One of Git’s core features is “rewriting history”, i.e., “altering” existing commits. I’m using quotation marks, because — despite the appearance — the Git history is immutable. It is by design impossible to modify an existing commit with regular Git commands.

And yet, amongst many Git users there exists this fear of losing some changes or simply creating a mess. In reality, any commited change can be considered safe, i.e., it can be retrieved even after “altering” the history.

What does rewriting really mean?

Why would you ever want to alter the Git history anyway? Let’s imagine you’ve made a typo in the most recent commit:

* 5c7a782 - (HEAD -> master) Fix deefct 47
* fb2546f - Add checkout page

Usually, you can use commands like reset or rebase (-i) to “rewrite” the Git history. However, correcting the last commit is fairly common, so there is an easier alternative:

$ git commit --amend -m "Fix defect 47"

Another look into the Git history shows the correct message:

* 2b049ea - (HEAD -> master) Fix defect 47
* fb2546f - Add checkout page

It appears as if we have just altered the last commit. However, the hash has changed from 5c7a782 to 2b049ea, which means that we have created a new commit. The reason is quite simple:

For every commit a unique hash is being generated and assigned to the commit. It is based on the commit message, content, author, parent commit etc. Changing any of those properties leads to a new hash and hence a new commit.

Since we’ve just changed the commit message, a new commit with a different hash was created. Same thing happens when “moving” (e.g. rebasing) commits, because the parent commit changes.

But where did the other commit go? By default, git log hides every commit that is not reachable from HEAD or any named branch. Those unreachable commits can be displayed with the --reflog flag:

$ git log --graph --oneline --reflog
* 2b049ea - (HEAD -> master) Fix defect 47
| * 5c7a782 - Fix deefct 47
|/
* fb2546f - Add checkout page

As we can see, “altering” history is nothing else than creating new commits and moving the HEAD and master pointers. Hence, the term “alternative history” is more suitable. It also means that we can easily revert our “destructive” commit --amend command if required:

$ git reset --hard 5c7a782

Garbage collection

Previously, I’ve claimed that all commits can be considered safe. However, there is a limitation:

The Git garbage collector will remove all unreachable commits automatically after a certain period of time (30 days by default).

Especially in a rebase flow, you will create and copy a lot of commits. The garbage collector does some housekeeping and removes all abandoned commits after a certain time. In my daily work, I rarely want to keep those commits. If you do, simply assign a branch to it:

$ git branch my-branch 5c7a782

Altering public history

As long as we are “altering” commits that are not public (i.e. they were never pushed to a remote repository), you’re free to do whatever you want. Things get tricky when we want to move a public branch around.

A common Git best practice states:

Do not alter public history.

I think this is great advice for Git beginners, but it can be limiting if you and your collegues know what you’re doing.

Let’s first see the consequences of rewriting public Git history. Let’s assume the flawed commit from the last section had been already pushed to the origin remote. After running git commit --amend the log would look like this:

* 2b049ea - (HEAD -> master) Fix defect 47
| * 5c7a782 - (origin/master) Fix deefct 47
|/
* fb2546f - Add checkout page

If we try to push our corrected commit to origin, we’ll get an error:

$ git push
To ../origin/
! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to '../origin/'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

This is the expected behavior, because by default a git push is only allowed to add new commits onto the latest “tip” of the remote (origin). Git suggests a simple solution: “integrating the remote changes”. In our case, git pull is equivalent to git merge origin/master, which is not what we want. Instead, we want to replace the remote commit. This can be achieved with a force push:

$ git push -f

Now everything looks fine:

* 2b049ea - (HEAD -> master, origin/master) Fix defect 47
* fb2546f - Add checkout page

But now Janet and Steve and all your other collegues will get the same issue as you’ve had before. That’s why projects often disallow a force push for the main branches (e.g. master, develop).

How do we deal with this situation?

When in doubt, follow the best practice “Do not alter public history”. A typo looks bad, but do you really want to go through all the hassle to fix it?

If you do have to force push a public branch, the first and most important step is Communication. All your collegues working on the same branch should know to expect an issue when interacting with the shared repository. They also should know how to fix this situation. For amended changes the solution is easy:

$ git pull --rebase

This command will fetch the remote branch and rebase all local commits on top of it. Amended commits will be resolved automatically (so the typo commit will be skipped).

Other use cases may be harder to fix, including (interactive) rebasing and cherry-picking. Always consider the trade-offs before force-pushing.

What if you’re working alone on a public branch? Then you’re usually free to force-push as much as you like. Again, communication is key here.

Conclusion

Version control is an underrated skill. Most software engineers use it daily, and yet, many are not willing to invest more than necessary to learn it. That’s fine, but knowing more than commit/push/pull will at least make you more efficient. It will also help you solve issues you (and your collegues) may encounter.

I hope this article explains Git’s default “safety net” behavior and motivates you to try out some advanced features.

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