When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
3.4.2 Special Parameters
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*
($*) Expands to the positional parameters, starting from one. When the ex- pansion is not within double quotes, each positional parameter expands to a separate word. In contexts where it is performed, those words are subject to further word splitting and pathname expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each param- eter separated by the first character of the IFS special variable. That is, "$*" 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 ex- pansion occurs within double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" .... If the double-quoted ex- pansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@" 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 back- ground, whether executed as an asynchronous command or using the bg builtin
(see Section 7.2 [Job Control Builtins], page 98).
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 (see Section 3.8 [Shell
Scripts], page 39), $0 is set to the name of that file. If Bash is started with the
-c option (see Section 6.1 [Invoking Bash], page 80), 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