Raspberry Pi Shutdown Button

listen-for-shutdown.py

#!/usr/bin/env python


import RPi.GPIO as GPIO
import subprocess
import time


GPIO.setmode(GPIO.BCM)
GPIO.setup(3, GPIO.IN, pull_up_down=GPIO.PUD_UP)
while True:
	GPIO.wait_for_edge(3, GPIO.FALLING, bouncetime=100)
	if GPIO.wait_for_edge(3, GPIO.RISING, timeout=2900) is None:
		subprocess.call(['wall.sh', "THE SYSTEM IS SHUTTING DOWN"], shell=False)
		time.sleep(3)
		subprocess.call(['shutdown', '-h', 'now'], shell=False)

wall.sh

#!/bin/sh
for f in /dev/pts/[0-9]*
do
	echo "Broadcast message from $(whoami)@$(hostname) ($(date +'%a %b %d %H:%M:%S %Y')):" > $f
	echo $* > $f
done 

This modification is made on this article https://howchoo.com/g/mwnlytk3zmm/how-to-add-a-power-button-to-your-raspberry-pi , in order to require the button to be pressed for 3 seconds and broadcast the message to the virtual terminals on GUI before shutting down.