"NUT Basics
The premiere UPS monitoring tool on Linux is Network UPS Tools (NUT), a free software tool that supports hundreds of UPS models from a wide range of vendors. NUT can monitor UPSes connected via serial cable or USB, and is designed for networked operation to control multiple servers and workstations.
"Since the vast majority of UPSes include only one USB or serial connection, NUT can be configured to run in "slave" mode, listening on a TCP port for a shutdown notice from the directly-connected computer. That way, you can connect your primary server to the UPS with the cable in "master" mode, but ensure that other devices -- such as a workstation, firewall, or NAS box -- receive the message that power is out and shut down cleanly. You can even configure NUT to shut down the slave devices immediately, preserving more battery power for the master, in case power is restored.
"NUT consists of three main pieces: the drivers that interface with the actual UPS hardware, decoding its status and reports to determine whether power is present and how much battery charge remains, the upsmon client that monitors UPS status and responds to power events by logging, sending notification messages, and shutting down the system when needed, and the upsd daemon that passes information between upsmon and the various drivers."
