man bash:
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 sin‐
gle word with the value of each parameter separated by the first
character of the IFS special variable. That is, "$*" is equiva‐
lent to "$1c$2c...", where c is the first character of the value
of the IFS variable. If IFS is unset, the parameters are sepa‐
rated 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, "[email protected]" is equivalent to "$1"
"$2" ... If the double-quoted expansion occurs within a word,
the expansion of the first parameter is joined with the begin‐
ning 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, "[email protected]" and [email protected] 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 fore‐
ground pipeline.
- Expands to the current option flags as specified upon invoca‐
tion, 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 current shell, not the sub‐
shell.
! Expands to the process ID of the most recently executed back‐
ground (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 com‐
mands, $0 is set to the name of that file. If bash is started
with the -c option, then $0 is set to the first argument after
the string to be executed, if one is present. Otherwise, it is
set to the file name used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute pathname used to invoke
the shell or shell script being executed as passed in the envi‐
ronment 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 check‐
ing mail, this parameter holds the name of the mail file cur‐
rently being checked.
http://blog.sina.com.cn/s/blog_6739945f0100sosu.html
特殊的shell变量:
$0 获取当前执行的shell脚本的文件名
$n 获取当前执行的shell脚本的第n个参数值，n=1..9
$* 获取当前shell的所有参数 “$1 $2 $3 …，受IFS控制
$# 获取当前shell命令行中参数的总个数
$$ 获取当前shell的进程号（PID）
$! 执行上一个指令的PID
$? 获取执行的上一个指令的返回值(0 为成功， 非零为失败)
[email protected] 这个程序的所有参数 “$1″ “$2″ “$3″ “…”，不受IFS控制
注：
IFS:IFS(Internal Field Seperator)在Linux的shell中预设的分隔符，用来把command line分解成word（字段）。IFS可以是White Space（空白键）、Tab（ 表格键）、Enter（ 回车键）中的一个或几个。
对于[email protected]与$*的区别如下：
这两个参数的不同点主要在于它们扩展参数的方式不同。当使用$*时，只是简单地在不保留引用的情况下扩展每个参数，这样会存在问题：当参数中存在空格时，该参数将被识别为多个参数使用，如：mytar
-t "my tar
file.tar"，使用$*时将被识别成my、tar、file.tar坧个文件，而使用[email protected]时则识别成一个文件。