TIL: Find shared lines with `comm`

diff is a well-known tool to see what lines are different between files; comm is a less-known tool to see the common lines. I found comm when I was looking for a way to identify lines that are shared (or not) between files. Specifically, I had two lists: one with all the APIs in my codebase, and another with all the tested APIs. From this I wanted to get a list of untested APIs. As an aside, my…