GIT Rebasing and Merging
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The document discusses the differences between Git merging and rebasing. Merging creates a merge commit to join branches, keeping the original branch histories intact. Rebasing rewrites history by applying commits from one branch onto another branch, creating linear history but losing original commit context. Rebasing is best for local branches while merging is safer for public branches being collaborated on. Interactive rebasing allows altering commits before moving them to another branch, enabling history cleanup.
![AshokKumarSatuluri
11 | P a g e
If you’re not entirely comfortable with gitrebase, you can always perform the rebase in a
temporary branch. That way, if you accidentally mess up your feature’s history, you can
check out the original branch and try again. For example:
git checkout feature
git checkout -b temporary-branch
git rebase -i master
# [Clean up the history]
git checkout master
git merge temporary-branch
Summary
And that’s all you really need to know to start rebasing your branches. If you would prefer a
clean, linear history free of unnecessary merge commits, you should reach for git
rebaseinstead of gitmergewhen integrating changes from another branch.
On the other hand, if you want to preserve the complete history of your project and avoid
the risk of re-writing public commits, you can stick with gitmerge. Either option is perfectly
valid, but at least now you have the option of leveraging the benefits of gitrebase.](https://image.slidesharecdn.com/269db8cc-0f61-4270-8f55-313f96e00a19-160127053925/85/GIT-Rebasing-and-Merging-11-320.jpg)
GIT Rebasing and Merging
- 1. AshokKumarSatuluri 1 | P a g e GIT Merging Vs Rebasing Overview: The first thing to understand about gitrebaseis that it solves the same problem as gitmerge. Both of these commands are designed to integrate changes from one branch into another branch—they just do it in very different ways Consider what happens when you start working on a new feature in a dedicated branch, then another team member updates the masterbranch with new commits. This results in a forked history, which should be familiar to anyone who has used Git as a collaboration tool. Now, let’s say that the new commits in master are relevant to the feature that you’re working on. To incorporate the new commits into your featurebranch, you have two options: merging or rebasing. The Merge Option The easiest option is to merge the master branch into the feature branch using something like the following: git checkout feature git merge master Or, you can condense this to a one-liner: git merge master feature This creates a new “merge commit” in the featurebranch that ties together the histories of both branches, giving you a branch structure that looks like this:
- 2. AshokKumarSatuluri 2 | P a g e Merging is nice because it’s a non-destructive operation. The existing branches are not changed in any way. This avoids all of the potential pitfalls of rebasing (discussed below). On the other hand, this also means that the featurebranch will have an extraneous merge commit every time you need to incorporate upstream changes. If master is very active, this can pollute your feature branch’s history quite a bit. While it’s possible to mitigate this issue with advanced gitlogoptions, it can make it hard for other developers to understand the history of the project. The Rebase Option As an alternative to merging, you can rebase the featurebranch onto master branch using the following commands: git checkout feature git rebase master This moves the entire featurebranch to begin on the tip of the master branch, effectively incorporating all of the new commits in master. But, instead of using a merge commit, rebasing re-writes the project history by creating brand new commits for each commit in the original branch.
- 3. AshokKumarSatuluri 3 | P a g e The major benefit of rebasing is that you get a much cleaner project history. First, it eliminates the unnecessary merge commits required by gitmerge. Second, as you can see in the above diagram, rebasing also results in a perfectly linear project history—you can follow the tip of featureall the way to the beginning of the project without any forks. This makes it easier to navigate your project with commands like git log, gitbisect, and gitk. But, there are two trade-offs for this pristine commit history: safety and traceability. If you don’t follow the Golden Rule of Rebasing, re-writing project history can be potentially catastrophic for your collaboration workflow. And, less importantly, rebasing loses the context provided by a merge commit—you can’t see when upstream changes were incorporated into the feature. Interactive Rebasing Interactive rebasing gives you the opportunity to alter commits as they are moved to the new branch. This is even more powerful than an automated rebase, since it offers complete control over the branch’s commit history. Typically, this is used to clean up a messy history before merging a feature branch into master. To begin an interactive rebasing session, pass the i option to the gitrebasecommand: git checkout feature git rebase -i master This will open a text editor listing all of the commits that are about to be moved: pick 33d5b7a Message for commit #1 pick 9480b3d Message for commit #2 pick 5c67e61 Message for commit #3 This listing defines exactly what the branch will look like after the rebase is performed. By changing the pick command and/or re-ordering the entries, you can make the branch’s
- 4. AshokKumarSatuluri 4 | P a g e history look like whatever you want. For example, if the 2nd commit fixes a small problem in the 1st commit, you can condense them into a single commit with the fixup command: pick 33d5b7a Message for commit #1 fixup 9480b3d Message for commit #2 pick 5c67e61 Message for commit #3 When you save and close the file, Git will perform the rebase according to your instructions, resulting in project history that looks like the following: Eliminating insignificant commits like this makes your feature’s history much easier to understand. This is something that git mergesimply cannot do. The Golden Rule of Rebasing Once you understand what rebasing is, the most important thing to learn is when not to do it. The golden rule of git rebaseis to never use it on public branches. For example, think about what would happen if you rebased masteronto your featurebranch:
- 5. AshokKumarSatuluri 5 | P a g e The rebase moves all of the commits in master onto the tip of feature. The problem is that this only happened in your repository. All of the other developers are still working with the original master. Since rebasing results in brand new commits, Git will think that your master branch’s history has diverged from everybody else’s. The only way to synchronize the two master branches is to merge them back together, resulting in an extra merge commit and two sets of commits that contain the same changes (the original ones, and the ones from your rebased branch). Needless to say, this is a very confusing situation. So, before you run gitrebase, always ask yourself, “Is anyone else looking at this branch?” If the answer is yes, take your hands off the keyboard and start thinking about a non- destructive way to make your changes (e.g., the gitrevertcommand). Otherwise, you’re safe to re-write history as much as you like. Force-Pushing If you try to push the rebased master branch back to a remote repository, Git will prevent you from doing so because it conflicts with the remote master branch. But, you can force the push to go through by passing the --forceflag, like so: # Be very careful with this command! git push --force This overwrites the remote master branch to match the rebased one from your repository and makes things very confusing for the rest of your team. So, be very careful to use this command only when you know exactly what you’re doing. One of the only times you should be force-pushing is when you’ve performed a local cleanup after you’ve pushed a private feature branch to a remote repository (e.g., for backup purposes). This is like saying, “Oops, I didn’t really want to push that original
- 6. AshokKumarSatuluri 6 | P a g e version of the feature branch. Take the current one instead.” Again, it’s important that nobody is working off of the commits from the original version of the feature branch. Workflow Walkthrough Rebasing can be incorporated into your existing Git workflow as much or as little as your team is comfortable with. In this section, we’ll take a look at the benefits that rebasing can offer at the various stages of a feature’s development. The first step in any workflow that leverages gitrebaseis to create a dedicated branch for each feature. This gives you the necessary branch structure to safely utilize rebasing: Local Cleanup One of the best ways to incorporate rebasing into your workflow is to clean up local, in- progress features. By periodically performing an interactive rebase, you can make sure each commit in your feature is focused and meaningful. This lets you write your code without worrying about breaking it up into isolated commits—you can fix it up after the fact. When calling git rebase, you have two options for the new base: The feature’s parent branch (e.g., master), or an earlier commit in your feature. We saw an example of the first option in the Interactive Rebasing section. The latter option is nice when you only need to fix up the last few commits. For example, the following command begins an interactive rebase of only the last 3 commits. git checkout feature git rebase -i HEAD~3
- 7. AshokKumarSatuluri 7 | P a g e By specifying HEAD~3 as the new base, you’re not actually moving the branch—you’re just interactively re-writing the 3 commits that follow it. Note that this will not incorporate upstream changes into the featurebranch. If you want to re-write the entire feature using this method, the gitmerge-basecommand can be useful to find the original base of the featurebranch. The following returns the commit ID of the original base, which you can then pass to git rebase: git merge-base feature master This use of interactive rebasing is a great way to introduce gitrebaseinto your workflow, as it only affects local branches. The only thing other developers will see is your finished product, which should be a clean, easy-to-follow feature branch history. But again, this only works for private feature branches. If you’re collaborating with other developers via the same feature branch, that branch is public, and you’re not allowed to re- write its history. There is no gitmergealternative for cleaning up local commits with an interactive rebase. IncorporatingUpstream Changes Into a Feature In the Conceptual Overview section, we saw how a feature branch can incorporate upstream changes from master using either gitmergeor gitrebase. Merging is a safe option that preserves the entire history of your repository, while rebasing creates a linear history by moving your feature branch onto the tip of master. This use of gitrebaseis similar to a local cleanup (and can be performed simultaneously), but in the process it incorporates those upstream commits from master.
- 8. AshokKumarSatuluri 8 | P a g e Keep in mind that it’s perfectly legal to rebase onto a remote branch instead of master. This can happen when collaborating on the same feature with another developer and you need to incorporate their changes into your repository. For example, if you and another developer named John added commits to the featurebranch, your repository might look like the following after fetching the remote featurebranch from John’s repository: You can resolve this fork the exact same way as you integrate upstream changes from master: either merge your local featurewithjohn/feature, or rebase your local featureonto the tip of john/feature.
- 9. AshokKumarSatuluri 9 | P a g e Note that this rebase doesn’t violate the Golden Rule of Rebasingbecause only your local featurecommits are being moved—everything before that is untouched. This is like saying, “add my changes to what John has already done.” In most circumstances, this is more intuitive than synchronizing with the remote branch via a merge commit. By default, the gitpull command performs a merge, but you can force it to integrate the remote branch with a rebase by passing it the --rebaseoption. Reviewing a Feature With a Pull Request If you use pull requests as part of your code review process, you need to avoid using git rebaseafter creating the pull request. As soon as you make the pull request, other developers will be looking at your commits, which means that it’s a public branch. Re- writing its history will make it impossible for Git and your teammates to track any follow- up commits added to the feature. Any changes from other developers need to be incorporated with gitmergeinstead of git rebase. For this reason, it’s usually a good idea to clean up your code with an interactive rebase before submitting your pull request.
- 10. AshokKumarSatuluri 10 | P a g e Integratingan Approved Feature After a feature has been approved by your team, you have the option of rebasing the feature onto the tip of the master branch before usinggitmergeto integrate the feature into the main code base. This is a similar situation to incorporating upstream changes into a feature branch, but since you’re not allowed to re-write commits in themaster branch, you have to eventually use gitmergeto integrate the feature. However, by performing a rebase before the merge, you’re assured that the merge will be fast-forwarded, resulting in a perfectly linear history. This also gives you the chance to squash any follow-up commits added during a pull request.
- 11. AshokKumarSatuluri 11 | P a g e If you’re not entirely comfortable with gitrebase, you can always perform the rebase in a temporary branch. That way, if you accidentally mess up your feature’s history, you can check out the original branch and try again. For example: git checkout feature git checkout -b temporary-branch git rebase -i master # [Clean up the history] git checkout master git merge temporary-branch Summary And that’s all you really need to know to start rebasing your branches. If you would prefer a clean, linear history free of unnecessary merge commits, you should reach for git rebaseinstead of gitmergewhen integrating changes from another branch. On the other hand, if you want to preserve the complete history of your project and avoid the risk of re-writing public commits, you can stick with gitmerge. Either option is perfectly valid, but at least now you have the option of leveraging the benefits of gitrebase.