问题：
I am confused by the usage of brackets, parentheses, curly braces in Bash, as well as the difference between their double or single forms. 我对Bash中括号，括号，花括号的使用以及它们的双精度或单精度之间的区别感到困惑。 Is there a clear explanation? 有明确的解释吗？
解决方案：
参考一： https://stackoom.com/question/9BFX/如何使用双括号或单括号-括号-花括号参考二： https://oldbug.net/q/9BFX/How-to-use-double-or-single-brackets-parentheses-curly-braces
来源：oschina
链接：https://my.oschina.net/u/4438370/blog/4358038