Simple Python Script to shutdown your Raspberry Pi.

Connect a button to GPIO-21 and press it and the Pi shuts down cleanly…

#!/usr/bin/python
#Simple script for shutting down the Raspberry Pi at the press of a button.
#by Inderpreet Singh

import RPi.GPIO as GPIO
import time
import os

# Use the Broadcom SOC Pin numbers
# Setup the pin with internal pullups enabled and pin in reading mode.

GPIO.setmode(GPIO.BCM)
GPIO.setup(21, GPIO.IN, pull_up_down=GPIO.PUD_UP)

# Our function on what to do when the button is pressed

def Shutdown(channel):
    print("Shutting Down")
    time.sleep(5)
    os.system("sudo shutdown -h now")

# Add our function to execute when the button pressed event happens

GPIO.add_event_detect(21, GPIO.FALLING, callback=Shutdown, bouncetime=2000)

# Now wait!
while 1:
    time.sleep(1)