Cherry-picking git commits amongst different repositories

I am trying how the git cherry-pick commands works and, in the specific, I am wondering if it is suitable to copy commits amongst two different repositories that are sharing only the first part of the git log history. Let’s start by creating two new repositories named repo_A and repo_B.

➜ cherry-pick-tutorial mkdir repo_{A,B}; ls | parallel 'cd {}; git init; cd ..'

I am now going to store the “About the Teeth of Sharks” poem (by John Ciardi) in the repo_A repository, but I’ll do that triggering one commit for each line.

1. The thing about a shark is—teeth,
2. One row above, one row beneath.
3. Now take a close look. Do you find
4. It has another row behind?
5. Still closer—here, I’ll hold your hat:
6. Has it a third row behind that?
7. Now look in and…Look out! Oh my,
8. I’ll never know now! Well, goodbye.

Start building repo_A

This is the initial repository named repo_A. Let’s perform 2 commits into this one with the first two lines. The result would be this log history

* 9e9e702 (HEAD -> master) second poem line

Push everything from repo_A to repo_B

We would like to push every commit done in repo_A into repo_B. So, cd into the repo_B folder and trigger: git config receive.denyCurrentBranch updateInstead. This will allow us to perform the following push operation from the repo_A folder:

➜  repo_A git:(master) git push --mirror ../repo_B/.git

Now, if you check the repo_B git log history, You’ll notice that the destination repository have the same commits and each of them has the same commit hash.

To easily remember which is the latest repo_A git commit hash included in repo_B, I suggest creating a tag pointing to 9e9e702. This can be done with git tag -a -m "repo_B started here" first_sync 9e9e702ff99845a1a0543ed84d1fb9503014cd65.
An alternative would be to use the git cherry command which I am not covering in this post.

Continue building repo_A

Let’s keep up with the good work on the first repository and commit an additional line:

* 828d233 (HEAD -> master) third poem line

Cherry-pick one single commit from repo_A to repo_B

Ok, here we are. Now we can perform the cherry-picking operation. First of all, we would like to set repo_A to be a remote of repo_B. So, cd into the repo_B folder and trigger git remote add repo_a ../repo_A/.git. Now repo_a is the remote name for the repo_A repository. Please, remember to git fetch repo_a before starting using it.

➜  repo_B git:(master) git remote -v                       

Then, we can cherry-pick 828d233 from repo_A in this way:

➜  repo_B git:(master) git cherry-pick 828d233ee36d640638e8b3c0a1e49c2acb42bec2

The repo_B log history has now the additional commit:

* 2a542c9 (HEAD -> master) third poem line

Please, note how the commit hash for the third poem line is changing after the cherry-pick. This means that we are cherry-picking the commit content into a new commit stored in the repo_B.

For our convenience, let’s annotate the repo_A commit we just picked by creating a tag pointing to 828d233. This can be done with: repo_A git:(master) git tag -a -m "I picked this commit" second_sync 828d233ee36d640638e8b3c0a1e49c2acb42bec2.

Finishing our poem

Let’s finish the poem in repo_A by adding the remaining lines. The result will be to Now we have this repo_A log history:

* 4a6a6c3 (HEAD -> master) eighth poem line

Copy all the remaining commits

You should be a master in cherry-picking now! Go ahead and cherry pick from dd7fc45 to 4a6a6c3:

➜  repo_B git:(master) git cherry-pick dd7fc45d5da43a8711ea1f340b3266aebe3e14bc^..4a6a6c34422cbce23f11a0e61104608de357a170

What? Four new commits in one single git command? Yes.

The basic form to cherry-pick a range of commits is git cherry-pick 828d233..4a6a6c3. The first hash in the range is the oldest commit and the last hash in the range is the newest commit. The confusion with this form of the cherry-pick merge is that the first hash listed in the range is not included in the commit. That’s why I opted for the form: git cherry-pick dd7fc45^..4a6a6c3 that includes the dd7fc45 commit and seems more intuitive to me.

This is the resulting repo_B log history:

* 1ab66d5 (HEAD -> master) eighth poem line

As a final note, what I did here was to merely copy every commit. Most probably, I would have reached the same result with a common git merge operation. The cherry-pick command would come in handy when you want to selectively select the commits to port over another branch.