Mine

#!/bin/bash

while [ true ]
do
    ip1=`ifconfig | grep -C 1 ppp0 | grep inet | cut -d ":" -f 2 | cut -d " " -f 1`
    sleep 10
    ip2=`ifconfig | grep -C 1 ppp0 | grep inet | cut -d ":" -f 2 | cut -d " " -f 1`
    
    if [ $ip1 != $ip2 ]
    then
	ddclient
    fi
done