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Assuming you've just entered the command shutdown -h now (or something equivalent), is there realistically anything you can do (if you're quick enough) to prevent the machine shutting down?

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Once you invoke shutdown -h now, I believe you lose command line access immediately similarly to halt -p so there isn't much you can do. – Ken Jan 18 '11 at 16:41
@Ken: on Ubuntu and Centos connected via putty the command line remains available for a few seconds. – Iain Jan 18 '11 at 18:43
Why say "now" if you don't mean "right now"? – Fred Nurk Jan 18 '11 at 20:27

2 Answers

up vote 6 down vote

You can run shutdown -c to cancel an already running shutdown.

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Did not believe it at first, but yes. More specifically ctrl+c – Ken Jan 18 '11 at 16:45
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@Ken: shutdown -c and [Ctrl]+[c] are entirely different. shutdown -c is a command which cancels a previously scheduled shutdown (even "now" is scheduled in this sense --- but whether the cancellation will prevent the reboot is a bit of a race in that case). [Ctrl]+[c] will cause you terminal to generate a SIGINT (interrupt signal) to whichever process was currently attached to it. This might work on a running shutdown command but that's rather incidental to your question. – Jim Dennis Jan 18 '11 at 17:25
@Jim Dennis: I'm aware of the difference between ctrc+c and shutdown -c. But given the speed of the -now option rather than its scheduled sibling, ctrl+c would be the fastest route to that end. I didn't believe you could stop an immanent shutdown, even more surprised that you can interrupt it, but apparently you can, but it's a race as you stated. – Ken Jan 18 '11 at 17:45
up vote 5 down vote

If you are fast enough you can issue an init 2 (or whatever runlevel you want) and that will likely stop the shutdown. Anything involving killing the shutdown command will fail as the command runs too quickly

I tried this with the script below and and even it's not fast enough to stop the shutdown

#!/bin/bash
shutdown -h now
shutdown -c "Aborting Shutdown"

Broadcast message from iain@iain-ubuntu
        (/dev/pts/0) at 16:50 ...

The system is going down for halt NOW!
shutdown: Cannot find pid of running shutdown

Where as interactively

shutdown -h now
init 2

the system stays up.

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