Posts tagged 'rebase'

Use git reflog to split two squashed commits

published on February 08, 2019.

Today I was using interactive git rebase to squash some commits together to clean up the commit history of a git branch. At one point I went a bit overboard with it and squashed together two commits by mistake.

I’ve heard somewhere from someone that in git you pretty much can’t lose code because everything is in the git reflog. Today I decided to put that to the test.

Spoiler alert: git reflog saved the day.

The setup

We start with two separate commits, “Commit 1” and “Commit 2”:

> git hist
* c3fa1d6 -  (HEAD -> reflog-undo-squash) Commit 2 [Robert Basic 4 seconds ago]
* db73960 -  Commit 1 [Robert Basic 56 seconds ago]

Using git rebase -i HEAD~2 we start the interactive rebase for the last two commits:

pick db73960 Commit 1
f c3fa1d6 Commit 2

We choose to fixup “Commit 2” with “Commit 1”, which means that the second commit will be squashed with the first commit, discarding the commit message of the second commit.

Uh oh, that was a mistake, and our commit history now looks like this:

> git hist
* 786865f -  (HEAD -> reflog-undo-squash) Commit 1 [Robert Basic 5 minutes ago]

Our nice little “Commit 2” is now gone and before we start panicking and making more damage, let’s take a look at the git reflog to see what we have there.

Looking at the reflog

The git reflog right after the bad rebase shows us the following:

> git reflog
786865f (HEAD -> reflog-undo-squash) HEAD@{0}: rebase -i (finish): returning to refs/heads/reflog-undo-squash
786865f (HEAD -> reflog-undo-squash) HEAD@{1}: rebase -i (fixup): Commit 1
0a8f291 HEAD@{2}: rebase -i (pick): Commit 1
684d689 HEAD@{3}: rebase -i (pick): Commit 1
5761a21 HEAD@{4}: rebase -i (start): checkout 5761a21b0c0a1f12ab1c60bfef1f1d111ba699c0
c3fa1d6 HEAD@{5}: commit: Commit 2
db73960 HEAD@{6}: commit (initial): Commit 1

The thing at HEAD@{6} was the first thing that happened and the thing at HEAD@{0} is whatever is going on right now.

What we want the current state to be is the state before we started the rebase process, that is the state at HEAD@{5}. To do that we “reset” our state to that point in time:

> git reset HEAD@{5}

And now our git history is back to it’s pre-rebase state:

> git hist
* c3fa1d6 -  (HEAD -> reflog-undo-squash) Commit 2 [Robert Basic 15 minutes ago]
* db73960 -  Commit 1 [Robert Basic 16 minutes ago]

Note that even the commit shas are back to their pre-rebase values, db73960 and c3fa1d6.

The Atlassian git reflog tutorial goes into more detail, so make sure to read that as well.

Happy hackin’!

Tags: git, reflog, squash, rebase.
Categories: Development, Software.
Robert Basic

Robert Basic

Software engineer, consultant, open source contributor.

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