https://www.jianshu.com/p/2237f029c385
https://www.cnblogs.com/chenpython123/p/11052276.html
https://blog.csdn.net/weixin_33907511/article/details/85756886
https://www.cnblogs.com/xunbu7/p/6187017.html
https://www.jianshu.com/p/919ee8129aa0
Shell—详解$( )、$(( ))、``与${ }的区别
原文：https://www.cnblogs.com/liuhaidon/p/11984189.html