Customize existing keyboard layout

I need to invert the behaviour of 2 key in my keyboard ("<" is inverted with "\").

I can do this easily with "Karabiner", but for me is a solution without an explanation.

I'm linux user and usually a do this directly from "terminal".

So, if Karabiner can do the trick, I suppose that I can do it too!

How can I modify a layout "programmatically" from terminal? Where layout configuration are stored?