rewinding git commit --amend

2012/06/25

It may come to pass that you will run git commit --amend by mistake. When this happens, you’ll want to unwind the operation you just did.

In some cases the changes are simple enough that you can use git reset -p to remove those lines from the commit. However, sometimes git reset -p isn’t up to the task, as in the case when the changeset is very large. Luckily, git has a ticker tape of the changes you make to each branch, which is called the reflog.

The reflog records when the tip of a branch is updated. The tip is updated any time you create a new commit, amend a commit, reset a commit, switch branches, etc. Basically, any time HEAD changes, you will get a reflog entry. The reflog therefore is a great tool for understanding how the repository came to be in a particular state.

git reflog -2 will give you the last two operations that Git performed. In the case of an amend, it will look something like this:

C HEAD@{0}: commit (amend): Something something something commit message
B HEAD@{1}: reset: moving to HEAD~1

git commit --amend is kind of shorthand for the following, given changes have been made, and are either in the index or in the working directory:

$ git stash
$ git reset HEAD~1
$ git stash pop
$ git add .
$ git commit

Or, in English:

Thus, the last two operations in the reflog are reset and commit.

So, what can we do with this? Well, B was HEAD before the amend happened. C is the amended commit. git diff C..B will show you what changes were applied as part of the amend:

$ git diff C..B

From here you can use git apply to apply the reverse of what you amended earlier to your working tree:

$ git diff C..B | git apply -

Now we can do another amend to put the commit back to where it was before we did the previous amend:

$ git commit -a --amend -CHEAD

And then, by reversing the order of the refs to git diff, get the changes we want to apply to the correct commit back:

$ git diff B..C | git apply -

And commit as necessary, this time using –fixup to indicate the correct commit (in this example, A):

$ git commit -a --fixup A

Then you can rebase either now or at a later time to do the ‘amend’ you had originally intended:

$ git rebase --i --autosquash A~1

So don’t fret when you do an accidental amend. It’s just a couple commands away from being unwound and applied to the correct commit.