JJJ：shell指令man

通过man指令可以查看：系统调用、shell指令、libc库等接口的调用说明，详细如下：

       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, e.g. /etc/passwd
       6   Games
       7   Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7), man-pages(7)
       8   System administration commands (usually only for root)
       9   Kernel routines [Non standard]

上述结果通过:man man获取

常用的就是：
man 1 shell_cmd：查看某个shell指令或者可执行文件的用法
man 2 sys_call：查看某个系统调用接口的用法，如read
man 3 lib_call：查看某个libc库的函数接口的用法，如fork