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How to get the primary IP address of the local machine on Linux and OS X?

I am looking for a command line answer that could go back me the number one (first) IP cope with of the localhost, other than 127.0.0.1

The solution should work at least for Linux (Debian and RedHat) and OS X 10.7+

I am conscious that ifconfig is to be had on both however its output isn't so consistent among these platforms.