What *nix based command can I use to find my external IP?

curl http://myip.dnsomatic.com

dig +short myip.opendns.com

This only works if you are using OpenDNS as your dns server.

If you aren't, one of these should work:

dig +short myip.opendns.com @208.67.222.222
dig +short myip.opendns.com @208.67.220.220
dig +short myip.opendns.com @208.67.222.222 @208.67.220.220

STUN is the proper solution.

% stun -v stun.ekiga.net
...
MappedAddress = 88.189.152.187:18009

• a list of public STUN servers
• a free software STUN client
• The STUN standard

You can use curl to get the page from something like whatismyip and then get the pieces out. I used whatismyipaddress.com in this example...obviously the fields will differ with different services.

curl -s http://whatismyipaddress.com/ | grep LOOKUPADDRESS | awk '{ print $4 }'