man手册：

man 1,用户命令手册
man 2,Linux内核系统调用手册
man 3,标准库函数手册

man 1 pwd ---------用户命令手册
NAME
pwd - print name of current/working directory
pwd [OPTION]…

man 2 exit ----Linux内核系统调用手册
_EXIT(2) Linux Programmer’s Manual
NAME
_exit, _Exit - terminate the calling process //退出系统调用 open read write

man 3 exit 标准库函数手册
EXIT(3) Linux Programmer’s Manual fopen fread fwrite
NAME
exit - cause normal process termination