Diffing and merging in Emacs

Diffing files you've got open in your Emacs

M-x ediff-buffers

Diffing revisions of a file

C-x v = (vc-diff)

You can also view the changes with ediff using M-x ediff-revision

Inside an ediff session

Once inside an ediff session, put focus in the ediff navigator and use e.g.:

Resolving merge conflicts in files with merge conflict markers

Out of the box Emacs gives us an easy interface to working with merge conflicts, called smerge (as in "simple merge").

When there's a merge conflict, the version control system writes the diff markers into the source files themselves and Emacs will highlight these automatically. It will recognise which parts are "theirs" and which are "mine" — as well as "ancestor" if you've set up 3 way merge.

resolving 3 way diffs in Emacs

To select the version at the bottom, I just put my pointer on it, and run M-x smerge-keep-current.

Telling Git to give us 3 way diff instead of two way

By default git sets up 2 way diffs when there's a merge conflict ("my" version of the file and "their" version). However, when resolving merge conflicts, it's often useful to in addition see the most recent common ancestor of the two versions of the file you're merging.

This is so useful, you may just want to set it globally for all your projects by adding the following to your ~/.gitconfig:

    conflictstyle = diff3


Tested on version Emacs version 25.1

P4 specific setup

in .p4enviro:


in .emacs:


in /home/torstein/bin/emerge (Gentoo users, note the full path ;-)


emacsclient -e "(ediff-merge-files-with-ancestor \"${yours}\" \"${theirs}\" \"${ancestor}\" () \"${merge_result}\")"
emacsclient "${merge_result}"

The full source of p4-emacs-merge can be found here: https://github.com/skybert/my-little-friends/blob/master/bash/vcs/p4-emacs-merge

P4 specifc post-merge notes

After you're finished with the ediff session and you're back at the p4/shell prompt, choose ae (accept edit) Here's one merge conflict, with the two version presented (p4 "yours" is A and p4 "theirs" is B):

ediff before

Now, I want "their" version, so I hit b. Emacs then replaces the merge conflict markers/block with the contents of the B hunk:

ediff after

Licensed under CC BY Creative Commons License ~ ✉ torstein.k.johansen @ gmail ~ 🐘 @skybert@emacs.ch ~ 🐦 @torsteinkrause