I used to rely on fast-forward merges, but recently Git started throwing this error when I run:
git merge --ff-only branch-name
It fails with:
fatal: not possible to fast-forward, aborting
I’m aware that --no-ff has its use cases, but I’m confused why a fast-forward merge isn’t possible anymore. The branch hasn’t had any conflicts or manual merges as far as I can tell. What are the common reasons for seeing the fatal: not possible to fast-forward, aborting message, and how can I check or fix it?
Any insights would be really helpful!
Ah, I’ve seen this happen a few times. Basically, it means your current branch has diverged from the target branch, so Git can’t fast-forward because it would lose some commits. To verify, try this:
git log --oneline --graph --decorate --all
Check for any commits on your current branch that aren’t in the target branch. If you see them, that’s the issue. You can’t do a fast-forward merge because the histories don’t align cleanly. If you still want to merge but avoid merge commits, try using --ff instead of --ff-only. It will fast-forward if possible, and if not, it will fall back without failing unlike --ff-only which requires a fast-forward merge. So, when you get fatal: not possible to fast-forward, aborting, that’s likely the cause.
Exactly, @panchal_archanaa ! Another thing to watch for is local commits that you made after branching off. Even if there’s no diff or conflict, a fast-forward merge won’t work if your branch has commits that don’t exist in the branch you’re merging into. You can quickly check this with:
git status
or to see the differences:
git log HEAD..branch-name
If this returns nothing, but you notice commits when you run git log branch-name..HEAD, it means your branch is ahead. That’s why you’re hitting fatal: not possible to fast-forward, aborting. In this case, consider rebasing your branch with:
git rebase branch-name
Then retry the merge, and it should work if you want to keep your history linear.
Another reason for this error could be that you haven’t pulled the latest changes from the remote branch. If the branch was updated remotely, but your local copy is outdated, Git might block the fast-forward merge. This happens because Git doesn’t have the full context to perform the merge
git fetch origin
After fetching, re-run the merge. I’ve run into this issue many times when trying to merge a stale branch. Keeping everything up-to-date with git fetch and checking the diff with:
git diff branch-name origin/branch-name
helps avoid this kind of confusion. So, if you’re still seeing fatal: not possible to fast-forward, aborting, make sure your branches are synced!