From time to time you may find that you want to make a new repository from a subpath of an existing repository. Perhaps you're moving some code out into a library or just want to have a common submodule across projects. Thanks to git, it's easy to do this without losing the history of that subpath in the process.

The Good Stuff

Splitting a subpath into a repository is a fairly straightforward process, even if the command is hard to remember. For this example, we split lib/ out of the GitHub gem repository, removing empty commits but retaining the path's history.

git clone git://github.com/defunkt/github-gem.git
# Clone the repository we're going to work with
# Initialized empty Git repository in /Users/tekkub/tmp/github-gem/.git/
# remote: Counting objects: 1301, done.
# remote: Compressing objects: 100% (769/769), done.
# remote: Total 1301 (delta 724), reused 910 (delta 522)
# Receiving objects: 100% (1301/1301), 164.39 KiB | 274 KiB/s, done.
# Resolving deltas: 100% (724/724), done.

cd github-gem/
# Change directory into the repository

git filter-branch --prune-empty --subdirectory-filter lib master
# Filter the master branch to the lib path and remove empty commits
# Rewrite 48dc599c80e20527ed902928085e7861e6b3cbe6 (89/89)
# Ref 'refs/heads/master' was rewritten

Now we have a re-written master branch that contains the files that were in lib/. We can add a remote to the new repository and push, or do whatever we want with the repository.