在linux下查看手册的时候，manual 后面总带着括号起来的数字，这个数字就是 manual section 的意思。一般表示如下的意义：
- General commands
- System calls
- C library functions
- Special files (usually devices, those found in /dev) and drivers
- File formats and conventions
- Games and screensavers
- Miscellanea
- System administration commands and daemons
维基百科有更详细的解释，https://en.wikipedia.org/wiki/Man_page