1. 查看帮助文档——man man

       1   Executable programs or shell commands
       2   System calls (functions provided by the kernel)
       3   Library calls (functions within program libraries)
       4   Special files (usually found in /dev)
       5   File formats and conventions eg /etc/passwd
       6   Games
       7   Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
       8   System administration commands (usually only for root)
       9   Kernel routines [Non standard]

　　1、2、3、5 需要了解

2. alias 应用别名

3. echo显示一个字符串

　　echo "hello world"

4.shutdown等关机命令