[Nut-upsuser] testing shutdown: pc not restarting; and "ups unavailable" messages

nut.user.u830 at neverbox.com nut.user.u830 at neverbox.com
Thu Feb 1 15:58:09 UTC 2018

> Out of curiosity: if the PC has already been switched off from software by the shutdown script, how would it know that the power has been cut while it was off? Are you telling me that the power supply of the PC keeps checking its incoming AC supply even while off, and that with the BIOS setting I selected it will wake up the PC even after a power cut DURING WHICH the PC was already off? I never imagined that... I guess it's something I could try independently, once I find a quiet time with no disk activity when it's OK to shut down that computer.

It actually occurred to me that I could try this with another (any other) computer here at work, so I did. I set the BIOS to "always on" after power cut. I booted ubuntu. I did "sudo shutdown -h now" from the console. The pc switched off as expected. Then I removed the power cable and then plugged it back in. According to the theory above, this should have caused the computer to detect a power loss and turn itself back on. Instead it remained off (as I expected).

So clearly I am missing something here. Once nut has turned off the pc completely with a sequence of unix commands, how can anything the ups might do to to the power line, including cutting the power and restoring, cause the pc to come back on?

I would have expected the pc only to come back on if either

a) power had been cut off (not from software) while the computer was still running, AND the bios setting said "always on"


b) the UPS simulated some kind of "power button keypress" or the like through the USB link.

If my understanding is flawed then please enlighten me. What does actually happen when things work?

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