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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。
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。
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/