What does it mean in shell script？
https://stackoverflow.com/questions/23489934/echo-2-some-text-what-does-it-mean-in-shell-scripting
shell 1>&2 2>&1 &>filename重定向的含义和区别
https://www.cnblogs.com/liuchaogege/p/6124669.html
shell重定向输出(1>&2 2>&1 &>file >&file)
https://www.jianshu.com/p/7ad21d9b28f0
Why redirect output to 2>&1 and 1>&2?
https://superuser.com/questions/436586/why-redirect-output-to-21-and-12
