As described there, I set up a cron job to back up our low-budget file server to an external USB drive and turn it off for the night.
After a while, it became painfully obvious that
shutdown -P now
was, at best, intermittently successful at turning off the power. The shutdown sequence would sometimes hang near the end, with a blank screen, after unmounting all the drives (so there are no logs), with the power on. Keyboard & mouse were dead, tapping the power button produced a display about acpid being unhappy, but nothing I could follow up.
Oddly, that same command issued from a terminal window would work perfectly for as long as I was willing to restart the machine.
Even more oddly, the box would shut off properly from the GUI or the GDM login scren.
After several days of tedious “try this” experimentation and rummaging through the scripts in /etc/init.d/, it seems this command works in the cron job the way it’s supposed to
halt -p -f
The -p calls for a power-down and -f says to force the halt (rather than calling shutdown, which we know won’t work).
So, finally, I can hack 25% off the power bill for that thing.
Memo to self: some day, figure out exactly how the whole shutdown sequence works.