$[ ] 的妙用

[bob@dream ~]$ var='123'
[bob@dream ~]$ var2='var'
[bob@dream ~]$ echo $[var2]
123
[bob@dream ~]$ echo $[var]
123
[bob@dream ~]$ echo ${var}
123
[bob@dream ~]$ echo ${!var}

[bob@dream ~]$ echo ${!var2}
123
[bob@dream ~]$