Single file merge in Subversion

Hmm, not sure I agree with Nick re tag being similar to a branch. A tag is just a marker

• Trunk would be the main body of development, originating from the start of the project until the present.

• Branch will be a copy of code derived from a certain point in the trunk that is used for applying major changes to the code while preserving the integrity of the code in the trunk. If the major changes work according to plan, they are usually merged back into the trunk.

• Tag will be a point in time on the trunk or a branch that you wish to preserve. The two main reasons for preservation would be that either this is a major release of the software, whether alpha, beta, RC or RTM, or this is the most stable point of the software before major revisions on the trunk were applied.

In open source projects, major branches that are not accepted into the trunk by the project stakeholders can become the bases for forks -- e.g., totally separate projects that share a common origin with other source code.

The branch and tag subtrees are distinguished from the trunk in the following ways:

Subversion allows sysadmins to create hook scripts which are triggered for execution when certain events occur; for instance, committing a change to the repository. It is very common for a typical Subversion repository implementation to treat any path containing "/tag/" to be write-protected after creation; the net result is that tags, once created, are immutable (at least to "ordinary" users). This is done via the hook scripts, which enforce the immutability by preventing further changes if tag is a parent node of the changed object.

Subversion also has added features, since version 1.5, relating to "branch merge tracking" so that changes committed to a branch can be merged back into the trunk with support for incremental, "smart" merging.

The simple answer is that you svn export the file instead of checking it out.

But that might not be what you want. You might want to work on the file and check it back in, without having to download GB of junk you don't need.

If you have Subversion 1.5+, then do a sparse checkout:

svn checkout <url_of_big_dir> <target> --depth empty
cd <target>
svn up <file_you_want>

For an older version of SVN, you might benefit from the following:

• Checkout the directory using a revision back in the distant past, when it was less full of junk you don't need.
• Update the file you want, to create a mixed revision. This works even if the file didn't exist in the revision you checked out.
• Profit!

An alternative (for instance if the directory has too much junk right from the revision in which it was created) is to do a URL->URL copy of the file you want into a new place in the repository (effectively this is a working branch of the file). Check out that directory and do your modifications.

I'm not sure whether you can then merge your modified copy back entirely in the repository without a working copy of the target - I've never needed to. If so then do that.

If not then unfortunately you may have to find someone else who does have the whole directory checked out and get them to do it. Or maybe by the time you've made your modifications, the rest of it will have finished downloading...