Splitting a repository

This page describes instructions on how to split a git repository while preserving the relevant history, tags, and branches. For example, it allows you to take a repository like:

XXX/
  AAA/
  BBB/
  CCC/

And create a new repository that looks like:

YYY/
  AAA/
  CCC/

To create the repository YYY:

1. Clone your repository.

   $ git clone XXX YYY
   $ cd YYY

2. Preserve the branches you want and remove the origin to avoid having references back to the old repository.

   $ for i in BRANCH1 BRANCH2 BRANCH3; do git branch -t $i origin/$i; done
   $ git remote rm origin

3. Remove any unneeded tags. To remove all tags, use following command:

   $ git tag -l | xargs git tag -d

4. Filter your repository using filter-branch. Use --tag-name-filter cat to rewrite your tags and --prune-empty to remove empty commits.
   If you only want to keep a single subdirectory (e.g. keep BBB), you can use:

   $ git filter-branch --tag-name-filter cat --prune-empty --subdirectory-filter BBB -- --all

   Alternatively, if you want to keep multiple directories (e.g. keep AAA and CCC), use:

   $ git filter-branch --tag-name-filter cat --prune-empty --index-filter 'git rm -r --cached --ignore-unmatch BBB' -- --all

5. Run the following cleanup commands.

   $ git reset --hard
   $ git for-each-ref --format="%(refname)" refs/original/ | xargs -n 1 git update-ref -d
   $ git reflog expire --expire=now --all
   $ git gc --aggressive --prune=now