[]with[[]]

[brand@localhost ~]$ echo $name hello
hello
[brand@localhost ~]$ [$name = "hello"]
-bash: [: missing `]' #[  ] 与变量要有间隔
[brand@localhost ~]$ [ $name = "hello" ]
-bash: [: =: unary operator expected# 包含$,分词影响

[brand@localhost ~]$ [[ $name = "hello" ]]
[brand@localhost ~]$ echo $?

[brand@localhost ~]$ [[ "$name" = "hello" ]]
[brand@localhost ~]$ echo $?

[brand@localhost ~]$ [ "$name" = "hello" ]
[brand@localhost ~]$ echo $?

[brand@localhost ~]$ 

[]是shell内嵌的命令,会执行

[[]]是usr/bin/test

(( ))用于数字计算