Bash基础——命令替换

参考：Linux 下Shell 脚本几种基本命令替换区别

Command substitution

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