Mac Geekery

Answer

You need a UPS with a USB connection for this to work. Get an APC one and just throw away the software it comes with; Mac OS X can see and understand it just fine.

Once the unit is plugged in, go into the Energy Saver prefs and … well, let’s see what the manual says, on page 60:

Working With an Uninterruptible Power Supply (UPS)

If your server is connected to a UPS, you can set an interval before the UPS shuts down the system. You can also revise the script that runs automatically when the UPS shuts down the system.

To set an interval before UPS shutdown of the system:

• Use the UPS button in the Energy Saver pane of System Preferences to implement a shutdown interval.

A system administrator can use the command-line tools to modify the script that runs automatically just before the UPS shuts down the system. This script tests for all running Mac OS X Server processes and shuts them down cleanly.

To modify the UPS shutdown script:

• Locate the script at /usr/libexec/upsshutdown and modify it as necessary.

Ever the curious beast, I looked in my own /usr/libexec/upsshutdown on my MacBook Pro and, lo, there’s a script ripe for hacking.

#!/bin/sh


	
#
	

		
Pick up the email recipient from /etc/hostconfig, if defined.
	
#

. /etc/hostconfig


	
#
	

		
Initialize…
	
#

PATH=/bin:/usr/bin:/sbin:/usr/sbin

SI_PATH=/System/Library/StartupItems

MAIL=mail

MSG=“Shutting down due to power loss!”


	
logger -i -p daemon.emerg -t UPS “${MSG}”

	
#
	

		
Send notification to the same address used for IP Failover.
	
#

if ! [ “${FAILOVER_EMAIL_RECIPIENT:=-AUTOMATIC-}” = “AUTOMATIC“ ] ; then
        SUBJECT=“UPS Notification: ${MSG}”
        ${MAIL} -s “${SUBJECT}” ${FAILOVER_EMAIL_RECIPIENT} < /dev/null

fi


	
#
	

		
Wait for watchdog and take the system down cleanly.
	
#

shutdown -h now “${MSG}”

Now, really, have fun with that. If you need to do anything special here, do it before the shutdown command is run and make sure your command is blocking — that is, it doesn’t move to the background. If it does, it won’t stop the shutdown from happening. Most shell commands are blocking, for what it’s worth.