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Is it possible to get the reason of system wake-up (Wake-on-LAN, Power button, wake from USB) on Linux? I'm using Ubuntu, kernel Linux homeless 2.6.37-12-generic #26~lucid1-Ubuntu SMP Wed Jan 5 21:52:06 UTC 2011 i686 GNU/Linux

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I'm not a linux expert, but if this is possible it may be distribution specific so you might want to include which distribution you're working with. This is usually a good idea with any question you ask of any OS. –  squillman Jun 14 '11 at 15:18
I think it is not very distro-specific (maybe it is stored somewhere in /sys or /proc), but I am using: Linux homeless 2.6.37-12-generic #26~lucid1-Ubuntu SMP Wed Jan 5 21:52:06 UTC 2011 i686 GNU/Linux –  honzas Jun 14 '11 at 15:59
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This sorta explains it at the hardware level. What do you see when you dmesg | grep -i 'acpi'?

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Yes, I know abou /proc/acpi/wakeup and I'm using it on another machine and it selects which devices are allowed to wakeup the system. But after the wakeup I'm unable to say which of these devices causes the wakeup. –  honzas Jun 14 '11 at 16:02
Hence I ask: does grepping the kernel log not tell you what wakes up the device via ACPI? I tried grepping it on a desktop, so I obviously did not find anything fun there. –  ajstein Jun 14 '11 at 17:14
This a pretty old message about a kernel bug, it specifically shows while debugging ACPI issues in 2.6.17 and beyond that it will list the device by PCI bus designation (or whatever the hell that is), and say wakeup, no? –  ajstein Jun 14 '11 at 17:52
I have tried it but there is no difference between dmesg after WOL and power button. –  honzas Jun 14 '11 at 18:38
Interesting. When Googling about it, a lot of people complained about noisy debug logs. It might be they subsequently changed the logging level to be quieter by default, and you need to up the noise if you want to see the old stuff. –  ajstein Jun 14 '11 at 20:25
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