shell脚本中经常会看到$开头的特殊变量,那怎么理解它们呢?

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变量说明: 
$$
Shell本身的PID(ProcessID)
$!
Shell最后运行的后台Process的PID
$?
最后运行的命令的结束代码(返回值)
$-
使用Set命令设定的Flag一览
$*
所有参数列表。如"$*"用「"」括起来的情况、以"$1 $2$n"的形式输出所有参数。
[email protected]
所有参数列表。如"[email protected]"用「"」括起来的情况、以"$1" "$2" … "$n" 的形式输出所有参数。
$#
添加到Shell的参数个数
$0
Shell本身的文件名
$1$n
添加到Shell的各参数值。$1是第1参数、$2是第2参数…。

示例

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 1 #!/bin/bash
2 #
3 printf "The complete list is %s\n" "$$"
4 printf "The complete list is %s\n" "$!"
5 printf "The complete list is %s\n" "$?"
6 printf "The complete list is %s\n" "$*"
7 printf "The complete list is %s\n" "[email protected]"
8 printf "The complete list is %s\n" "$#"
9 printf "The complete list is %s\n" "$0"
10 printf "The complete list is %s\n" "$1"
11 printf "The complete list is %s\n" "$2

结果:

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[[email protected] ~]$ bash params.sh 123456 QQ
The complete list is 24249
The complete list is
The complete list is 0
The complete list is 123456 QQ
The complete list is 123456
The complete list is QQ
The complete list is 2
The complete list is params.sh
The complete list is 123456
The complete list is QQ