Saturday, August 15, 2015

Keeping a GitHub fork up-to-date with the original repo

Forking is a standard way to make changes to a third party repository on GitHub. Typically a developer makes a fork (essentially a clone on the GitHub server side) of someone else's repo in order to make their own changes to the project. These changes might be for private use, or they could subsequently be submitted back to the original repo for evaluation and possible inclusion via the pull request mechanism.

However: once the fork is created, any further commits or other changes made to the original repo will not automatically be reflected in the fork. This post describes how to keep the fork up-to-date with the original repository (which is normally referred to as the upstream repo) by pulling in the changes manually via a clone of the fork, using the process described below.

The update procedure is:

0. Make a clone of your fork onto your local machine (or use one you've already made), and move into the cloned directory.

1. Add a new remote repository to the clone which points to the upstream repo. The general syntax to do this is:

git remote add upstream

A remote is simply a version of the project which is hosted on the Internet or elsewhere on the network, and a repo can have multiple remotes (use git remote -v to list all the ones that are defined; you should that origin is also a remote). Conventionally the remote used for this syncing is given the name upstream, however it could be called anything you like.

For example:

git remote add upstream

This step only needs to be done once for any given clone.

2. Update the local clone by fetching and merging in any changes from the upstream repo, using the procedure:

git fetch upstream
git checkout master
git merge upstream/master

If you have made your changes in your own dedicated branches and avoided making commits to the fork's master branch then the upstream changes should merge cleanly without any conflicts. (It's considered best practice that changes made to the fork should always be done in a purpose-made branch; working directly in master makes it harder to make clean pull requests and can cause conflicts when trying to merge the upstream changes into your fork.)

3. Push the updated master branch back to your fork on GitHub using:

git push origin master

Once again, if you have avoided committing local changes to master then the push should be drama-free.

Finally, you will need to repeat steps 2 and 3 in order to stay up to date whenever new changes are made to the upstream repo.

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