Here's a good link on using nut (ups monitoring software for Linux): http://fedoranews.org/contributors/kazutoshi_morioka/nut/
Also, the nut project page: http://www.networkupstools.org/
Functionality seems to be dependent a bit on what Vendor / Model of UPS you are utilizing but at a minimum a clean system shutdown will be had.
Steps:
The following example is for a Liebert PowerSure UPS
Connect UPS Serial cable to COM1:
Login to the server as root.
Install nut and the nut client 'yum 'y install nut nut-client'
Edit /etc/sysconfig/ups 'nano -w /etc/sysconfig/ups'
Find 'MODEL=' line and set to 'MODEL=liebert'
Find 'DEVICE=' line and set to 'DEVICE=/dev/ttyS0' or use ttyS1 for COM2:
Save file 'Ctl-X, y'
Set proper file permissions because the install doesn't.
Cd /etc/ups
Chown root:nut ups*
Chmod 640 ups*
Edit /etc/ups/ups.conf 'nano -w /etc/ups/ups.conf'
Make the following entries in the conf file:
liebert
driver = liebert
port = /dev/ttyS0 (or ttyS1 for COM2
sdtype = 0
Edit /etc/ups/upsd.users 'nano -w /etc/ups/upsd.users'
Make the following entries in the users file:
monuser
password = YourFavPasswd
allowfrom = localhost
upsmon master
Edit /etc/ups/upsmon.conf 'nano -w /etc/ups/upsmon.conf'
Make the following line in the conf file:
MONITOR liebert@localhost 1 monuser YourFavPasswd master
Start upsd '/etc/init.d/ups start'