# Git Branch Rename

So, you want to rename a branch on Git/GitHub. The most common case of this is to rename the default "master" branch to something else, such as "main". We are going to use this as our example, but you may have a different use case in mind. If that's the case, just replace "master" and "main" to the source and target branches you have in mind.

##
Table of Contents

##
New Project

If you are starting a brand new project, you can follow these steps:

1. Create a new repository on GitHub to host your project with the following settings:

   - Repository template: No template
   - Initialize this repository with a README: Leave unchecked
   - Add .gitignore: None
   - Add a license: None

   If you wish to use any of these features (i.e. with settings different than the above), that is not a problem. However, GitHub will create the default "master" branch for you. In that case, clone the repository locally and follow the steps in the Simple Migration section instead.

2. Initialize an empty Git repository:

       $ mkdir my-git-project
       $ cd my-git-project
       $ git init .

3. Create the "main" branch:

4. Work on your project and commit some changes. For example, add a README:

       $ echo "# My Git Project" >> README.md
       $ git add README.md
       $ git commit -m "Initial commit"

5. Push your commits to GitHub:

       $ git remote add origin https://github.com/your-username/my-git-project.git
       $ git push -u origin main

That's all! Since "main" was the first branch you pushed to GitHub, it will be set correctly as the default branch. We also never committed any changes to the "master" branch locally, it effectively never existed so there is nothing left to clean up. Enjoy your new project!

##
Simple Migration

If you have a small personal project and you are comfortable with just doing the rename right away, here are the steps you can take.

Note that if this is a GitHub Pages repository and the content is stored on the "master" branch, then you cannot perform the migration at this time since GitHub Pages does not seem to support alternative default branch names yet. If you want, you may follow the steps in the Local Migration or Gradual Migration section partially to get your project ready.

1. Pull the latest commits from the "master" branch into your local repository:

       $ cd my-git-project
       $ git checkout master
       $ git pull origin master

2. Rename the local "master" branch to "main":

3. Push the "main" branch to GitHub:

       $ git push -u origin main

4. Change the default branch on GitHub to "main".

5. If there are existing pull requests open against the "master" branch that you would like to keep, update their base branch to the "main" branch. Otherwise, they will be closed automatically when we delete the remote "master" branch on GitHub.

6. Delete your local "master" branch:

7. Delete the remote "master" branch:

       $ git push origin :master

At this point, the migration is complete. However, as you work on the project, you may discover additional settings that you need to tweak to account for the rename. For example, you may have update the branch names in your CI config.

##
Gradual Migration

If you have a work or open-source repository with multiple contributors and forks, you may prefer to perform the migration gradually.

These steps are for you if you find yourself in a similar scenario, where it is important to give everyone ample of time to prepare for the migration, changing local configs, adapting workflows, scripts and other automation to ensure a seamless migration.

This gradual migration plan is intended to be spread out over a long period of time – weeks, or months. It uses all available tools on GitHub to provide as much advance notice as possible.

For most organizations, this plan may be overly cautious and some of the steps may not be necessary. On the other hand, for a popular open-source project, it may not be feasible to get to the end and fully deprecate and remove the legacy "master" branch at all, due to compatibility requirements. Treat this plan as a template and adapt it for your own needs.

Unlike the other sections, the steps here are a bit less exact and is intended for someone with a some prior experience with Git and GitHub. In practice, you will probably run into situations that causes deviations from the plan which would require some manual repair and adjustments. For example, pushes during a partial GitHub outage may cause the two branches to diverge and requires you to determine how to best reconcile the differences.

###
Phase 1: Mirror "master" and "main"

The goal of this phase is to make it possible to use the "main" branch name as an alternative. This allows some early adopters to start testing the new setup.

1. As always, make sure your local "master" branch is up-to-date.

2. Create the new "main" branch:

3. Add a GitHub Actions workflow file at .github/workflows/mirror-master-and-main.yml with the following:

       name: Mirror "master" and "main" branches
       on:
         push:
           branches:
             - master
             - main

       jobs:
         mirror:
           runs-on: ubuntu-latest
           steps:
             - name: Mirror to "master"
               uses: zofrex/mirror-branch@v1
               with:
                 target-branch: master
                 force: false
             - name: Mirror to "main"
               uses: zofrex/mirror-branch@v1
               with:
                 target-branch: main
                 force: false

4. Commit and your changes to the local "main" branch.

5. Push your changes to the remote "main" branch:

       $ git push -u origin main

If things are working correctly, you should see the same commit pushed to the "master" branch shortly. You can monitor the progress and unexpected errors in the "Actions" tab in your repository.

####
About GitHub Actions

You do not have to be already using GitHub Actions for this to work. You do not need to activate or enable the feature for your repository, simply pushing the workflow file to the is sufficient. All open-source repositories have unlimited GitHub Actions minutes, and all personal and team accounts comes with 2000-3000 free GitHub Actions minutes for private repositories.

If you already regularly uses up your free minutes, this may slightly increase your GitHub bills, but in practice, the workflow we added runs very quickly so the impact is very minimal. For reference, GitHub Actions are billed at $0.008 USD per minute for private repositories, after the free quota is exhausted.

This workflow will be triggered when commits are pushed to either the "master" or "main" branch. It uses the Mirror Branch action to update the branches using GitHub's API.

####
Interaction with Branch Protection

Unfortunately, if you have enabled branch protection on the "master" branch, the push from the mirroring action may be rejected. For example, if you had enabled "Require pull request reviews before merging", then this would not work as the changes are expected to be submitted via a pull request with reviews.

A possible workaround is to disable the "Include administrators" checkbox in the branch protection settings for the "master" branch and configure the script to push the commits as an administrator:

1. Login as an administrator on GitHub. You may also want to consider creating a new account specifically for this purpose.

2. Generate a personal access token with the "repo" scope (and the "public_repo" scope, if needed).

3. Add it as a secret to the repository.

4. Change the "Checkout" step in the mirror workflow to use the new deploy key:

       - name: Checkout
         uses: actions/checkout@v2
         with:
           token: ${{ secrets.DEPLOY_TOKEN }}

   Here, `DEPLOY_TOKEN` is the name you picked from step 3.

Alternatively, an SSH key for the administrator can be used instead of a personal access token, via the `ssh-key` argument. See the documentation for the checkout action for more details.

####
Interaction with Other GitHub Action Workflows

By default, when pushing commits from within the GitHub Actions job, it does not trigger additional GitHub Actions workflow to run. For example, when a contributor pushes to the "main" branch, it will trigger any GitHub Actions that normally runs on the "main" branch's push events, including the one we added here. However, when our mirror workflow pushes the same commit to the "master" branch, it will not trigger any workflow that normally runs on the "master" branch's push events.

For this reason, you may want to update existing workflows that runs on the "master" branch to also run on the "main" branch, as we did in our mirror workflow file (the `on.push.branches` config key). This is the recommended approach as it ensures only a single build per push.

Alternatively, if it is important to you that workflows are triggered by pushes from the mirror workflow, you can accomplish this by supplying an alternative SSH key to the checkout action:

1. Generate a new SSH key locally. Don't worry about adding it to the ssh-agent.

2. Find the generated public key and add it as a deploy key to the repository. Be sure to select "Allow write access".

3. Find the generated private key and add it as a secret to the repository.

4. Change the "Checkout" step in the mirror workflow to use the new deploy key:

       - name: Checkout
         uses: actions/checkout@v2
         with:
           ssh-key: ${{ secrets.DEPLOY_KEY }}

   Here, `DEPLOY_KEY` is the name you picked from step 3.

With this, the mirror workflow will authenticate with GitHub using the deploy key instead of the default token when pushing commits, triggering any workflows as if a regular user had pushed those commits. This does not change the author or committer on the commits.

####
Real World Example

See this commit and the resulting workflow run for an example of this working in action. Note that the code in the commit may be outdated by the time you read this – refer to the above for the latest instructions.

####
Next Steps

After verifying that everything is working as intended, you can start inviting the early adopters to start pushing to the "main" branch. The easiest way to do this is to rename the local "master" branch:

    $ cd my-git-project
    $ git checkout master
    $ git branch -m main
    $ git branch -u origin/main

This would also be a good time to start changing any automation or external services to the "main" branch to ensure that everything is working as expected.

###
Phase 2: Change the default branch to "main", deprecate "master"

After verifying the viability of the rename during the previous phase, the goal of this phase is to set "main" as the default branch and start issuing deprecation warnings when the legacy "master" branch is used.

1. Change the default branch on GitHub to "main".

2. Add a GitHub Actions workflow file at .github/workflows/deprecate-master-branch.yml with the following:

       name: Deprecate "master" branch
       on:
         push:
           branches:
             - master
         pull_request:
           branches:
             - master

       jobs:
         on-push:
           runs-on: ubuntu-latest
           if: ${{ github.event_name == 'push' }}
           steps:
             - name: Deprecation
               uses: peter-evans/commit-comment@v1
               with:
                 body: |
                   Hello @${{ github.event.sender.login }}!

                   I see that you have pushed some commits to the "master" branch. We are in the process of renaming the "master" branch to "main" in this repository.

                   :warning: **The "master" branch is deprecated and will be removed from this repository in the future.**

                   Please migrate your local repository by renaming the "master" branch to "main":

                   ```bash
                   $ cd my-git-project
                   $ git checkout master
                   $ git branch -m main
                   $ git branch -u origin/main
                   ```

                   Before merging pull requests, ensure their base branch is set to "main" instead of "master". For more information on how to do this, refer to [this GitHub support article][1].

                   [1]: https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/changing-the-base-branch-of-a-pull-request

         on-pull-request:
           runs-on: ubuntu-latest
           if: ${{ github.event_name == 'pull_request' }}
           env:
             DEPRECATION_MESSAGE: |
               Hello @${{ github.event.sender.login }}!

               I see that you have opened a pull request against the "master" branch. We are in the process of renaming the "master" branch to "main" in this repository.

               :warning: **The "master" branch is deprecated and will be removed from this repository in the future.**

               Please migrate your local repository by renaming the "master" branch to "main":

               ```bash
               $ cd my-git-project
               $ git checkout master
               $ git branch -m main
               $ git branch -u origin/main
               ```

               Please also set the base branch for this pull request to "main" instead of "master". For more information on how to do this, refer to [this GitHub support article][1].

               [1]: https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/changing-the-base-branch-of-a-pull-request
           steps:
             - name: Deprecation
               if: ${{ github.event.pull_request.head.repo.fork == false }}
               uses: peter-evans/create-or-update-comment@v1
               with:
                 issue-number: ${{ github.event.number }}
                 body: ${{ env.DEPRECATION_MESSAGE }}
             - name: Deprecation
               if: ${{ github.event.pull_request.head.repo.fork == true }}
               run: |
                echo "$DEPRECATION_MESSAGE"
                echo '::error::Please set the base branch for this pull request to "main" instead of "master".'
                exit 1

3. Commit and push your changes to the "main" branch.

We added a workflow file that triggers whenever a contributor pushes to or opens a pull request against the "master" branch.

The workflow adds a comment to the commit or pull request, notifying the contributor that the "master" branch has been deprecated, along with the steps they need to take to migrate their local repository and changes they need to make it to the pull request.

It is recommended that you customize the messages with additional information relevant for your organization. For example, you may want to include a link to a tracking issue for additional context, or ways for the contributor to ask for additional assistance if needed.

####
Limitations of GitHub Actions in Forks

Unfortunately, due to limitations of GitHub Actions, it is not possible to add a pull request comment from the workflow when the pull request originated from a fork, which is very common in open-source repositories. As a workaround, the workflow prints the deprecation message to the logs and fails the build.

####
Real World Example

See this commit and this pull request for an example of this working in action. Note that the code in the commit may be outdated by the time you read this – refer to the above for the latest instructions.

####
Next Steps

This would be a good time to start updating any internal and external links to the "master" branch. A common example would be automatically generated links to source code from the API documentation.

###
Phase 3: Complete the migration

After a successful phase 2 rollout, it is time to plan for completing the migration. However, what completion means is going to be different depending on your situation.

For private work repositories, the legacy "master" branch can likely be removed from the repository after giving team members a few weeks to migrate. Don't forget to remove the workflow files as well.

For open-source repositories with lots of contributors, you may want to move a lot slower. Monitor the Actions tab of the repository to see how often the deprecation workflow is triggered. When the activity diminishes, it may be good indication that the legacy "master" branch is no longer needed.

####
Soft Removal

As an alternative to removing the branch right away, you may want to consider pushing a final commit to the branch, removing all files but leave behind a README file explaining that branch has been moved, along with the steps they need to take in order to migrate their local repository.

####
URL Considerations

It is also important to consider URL breakages, as links that points to the files on the "master" branch will stop working once the branch is removed, including with the soft-removal method described above.

The impact of this has to be evaluated contextually, but it is important to note that the scope of the breakage is fairly limited, as this only directly impacts URLs that links to the "master" branch directly. Links pointing to the active development branch of repository is quite fragile (especially with line numbers) and often breaks for other reasons.

For example, a refactor that moves around files or switching from JavaScript to TypeScript would invalidate these URLs. This sort of activity is fairly common on an active code repository and is usually performed without taking the URL consideration in mind. For this reason, it is considered a best practice to use SHA-based permanent links in most situations. These links are unaffected by the branch rename.

####
Packages Considerations

For repositories containing installable packages, there are some additional considerations. Many package managers allow for installing packages from a Git repository. For example, in npm and yarn, dependencies can be a string like "username/repo" or "username/repo#branch" in lieu of version range. Likewise, GitHub Actions are installed using repository and branch references, and it is a relatively common practice to point an action at the "master" branch.

In these cases, the decision on whether to remove the legacy "master" branch has to be made carefully. Here are some examples of things to investigate and consider:

- When the branch is omitted from the specifier (e.g. "username/repo"), does the package manager in your ecosystem hard-code the default to "master" on the client, or does it respect the remote HEAD ref?

- Does the package manager use a lockfile, and if so, does it serialize the branch name (as opposed to the resolved SHA) into the lockfile?

- Does the package manager maintain a cache of cloned repositories, and if so do they recover gracefully to a remote branch disappearing?

The answer to these questions affects the potential impact to your end-users if the legacy "master" branch is removed from the repository. For some, the end-state of the migration may be to keep the "master" branch permanently as a read-only mirror, or it may be sufficient to freeze the branch's content and stop providing updates there. For others, the potential breakage maybe small enough that it is can be easily justified.

While the workflows added in phase 2 are effective for deprecating writes to the legacy "master" branch. Unfortunately, Git and GitHub do not offer the ability to do the same for reads to the branch.

However, you may be able to use features from the package manager to accomplish a similar result. For example, instead of mirroring the "main" branch to the "master" branch exactly, you could add a post-install hook to the version on "master" to issue the deprecation message for any potential consumers.

####
A Concrete Example

Using the Node.js ecosystem as an example, we can put these considerations into context, based on preliminary testing with npm and yarn classic (v1). If you found different results in your own testing, or with other package managers (pnpm, yarn v2, etc), please file an issue here.

- The popular package managers in the ecosystem support installing packages from git, by specifying "username/repo" or "username/repo#branch" in the dependencies section of package.json.

- When a branch name is not specified, the popular package managers defaults to the remote HEAD ref, which is set by the default branch on GitHub, instead of hardcoding to the "master" branch in the client.

- The popular package managers uses a lockfile by default. They serialize the resolved SHA into the lockfiles, as long as the commits referred to by these SHAs remain reachable on the remote repository, they will install just fine when using a lockfile. In the case of a branch rename, this is not an issue, as the history and commits will remain intact.

- When installing without a lockfile, the popular package managers will fetch the latest commit from the repository. Renaming the remote branch did not appear to cause any issues, either because the repositories are not cached, or the clients are able to recover gracefully from a "missing" remote branch.

- Post-install hooks are supported. However, yarn classic hides the output produced by these scripts if they are successful (exit cleanly with exit code 0). However, when they fail, they output is shown to the user.

- Generally speaking, the ecosystem has strong norms and expectations around adhering to semantic versioning. Usually, breaking changes are only expected on major version bumps.

- By using a Git dependency instead of specifying a semver range, they are explicitly opting out of the normal semver guarantee, and breakages are to be expected. No one could reasonably expect that pointing a dependency to the active development branch without using a lockfile will result in a stable system.

Given these findings, here is a concrete proposal for a maximally graceful completion plan:

1. Stop providing updates to the "master" branch by removing the mirroring workflow added in phase 1.

2. Push a commit the "master" branch, updating the README as well as adding a deprecation message to the runtime code (the `main` entry point):

       // index.js

       let command = 'npm update my-git-project';

       if (process.env.npm_execpath && process.env.npm_execpath.indexOf('yarn') !== -1) {
         command = 'yarn upgrade my-git-project';
       }

       console.warn(
         `You are running a deprecated copy of the "my-git-project" installed ` +
         `from the "master" branch of our repository. The "master" branch is ` +
         `deprecated and no longer receives any updates. The branch will soon ` +
         `be removed from our repository, at which point the package will fail ` +
         `to install.\n\n` +
         `To fix this issue, please modify your package.json and change the ` +
         `"my-git-project" dependency from "my-repo/my-git-project#master" to ` +
         `"my-repo/my-git-project" or a valid semver range. After making the ` +
         `change, run "${command}" to update the package.`
       );

       // ...rest of index.js

3. When releasing the next major version of the package, push another commit to the "master" branch, remove all files from the branch, leaving behind only a minimal README file, package.json and index.js:

       // package.json
       {
         "name": "my-git-project",
         "scripts": {
           "postinstall": "node index.js"
         }
       }

       // index.js

       let command = 'npm update my-git-project';

       if (process.env.npm_execpath && process.env.npm_execpath.indexOf('yarn') !== -1) {
         command = 'yarn upgrade my-git-project';
       }

       console.error(
         `You have installed "my-git-project" from the "master" branch of our ` +
         `repository. The "master" branch has been official retired.\n\n` +
         `To fix this issue, please modify your package.json and change the ` +
         `"my-git-project" dependency from "my-repo/my-git-project#master" to ` +
         `"my-repo/my-git-project" or a valid semver range. After making the ` +
         `change, run "${command}" to update the package.`
       );

       process.exit(1);

4. After some time, the master branch can be removed from the repository. This does not constitute a breaking change, as the package already ceased to function as of the previous commit. It was just a courteous message to ease confusion and provide actionable instructions for fixing the issue.

For most projects and organizations, this amount of notice is probably not necessary or warranted, but it showcases the available tools and techniques, and demonstrates that there can be a good migration path even under very strict compatibility requirements. As always, use these steps as a template and tailor them to your own needs.

##
Local Migration

Finally, if you are working on a repository you don't control, and you would like to refer to your local branch with a different name without making any changes to the upstream project, you can rename your local "master" branch to "main" with these steps:

    $ cd my-git-project
    $ git checkout master
    $ git branch -m main
    $ git branch -u origin/master

This renames the local branch to "main" but sets the remote tracking branch to "master".

##
License

This content in this repository, including this documentation and code examples are licensed under the CC0 "No Rights Reserved" public domain license. Feel free to reproduce and adapt this work into your own proposals, documentation, etc.

Attribution is not necessary. However, this guide receives constant updates to reflect current best-practice and solutions based implementation feedback, so a reference to this repository may be helpful.