Undoing the merge in Git so that no commits from the other branch exist

Question

I've two branches: development, and bug_fixes, and I accidentally merged development branch in bug_fixes two to three weeks ago, and have been working on bug_fixes since then, and I've pushed the changes in bug_fixes.

Now, I need to un-merge the development branch. I want a pure bug_fixes branch with no commits form development branch. By googling, I've come up with the following scenarios:

1) Revert the commit that merged development branch in bug_fixes branch, but it partially did the job. The commits from development branch can still be seen in bug_fixes branch, something that isn't required.

2) Rebase, and delete the commit that did the merging, but it still has the same issue described in point 1. bug_fixes branch still contains the commits from development branch.

Is there a way I can delete the merging commit, and all those commits that came in bug_fixes branch form development branch, would be deleted as well? Or, it isn't possible?

Answer 1

IF you know which commits are coming from the development branch (in addition of the merge commit), you can do a git rebase -i (interactive rebase) from the last good commit of bug fixes:

 b--b--(B)--M--d--b--d--b--b (bugfix)
           /
    d--d--d  (devel)

git checkout bugfix
git rebase -i (B)
# drop M, d, and d

 b--b--(B)--b'--b'--b' (bugfix)

    d--d--d  (devel)

The more complex alternative would be a git filter-branch, but hopefully, the interactive rebase is enough.

In the OP's case, the full command was:

git rebase -p -i (b)

-p allows to preserve the merge commit.

Answer 2

I would try the following approach:

First of all, make a list of the commits in both branches:

git checkout development
git log --oneline > log-devel.txt

git checkout bug_fixing
git log --oneline > log-bugfixing.txt

Second, identify in each branch the id of the last commit before executing the merge. Let's name these commits devel-0 and bugfixing-0 Then, take your branches to the status they were before the accidental merge:

git checkout development
git reset --hard <devel-0-hash>

git checkout bug_fixing
git reset --hard <bugfixin-0-hash>

At this point you have two clean but outdated git branches. Now, you should take from log-devel.txt and log-bugfixing.txt all the commits that are not included in the current branches, and add them to the corresponding branch. In order to add a commit with id abc0123 to your branch development you can use cherry pick:

git checkout development
git cherry-pick `abc0123`

Take into account that the commits should better be added to the clean branches in the same order they were added to the bug fixing branch, in order to minimize the number of conflicts appearing.

Answer 3

you need to know your latest commit message for bug_fixes before merge, if you know the sha hash then evne better.

now first you need to find the sha hash using the command

$ git reflog

the output is something like

1fb5738 HEAD@{10}: commit...

4d47df6 HEAD@{11}: commit...

5f32c4b HEAD@{12}: merge...

428f91d HEAD@{13}: checkout...

5f32c4b HEAD@{14}: checkout...

the first part is the sha hash. In this case the state before merge has hash value 428f91d

so now, in bug_fixes branch we reset to this hash value

git reset --hard 428f91d

now it is undone