I recently ran into a task where I needed to rename author and committer information of a commit range in a git repository, but only if the author or comitter fields matched a certain value. This tutorial is a quick overview of how I managed to solve this.

git filter-branch is a powerful and versatile tool in Git used to rewrite the history of a repository. It’s primarily used for applying complex changes to the repository’s history, such as renaming files or directories, removing sensitive data, or, as in my case, modifying authorship information.

In summary, git filter-branch essentially goes through each commit in the specified range (e.g., all commits, commits on a specific branch, etc.) and applies a specified set of filters to rewrite that commit. This includes rewriting the commit message, modifying the author or committer information, filtering files, and more.

I knew the commit range that I wanted to examine and rewrite and then was able to put this command together:

git filter-branch --env-filter '
if [ "$GIT_COMMITTER_NAME" = "spongebob" ]; then
    export GIT_COMMITTER_NAME="SpongeBob"
    export GIT_COMMITTER_EMAIL="spongebob@gmail.com"
fi
if [ "$GIT_AUTHOR_NAME" = "spongebob" ]; then
    export GIT_AUTHOR_NAME="SpongeBob"
    export GIT_AUTHOR_EMAIL="spongebob@gmail.com"
fi
' -f -- master 2a12a6cc..7ade6a

This git filter-branch command is rewriting the history of the specified range of commits on the master branch. In particular, -- master -- master 2a12a6cc..7ade6a, it starts from commit 2a12a6cc and ends at commit 7ade6a on the master branch.

The --env-filter is used to modify the environment in which the commit will be performed since we want to rewrite the author/committer name/email/time environment variables. Note that -- is used to separate the revision range and the options.

Remember that after running the above command, you need to force push the changes to update the remote repository and once you’re sure the changes are as you expect, you can remove the backup created:

git update-ref -d refs/original/refs/heads/master

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So…

I hope this review was of some help to you and I am sure that both this post as well as the functionality it attempts to explain can be improved in any number of ways.

Happy Coding,

Misagh Moayyed