Learning immediately: https://edu.csdn.net/course/play/26264/326637?utm_source=blogtoedu
man command help, including nine most 1, user commands, 2, or kernel system call function, 3, and other library functions. man ls
man mkdir (mkdir there are two default user command)
man 3 mkdir (see library functions use this function, it is 3)
/ First find highlighted, n skip to the next
