Both git merge and git rebase are used to integrate changes from one branch into another in Git, but they do so in different ways. Here’s a breakdown of how each command works and when you might use them.

1. Git Merge

git merge integrates changes from one branch into another by creating a new “merge commit” that combines the histories of the two branches.

Key Characteristics:

• Non-linear history: git merge creates a new commit that merges the changes from the source branch into the target branch while preserving the full history of both branches.
• Preserves commits: All commits from both branches remain in the history, and the two lines of development (branches) remain intact.
• Merge commit: A new commit is created, combining both branches’ commits with metadata showing both parent branches.

Example:

# Assuming you are on the “main” branch
git checkout main
git merge feature-branch

This command would merge the feature-branch into the main branch, creating a new merge commit that references both branches’ histories.

When to Use git merge:

• When you want to preserve the full history of both branches, including when and where branches diverged.
• When working in a collaborative environment where the history of changes and branch points is important for team members to track.
• When you’re okay with a potentially more complex history, which shows multiple branches being merged over time.

2. Git Rebase

git rebase integrates changes by “replaying” the commits of one branch on top of another, effectively rewriting the commit history.

Key Characteristics:

• Linear history: git rebase creates a clean, linear history by placing the commits of one branch onto another as if the work was done in sequence.
• Rewrites commits: Instead of merging branches, git rebase applies each commit from the feature branch on top of the target branch, effectively rewriting the commit history.
• No merge commit: Since rebase doesn’t create a merge commit, it avoids the creation of an additional commit and makes the history linear and cleaner.

Example:

# Assuming you are on the “feature-branch”
git checkout feature-branch
git rebase main

This command takes the commits from the feature-branch and rebases them on top of the main branch, as if the work in feature-branch was started after the latest commit on main.

When to Use git rebase:

• When you want to keep a clean, linear history without the complexity of merge commits.
• When you’re working on a feature branch that hasn’t yet been shared with others and you want to integrate changes from the main branch without creating a merge commit.
• When you’re preparing for a pull request and want a simple, linear history that looks as if the feature branch was built on top of the latest version of the main branch.

A Visual Example:

Merge:

C—D—E  (feature-branch)
/          \
A—B—-F—G——–H  (main)

After a git merge, you get:

C—D—E
/          \
A—B—-F—G——–H  (main, merge commit)

Rebase:

C—D—E  (feature-branch)
/
A—B—-F—G  (main)

After a git rebase, you get:

A—B—-F—G—C’—D’—E’  (feature-branch rebased onto main)

Pros and Cons:

	Git Merge	Git Rebase
History	Non-linear, keeps all branch commits	Linear, rewrites commit history
Commit Logs	Preserves all commits and branch info	Rewrites commits with new hash values
Conflict Resolution	Happens once during the merge	Might have to resolve conflicts at each commit during rebase
Collaborative Work	Safer, since history is preserved	Risky if rebasing shared branches (could cause confusion)
Ease of Use	Easier to understand full history	Cleaner, but can rewrite history

When Not to Use git rebase:

• On public/shared branches: Rewriting history on branches shared with others can lead to confusion and force them to reconcile with diverging histories.
• If history integrity is important: If you want to preserve the complete history with branching information, merging is better suited.

In Summary:

• Use git merge when you want to preserve the full context of both branches and are fine with a more complex history.
• Use git rebase when you want a clean, linear history and are not collaborating on the branch with others.