Git Rebase vs. Merge: Understanding Differences and When to Use

In the world of version control, Git offers powerful tools for managing code changes. Among the most fundamental and frequently debated are git merge and git rebase. Both commands are used to integrate changes from one branch into another, but they achieve this in vastly different ways, leading to distinct effects on your project's commit history. Understanding these differences is crucial for maintaining a clean, understandable, and collaborative codebase.

This article will demystify git rebase and git merge. We will explore their core functionalities, analyze their impact on commit history, and provide practical guidance on when to use each command. By the end, you'll be equipped to make informed decisions that contribute to a more organized and efficient Git workflow, especially in collaborative environments.

What is `git merge`?

git merge is the most common and straightforward way to integrate changes from one branch into another. When you merge branch B into branch A, Git looks for a common ancestor commit between A and B. It then creates a new commit (a merge commit) on branch A that has two parents: the tip of A and the tip of B. This merge commit encapsulates all the changes introduced in B since the common ancestor.

Key characteristics of git merge:

Preserves History: git merge creates a merge commit, which explicitly records when and where two branches were joined. This preserves the historical context of your development, showing the branching and merging points.
Non-destructive: It does not rewrite existing commits. The original commits on both branches remain untouched.
Creates Merge Commits: Each merge results in a new commit, which can lead to a more complex and non-linear commit history, often visualized as a graph with multiple branches diverging and converging.

Example of `git merge`:

Let's say you have a main branch and you create a feature branch from it. You make some commits on feature, and meanwhile, new commits are added to main.

# Initial state:
# A -- B -- C (main)
#      \n#       D -- E (feature)

# Switch to the main branch
git checkout main

# Merge the feature branch into main
git merge feature

# Resulting state:
# A -- B -- C -- F (main)
#      \n#       D -- E (feature)
# Where F is the merge commit with parents C and E

In this scenario, the commit F is a merge commit that brings the changes from E into main. The feature branch still exists independently.

What is `git rebase`?

git rebase, on the other hand, is a way to integrate changes from one branch onto another by rewriting your commit history. When you rebase branch B onto branch A, Git takes the commits that are unique to B, temporarily stores them, resets B to the tip of A, and then reapplies the stored commits one by one on top of A.

Key characteristics of git rebase:

Rewrites History: git rebase creates new commits with the same content as the original ones, but with new commit IDs. This makes the commit history appear linear, as if the feature branch was developed sequentially after the latest changes on the target branch.
Avoids Merge Commits: It generally avoids creating merge commits, leading to a cleaner, linear history.
Can be Destructive: Since it rewrites history, git rebase should be used with caution, especially on branches that have been shared with others.

Example of `git rebase`:

Using the same scenario as above:

# Initial state:
# A -- B -- C (main)
#      \n#       D -- E (feature)

# Switch to the feature branch
git checkout feature

# Rebase the feature branch onto main
git rebase main

# Resulting state:
# A -- B -- C (main)
#           \n#            D' -- E' (feature)
# Where D' and E' are new commits with the same content as D and E

After rebasing feature onto main, the commits D and E are replayed on top of commit C. The feature branch now starts from the latest commit in main, and the history is linear. The original commits D and E are effectively abandoned (though recoverable for a time).

Rebase vs. Merge: Key Differences Summarized

Feature	`git merge`	`git rebase`
History	Preserves original history; creates merge commits	Rewrites history; creates a linear history
Commit IDs	Original commits remain unchanged	New commits are created; old ones are abandoned
Collaboration	Safe for shared branches	Risky for shared branches; use on local/private branches
Complexity	Can lead to a complex, non-linear history	Creates a simpler, linear history
Purpose	Integrates changes while retaining context	Integrates changes by reapplying them sequentially

When to Use `git merge`

git merge is generally the safer and more common choice, especially for integrating changes into long-lived branches or when collaborating with a team on a shared branch.

Integrating into main/master: When you want to bring a completed feature branch into your main development line (main or master), merging is often preferred. This preserves the context of the feature branch's development and explicitly marks its integration point.
Shared Branches: If you are working on a branch that is shared with other team members, git merge is almost always the correct choice. Rebasing a shared branch can cause significant problems for your collaborators as it rewrites history they may have already based their work on.
Preserving Release History: For important branches like release branches, maintaining a clear, immutable history with merge commits can be beneficial for auditing and understanding past releases.

Scenario: Merging a completed feature into `main`

# Suppose you're on the 'main' branch and your feature branch is up-to-date
git checkout main
git merge feature-branch-name

This will create a merge commit on main incorporating all changes from feature-branch-name.

When to Use `git rebase`

git rebase is powerful for keeping your local branches up-to-date with a main branch and for cleaning up your own commit history before sharing it.

Updating Local Feature Branches: If you've created a feature branch and the main branch has moved forward, rebasing your feature branch onto main allows you to incorporate those upstream changes without creating an immediate merge commit. This keeps your feature branch commits logically sequential.
Cleaning Up Local History (Interactive Rebase): git rebase -i (interactive rebase) is invaluable for tidying up your own commits before pushing them. You can squash multiple small commits into one, reorder commits, edit commit messages, or even delete commits.
Maintaining a Linear Project History: If your team adopts a workflow that prioritizes a clean, linear history, rebasing feature branches onto main before merging can achieve this. However, this requires strict adherence to the rule of not rebasing shared branches.

Scenario: Updating your feature branch with upstream changes

# Suppose you're on your 'feature' branch and 'main' has new commits
git checkout main             # Switch to main
git pull origin main        # Ensure main is up-to-date
git checkout feature        # Switch back to your feature branch
git rebase main             # Replay your feature commits on top of the latest main

Now, your feature branch is based on the latest main, and when you eventually merge feature back into main, it will be a fast-forward merge (no merge commit needed if no new commits have been made to main since your rebase).

Scenario: Cleaning up your local commits (Interactive Rebase)

# Suppose you made several small commits on your feature branch
git checkout feature-branch-name
git rebase -i HEAD~3      # Rebase the last 3 commits interactively

This will open an editor where you can choose to pick, reword, edit, squash, fixup, or drop your commits, allowing you to consolidate them into a more meaningful set.

Best Practices and Warnings

NEVER Rebase Shared/Public Branches: This is the golden rule. Rebasing branches that others have already pulled and based their work on will cause their history to diverge from yours, leading to confusion and difficult merges for them. Always use git merge for public or shared branches.
Rebase on Your Own Branches: Rebasing is excellent for your local, private feature branches to keep them clean and up-to-date. Once you're satisfied with your local changes, you can then merge them into a shared branch.
Understand the Impact: Before running git rebase, ensure you understand that it rewrites history. If you're unsure, git merge is always the safer option.
Consider Your Team's Workflow: Discuss with your team which strategy (merge vs. rebase) they prefer or what your defined workflow dictates.
Clean History is Important: While git merge preserves history, a history full of many small, insignificant merge commits can become noisy. git rebase can help create a cleaner, more readable history, especially for feature branches before they are merged.

Conclusion

Both git merge and git rebase are essential tools for managing code changes in Git. git merge is about preserving history and integrating changes by creating merge commits, making it safe for shared branches. git rebase is about rewriting history to create a linear, cleaner commit log, which is ideal for local cleanup and updating feature branches before they are shared.

Choosing between them depends on your specific situation, the branch you're working on, and your team's workflow. By understanding their fundamental differences and following best practices, you can effectively leverage both commands to maintain a healthy and understandable project history.