关键词是 shell扩展！所以我强烈建议你看看在你的bash参考手册中的部分 shell扩展。

The keyword is shell expansion! So i highly recommend you to read the section Shell Expansions in your bash reference manual.

您可以在线访问这一部分[1]。

You can access this section online [1].

的下列规定形式的名称是：

The names of the following stated forms are:

• $ {} 参数扩展


• $（）命令替换

• ${} Parameter expansion
• $() Command substitution

表格 $ [] 未在说明书中说明实际，所以我不会使用它。

the form $[] isn't actually stated in the manual so i wouldn't use it.

[1] http://www.gnu.org /software/bash/manual/bashref.html#Shell-Expansions