Reverting Merge Commits in Git

Merge commits are essential for integrating changes from different branches in Git. However, sometimes a merge introduces unintended consequences or errors, and you need to undo it. This tutorial explains how to revert a merge commit, particularly when it has already been pushed to a remote repository.

Understanding the Challenge

Reverting a regular commit is straightforward using git revert. However, merge commits are different. They have multiple parents – the commit on your main branch and the commit at the tip of the merged branch. This means Git needs to know which parent represents the mainline (the branch you want to preserve) and which represents the branch you want to “un-merge.”

The `git revert -m` Command

The key to reverting a merge commit is the -m option with the git revert command. This option specifies the parent number to use as the mainline.

The general syntax is:

git revert -m <parent_number> <merge_commit_hash>

<parent_number>: Indicates which parent of the merge commit should be considered the mainline. Usually, 1 represents the mainline and 2 represents the branch that was merged.
<merge_commit_hash>: The SHA-1 hash of the merge commit you want to revert.

Identifying the Correct Parent Number

Before reverting, you need to identify the correct parent number. You can do this by examining the merge commit’s history using git log.

For example, if you run git log and see output like this:

commit 8f937c683929b08379097828c8a04350b9b8e183
Merge: 8989ee0 7c6b236
Author: Ben James <[email protected]>
Date:   Wed Aug 17 22:49:41 2011 +0100

Merge branch 'gh-pages'

Conflicts:
    README

The Merge: line tells you the parent commit hashes: 8989ee0 and 7c6b236.

If you want to revert to the state before the gh-pages branch was merged (keeping your main branch as is), use -m 1. This instructs Git to treat 8989ee0 as the mainline.
If you want to revert to the state where the gh-pages branch was not merged and effectively reinstate the gh-pages branch, use -m 2.

Example

Let’s say you have a merge commit with hash 8f937c6 and you want to revert it, treating the first parent as the mainline. The command would be:

git revert -m 1 8f937c6

This will create a new commit that effectively undoes the changes introduced by the merge.

Pushing the Reverted Commit

After successfully reverting the merge commit locally, you need to push the changes to the remote repository:

git push origin <branch_name>

Important Considerations

History Rewriting: Reverting a merge commit creates a new commit. It doesn’t erase the original merge commit from the history. While safer than rewriting history with git reset, it’s still important to coordinate with your team.
Future Merges: When you revert a merge commit, you’re declaring that you don’t want the changes brought in by that merge ever. Future merges won’t automatically include those changes. Be certain this is the desired outcome.
Alternative: Resetting (Caution!) If you are working on a private branch and haven’t shared it with anyone, you can consider using git reset to revert to a previous commit. However, this rewrites history and should never be done on shared branches. If you choose to use git reset, you’ll likely need to force-push (git push -f) your changes, which can cause problems for collaborators.

By understanding the git revert -m command and carefully considering the implications, you can effectively revert merge commits and maintain a clean and accurate project history.