#! /usr/bin/env python
import os
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
# GPIO21 (pin 40) set up as input. It is pulled up
GPIO.setup(21, GPIO.IN, pull_up_down=GPIO.PUD_UP)

try:
    while True:
        # wait for the pin to be shorted with GND.. 
        # ..via reed-relais and if so, halt the system
        GPIO.wait_for_edge(21, GPIO.FALLING)
        print ("Shutting down...")
        os.system("sudo /sbin/shutdown -h now")
except:
        GPIO.cleanup()