How do I change the author and committer name/email for multiple commits?

Changing the author and committer name/email for multiple commits in a Git repository is a task that involves rewriting the repository’s history. This can be necessary for various reasons, such as correcting mistakes in commit metadata, ensuring consistency in author information, or complying with organizational policies.

⚠️ _Important Warning:_**

Rewriting history alters commit hashes and affects all descendants of the rewritten commits.
If you’ve already pushed these commits to a shared repository, collaborating team members will encounter issues.
Proceed with caution and ensure that all collaborators are informed before making such changes.

This comprehensive guide will walk you through multiple methods to change the author and committer information for multiple commits, including using git filter-repo, git filter-branch, and interactive rebasing. Additionally, it covers best practices, example scenarios, and troubleshooting tips to ensure a smooth and safe process.

Table of Contents

1. Understanding Author vs. Committer
2. Prerequisites
3. Method 1: Using git filter-repo
4. Method 2: Using git filter-branch
5. Method 3: Interactive Rebase (git rebase -i)
6. Method 4: Using BFG Repo-Cleaner
7. Best Practices
8. Example Scenarios
9. Troubleshooting Common Issues
10. Additional Resources
11. Conclusion

Understanding Author vs. Committer

Before diving into the methods, it's essential to understand the difference between Author and Committer in Git:

• Author:
  • The person who originally wrote the changes.
  • Recorded in the commit metadata.
• Committer:
  • The person who last applied the changes.
  • Can be different from the author, especially when patches are applied by someone else.

Both Author and Committer have associated name and email fields.

Prerequisites

Before attempting to change author and committer information, ensure you have the following:

• Git Installed: Verify by running:

  git --version

  If not installed, download it from the official Git website.

• Backup Your Repository:

  Since these operations rewrite history, it's prudent to back up your repository.

  git clone --mirror <repository-url> backup-repo.git

• Familiarity with Git Commands:

  Understanding basic Git operations will help in executing these methods effectively.

• Install git filter-repo (For Method 1):

  git filter-repo is a powerful and efficient tool for rewriting Git history. It's not bundled with Git by default.

  • Installation:

    • Using pip:

      pip install git-filter-repo

    • From Source:

      Follow the instructions on the git-filter-repo GitHub repository.

Method 1: Using git filter-repo

Recommended Method

git filter-repo is a versatile and efficient tool for rewriting Git history. It supersedes git filter-branch and offers better performance and safety.

Steps:

1. Navigate to Your Repository:

   cd path/to/your/repository

2. Install git filter-repo:

   Ensure that git filter-repo is installed (see Prerequisites).

3. Create a Replacement Configuration:

   Prepare a Python script or use inline arguments to specify the changes. Here's an example using a Python script:

   # change_author.py
   import sys
   from git_filter_repo import FilterRepo
   
   # Define the old and new author/committer details
   OLD_EMAIL = "old.email@example.com"
   NEW_NAME = "New Name"
   NEW_EMAIL = "new.email@example.com"
   
   def replace_author(commit):
       if commit.author_email.decode('utf-8') == OLD_EMAIL:
           commit.author_name = NEW_NAME.encode('utf-8')
           commit.author_email = NEW_EMAIL.encode('utf-8')
       if commit.committer_email.decode('utf-8') == OLD_EMAIL:
           commit.committer_name = NEW_NAME.encode('utf-8')
           commit.committer_email = NEW_EMAIL.encode('utf-8')
   
   repo = FilterRepo()
   repo.commit_callback = replace_author
   repo.run()

4. Run git filter-repo with the Script:

   git filter-repo --force --replace-text change_author.py

   Note: The --force flag is used to bypass safety checks. Use it cautiously.

5. Alternative: Using Inline Arguments:

   If you prefer not to use a script, you can use the --mailmap feature or multiple --replace-email options. However, git filter-repo primarily uses scripting for complex replacements.

6. Verify the Changes:

   git log --pretty=fuller

   Ensure that the author and committer information has been updated as intended.

7. Force Push the Rewritten History (If Necessary):

   ⚠️ Warning: Force pushing rewritten history can disrupt collaborators. Ensure that all team members are informed.

   git push --force --all
   git push --force --tags

Advantages:

• Performance: Faster and more efficient than git filter-branch.
• Safety: Designed to prevent common mistakes in history rewriting.
• Flexibility: Highly customizable through scripting.

Limitations:

• Installation Required: Not included by default with Git.
• Requires Scripting Knowledge: Customizations may require writing Python scripts.

Method 2: Using git filter-branch'

Deprecated Method (Use git filter-repo Instead)

While git filter-branch is a traditional method for rewriting history, it's now considered slow and error-prone compared to git filter-repo. However, it's still available and can be used if git filter-repo isn't an option.

Steps:

1. Navigate to Your Repository:

   cd path/to/your/repository

2. Backup Your Repository:

   git clone --mirror <repository-url> backup-repo.git

3. Run git filter-branch with Environment Variables:

   Replace <OLD_EMAIL>, <NEW_NAME>, and <NEW_EMAIL> with your specific details.

   git filter-branch --env-filter '
   OLD_EMAIL="old.email@example.com"
   CORRECT_NAME="New Name"
   CORRECT_EMAIL="new.email@example.com"
   
   if [ "$GIT_COMMITTER_EMAIL" = "$OLD_EMAIL" ]
   then
       export GIT_COMMITTER_NAME="$CORRECT_NAME"
       export GIT_COMMITTER_EMAIL="$CORRECT_EMAIL"
   fi
   if [ "$GIT_AUTHOR_EMAIL" = "$OLD_EMAIL" ]
   then
       export GIT_AUTHOR_NAME="$CORRECT_NAME"
       export GIT_AUTHOR_EMAIL="$CORRECT_EMAIL"
   fi
   ' --tag-name-filter cat -- --branches --tags

4. Verify the Changes:

   git log --pretty=fuller

5. Force Push the Rewritten History:

   ⚠️ Warning: This operation rewrites history. Communicate with your team before force pushing.

   git push --force --all
   git push --force --tags

Advantages:

• Built-In: Available in all Git installations.
• Customizable: Allows for various filters and modifications.

Limitations:

• Performance: Slower, especially on large repositories.
• Risk of Errors: More prone to mistakes compared to git filter-repo.
• Deprecated: Not recommended for new projects.

Method 3: Interactive Rebase (git rebase -i)

Suitable for Changing a Small Number of Commits

Interactive rebasing allows you to rewrite commit history, modifying multiple commits as needed. This method is best for repositories with a manageable number of commits to alter.

Steps:

1. Navigate to Your Repository:

   cd path/to/your/repository

2. Identify the Range of Commits:

   Determine how many commits you need to rewrite. For example, to rewrite the last 5 commits:

   git log --oneline

3. Start Interactive Rebase:

   git rebase -i HEAD~5

4. Mark Commits for Editing:

   An editor will open with a list of commits. Change pick to edit (or e) for each commit you want to modify.

   edit a1b2c3d Commit message 1
   edit e4f5g6h Commit message 2
   pick d7e8f9g Commit message 3
   pick f6g7h8i Commit message 4
   pick j9k0l1m Commit message 5

5. Modify Author and Committer Information for Each Commit:

   For each commit marked edit, perform the following:

   a. Set the Correct Author Information:

   git commit --amend --author="New Name <new.email@example.com>" --no-edit

   • --author: Specifies the new author name and email.
   • --no-edit: Retains the existing commit message.

   b. Set the Correct Committer Information:

   GIT_COMMITTER_NAME="New Name" GIT_COMMITTER_EMAIL="new.email@example.com" git commit --amend --no-edit --reset-author

   • --reset-author: Updates the committer information to match the provided environment variables.

   c. Continue Rebase:

   git rebase --continue

6. Repeat for Each Commit:

   The rebase process will pause at each edit commit. Repeat step 5 until all desired commits are modified.

7. Force Push the Rewritten History:

   git push --force

   ⚠️ Warning: Rewriting history requires force pushing, which can disrupt collaborators.

Advantages:

• Granular Control: Modify specific commits as needed.
• No External Tools Required: Utilizes Git's built-in features.

Limitations:

• Manual Process: Time-consuming for many commits.
• Potential for Errors: Mistakes during rebase can complicate history.
• Not Ideal for Large Repositories: Inefficient for altering numerous commits.

Method 4: Using BFG Repo-Cleaner

An Alternative to git filter-repo and git filter-branch

BFG Repo-Cleaner is a tool designed to simplify and speed up the process of cleaning Git repositories, including changing author information.

Steps:

1. Download BFG Repo-Cleaner:

   Download the latest bfg.jar from the official website.

2. Navigate to Your Repository:

   cd path/to/your/repository

3. Backup Your Repository:

   git clone --mirror <repository-url> backup-repo.git

4. Create a File with Replacement Rules:

   For example, create users.txt with the following content:

   old.email@example.com=New Name <new.email@example.com>
   another.old@example.com=Another Name <another.new@example.com>

5. Run BFG to Replace Author Information:

   java -jar bfg.jar --replace-text users.txt

6. Clean Up and Compact the Repository:

   git reflog expire --expire=now --all && git gc --prune=now --aggressive

7. Force Push the Rewritten History:

   git push --force

   ⚠️ Warning: As with other history-rewriting methods, force pushing can disrupt collaborators.

Advantages:

• Speed: Faster than git filter-branch.
• Simplicity: Easier to use for specific tasks like replacing text.

Limitations:

• Less Flexible: Not as customizable as git filter-repo.
• External Tool Required: Requires downloading and using an external JAR file.

Best Practices

1. Communicate with Your Team:

   • Inform all collaborators before rewriting history to prevent confusion and conflicts.
   • Coordinate to ensure everyone resets their local repositories accordingly.

2. Backup Before Proceeding:

   • Always create backups of your repository before performing history-rewriting operations.

   git clone --mirror <repository-url> backup-repo.git

3. Use Descriptive Commit Messages:

   • When modifying commits, ensure commit messages remain clear and accurate to maintain repository history integrity.

4. Test Changes Locally:

   • Before pushing changes to the remote repository, verify that the author and committer information has been correctly updated.

5. Minimize Rewrites:

   • Avoid rewriting history frequently to reduce the risk of repository inconsistencies and conflicts.

6. Understand the Tools:

   • Familiarize yourself with the tools and commands you plan to use (git filter-repo, git filter-branch, interactive rebasing) to use them effectively and safely.

Example Scenarios

Scenario 1: Correcting a Single Author’s Email Across All Commits

Objective: Update the email address of a specific author in all past commits.

Steps:

1. Using git filter-repo:

   git filter-repo --commit-callback '
   if commit.author_email.decode("utf-8") == "old.email@example.com":
       commit.author_name = "New Name".encode("utf-8")
       commit.author_email = "new.email@example.com".encode("utf-8")
   if commit.committer_email.decode("utf-8") == "old.email@example.com":
       commit.committer_name = "New Name".encode("utf-8")
       commit.committer_email = "new.email@example.com".encode("utf-8")
   '

2. Verify Changes:

   git log --pretty=format:"%h %an <%ae>"

3. Force Push:

   git push --force --all
   git push --force --tags

Scenario 2: Changing Author Information for a Range of Recent Commits

Objective: Update the author and committer information for the last 3 commits.

Steps:

1. Start Interactive Rebase:

   git rebase -i HEAD~3

2. Mark Commits for Editing:

   In the editor that opens, change pick to edit for each of the last 3 commits.

   edit a1b2c3d Commit message 1
   edit e4f5g6h Commit message 2
   edit d7e8f9g Commit message 3

3. Amend Each Commit:

   For each commit:

   a. Set Author and Committer:

   git commit --amend --author="New Name <new.email@example.com>" --no-edit

   GIT_COMMITTER_NAME="New Name" GIT_COMMITTER_EMAIL="new.email@example.com" git commit --amend --no-edit --reset-author

   b. Continue Rebase:

   git rebase --continue

4. Force Push:

   git push --force

Scenario 3: Rewriting All Commits to Change a Single Author’s Information

Objective: Change all commits authored by old.email@example.com to New Name <new.email@example.com>.

Steps:

1. Using git filter-repo:

   git filter-repo --commit-callback '
   if commit.author_email.decode("utf-8") == "old.email@example.com":
       commit.author_name = "New Name".encode("utf-8")
       commit.author_email = "new.email@example.com".encode("utf-8")
   if commit.committer_email.decode("utf-8") == "old.email@example.com":
       commit.committer_name = "New Name".encode("utf-8")
       commit.committer_email = "new.email@example.com".encode("utf-8")
   '

2. Verify Changes:

   git log --pretty=format:"%h %an <%ae>"

3. Force Push:

   git push --force --all
   git push --force --tags

Troubleshooting Common Issues

Issue 1: Conflicts During Rewriting History

Symptom: During interactive rebasing or filter-branch operations, merge conflicts arise.

Solutions:

1. Resolve Conflicts Manually:

   • Open conflicted files in your editor.

   • Look for conflict markers (<<<<<<<, =======, >>>>>>>) and resolve differences.

   • Stage resolved files:

     git add <file>

   • Continue the process:

     git rebase --continue

   • Or, if using git filter-repo, abort and retry after resolving issues.

2. Abort the Operation:

   If conflicts are too complex, abort the rebase or filter operation:

   • Interactive Rebase:

     git rebase --abort

   • git filter-branch:

     git filter-branch --abort

   • git filter-repo:

     Unfortunately, git filter-repo doesn't support aborting mid-operation. If issues occur, refer to git filter-repo's documentation.

Issue 2: Unintended Changes After Rewriting

Symptom: After rewriting history, unexpected changes appear in commits.

Solutions:

1. Review the Rewriting Commands:

   Ensure that your scripts or commands accurately target the intended commits and modify only the desired fields.

2. Check for Overlapping Conditions:

   Verify that your filters don't unintentionally match other commits or authors.

3. Use git log to Inspect Commits:

   git log --pretty=fuller

   Ensure that only the intended commits have updated author/committer information.

4. Restore from Backup:

   If unintended changes are significant, restore from the backup created before rewriting history.

Issue 3: Permission Denied or Access Issues During Push

Symptom: Attempting to push rewritten history results in permission errors.

Solutions:

1. Ensure Correct Permissions:

   Verify that your user has the necessary rights to push to the repository.

2. Authenticate Properly:

   Use SSH keys or correct HTTPS credentials as required by the repository.

3. Force Push Correctly:

   Use the appropriate flags to force push:

   git push --force --all
   git push --force --tags

4. Check Remote URL:

   Ensure that the remote repository URL is correct.

   git remote -v

Additional Resources

• Official Git Documentation:

  • git filter-repo
  • git filter-branch
  • git rebase
  • git commit

• Tutorials and Guides:

  • Atlassian Git Tutorials: Changing Author Info
  • GitHub Docs: Changing Author Info

• Books:

  • Pro Git by Scott Chacon and Ben Straub – Available for free online.

• Interactive Learning:

  • Learn Git Branching – An interactive tool to visualize and practice Git commands, including history rewriting.

• Git GUI Tools:

  • GitKraken
  • SourceTree
  • GitHub Desktop
  • Visual Studio Code with Git Extensions

Conclusion

Changing the author and committer information for multiple commits in Git is a delicate operation that requires careful execution to maintain repository integrity and collaboration harmony. By utilizing tools like git filter-repo, git filter-branch, and interactive rebasing, you can effectively update commit metadata to reflect accurate authorship.

Key Takeaways:

• Choose the Right Tool: git filter-repo is recommended for its performance and flexibility, but git filter-branch and interactive rebasing are viable alternatives for specific scenarios.

• Backup Before Rewriting: Always create backups of your repository before performing history-altering operations to safeguard against unintended consequences.

• Communicate with Collaborators: Inform your team about history rewrites to prevent synchronization issues and ensure a smooth transition.

• Verify Changes Thoroughly: After rewriting history, inspect the commit log to confirm that author and committer information has been correctly updated.

• Understand the Implications: Rewriting history affects commit hashes and can disrupt shared workflows. Use these methods judiciously and only when necessary.

By adhering to these practices and understanding the underlying mechanisms, you can confidently manage and correct commit metadata in your Git repositories, ensuring that your project's history remains accurate and meaningful.