I recently upgraded from Ubuntu 11.10 to 12.04 on my Dell Latitude D630C.
Now, when I want to shut down the system does shut down but then starts up again immediately. It's like it does a reboot instead of a shut down.
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I have had exactly the same problem on my Acer V5-571G: the laptop shuts down, turns off its power LED, and then starts again after a few seconds.
The problem seems to arise from incorrect work of pm-utils (which are replacement for laptop-mode-tools). But if you write
/sys/bus/*/devices/*/power/control, the kernel takes control over power management at the specified devices - and somehow this makes the hardware to shut down correctly.
So I have written a script:
#!/bin/bash case "$1" in stop) for i in /sys/bus/*/devices/*/power/control ; do echo on > $i ; done ;; esac exit 0
and to make it being executed every time before shutdown I've placed it under name K32power-control-on into /etc/rc0.d directory (don't forget to make the script executable, sudo chmod +x K32power-control-on). After that the laptop shuts down ok.
I think that there are only a few buses/devices which need to have 'on' value written to their power/control, so the list of devices in the script can be restricted, but I just didn't investigate further.
I had same problem in with my PC, but the first time it can be normally shutdown. Then, after a few days it can't be shutdown and always restart. After I had reinstalled the grub using boot-repair, because I formatted my windows, my PC could be shutdown normally as the first time.
You can obtain boot-repair here:
Hope this answer help you.
NB: I used boot-repair with live-usb and sorry if my english doesn't very good.