Once again, svn copy comes to the rescue. If you want to create a snapshot of /calc/trunk exactly as it looks in the HEAD revision, make a copy of it:

$ svn copy http://svn.example.com/repos/calc/trunk 
           http://svn.example.com/repos/calc/tags/release-1.0 
      -m "Tagging the 1.0 release of the 'calc' project."

Committed revision 902.

This example assumes that a /calc/tags directory already exists. (If it doesn’t, you can create it using svn mkdir.) After the copy completes, the new release-1.0 directory is forever a snapshot of how the /trunk directory looked in the HEAD revision at the time you made the copy. Of course, you might want to be more precise about exactly which revision you copy, in case somebody else may have committed changes to the project when you weren’t looking. So if you know that revision 901 of /calc/trunk is exactly the snapshot you want, you can specify it by passing -r 901 to the svn copy command.

But wait a moment: isn’t this tag creation procedure the same procedure we used to create a branch? Yes, in fact, it is. In Subversion, there’s no difference between a tag and a branch. Both are just ordinary directories that are created by copying. Just as with branches, the only reason a copied directory is a “tag” is because humans have decided to treat it that way: as long as nobody ever commits to the directory, it forever remains a snapshot. If people start committing to it, it becomes a branch.

If you are administering a repository, there are two approaches you can take to managing tags. The first approach is “hands off”: as a matter of project policy, decide where your tags will live, and make sure all users know how to treat the directories they copy. (That is, make sure they know not to commit to them.) The second approach is more paranoid: you can use one of the access control scripts provided with Subversion to prevent anyone from doing anything but creating new copies in the tags area (see Chapter 6). The paranoid approach, however, isn’t usually necessary. If a user accidentally commits a change to a tag directory, you can simply undo the change as discussed in the previous section. This is version control, after all!