This script can be used to update a Namecheap dynamic DNS A record from a cron job. This is useful when the host does not have a static IP e.g. when sitting behind a DSL modem and being assigned an IP by the ISP via DHCP.

ddns.sh

#!/bin/bash

# This script makes it possible to update a Namecheap dynamic DNS 
# A record automatically using a cron job

# configure these
dnshost="host"
dnsdomain="example.com"
dnspw="longasspasswd"

# DNS A record is updated by issuing a GET request to this URL
CURLARGS="https://dynamicdns.park-your-domain.com/update?host=$dnshost&domain=$dnsdomain&password=$dnspw"

# if this script was run before there should be a log file 
# containing last run's IP
if [ -f $(pwd)/ip.log ]; then
	oldip=$(cat $(pwd)/ip.log)
else
	oldip="0.0.0.0"
fi

# get the current IP
newip=$(curl -4 https://icanhazip.com/s)

# if the IP has changed since the last run then update the Namecheap A record 
if [ $oldip != $newip ]; then
	oldip=$newip
	echo $oldip > $(pwd)/ip.log
	curl $CURLARGS > $(pwd)/curl.log
fi