On IP change with bash

on_change_ip.sh

#! /bin/bash
# path to save your ip (it shall always contain only one line, if not it can cause problems)
FILE_PATH=/tmp/myip.txt
# get your IP from an external server (with a regex to verify that it is an ip)
EXT_IP=$(curl http://ipecho.net/plain | grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}")
# if the file doesn't exist, create it
[ ! -f $FILE_PATH ] && touch $FILE_PATH
# if your ip changed, do something
if [ -n "$EXT_IP" ] && [ "$EXT_IP" != "$(cat $FILE_PATH)" ]; then
    echo $EXT_IP > $FILE_PATH
    echo "action when IP changed"
    echo "your new IP is $EXT_IP"
fi

The action can be to send you an email for example. If you have a dynamic IP at your house, it will notify you when you change your IP. This function runs as a cron function.