参数间接

Bash indirection 允许获取名称包含在另一个变量中的变量的值。变量示例：

$ red="the color red"
$ green="the color green"

$ color=red
$ echo "${!color}"
the color red
$ color=green
$ echo "${!color}"
the color green

一些演示间接扩展用法的示例：

 $ foo=10
 $ x=foo
 $ echo ${x}      #Classic variable print  
 foo  
 
 $ foo=10
 $ x=foo
 $ echo ${!x}     #Indirect expansion
 10

还有一个例子：

$ argtester () { for (( i=1; i<="$#"; i++ )); do echo "${i}";done; }; argtester -ab -cd -ef 
1   #i expanded to 1 
2   #i expanded to 2
3   #i expanded to 3

$ argtester () { for (( i=1; i<="$#"; i++ )); do echo "${!i}";done; }; argtester -ab -cd -ef 
-ab     # i=1 --> expanded to $1 ---> expanded to first argument sent to function
-cd     # i=2 --> expanded to $2 ---> expanded to second argument sent to function
-ef     # i=3 --> expanded to $3 ---> expanded to third argument sent to function