a. 開機按 shift 進入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!
a. grub
b. "e" ， 末尾輸入：空格 1
c. "b" boot
<a target="_blank" href="http://www.itzgeek.com/how-tos/linux/centos-how-tos/single-user-mode-in-centos-7-rhel-7.html">boot your system in single user mode.</a>
chroot your system
reset the root password.
update selinux information.
exit chroot
reboot your system.
login with new password.
that’s all.