命令替换Command substitution
https://www.jb51.net/article/95585.htm
https://www.cnblogs.com/f-ck-need-u/archive/2017/08/20/7401591.html
https://www.cnblogs.com/itxdm/p/something_of_shellscirpt.html
原文：https://www.cnblogs.com/kelamoyujuzhen/p/9830519.html