What To Do When Your Ubuntu System Hangs On Reboot Or Shutdown?

When Linux hangs or freezes during the shutdown or reboot sequence, especially during remote sessions where no physically attached keyboard can be accessed, you may want to make sure the server will reboot no matter what and apply the -f parameter to force immediate shutdown: 
reboot -f
In other situations where you may have a keyboard at hand and want to debug shutdown hangs, you may find the not so well known Magic SysRq keyboard shortcuts come in handy. Pressing CTRL+PRINT+B will immediately reboot your system without invoking the init system and even when the systemd debug console won't accept any keyboard inputs anymore.

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