dig +short myip.opendns.com @resolver1.opendns.com
# or
dig TXT +short o-o.myaddr.l.google.com @ns1.google.com

Store it in a shell variable

myip="$(dig +short myip.opendns.com @resolver1.opendns.com)"
echo "My WAN/Public IP address: ${myip}"

http://www.cyberciti.biz/faq/how-to-find-my-public-ip-address-from-command-line-on-a-linux/