Subversion tracks tree structures, not just file contents. It's one of the biggest reasons Subversion was written to replace CVS.

Here's what this means to you, as a former CVS user:

  • The svn add and svn delete commands work on directories now, just as they work on files. So do svn copy and svn move. However, these commands do not cause any kind of immediate change in the repository. Instead, the working items are simply scheduled for addition or deletion. No repository changes happen until you run svn commit.

  • Directories aren't dumb containers anymore; they have revision numbers like files. (Or more properly, it's correct to talk about directory foo/ in revision 5.)

Let's talk more about that last point. Directory versioning is a hard problem; because we want to allow mixed-revision working copies, there are some limitations on how far we can abuse this model.

From a theoretical point of view, we define revision 5 of directory foo to mean a specific collection of directory entries and properties. Now suppose we start adding and removing files from foo, and then commit. It would be a lie to say that we still have revision 5 of foo. However, if we bumped foo's revision number after the commit, that would be a lie too; there may be other changes to foo we haven't yet received, because we haven't updated yet.

Subversion deals with this problem by quietly tracking committed adds and deletes in the .svn area. When you eventually run svn update, all accounts are settled with the repository, and the directory's new revision number is set correctly. Therefore, only after an update is it truly safe to say that you have a perfect revision of a directory. Most of the time, your working copy will contain imperfect directory revisions.

Similarly, a problem arises if you attempt to commit property changes on a directory. Normally, the commit would bump the working directory's local revision number. But again, that would be a lie, as there may be adds or deletes that the directory doesn't yet have, because no update has happened. Therefore, you are not allowed to commit property changes on a directory unless the directory is up to date.

For more discussion about the limitations of directory versioning, see the section called “Mixed-revision working copies”.