https://lyjhc.com/2014/12/linux%e4%b8%8bshell%e6%98%be%e7%a4%ba-bash-4-1%e4%b8%8d%e6%98%be%e7%a4%ba%e8%b7%af%e5%be%84%e8%a7%a3%e5%86%b3%e6%96%b9%e6%b3%95.html