Linux system has several running levels:
0 - shutdown
1 - single user
2 - multi user without Internet services
3 - multi user with Internet services
4 - unused and reservered
5 - GUI
6 - Reboot
We can use the "Init *" command to change running level of the system.
To change the default setting of running level, we need to edit the file "/etc/inittab":
"id:*:initdefault".
Often, we may have probles of setting a wrong default running level. We need to press "e" on the interface of grub guidence. Then we need to add 1 to the second line. The reason is that Linux will read inittab file before boot except for running level 1. We can go to running level 1 and then change the "/etc/inittab" file.
Basically, this is a bug for the Linux system and it is necessary for system admin to set passwords for grub interface.
In running level 1 - single user mode, we can directly change the password of root.