I frequently have my machines connected via multiple interfaces. I put together a little simple C program that can be run as a script.

https://github.com/colindean/getip

I've not actually used it as a script in a while, and it appears to be broken in that regard at the moment (something about ld not being able to write to the tmp directory). However, its Makefile is still operational. Maybe someone will see this and submit a PR that fixes it?