I know there are lots of ways to do this. This is not a HOW-TO, but just sharing a script I made for doing this. It's a decent example of how to write a command-line utility in Python.
Since I write my cover letters and resumes in LaTeX I always need to concatenate the two together before sending it to an employer. This is very important so that the person on the other end prints out both and it makes their life easier by not having to worry about two files. For the longest time I just had a simple script that looked like this: