First, consider the concept of single-user mode: Http://baike.baidu.com/link?url= Hcy6fmjdxmtvnlih9a9l2hc5ndhpzs0onafqwmzc0e5iuy5onkfd5vhaukzwhq3bgi9bypwwvunv_qu-yxxrbk
Secondly, one of the most prominent and practical advantages of single-user is: can login without password, so, in the forgotten password, you can enter the single-user mode to modify the login password.
Finally, take a look at how to enter Linux single-user mode:
1, when the system starts, press "E" on the keyboard until you enter the following page:
2. Choose kernel ... (i.e. the second item), press "E", enter at the end of the line: space single, then enter to determine
3. Press "B" on the keyboard to reboot the system
4, then into the single-user mode, you can use the root function of things, change the file you want to change after reboot can
Ext.: https://www.cnblogs.com/comeonbaby/p/5276096.html
Single-user mode for Linux