: Learning GNU Emacs, 3rd Edition

Chapter12.Version Control

12.1 The Uses of Version Control

If you write either large programs or long documents, you have probably been caught at least once in a situation where you've made changes that turned out to be a bad thing, only to be confused and stymied because you weren't sure exactly how to reverse them and get back to a known good state. Or, perhaps you've released a program or document to someone else, then gotten a bug fix or a comment that you couldn't integrate properly because you couldn't recover the old version that person was working with. Perhaps you're a member of a development or documentation team and have felt the need for some way to keep change histories, indicating who was responsible for each change.

These common kinds of problems can be addressed with a version control system. A version control system gives you automated help at keeping a change history for a file or group of files. It allows you to recover any stage in that history, and it makes getting reports on the differences between versions easy.

Today a variety of version control systems are widely available on machines that run Emacs. Some are commercial, but there are a wealth of free, open, and powerful choices, and it seems appropriate for our discussion to focus on these. Historically, Emacs evolved largely in a Unix environment alongside the SCCS and RCS systems, and its built-in support for version control reflects their approach and terminology. Today the most popular by far is CVS (which builds on RCS, giving it more flexibility and power), and there is a new system called Subversion that is starting to catch on. Preliminary support for working with Subversion shipped with Emacs 21.3.5; its documentation suggests you check the Subversion site, http://subversion.tigris.org/, for updates.

Given that when you need version control, you generally need it very badly (and you have enough other challenges to occupy your mind), it's not surprising that most integrated development environments today offer automated support for these tools. And if any other IDE does it, by now you can certainly predict that Emacs does too!

In this chapter, we'll introduce you to the Emacs facility called VC, an Emacs Lisp minor mode that makes using version control systems very easy. VC runs all version control commands for you (using Emacs' subprocess facilities in the same way that compiler modes do). VC hides almost all the details of their interfaces from you; instead, you can trigger most basic version control operations with a single command, with Emacs correctly deducing what needs to be done next.

As noted above, the VC architecture was designed with the behavior of RCS in mind. So as we explain VC, we'll explain the RCS terminology and behavior as Emacs presents it. Where needed, we'll point out key differences in the way CVS behaves. Subversion, in turn, is being designed as a more modern version of CVS, and acts like CVS with respect to its interactions with Emacs.

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