1. Create the shtdwn.py script as follows:

#!/usr/bin/python3 
#shtdwn.py 
import time 
import RPi.GPIO as GPIO 
import os 
 
# Shutdown Script 
DEBUG=True #Simulate Only 
SNDON=True 
#HARDWARE SETUP 
# GPIO 
# 2[==X==L=======]26[=======]40 
# 1[===1=========]25[=======]39 
 
#BTN CONFIG - Set GPIO Ports 
GPIO_MODE=GPIO.BOARD 
SHTDWN_BTN = 7 #1 
LED = 12       #L 
 
def gpio_setup(): 
  #Setup the wiring 
  GPIO.setmode(GPIO_MODE) 
  #Setup Ports 
  GPIO.setup(SHTDWN_BTN,GPIO.IN,pull_up_down=GPIO.PUD_UP) 
  GPIO.setup(LED,GPIO.OUT) 
 
def doShutdown(): 
  if(DEBUG):print("Press detected") 
  time.sleep(3) 
  if GPIO.input(SHTDWN_BTN): 
    if(DEBUG):print("Ignore the shutdown (<3sec)") 
  else: 
    if(DEBUG):print ("Would shutdown the RPi Now") 
    GPIO.output(LED,0) 
    time.sleep(0.5) 
    GPIO.output(LED,1) 
    if(SNDON):os.system("flite -t 'Warning commencing power down 3 2 1'") 
    if(DEBUG==False):os.system("sudo shutdown -h now") 
    if(DEBUG):GPIO.cleanup() 
    if(DEBUG):exit() 
 
def main(): 
  gpio_setup() 
  GPIO.output(LED,1) 
  while True: 
    if(DEBUG):print("Waiting for >3sec button press") 
    if GPIO.input(SHTDWN_BTN)==False: 
       doShutdown() 
    time.sleep(1) 
 
try: 
  main() 
finally: 
  GPIO.cleanup() 
  print("Closed Everything. END") 
#End

2. To get this script to run automatically (once we have tested it), we can place the script in the ~/bin (we can use cp instead of mv if we just want to copy it) and add it to crontab with the following code:

mkdir ~/bin 
mv shtdwn.py ~/bin/shtdwn.py  
crontab -e 

3. At the end of the file, we add the following code:

@reboot sudo python3 ~/bin/shtdwn.py