#!/bin/bash

IP=$(curl -s http://www.ipchicken.com/ | awk '/[0-9]*\.[0-9]*\.[0-9]*\.[0-9]*/ {print $1}' | uniq)
FILE="/dev/shm/ip.dat"

trigger_notification () {
  echo $IP > $FILE
  # see http://www.nixtutor.com/linux/send-mail-with-gmail-and-ssmtp/
  /sbin/ifconfig | mail -s "Notification: IP address $IP" my-email@addr.com
}

if [ ! -f "$FILE" ]; then
  # data file does not exist, create one and notify the user
  trigger_notification
else
  if [ $IP != "$(cat $FILE)" ]
  then
    # file already exists but ip doesn't match
    trigger_notification
  fi
fi