Git Merge vs. Git Rebase: A Beginner’s Guide to Integrating Changes

Welcome to the world of Git! As you start collaborating on code projects, you’ll quickly encounter the need to bring changes from one branch into another. Two primary commands for this are git merge and git rebase. While both achieve the goal of integration, they do so in fundamentally different ways, impacting your project’s history.

Understanding Git Merge vs Git Rebase is crucial for clean and manageable version control, especially for beginners. Let’s break down these two core concepts.

What is Git Merge?

Think of git merge as bringing two diverging timelines together. When you merge branch ‘feature’ into branch ‘main’, Git finds the common ancestor commit, then takes all the commits made on the ‘feature’ branch since that ancestor and all the commits made on the ‘main’ branch, and combines them. It creates a new commit, called a merge commit, on the target branch (in this case, ‘main’) that ties together the histories of both branches.

Here’s what happens:

• Git identifies the latest commit on each branch and their most recent common ancestor.
• It combines the changes from both branches.
• A new merge commit is created on the target branch. This commit has two parent commits – the last commit of the target branch and the last commit of the branch being merged.
• The branch pointers are updated.

[Hint: Insert image/video illustrating a 3-way merge process with a new merge commit]

Key characteristics of git merge:

• Preserves History: The most significant aspect of merging is that it keeps the original history intact. You can see exactly when and where branches diverged and were joined.
• Non-Destructive: It doesn’t rewrite existing commits. It simply adds a new merge commit.
• Safe and Common: It’s generally considered the “safe” and default way to integrate changes.
• Merge Commits: Can lead to a history graph with many branching and merging lines, sometimes referred to as “octopus merges” if many branches are merged simultaneously.

If Git can combine the changes automatically, it performs a “fast-forward” merge (if the target branch hasn’t moved since the feature branch diverged) or a standard three-way merge. If there are conflicting changes in the same part of a file, you will encounter a merge conflict that you need to resolve manually before the merge commit can be finalized.

What is Git Rebase?

git rebase, on the other hand, offers a different approach focused on creating a linear history. When you rebase your ‘feature’ branch onto ‘main’, Git doesn’t merge the branches. Instead, it takes the commits from your ‘feature’ branch and reapplies them one by one onto the latest commit of the ‘main’ branch.

Imagine lifting your feature branch’s changes off their starting point and transplanting them onto the tip of ‘main’.

Here’s the process:

• Git goes back to the common ancestor of ‘feature’ and ‘main’.
• It saves the changes from each commit on the ‘feature’ branch temporarily.
• It moves the ‘feature’ branch pointer to the latest commit of the ‘main’ branch.
• It reapplies the saved changes (commits) from the ‘feature’ branch one by one onto the ‘main’ branch. Because these are new commits created by reapplying the changes, they will have different commit IDs than the originals.

[Hint: Insert image/video illustrating the rebase process, showing commits being reapplied linearly]

Key characteristics of git rebase:

• Rewrites History: This is the most significant difference. Rebase creates new commits with new IDs for the changes it reapplies. The original commits on the feature branch are effectively replaced.
• Linear History: The result is a clean, straight history line, making it easier to follow the sequence of changes.
• “Destructive” Operation: Because it rewrites history, rebase is considered a “destructive” operation compared to merge. This is particularly important if you’ve pushed your branch to a remote repository, as rebasing and force-pushing can cause issues for collaborators.
• Conflict Resolution: You might encounter conflicts during the rebase process. You resolve them commit by commit as Git reapplies them.

A common use case for rebase is to keep your local feature branch up-to-date with the main development branch before merging. Instead of merging ‘main’ into your ‘feature’ branch (which would add merge commits to your feature branch history), you can rebase ‘feature’ onto ‘main’. This incorporates the latest ‘main’ changes into your ‘feature’ branch with a clean, linear history before you eventually merge ‘feature’ back into ‘main’ (often with a fast-forward merge or a squash merge).

Git Merge vs Git Rebase: Choosing the Right Tool

So, when should you use which?

• Use git merge when:
  • You want to preserve the exact history of your project, showing where feature branches diverged and merged.
  • You are working on a public or shared branch that others are also using. Rewriting history on shared branches can cause significant problems for collaborators.
  • You prefer a history graph that reflects the actual development flow, including all integration points.
• Use git rebase when:
  • You want a clean, linear project history without extraneous merge commits.
  • You are working on a local, private feature branch that no one else has based their work on yet.
  • You want to integrate changes from a main branch into your feature branch and prefer to keep your feature branch’s history tidy before merging it back.
  • You want to clean up a messy history by squashing multiple small commits into a single, meaningful one before integrating them (often done interactively with git rebase -i).

[Hint: Insert image comparing the history graphs produced by merge vs rebase]

Many teams adopt specific workflows around these commands. Some prefer a strictly linear history facilitated by rebasing, while others prioritize an accurate historical record provided by merging. There’s no single “right” answer; the best choice often depends on team preference and project needs.

Both commands also support squashing commits using the --squash flag, which can combine multiple commits into one before integration, further streamlining the history regardless of whether you merge or rebase.

In Summary

Ultimately, both git merge and git rebase are powerful tools for integrating changes in Git. Merge preserves history and is non-destructive, creating merge commits. Rebase rewrites history for a cleaner, linear look but should be used cautiously on shared branches. Understanding their fundamental differences and the impact they have on your commit history is key to using Git effectively as a beginner.

As you gain more experience, you’ll develop a better intuition for which command suits different scenarios. For a deeper dive into the foundations of version control, check out our guide What is Version Control? An Introduction to Git for Beginners.

For more detailed documentation, refer to the official Git documentation.