Reference:

1. 鳥哥
   http://linux.vbird.org/linux_basic/0210filepermission.php
2. chown、chgrp和chmod的用法
   https://www.itread01.com/content/1523008588.html
3. https://askubuntu.com/questions/30629/how-can-i-recursively-change-the-permissions-of-files-and-directories
4. https://en.wikipedia.org/wiki/Ls