Learning GNU Emacs, 3rd Edition

Comparing with Ediff

In working with any version control system, you sometimes want to compare different revisions of a file. Often you're interested in what's changed in the current working revision, but sometimes you're after more historical information. The most challenging situation arises when an optimistic strategy like CVS is proved wrong, and you need to merge incompatible changes made by multiple developers to the same section of a file.

We've already described vc-diff, VC's built-in facility for helping with these tasks. We would be remiss, however, if we did not introduce you to Ediff, an even more powerful facility that is available in current releases of GNU Emacs. Ediff is extremely rich; it almost feels like another program that "takes over" your Emacs session for a while. Full coverage would require an entire chapter, or perhaps even its own handbook, but this introduction will get you started and point you at the built-in manual if you want to delve deeper.

Starting Ediff

For the most part, you launch Ediff as an independent entity rather than having it invoked automatically by the version control interface. The exception (as mentioned above) is if you ask Emacs for help resolving conflicts when they occur during a check-in operation or manually invoke vc-resolve-conflicts while visiting a buffer containing such conflicts.

If you want to use Ediff to compare two nonconflicting revisions of a file, choose Tools → Compare (Ediff) → File with revision, or type M-x ...