当CentOS操作系统崩溃，无法正常启动时，可以通过救援模式或者紧急模式进行系统登录。

启动CentOS, 当出现下面界面时，按e进入编辑界面。

1. 在编辑界面里，加入参数：systemd.unit=rescue.target ，然后Ctrl-X启动进入救援模式。

2. 在编辑界面里，加入参数：systemd.unit=emergency.target， 然后Ctrl-X进入紧急模式。

救援模式与紧急模式可以相互切换：

Switch to Emergency mode(target)

To switch to Emergency target, simply run following command as root:

# systemctl emergency

Broadcast message from root@geeklab on pts/1 (Mon 2016-08-17 00:44:58 EDT):

The system is going down to emergency mode NOW!

Switch to Rescue mode(target)

To switch to rescue target, simply run following command as root:

# systemctl rescue

Broadcast message from root@geeklab on pts/0 (Mon 2016-08-17 00:22:44 EDT):

The system is going down to rescue mode NOW!

参考：https://www.thegeekdiary.com/centos-rhel-7-how-to-boot-into-rescue-mode-or-emergency-mode/