In Git, there are two main ways to integrate changes from one branch into another: the merge and the rebase.

1. fast-forward vs three-way merge#

There are two type of merge:

  • three-way merge,
    • happens when the branches’ commit have diverged
    • will result a merge commit
  • fast-forward merge,
    • happens when there is a liner structure in the commit history
    • we usually prevent fast-forward merge, because there is no merge commit.
    • can be suppressed with the --no-ff option

The liner structure commit structure looks like this:

      A---B fixbug
     /
...--E master

Branches’ diverged commit structure looks like this:

      A---B fixbug
     /
...--E--F master

<div style="position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden;">
  <iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen="allowfullscreen" loading="eager" referrerpolicy="strict-origin-when-cross-origin" src="https://www.youtube.com/embed/zOnwgxiC0OA?autoplay=0&amp;controls=1&amp;end=0&amp;loop=0&amp;mute=0&amp;start=0" style="position: absolute; top: 0; left: 0; width: 100%; height: 100%; border:0;" title="YouTube video"></iframe>
</div>

Video: https://youtu.be/zOnwgxiC0OA

2. git rebase#

The Golden Rule of Rebasing reads: “Never rebase while you’re on a public branch.”

Source: https://www.gitkraken.com/blog/golden-rule-of-rebasing-in-git

On main branch:

* commit 41bc8e89e60d75e21e034aaabcbd20103a61fca4 (HEAD -> main)
|     main: Fri 15 Sep 2023 13:01:02 ADT
| 
* commit 2220b67f9c8ccf5f47e51bff7bd3a3fca6e141b6
      main: Fri 15 Sep 2023 11:47:54 ADT

On fixbug branch:

* commit 0096a2dfa2b4e4c40011213b6cce12ee73833ca9 (HEAD -> fixbug)
|     fixbug: Fri 15 Sep 2023 11:48:53 ADT
| 
* commit fd900fa0c05632757f45fd6fee236eb84c99bb94
|     fixbug: Fri 15 Sep 2023 11:48:27 ADT
| 
* commit 2220b67f9c8ccf5f47e51bff7bd3a3fca6e141b6
      main: Fri 15 Sep 2023 11:47:54 AD

Now the commit history of all branches looks like this:

      A---B fixbug
     /
...--E---F main

Go to main branch and make rebase.

$ git switch main
$ git rebase fixbug
Successfully rebased and updated refs/heads/main.

Check the log, you can see, these two branches combined but the commit histroy structure is linear:

* commit 918260ae4f0b7f7d573a9a640ce380ea0f861a6a (HEAD -> main)
|     main: Fri 15 Sep 2023 13:01:02 ADT
| 
* commit 0096a2dfa2b4e4c40011213b6cce12ee73833ca9 (fixbug)
|     fixbug: Fri 15 Sep 2023 11:48:53 ADT
| 
* commit fd900fa0c05632757f45fd6fee236eb84c99bb94
|     fixbug: Fri 15 Sep 2023 11:48:27 ADT
| 
* commit 2220b67f9c8ccf5f47e51bff7bd3a3fca6e141b6
      main: Fri 15 Sep 2023 11:47:54 ADT

Now the commit history of main branch looks like this (and you can delete fixbug branch):

      A---B---F' main
     /
...--E

As you can see, rebase just make a fast-forward merge happen even the branches has diverged.

And note that the F' mean it’s a copy of original commit F (git will do this internally), this is why we should not use rebase on a shared branch, main for example, because rebase will “change” the commit history of branch main.

<div style="position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden;">
  <iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen="allowfullscreen" loading="eager" referrerpolicy="strict-origin-when-cross-origin" src="https://www.youtube.com/embed/zOnwgxiC0OA?autoplay=0&amp;controls=1&amp;end=0&amp;loop=0&amp;mute=0&amp;start=0" style="position: absolute; top: 0; left: 0; width: 100%; height: 100%; border:0;" title="YouTube video"></iframe>
</div>

3. git rebase vs merge#

  • Merge preserves commit history.
    • Use merge on a public branch.
  • Rebase rewrites (makes copy) history.
  • git push --force on shared branches is an absolute no-no.

Note that we say use rebase on a private branch means we can use the commandgit rebase master on a private, please don’t use git rebase fixissue on master branch which apparently is a publick branch.

Learn more: https://youtu.be/zOnwgxiC0OA?si=CwbvoPI35pHgJ1pn&t=401

<div style="position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden;">
  <iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen="allowfullscreen" loading="eager" referrerpolicy="strict-origin-when-cross-origin" src="https://www.youtube.com/embed/CRlGDDprdOQ?autoplay=0&amp;controls=1&amp;end=0&amp;loop=0&amp;mute=0&amp;start=0" style="position: absolute; top: 0; left: 0; width: 100%; height: 100%; border:0;" title="YouTube video"></iframe>
</div>

References: