It’s a little known feature of a BIOS: You can set it to start the PC at a specified time and date, practically like an alarm clock. Even lesser known is the fact that you can set this same setting in Linux by writing a date and time to this special file (as root):
$ echo "+00-00-00 00:05:00" > /sys/class/rtc/rtc0/wakealarm
Now shutdown your PC. It should boot up again 5 minutes after you executed that line.
Let’s harness this awesome power and turn it into a poor man’s complex alarm clock:
- sudo sh -c ‘echo “2008-05-24 06:59:00″ > /sys/class/rtc/rtc0/wakealarm’
- Start a media-player and make it play a song
- Quickly put your PC in hibernate
Now when your PC is booting in the morrow, it’ll return from hibernate and continue playing the song, waking you up in the process. Yay.
Update: Since kernel 2.6.22 (and Ubuntu Intrepid) the location of the alarm file has changed. I’ve updated the path, but if you’re using a kernel version lower, or an Ubuntu version older than these, you should echo to the file “/proc/acpi/alarm” instead of “/sys/class/rtc/rtc0/wakealarm“.