Christoph Rüegg

Math.NET, distributed computing and how an electrical engineer sees the world of complex software

Git Howto: Revert a Commit Already Pushed to a Remote Repository

So you’ve just pushed your local branch to a remote branch, but then realized that one of the commits should not be there, or that there was some unacceptable typo in it. No problem, you can fix it. But you should do it rather fast before anyone fetches the bad commits, or you won’t be very popular with them for a while ;)

First two alternatives that will keep the history intact:

Alternative: Correct the mistake in a new commit

Simply remove or fix the bad file in a new commit and push it to the remote repository. This is the most natural way to fix an error, always safe and totally non-destructive, and how you should do it 99% of the time. The bad commit remains there and accessible, but this is usually not a big deal, unless the file contains sensitive information.

Alternative: Revert the full commit

Sometimes you may want to undo a whole commit with all changes. Instead of going through all the changes manually, you can simply tell git to revert a commit, which does not even have to be the last one. Reverting a commit means to create a new commit that undoes all changes that were made in the bad commit. Just like above, the bad commit remains there, but it no longer affects the the current master and any future commits on top of it.

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$ git revert dd61ab32

About History Rewriting

People generally avoid history rewiriting, for a good reason: it will fundamentally diverge your repository from anyone who cloned or forked it. People cannot just pull your rewritten history as usual. If they have local changes, they have to do some work to get in sync again; work which requires a bit more knowledge on how Git works to do it properly.

However, sometimes you do want to rewrite the history. Be it because of leaked sensitive information, to get rid of some very large files that should not have been there in the first place, or just because you want a clean history (I certainly do).

I usually also do a lot of very heavy history rewriting when converting some repository from Subversion or Mercurial over to Git, be it to enforce internal LF line endings, fixing committer names and email addresses or to completely delete some large folders from all revisions. I recently also had to rewrite a large git repository to get rid of some corruption in an early commit that started causing more and more problems.

Yes, you should avoid rewriting history which already passed into other forks if possible, but the world does not end if you do nevertheless. For example you can still cherry-pick commits between the histories, e.g. to fetch some pull requests on top of the old history.

In opensource projects, always contact the repository maintainer first before doing any history rewriting. There are maintainers that do not allow any rewriting in general and block any non-fastforward pushes. Others prefer doing such rewritings themselves.

Case 1: Delete the last commit

Deleting the last commit is the easiest case. Let’s say we have a remote mathnet with branch master that currently points to commit dd61ab32. We want to remove the top commit. Translated to git terminology, we want to force the master branch of the mathnet remote repository to the parent of dd61ab32:

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$ git push mathnet +dd61ab32^:master

Where git interprets x^ as the parent of x and + as a forced non-fastforward push. If you have the master branch checked out locally, you can also do it in two simpler steps: First reset the branch to the parent of the current commit, then force-push it to the remote.

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$ git reset HEAD^ --hard
$ git push mathnet -f

Case 2: Delete the second last commit

Let’s say the bad commit dd61ab32 is not the top commit, but a slightly older one, e.g. the second last one. We want to remove it, but keep all commits that followed it. In other words, we want to rewrite the history and force the result back to mathnet/master. The easiest way to rewrite history is to do an interactive rebase down to the parent of the offending commit:

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$ git rebase -i dd61ab32^

This will open an editor and show a list of all commits since the commit we want to get rid of:

pick dd61ab32
pick dsadhj278
...

Simply remove the line with the offending commit, likely that will be the first line (vi: delete current line = dd). Save and close the editor (vi: press :wq and return). Resolve any conflicts if there are any, and your local branch should be fixed. Force it to the remote and you’re done:

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$ git push mathnet -f

Case 3: Fix a typo in one of the commits

This works almost exactly the same way as case 2, but instead of removing the line with the bad commit, simply replace its pick with edit and save/exit. Rebase will then stop at that commit, put the changes into the index and then let you change it as you like. Commit the change and continue the rebase (git will tell you how to keep the commit message and author if you want). Then push the changes as described above. The same way you can even split commits into smaller ones, or merge commits together.

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