If you use multiple git repositories, it’s only a matter of time until you’ll want to refactor some files from one project to another.  Today at Pulse we reached the point where it was time to split up a very large repository that was starting to be used for too many different sub-projects.

After reading some suggested approaches, I spent more time than I would have liked fighting with Git to actually make it happen. In the hopes of helping someone else avoid the same trouble, here’s the solution that ended up working best. The solution is primarily based on ebneter’s excellent question on Stack Overflow.

Another solution is Linus Torvald’s “The coolest merge, EVER!“ Unfortunately, his approach seems to require more manual fiddling than I would like and results in a repository with two roots. I don’t completely understand the implications of this, so I opted for something more like a standard merge.

Goal:

• Move directory 1 from Git repository A to Git repository B.

Constraints:

• Git repository A contains other directories that we don’t want to move.
• We’d like to perserve the Git commit history for the directory we are moving.

Get files ready for the move:

Make a copy of repository A so you can mess with it without worrying about mistakes too much.  It’s also a good idea to delete the link to the original repository to avoid accidentally making any remote changes (line 3).  Line 4 is the critical step here.  It goes through your history and files, removing anything that is not in directory 1.  The result is the contents of directory 1 spewed out into to the base of repository A.  You probably want to import these files into repository B within a directory, so move them into one now (lines 5/6). Commit your changes and we’re ready to merge these files into the new repository.

git clone <git repository A url>
cd <git repository A directory>
git remote rm origin
git filter-branch --subdirectory-filter <directory 1> -- --all
mkdir <directory 1>
mv * <directory 1>
git add .
git commit

Merge files into new repository:

Make a copy of repository B if you don’t have one already.  On line 3, you’ll create a remote connection to repository A as a branch in repository B.  Then simply pull from this branch (containing only the directory you want to move) into repository B.  The pull copies both files and history.  Note: You can use a merge instead of a pull, but pull worked better for me. Finally, you probably want to clean up a bit by removing the remote connection to repository A. Commit and you’re all set.

git clone <git repository B url>
cd <git repository B directory>
git remote add repo-A-branch <git repository A directory>
git pull repo-A-branch master
git remote rm repo-A-branch

Update: Removed final commit thanks to Von’s comment.