parameters may only be referenced;
assignment to them is not allowed.
$*
- Expands to the positional parameters, starting from one. When the
expansion occurs within double quotes, it expands to a single word with the value of each
parameter separated by the first character of the
IFS
special variable. That
is, "$*"
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" "$2" ...
.
When there are no positional parameters, "$@"
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 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 (see section 3.8
Shell Scripts),
$0
is set to the name of that file. If Bash is started
with the `-c' option (see section 6.1
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 filename of
the shell or shell script being executed as passed in the argument list. Subsequently,
expands to the last argument to the previous command, after expansion. Also set to the
full pathname of each command executed and placed in the environment exported to that
command. When checking mail, this parameter holds the name of the mail file.
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