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Version Control with Subversion, 2nd Edition by Brian W. Fitzpatrick, Ben Collins-Sussman, C. Michael Pilato

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Traversing Branches

The svn switch command transforms an existing working copy to reflect a different branch. While this command isn’t strictly necessary for working with branches, it provides a nice shortcut. In our earlier example, after creating your private branch, you checked out a fresh working copy of the new repository directory. Instead, you can simply ask Subversion to change your working copy of /calc/trunk to mirror the new branch location:

$ cd calc

$ svn info | grep URL
URL: http://svn.example.com/repos/calc/trunk

$ svn switch http://svn.example.com/repos/calc/branches/my-calc-branch
U   integer.c
U   button.c
U   Makefile
Updated to revision 341.

$ svn info | grep URL
URL: http://svn.example.com/repos/calc/branches/my-calc-branch

Switching a working copy that has no local modifications to a different branch results in the working copy looking just as it would if you’d done a fresh checkout of the directory. It’s usually more efficient to use this command, because often branches differ by only a small degree. The server sends only the minimal set of changes necessary to make your working copy reflect the branch directory.

The svn switch command also takes a --revision (-r) option, so you need not always move your working copy to the HEAD of the branch.

Of course, most projects are more complicated than our calc example and contain multiple subdirectories. Subversion users often follow a specific algorithm when using branches:

  1. Copy the project’s entire trunk to a new branch directory

  2. Switch only part of the trunk working copy to mirror the branch

In other words, if a user knows that the branch work needs to happen on only a specific subdirectory, she uses svn switch to move only that subdirectory to the branch. (Or sometimes users will switch just a single working file to the branch!) That way, the user can continue to receive normal trunk updates to most of her working copy, but the switched portions will remain immune (unless someone commits a change to her branch). This feature adds a whole new dimension to the concept of a mixed working copy—not only can working copies contain a mixture of working revisions, but they can also contain a mixture of repository locations.

If your working copy contains a number of switched subtrees from different repository locations, it continues to function as normal. When you update, you’ll receive patches to each subtree as appropriate. When you commit, your local changes will still be applied as a single, atomic change to the repository.

Note that while it’s okay for your working copy to reflect a mixture of repository locations, these locations must all be within the same repository. Subversion repositories aren’t yet able to communicate with one another; that feature is planned for the future.[24]

Because svn switch is essentially a variant of svn update, it shares the same behaviors; any local modifications in your working copy are preserved when new data arrives from the repository.

Tip

Have you ever found yourself making some complex edits (in your /trunk working copy) when you suddenly realized, Hey, these changes ought to be in their own branch? A great technique to do this can be summarized in two steps:

$ svn copy http://svn.example.com/repos/calc/trunk \
           http://svn.example.com/repos/calc/branches/newbranch \
      -m "Create branch 'newbranch'."
Committed revision 353.
$ svn switch http://svn.example.com/repos/calc/branches/newbranch
At revision 353.

The svn switch command, like svn update, preserves your local edits. At this point, your working copy is now a reflection of the newly created branch, and your next svn commit invocation will send your changes there.



[24] You can, however, use svn switch with the --relocate option if the URL of your server changes and you don’t want to abandon an existing working copy. See “svn switch” in Chapter 9 for more information and an example.

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