shut down a system when it becomes idle - for aws spot instances

idleshutdown.sh

#!/bin/bash

IDLES=0
while sleep 30
do
  PCT=`top -bn 2 -d 0.1 | grep '^%Cpu' | tail -n 1 | tr :. '  ' | awk '{print$2}'`
  if [ $PCT -lt 10 ]
  then
    IDLES=$[$IDLES+1]
    #echo $IDLES $PCT
  else
    IDLES=0
  fi
  if [ $IDLES -ge 20 ]
  then
    sudo shutdown -h now
    break
  fi
done