man就是manual的缩写，用来查看系统中自带的各种参考手册，但是手册页分为好几个部分，如下所示：
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]
在shell中输入 man+数字+命令/函数 即可以查到相关的命令和函数。若不加数字，那man命令默认从数字较小的手册中寻找相关命令和函数。