How to rebase one Git repository onto another one?

Question

I had one Git repository (A) which contains the development of a project until a certain point. Then I lost the USB stick this repo A was on. Luckily I had a backup of the latest commit, so I could create a new repository (B) later where I imported the latest project's state and continue development. Now I recovered that lost USB stick, so I have two Git repositories.

I think I just have to rebase repo B onto repo A somehow, but I have no idea how to do that, maybe using fetch/pull and rebase?

Solution 1

If A and B are not the same repo (you created B by using the latest working copy you had), you have to use a graft to pretend that they have common history.

Let’s assume you’ve added A as a remote for B as per VonC’s answer, and the repo looks like this¹:

~/B$ git tnylog 
* 6506232 (HEAD, master) Latest work on B
* 799d6ae Imported backup from USB stick
~/B$ git tnylog A/master
* 33b5b16 (A/master) Head of A
* 6092517 Initial commit

Create a graft telling the root of B that its parent is the head of A:

echo '799d6aeb41095a8469d0a12167de8b45db02459c 33b5b16dde3af6f5592c2ca6a1a51d2e97357060' \
 >> .git/info/grafts

Now the two histories above will appear as one when you request the history for B. Making the graft permanent is a simple git filter-branch with no arguments. After the filter-branch, though, you aren’t on any branch, so you should git branch -D master; git checkout -b master.

---

¹ git tnylog = git log --oneline --graph --decorate

Solution 2

If A and B are the same repo (the first SHA1 are common), you can:

• declare A as a remote for B: git remote add A /path/to/A
• git fetch A to update all remote A branches on the B repo
• git checkout dev (on B, where you are developing)
• git rebase A/devBranch to replay B (i.e. what you develop or re-develop from your backup) on top of A/devBranch (the development you lost). A bit like this SO question.

The last step allows you to sync your dev with the one you lost.
But actually, once you have fetch from A, you are done: B now contains the "all" history (the one you lost and your current work)

Solution 3

2021 Update

git filter-branch has been deprecated since Git 2.24 (Q4 2019). Instead, for the scenario where A and B are not the same repo, you can use git replace --graft <commit> <parent>.

See:

• "How do I combine several Git repositories without breaking file history?"
• "Git commands that could break/rewrite the history"

Solution 4

First of all, start by making a working clone of repo A.

Then simply pull into it from B and merge. You might prefer to create a new branch, pull onto it, then merge the two branches. You might also need a forcing flag; I've done things like this in Mercurial (grafting two apparently-unrelated repositories together) and it needs "-f".

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Comments

[kroimon]

Thanks, seems to work, just have to resolve some conflicts during the merge/rebase now :)

[VonC]

@kroimon: conflicts were inevitable, I suppose, since you were re-developing in B some part of the code committed in A.

[VonC]

+1 for taking care of the "other scenario" were A and B differ completely in commit history.

[Sedate Alien]

This was a brilliant help; did absolutely what I wanted with a minimum of fuss. Thank you!

[Filip Dupanović]

I just used the subtree-merge approach to stitch several repositories into one. I successively failed at the attempt to rebase them one over the other. Using graft points, I've now been able to stitch them onto a single development timeline, thanks!

[Alberto]

+1 By the way, I edited your answer to reflect what your git-tnylog does (I was puzzled and took me a while until I found it was an alias of yours: 'Pimped tnylog a bit')

[Pim Jager]

How would you push this to a remote? Say I host my B on bitbucket and want to push this. Simply (--force) pushing a new commit to bitbucket doesn't (appear) to add the history of A to the the remote bitbucket.

[Pim Jager]

I already tried the command git filter-branch 33b5b16dde3af6f5592c2ca6a1a51d2e97357060..HEAD from link but then I can't push anymore since I get the error that the remote has unresolved deltas (remote: fatal: pack has 50 unresolved deltas).

[Pim Jager]

Update: after an hour of googling and using the rebasing option from this answer, I managed to create something which could be pushed to a new bare repo.

[Alison S]

I ended up in a detached HEAD, and had to reference this: stackoverflow.com/a/25670296/1807668

[VonC]

@AlisonS I agree: the rebase command was incorrect. If you have checked out dev, all you need is git rebase A/devBranch in order to replay dev on top of A/devBranch. I have edited the answer.

[Admin]

@PimJager (or anyone else with the same query), see: How to push a “git replace --graft”.

[Admin]

Related: How do git grafts and replace differ? (Are grafts now deprecated?).

[timgeb]

This worked well for me without A and B having a common first SHA1. I did an interactive rebase and dropped some initial commits.