https://www.unix.com/shell-programming-and-scripting/125947-difference-between-dev-null-2-1-2-1-dev-null.html
https://superuser.com/questions/1179844/what-does-dev-null-21-true-mean-in-linux
https://stackoverflow.com/questions/23489934/echo-2-some-text-what-does-it-mean-in-shell-scripting
【问题】Difference between ">/dev/null 2>&1" and "2>&1 >/dev/null"
原文：https://www.cnblogs.com/kelamoyujuzhen/p/9160219.html