1:passwd 修改口令 d delete the password for the named account (root only)
2:exit 退出系统 ctrl-d
3: man显示命令的联机帮助信息 空格：向下翻页，b：向上翻页，q：退出 Command --help
4:su run a shell with substitute user and group IDs
5:who 查看登录用户及其信息 q all login names and number of users logged on 来自：
http://blog.sina.com.cn/s/blog_3f3f808f0100bki3.html