1. Ubuntu
a. 开机按 shift 进入GRUB

In fact: "generic" or "recovery mode" whichever is OK!
b. "e"

c. ctrl+x or F10

d. poweroff

but: shell> reboot -f /// You can make it!
2. CentOS6
a. GRUB

b. "e" , 末尾输入:空格 1

c. "b" boot

3. CentOS7


Boot your system in single user mode.
chroot your system
reset the root password.
Update SELinux information.
Exit chroot
Reboot your system.
Login with new password.
That’s All.