Understanding git rebase: Does It Require a Common Commit Ancestor?

Explore the intricacies of `git rebase` and learn why it doesn't always require a common commit ancestor. Uncover the surprising flexibility of this powerful git command.
---
This video is based on the question https://stackoverflow.com/q/70412430/ asked by the user 'Jamie Julius' ( https://stackoverflow.com/u/3240980/ ) and on the answer https://stackoverflow.com/a/70412960/ provided by the user 'matt' ( https://stackoverflow.com/u/341994/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Does git rebase ever require a common commit ancestor?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/licensing
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding git rebase: Does It Require a Common Commit Ancestor?

When working with Git, you might often find yourself navigating through branches and commits, trying to streamline your development process. One crucial aspect of this management is the concept of rebasing branches, which can sometimes lead to confusion. A common question that arises is whether git rebase necessitates a common commit ancestor. Let’s delve into this subject, approach the mechanics behind rebasing, and clarify how it interacts with commit ancestors.

The Scenario: Branches Without a Common Ancestor

Imagine a situation where you've created two branches—master and other. You start on the master branch, and you attempt to merge other with a straightforward command:

[[See Video to Reveal this Text or Code Snippet]]

However, Git responds with an error: fatal: refusing to merge unrelated histories. This error is expected because merges typically require a shared history between the branches being merged. This raises your curiosity: will git rebase behave similarly?

You may be surprised to find that running the command:

[[See Video to Reveal this Text or Code Snippet]]

succeeds without issue. How can this be? Isn’t rebasing just like merging? Let’s break it down to understand this behavior better.

What is git rebase?

The concept of rebase can often be misunderstood, but a clear and comprehensive understanding reveals its functionality:

Rebase vs. Merge: While merging brings together two different histories, rebasing takes a branch and ‘replays’ its commits onto another branch. This means the result is a linear series of commits instead of a tree structure.

The Power of Cherry-Picking: Under the hood, rebase operates like a series of cherry-picks. Essentially, when you instruct Git to rebase, it creates new commits based on the differences from your current branch up to the target branch, bypassing the original commit history.

How Does the Rebase Command Work?

When you issue the command git rebase other, you might think of it as a shorthand for something more complex. The full command can be viewed as:

[[See Video to Reveal this Text or Code Snippet]]

Breaking Down the Components:

<target>: This is the branch where you want to apply the rebased commits.

<upstream>: Traditionally, this is the common ancestor where the two branches diverged.

<branch>: This is the current working branch being rebased.

In the situation of having no common ancestor, git cleverly opts to use the "root" or the "nothingness" when it tries to establish the upstream. This means that instead of failing due to the absence of a shared history, Git simply creates new commits from scratch.

Illustrated Example

Consider the following branches for clarification:

[[See Video to Reveal this Text or Code Snippet]]

When you execute git rebase one from two, Git will analyze each commit on two, starting at d. It will try to cherry-pick the changes between nothingness (since there's no ancestor) and commit c onto b, and then be able to take the changes from c to d and finish the rebase. The commits c and d become c' and d', effectively creating new versions that exist in a clean linear history without referencing their original branches:

[[See Video to Reveal this Text or Code Snippet]]

Note that while c and d remain available, they are not directly referenced, and thus will be collected by Git's cleanup routines later on.

Conclusion

In summary, git rebase does not inherently require a common commit ancestor, which can be a surprising discovery for many developers. The flexibility of rebase allows it to function even in cases where the histories of branches do not intersect. By u

Видео Understanding git rebase: Does It Require a Common Commit Ancestor? канала vlogize