How to Use Git Rebase to Squash a Sequence of Commits to a New Base Commit

May 4th 2021 new story

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What is a rebase? 📚

Rebasing is one of the two Git processes of integrating changes from one branch to another.

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While merging always moves a branch forward in history by simply adding a new commit representing each branch integration, rebasing can be much more powerful (and dangerous) as it allows us to rewrite history.

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Why do we rebase? 🧐

Rebasing is the process of moving or combining (i.e. squashing) a sequence of commits to a new base commit.

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Imagine you’re working on a feature branch and after some time you realise the main (aka "master") branch has been updated by someone else's code. This means your branch has now diverged from the main branch. At some point, you will want to include said changes in your feature branch. A common way of doing this would be to simply do a git pull from main which would add a merge commit to your feature branch.

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The issues with this method can be:

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• Merge commits

   can be very vague, providing very little information in the git tree which makes debugging a much harder task
• Updating a branch — by 

  pulling/ merging

   - multiple times can result in a git history similar to this:

Reading this history provides very little (if any) information on what changes have been made to the project.

The goal of rebasing is to keep a clean, clear project history. So instead of pulling the 

main

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Feature branch JIRA-123 is being rebased to the top of the master branch

By rebasing our feature branch to the top of the 

main

updates

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$ git rebase main

This will start the rebase process. If no conflicts are found you should see a success message and your feature branch is now up-to-date with the main branch and we're one step closer to having a project history similar to this:

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Interactive rebase 🔬

Interactive rebasing brings the power of rebasing to a whole new level! It allows us to interactively change multiple commits in different ways.

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You can start an interactive rebase by executing a rebase command followed by the -i parameter and the commit we want to modify:

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$ git rebase -i <commit hash | HEAD position>

Interactive rebase CLI showing the three last commits and always in order from oldest (top) to newest (bottom)

We can tell just by looking at the list of commands displayed in the CLI how powerful this tool can be! We can edit commits, squash them, remove them, amend them and so much more. We can even permanently change the commit order!

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Squashing

Squashing allows us to specify which commits we want to combine in order to help maintain clean history.

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Does something like this look familiar?

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Well, if you like committing a lot while you work you probably don’t like thinking about being super descriptive on every single commit message. Squashing can be extremely helpful in these situations because starting an interactive rebase you can then tell git which commits will be “merged” into one (or more) and then edit its commit message.

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Keep in mind that we must always squash a commit “into” another one.

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So say we have 6 commits and we think only two of them are relevant we could squash the other 4 like:

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$ git rebase -i HEAD~6

“s” is simply a shorthand name for “squash”

In this case we’re saying we’d like to merge commits 2–5 and include them in the first one ( 77497f5) and keep the last commit as-is.

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Next git will rebase each commit to their new base ( 77497f5) and if there are no conflicts, you'll be able to change their final commit messages, and voila! Only 2 commits total!

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What could go wrong? 😇

Conflicts

Similarly to what happens with a Git merge, updating a branch with another can result in code conflicts.

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A big difference in rebase conflicts is that we’re not dealing with one merge commit but with (possibly) multiple commits being moved.

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This means rebase conflict solving is an iterative process that goes through each commit being rebased.

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In a feature branch with two commits, if a conflict is found while rebasing commit A we fix it, tell git to continue the process to the next commit, fix the new conflicts on commit B and only then we’ll be done

Pushing local branch after rebasing

Rebasing a local feature branch means we’re rewriting history, so what this actually means is that we’re not cherry-picking the feature branch commits to the new base. Although visually everything looks the same, Git creates new commits (with new hashes) on the targeted branch.

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This means whenever we try simply pushing our changes to our remote repo we get an error stating the local and remote branches have diverged.

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To fix this problem it’s good practice to push using the --force-with-lease parameter.

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You can check -force vs -force with lease to learn more about this.

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“I think I made a mistake”

Have you heard of the beautiful command:

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$ git reflog

Reflog allows us to check every change we did to our local branch. Running the command will return something like this:

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Reflog output showing the last 3 actions

In this case, if I’d like to go back before I ever did the rebase I could jump back to HEAD~2 or using its hash instead:

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$ git checkout 682c215

At this point, I’m in the past and can undo the mistakes I’d made before. Time travel!! 🏎🔥

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If things get out of control

At any point in our interactive rebase if things get too messy and we no longer know how to fix them, we can always abort the whole process in the terminal and the interactive rebase will be canceled completely:

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$ git rebase — abort

Conclusion

As you can tell rebase can be used for a lot of different purposes, especially using the interactive rebase functionality. Hopefully, this article provides you with just enough to get started messing around with it.

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Don’t worry if you run into issues understanding some of the nuances of Git rebase. It has its learning curve and that’s why I included some escape methods so you can go back and try again 💪

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Also published on Alex's Medium

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