Check the status of the UPS using NUT (Network UPS Tools).
It can check if nut daemon runs, if the UPS is online or on battery, the charge of the battery, the temperature and the load of the UPS.

Tested on Ubuntu 6.10 and 8.04 and Fedora Core2.

Example to use this script:

command[check_ups]=/usr/lib/nagios/plugins/check_nut apc@localhost -w 50 -c 10 -t 45 -T 50 -l 30 -L 50

Check the UPS with Name "apc" attached on localhost.
Warning if battery lower as 50%, or temperature greater as 45°, or UPS load greater as 30.
Critical if battery lower as 10%, or temperature greater as 50°, or UPS load greater as 50.

You can add the parameter -p (--perfdata) to get performance data, too (useful for pnp4nagios, NagiosGrapher)