man命令的解释：
man的全称是manual,手册的意思，所以man命令有帮助解释其它命令的作用
基本用法：
man + 命令
比如：
man pwd
就会出现下面信息
PWD(1) User Commands PWD(1)
//注意括号中的数字
NAME //关于命令的名次和说明
pwd - print name of current/working directory
SYNOPSIS //命令的几本语法
pwd [OPTION]
DESCRIPTION //详细描述语法参数的使用方法
Print the full filename of the current working directory.
--help display this help and exit
--version
output version information and exit
NOTE: your shell may have its own version of pwd, which usually super-
sedes the version described here. Please refer to your shell's docu-
mentation for details about the options it supports.
AUTHOR //作者信息
Written by Jim Meyering.
REPORTING BUGS //作者的联系方式
Report bugs to <bug-coreutils&
