I recently had to give a coding demonstration. I planned to walk through a tutorial in front of the students. I practiced ahead of time, storing my code in a Git repository along the way. During the presentation, I coded from scratch so they could follow along, and I stored the results in completely new Git repository.

The resulting code ended up being a bit different than my trial run. After class, each repository had some code I wanted to appear in the final version. However, the Git repositories did not share a common commit/ancestor (since I created the second one from scratch). As far as I know, Git does not support merging files that are not part of the same repository.

So I looked for a Unix utility to do the merge for me. I ran across sdiff which looked like it had potential:

With sdiff, you can merge two files interactively based on a side-by-side -y format comparison see Side by Side. Use –output=file (-o file) to specify where to put the merged text. See Invoking sdiff, for more details on the options to sdiff.

So I ran the command

sdiff --output version3.js version1.js version2.js

but couldn't quite make sense of the interface after that. I had the urge to give up because I had seen a more interesting path a few pages earlier. The sdiff page had also mentioned,

Another way to merge files interactively is to use the Emacs Lisp package emerge.

Well, you don't need to tell me twice when it comes to Emacs-based solutions. I head to the emerge documentation and start reading.

_I read the docs for 30 minutes. Sometimes I have to fight away the thought that I'm taking longer than is needed. Is there not a simpler, brute force solution? Isn't there an xkcd comic about this? I also grab a snack since I'm a little hungry._

Ok, after reading "Overview of Emerge" and "Submodes of Emerge", I decide to run the Emacs command

M-x emerge-files

(I could have used M-x emerge-buffers too, I suppose.)

I'm prompted for two files I'd like to merge. I am shown the files side-by-side, with a merge buffer underneath. Initially, the merge buffer appears the same as the first file and says "diff 0 of 5" in the status bar.

I read the "Merge Commands" section and learn about the key commands available. I press:

  • n to go to the next difference (or, first, in this case)
  • a to choose the left version
  • b to chose the right version
  • p to go to the previous diff, and
  • q to quit, when I'm satisfied with my changes.

So there you go. Emacs' emerge is pretty cool. I didn't have to deal with manually merging individual diffs, but I assume I could just switch to the merge buffer and edit it like I would a normal buffer. I can't say I'm blown away, but I'll learn some more the next time I need to merge files without the help of git (and magit).