What is the difference between a Git merge and a Git rebase?

Both Git merge and Git rebase are commands used to integrate changes from one branch into another. While they achieve a similar outcome of combining work, they do so in fundamentally different ways, impacting the project's commit history and workflow.

Git Merge

Git merge integrates changes by creating a new 'merge commit'. This commit has two parent commits: the tip of the branch you are merging into, and the tip of the branch you are merging from. The merge commit explicitly records the fact that two branches were joined. This method preserves the original history of both branches exactly as it happened, making it a non-destructive operation.

When conflicts occur during a merge, you resolve them once and then commit the resolution as part of the new merge commit. Merging is generally preferred for integrating changes into public, shared branches because it maintains a clear, traceable history of when and where integrations occurred, without rewriting past commits.

Git Rebase

Git rebase integrates changes by moving or rewriting a series of commits to a new base commit. Instead of creating a new merge commit, rebase effectively replays the commits from your feature branch one by one onto the target branch's latest commit. This creates a linear history, making it look as if the feature branch started from the very latest state of the target branch.

Rebasing rewrites history by creating new commit objects with different SHAs for the rebased commits. This can lead to a cleaner, more linear project history without extra merge commits. However, because it rewrites history, rebasing should generally only be done on private, unpushed branches. Rebasing a public branch that others have already pulled from can cause significant issues and confusion for collaborators.

Key Differences and Considerations

• History: Merge preserves the exact history, including all merge commits. Rebase rewrites history to create a linear progression, removing merge commits and potentially changing commit SHAs.
• Commit Graph: Merge creates a diamond shape in the commit graph, clearly showing divergent development paths converging. Rebase creates a straight, linear commit graph, as if all work happened sequentially.
• Merge Commits: Merge introduces a new merge commit. Rebase avoids merge commits, resulting in a cleaner history.
• Destructive vs. Non-Destructive: Merge is non-destructive; it never changes existing commits. Rebase is destructive to the original commits being replayed as it creates new ones with different SHAs.
• Collaboration: Merge is safe for public/shared branches. Rebase is generally only safe for private, local branches that haven't been pushed to a remote repository or shared with others.

When to Choose Which?

Choose Merge when: You want to preserve an accurate, complete history of all development, including when and how branches diverged and merged. It's best for integrating feature branches into shared main branches (e.g., main, develop) and for teams that value immutable history.

Choose Rebase when: You want a clean, linear history, free of unnecessary merge commits. It's often used to clean up a feature branch before merging it into a main branch (e.g., rebasing your feature branch onto main before creating a pull request). Always ensure you are only rebasing branches that no one else is working on or has pulled.