A.I, Data and Software Engineering

WHAT IS GIT REBASE?

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Rebasing is the process of moving or combining a sequence of commits to a new base commit. Rebasing is most useful and easily visualized in the context of a feature branching workflow. The visualized general process is as the following:

From a content perspective, rebasing is changing the base of your branch from one commit to another making it appear as if you’d created your branch from a different commit. Internally, Git accomplishes this by creating new commits and applying them to the specified base. It’s very important to understand that even though the branch looks the same, it comprises of entirely new commits.

Usage

The primary reason for rebasing is to maintain a linear project history. For example, consider a situation where the master branch has progressed since you started working on a feature branch. You want to get the latest updates to the master branch in your feature branch, but you want to keep your branch’s history clean so it appears as if you’ve been working off the latest master branch. This gives the later benefit of a clean merge of your feature branch back into the master branch. Why do we want to maintain a “clean history”? The benefits of having a clean history become tangible when performing Git operations to investigate the introduction of a regression. A more real-world scenario would be:

  1. A bug is identified in the master branch. A feature that was working successfully is now broken.
  2. A developer examines the history of the master branch using git log because of the “clean history” the developer is quickly able to reason about the history of the project.
  3. The developer can not identify when the bug was introduced using git log so the developer executes a git bisect.
  4. Because the git history is clean, git bisect has a refined set of commits to compare when looking for the regression. The developer quickly finds the commit that introduced the bug and is able to act accordingly.

You have two options for integrating your feature into the master branch: merging directly or rebasing and then merging. The former option results in a 3-way merge and a merge commit, while the latter results in a fast-forward merge and a perfectly linear history. The following diagram demonstrates how rebasing onto the master branch facilitates a fast-forward merge.

Rebasing is a common way to integrate upstream changes into your local repository. Pulling in upstream changes with Git merge results in a superfluous merge commit every time you want to see how the project has progressed. On the other hand, rebasing is like saying, “I want to base my changes on what everybody has already done.”

Don’t rebase public history

As we’ve discussed previously in rewriting history, you should never rebase commits once they’ve been pushed to a public repository. The rebase would replace the old commits with new ones and it would look like that part of your project history abruptly vanished.

Git Rebase Standard vs Git Rebase Interactive

Git rebase interactive is when git rebase accepts an -- i argument. This stands for “Interactive.” Without any arguments, the command runs in standard mode. In both cases, let’s assume we have created a separate feature branch.

Git rebase in standard mode will automatically take the commits in your current working branch and apply them to the head of the passed branch.

This automatically rebases the current branch onto , which can be any kind of commit reference (for example an ID, a branch name, a tag, or a relative reference to HEAD).

Running git rebase with the -i flag begins an interactive rebasing session. Instead of blindly moving all of the commits to the new base, interactive rebasing gives you the opportunity to alter individual commits in the process. This lets you clean up history by removing, splitting, and altering an existing series of commits. It’s like Git commit --amend on steroids.

This rebases the current branch onto but uses an interactive rebasing session. This opens an editor where you can enter commands (described below) for each commit to be rebased. These commands determine how individual commits will be transferred to the new base. You can also reorder the commit listing to change the order of the commits themselves. Once you’ve specified commands for each commit in the rebase, Git will begin playing back commits applying the rebase commands. The rebasing edit commands are as follows:

Additional rebase commands

As detailed in the rewriting history page, rebasing is useful to change older and multiple commits, committed files, and multiple messages. While these are the most common applications, git rebase also has additional command options that can be useful in more complex applications.

  • git rebase -- d means during playback the commit will be discarded from the final combined commit block.
  • git rebase -- p leaves the commit as is. It will not modify the commit’s message or content and will still be an individual commit in the branches history.
  • git rebase -- x during playback executes a command-line shell script on each marked commit. A useful example would be to run your codebase’s test suite on specific commits, which may help identify regressions during a rebase.

Wrapping up

Interactive rebasing gives you complete control over what your project history looks like. This affords a lot of freedom to developers, as it lets them commit a “messy” history while they’re focusing on writing code, then go back and clean it up after the fact.

Most developers like to use an interactive rebase to polish a feature branch before merging it into the main code base. This gives them the opportunity to squash insignificant commits, delete obsolete ones, and make sure everything else is in order before committing to the “official” project history. To everybody else, it will look like the entire feature was developed in a single series of well-planned commits.

The real power of interactive rebasing is observable in the history of the resulting master branch. To everybody else, it looks like you’re a brilliant developer who implemented the new feature with the perfect amount of commits the first time around. This is how interactive rebasing can keep a project’s history clean and meaningful.

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