title: Find lines common to several files
body: <p>I'm trying to determine which header declares a specific function. I've used <code>grep</code> to find instances of the function's use; now, I want to find which header is included by all the files. I'm aware of the <a href="https://stackoverflow.com/questions/373810/unix-command-to-find-lines-common-in-two-files"><code>comm</code> utility</a>; however, it can only compare two <em>sorted</em> files. Is there a Unix utility that can find the common lines between an arbitrary number of <em>unsorted</em> files, or must I write my own?</p>