From https://github.com/rsp/scripts
Usage: internalip [TARGET]
Source: https://rawgit.com/rsp/scripts/master/internalip
Get internal IP address used for outgoing Internet connections. This is an IP address of your computer's network interface that would be used to connect to a given target.
The TARGET is optional and by default it is 8.8.8.8
(which is the Google's public DNS resolver).
It can be an IP address or a host name (if gethostip or getent are available on the system).
Note: this is not necessarily the same IP as the public IP that you visible under while connecting to the Internet (for that use externalip).
Unlike most of the solutions to get your IP from the output of programs like ifconfig or hostname that are usually recommended, often with a need to manually choose a network interface, this method has an advantage that the result is based on your routing tables and you get the IP address that is actually used to connect to a given target (the public Internet by default).
The idea was taken from this Stack Overflow answer by Collin Anderson.