How do I merge a sub directory in Git?

Question

Is it possible to merge only the changes for a sub-directory from a local Git branch to a remote Git branch or is it "all or nothing"?

For example, I have:<

Answer 1

Just as an alternative to the SO question "How do you merge selective files with git-merge?", I just found this GitHub thread which could be more adapted for merging a whole subdirectory, based on git read-tree:

• My repository => cookbooks
  My repository target directory => cookbooks/cassandra

• Remote repository => infochimps
  Remote repository source I want merged into cookbooks/cassandra => infochimps/cookbooks/cassandra

Here are the commands I used to merge them

• Add the repository and fetch it

git remote add -f infochimps git://github.com/infochimps/cluster_chef.git

• Perform the merge

git merge --allow-unrelated-histories -s ours --no-commit infochimps/master

(this performs a merge by using the 'ours' strategy (-s ours), which discards changes from the source branch. This records the fact that infochimps/master has been merged, without actually modifying any file in the target branch)

• Merge only infochimps/cookbooks/cassandra into cassandra

git read-tree --prefix=cassandra/ -u infochimps/master:cookbooks/cassandra

This reads the tree for only the required source subdirectory i.e. cookbooks/cassandra, on the upstream branch of the source repository.

Note that the target subdirectory name should also be cookbooks/cassandra, or you would see:

fatal: Not a valid object name

• Commit the change

 git commit -m 'merging in infochimps cassandra'

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Addendum

It's bizarre,^{[edit me]} — but the read-tree step can possibly fail like this:

error: Entry 'infochimps/cookbooks/cassandra/README' overlaps with 'cookbooks/cassandra/README'. Cannot bind.

... even when both files are identical. This might help:

git rm -r cassandra
git read-tree --prefix=cassandra/ -u infochimps/master:cookbooks/cassandra

But off course, verify manually that this does what you want.