Forking a sub directory of a repository on GitHub

Question

Apologies, I am very new to Git and GitHub, I've read through a few things but I'm not sure if what I'm trying to do is entirely possible.

Basically I want to fork the Confluence Skin used on XBMC and modify various elements located here:

https://github.com/xbmc/xbmc/tree/master/addons/skin.confluence

However I don't want to fork the entire XBMC repository, so a simple fork action won't do.

Here are my general requirements:

• I would like to take the contents of the skin.confluence folder and put it into a repository on my own GitHub account

• I need to be able to keep it linked within the original XBMC repo to receive upstream commits, as my modifications will generally be theme based, rather than functionality.

Thanks to the response posted by tbekolay, I have been able to do a subtree split to take only the skin.confluence part of repo and essentially create a branch, however I am unsure on how to keep it linked with the original XBMC repo while being pushed to my own repo with my modifications.

Answer 1

This is mostly possible with git, though it is not a typical use case, so there may be some rough edges as you do this (especially if you are new to git).

The tool that we'll use for this job is git subtree.

Setting up a repository

Start by cloning the whole XBMC repository.

git clone https://github.com/xbmc/xbmc.git
cd xbmc

We start on the master branch by default. We want to make our own master branch, so let's rename master to upstream-master.

git branch -m upstream-master

Now use git subtree split to only include the part that you want. We'll make the split off part a new branch called upstream-skin.

git subtree split --prefix=addons/skin.confluence -b upstream-skin
git checkout upstream-skin

This gives you a new upstream-skin branch that only contains the contents of addons/skin.confluence, and with a filtered history that contains only the commits that modified files in addons/skin.confluence.

Now, let's set up our remotes. Since you cloned xbmc/xbmc.git, the origin remote will point there. Let's rename that to upstream.

git remote rename origin upstream

Make a repository on Github to contain your modifications to addons/skin.confluence. As an example, I'll use tbekolay/xbmc-skin, but replace this with your own repo. Add this repo as a remote, and push your upstream-skin branch to it.

git remote add origin https://github.com/tbekolay/xbmc-skin.git
git fetch origin
git push -u origin upstream-skin

Finally, we'll make a new branch called master that will contain your changes.

git checkout -b master
git push -u origin master

You now have a "fork" of the addons/skin.confluence subdirectory.

Making changes to your repositories

When you're dealing with your own local and remote repositories, you can use normal git commands. Make sure to do this on the master branch (or some other branch, if you'd like) and not the upstream-skin branch, which should only ever contain commits from the upstream project.

git checkout master
echo "My XBMC Skin" > README
git add README
git commit -m "Added README"
git push

Receiving upstream commits

When you're dealing with the upstream repository, you will have to use a mix of git and git subtree commands. To get new filtered commits, we need to do it in three stages.

In the first stage, we'll update upstream-master to the current version of the XBMC repository.

git checkout upstream-master
git pull

This should pull down new commits, if there are any.

Next, we will update upstream-skin with the new filtered version of the commits. Since git subtree ensures that commit hashes will be the same, this should be a clean process. Note that you want to run these commands while still on the upstream-master branch.

git subtree split --prefix=addons/skin.confluence \
  --onto upstream-skin -b upstream-skin

With upstream-skin now updated, you can update your master branch as you see fit (either by merging or rebasing).

git checkout master
git rebase upstream-skin

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Note that the XBMC repository is gigantic, and the git subtree commands will take quite a bit of time to filter through all that history -- and since you're regenerating the split subtree each time you interact with the remote repository, it's quite an expensive operation. I'm not sure if this can be sped up.

This blog post goes into some more detail on the commands above. Also see the git-subtree docs for even more detail.