1、命令一般格式
COMMAND [ OPTION ] ARGUMENT
2、常见命令
[ifconfig](http://blog.51cto.com/2857034/2161663)
3、查询命令是否为内置命令
> type COMMAND
>type find
find is /usr/bin/find
>type cd
cd is a shell builtin
>type echo
echo is a shell builtin
4、查询命令帮助
内置命令：
help COMMAND
> help echo
外置命令：
COMMAND --help
>find --help
info信息页：
>info echo
>info find
man手册页：
默认搜索路径：/usr/share/man ，有9个section组成
各section作用：
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]
实例：
1、查询COMMAND手册页section位置
>whatis find
find (1) - search for files in a directory hierarchy
>whatis echo
echo (1) - display a line of text
2、查询对应手册页帮助
man [section] COMMAND
man 1 find
man 1 echo
