it will turn on the ACT (green) LED when the temperature is higher than <level 1> degrees celcius
and will make the ACT (green) LED blink when the temperature is higher than <level 2> degrees celcius
otherwise the ACT (green) LED will stay dark
works nice on retropi
as root:
copy the pi_temp_watcher.sh script to /usr/local/bin
chmod a+rx /usr/local/bin/pi_temp_watcher.sh
and add this line to /etc/rc.local (before the "exit 0" line):
/usr/local/bin/pi_temp_watcher.sh >/dev/null 2>/dev/null &
then reboot