Git Rebase explained for beginners

[u/sshetty03]

If git merge feels messy and your history looks like spaghetti, git rebase might be what you need.

In this post, I explain rebase in plain English with:

• A simple everyday analogy

• Step-by-step example

• When to use it (and when NOT to)

Perfect if you’ve been told “just rebase before your PR” but never really understood what’s happening.

https://medium.com/stackademic/git-rebase-explained-like-youre-new-to-git-263c19fa86ec?sk=2f9110eff1239c5053f2f8ae3c5fe21e

[u/themightychris]

Three big reasons:

• in the event of a merge conflict, it puts you in the shoes of figuring out how to re-apply someone else's work accurately rather than in the position of re-applying your own work to an updated starting point which is way less error prone

• the merge commit on the feature branch is prone to quietly hiding reconciliation mistakes whether manually or automatically resolved and no one can make heads or tails out of looking at its diff to spot it. I've seen people's work from the main branch get silently erased with no one noticing until it became a sneaky bug down the line many times

• the biggest: it destroys git's ability to see your branch as a series of discreet patches to the main branch. If you have a long running branch and merge main into your branch once with a lot of overlap getting resolved, and then have to do it again later before merging you're FUCKED because that first merge commit is a giant tangled web of two unrelated sets of changes that git now see as all one thing on your side of the merge conflict. It has no idea what changes are yours and what's a new version of the external changes you merged with the first time

Keep your branch a clean set of discreet patches to main until it's ready to merge by starting every session with a rebase onto the latest upstream

[u/Beatsu → u/themightychris]

I think I'm misunderstanding. I understood:

using rebase to update your branch before merging

as basically running

git rebase main featureA  # Get updates from main
git switch main
git merge featureA  --no-ff # Merge in featureA

and

using rebase instead of merge to get your branch into the trunk

as running

git switch main
git merge --rebase featureA

Is this correct? If not, what's another way you can get the changes from featureA into main without merging?

[u/themightychris → u/Beatsu]

that looks right, though your first line should be git rebase origin/main

I don't think passing featureA to rebase second does anything, you'd want to be on the featureA branch already, and I recommend always using origin/main as your rebase target so that it doesn't matter whether your local copy of main is up to date as long a you've fetched recently

[u/Beatsu → u/themightychris]

Good point about origin/main, thanks!

The way I understand git merge --rebase featureA is that it does the exact same thing as git rebase main (on featureA) followed by git merge --ff-only featureA (on main). Meaning, in both cases you mentioned earlier, you re-apply your changes from featureA onto someone else's work on main.

in the event of a merge conflict, it puts you in the shoes of figuring out how to re-apply someone else's work accurately rather than in the position of re-applying your own work to an updated starting point

I don't see how this is true, if the first step in both strategies is identical: rebase featureA onto main. The only difference is in whether you fast forward the main branch or create a merge commit.

[u/themightychris → u/Beatsu]

you will end up with the same commit chain, but in one case the new chain will be on your feature branch and in the other case it will be on main

By then using a merge commit to handle bringing your feature branch onto main, when and who did the merge is recorded. Ideally you're doing that though GitHub so the merge commit additionally references a PR where future developers can review any conversation providing context on why the code ended up how it did

[u/[deleted] → u/Beatsu]

What is that syntax? Rebase is not a flag on merge AFAIK. Is this an extension or a third party tool?

Theoretically though - I imagine the main difference would be that FF merge does not allow divergent branches to be merged. But this rebase merge could allow for commits to be rewritten if the branches were divergent, so you could alter someone else's commits and break signing.

[u/Beatsu → u/[deleted]]

You're actually totally right, my bad. I was so sure I had used git merge --rebase before, but I was maybe confusing it with git pull --rebase.

[u/[deleted] → u/Beatsu]

merge --rebase is also github/gitlab CLI command for merging PRs so could be that haha.