关于shell对特殊字符的处理

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.
-      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 current 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 (see JOB CONTROL below).
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, $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 filename 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 environment or argument list.  Subsequently, expands to the last argument to the previous command,  after  expan‐
sion.  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 currently being checked

以上内容摘自bash的manpage，不过也可以简单的介绍下，一些常用到的

# 表示参数的个数，如

cat test_bash.sh

#!/bin/bash

echo $#

sh test_bash.sh 1 23 3 a b c

6

@把所有的参数都列出来

# sh test_bash.sh 1 23 3 a b c

1 23 3 a b c

[root@kvm ~]# cat test_bash.sh

#!/bin/bash

echo $@

＊貌似和@区别不大

# cat test_bash.sh

#!/bin/bash

echo $*

# sh test_bash.sh 1 23 3 a b c "asasd_ 3"

1 23 3 a b c asasd_ 3

[root@kvm ~]# sh test_bash.sh 1 23 3 a b c "asa\"sd_ 3"

1 23 3 a b c asa"sd_ 3

_这个表示上次执行的命令

# w

00:51:39 up 22:07, 2 users, load average: 0.01, 0.08, 0.11

USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT

root pts/0 192.168.3.253 17:22 0.00s 0.38s 0.00s w

root pts/3 192.168.3.253 22:06 1:03m 0.03s 0.03s -bash

echo $_

w

?表示上个程序执行结束后的返回值

# kkk

-bash: kkk: command not found

[root@kvm ~]#

[root@kvm ~]# echo $?

127

[root@kvm ~]# ls > /dev/null

[root@kvm ~]# echo $?

0

$$ 当前进程的名字，下面的例子中，进程名语法是shell本身 -bash

# echo $$

9058

[root@kvm ~]# ps aux | grep 9058

root 9058 0.0 0.1 108408 2112 pts/0 Ss May06 0:00 -bash

root 15879 0.0 0.0 103304 924 pts/0 R+ 00:53 0:00 grep 9058

[root@kvm ~]# ps aux | grep 9058 | grep -v grep

root 9058 0.0 0.1 108408 2112 pts/0 Ss May06 0:00 -bash

！ 表示最近一个在后台运行的进程的pid

[root@kvm ~]# sleep 5000 &

[2] 15902

[root@kvm ~]# echo $!

15902

0的话，就是当前程序的名字了