Script that checks a monitor host (via MAC address) to see if it is alive. If not, it will SSH (assume keys have been setup) to another host and shut it down.

check-power.sh

#!/bin/bash
#
# A quick-and-dirty UPS shutdown monitor by Thanos
#
# Can perform shutdown of a "precious" machine that is on a UPS
# based on network reachability of a "inexpensive" machine that is
# not on a UPS. In my case, the "inexpensive" is a RPI that is always on.
#
# Cron entry for this script (uses "pi" user):
# */2 * * * * /home/pi/check-power.sh > /home/pi/powercheck.log 2>&1

# MAC address of system without UPS support
MAC="XX:YY:ZZ:BB:CC:DD"

# Host (and presumably user) to SSH to
HOST="admin@10.10.10.129"

# CMD to execute for shutdown
# QNAP uses "halt", usually Linux would be "shutdown -h now"
CMD="/sbin/halt"

# LOG file to keep shutdown commands
LOSSLOG=/home/pi/powerloss.log

# find the IP address from the MAC we got
IP=$(cat /proc/net/arp | grep $MAC | awk '{print$1}')

if [ "$IP" != "" ] ;  then
  echo -n "`date "+%d/%m/%y %H:%M:%S" ` monitor found at $IP "
  ping -c 5 -i 12 -w 70 $IP &> /dev/null
  RET=$?
  if [ $RET -gt 0 ]; then
    echo -n "down (`date "+%H:%M:%S"`)..."
    echo "`date "+%d/%m/%y %H:%M:%S" ` Will now issue $CMD to $HOST, $IP was unreachable" >> $LOSSLOG
    sync;sync # make sure we got the log entry written (about to lose power also :D)
    ssh $HOST $CMD
    break
  else
    echo "up!"
  fi
else
  echo -n "`date "+%d/%m/%y %H:%M:%S" ` monitor MAC not found on network"
fi