Getting your external IP address from the command line in both windows and linux is very easy with the OpenDNS resolver and a couple of easy commands found here.
In short, use dig on any unix variant (Linux, BSD, MacOS) as it is usually installed by default
dig +short myip.opendns.com @resolver1.opendns.com
and on Windows, if you don't have dig installed you can use nslookup
nslookup myip.opendns.com. resolver1.opendns.com
