How to rebase one Git repository onto another one?

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?

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

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)

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"

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".