This script check your public IP on checkip.dyndns.org and logs its changes

public-ip-tracker.sh

#/bin/bash

LOGFILE_NAME=whatismyip.log

GATEWAY_IP=192.168.0.1
IP_PAGE_URL=checkip.dyndns.org
LAST_IP=`curl -s ${IP_PAGE_URL} | sed -e 's/.*Current IP Address: //' -e 's/<.*$//'`
echo "Initial IP: ${LAST_IP}"

while true
do
    sleep 2
    CURR_TIME=`date '+%Y-%m-%d.%H.%M.%S'`
    echo "."
    CURR_IP_HTML=`curl -s ${IP_PAGE_URL}`
    if [ $? -ne 0 ]; then
        echo "[${CURR_TIME}] Couldn't connect to ${IP_PAGE_URL}" >> ${LOGFILE_NAME}
        GT_PING_STDOUT=`ping -c 1 ${GATEWAY_IP} 2>/dev/null`
        if [ $? -ne 0 ]; then
            echo "[${CURR_TIME}] Couldn't reach LAN Gateway (${GATEWAY_IP})" >> ${LOGFILE_NAME}
            continue
        fi
    fi
    CURR_IP=`echo ${CURR_IP_HTML} | sed -e 's/.*Current IP Address: //' -e 's/<.*$//'`
    if [ ${CURR_IP} != ${LAST_IP} ]; then
        echo "[${CURR_TIME}] IP changed, ${LAST_IP} => ${CURR_IP}" >> ${LOGFILE_NAME}
    fi
    LAST_IP=${CURR_IP}
done