Git — Merge

What is Merge?

Merging takes changes from one branch and integrates them into another. It's how you bring feature work back into the main codebase.

# Merge feature-branch into current branch
git merge feature-branch

Types of Merges

Fast-Forward Merge

When the target branch has no new commits since the feature was created:

# Before:
# main: A → B → C
# feature: A → B → C → D → E

git switch main
git merge feature

# After:
# main: A → B → C → D → E

No new commit is created — main just moves forward.

Three-Way Merge

When both branches have new commits:

# Before:
# main: A → B → C → F
# feature: A → B → C → D → E

git switch main
git merge feature

# After:
# main: A → B → C → F → M (merge commit)
#                   ↘ D → E ↗

A new merge commit M is created, combining both histories.

Merge Conflicts

When Git can't automatically merge changes, you get a conflict:

# Automatic merge failed
CONFLICT (content): Merge conflict in index.html
Automatic merge failed; fix conflicts and then commit the result.

What Conflict Looks Like

<<<<<<< HEAD
<p>Welcome to our website!</p>
=======
<p>Welcome to our new website!</p>
>>>>>>> feature-update

• <<<<<<< HEAD — your current branch's version
• ======= — separator
• >>>>>>> feature-update — incoming branch's version

Resolving Conflicts

# 1. Open the conflicted file
# 2. Edit to keep what you want
<p>Welcome to our new website!</p>

# 3. Remove conflict markers (<<<<<<< ======= >>>>>>>)

# 4. Stage the resolved file
git add index.html

# 5. Commit
git commit -m "Resolve merge conflict in index.html"

Abort a Merge

# Start over if conflicts are too complex
git merge --abort

Merge Strategies

--no-ff (No Fast-Forward)

Always creates a merge commit, even for fast-forward:

git merge --no-ff feature-branch -m "Merge feature-branch"

This preserves the branch history, making it easier to see what was a feature branch.

Squash Merge

Combines all commits into one:

git merge --squash feature-branch
git commit -m "Add complete feature"

Useful for cleaning up messy commit histories.

Viewing Merge History

# Show merge commits
git log --merges

# Show graph
git log --graph --oneline --all

# See what was merged
git log --oneline main..feature-branch

Practical Example

# Create feature branch
git switch -c feature/contact-form

# Make changes
echo "contact form" > contact.html
git add contact.html
git commit -m "feat: add contact form"

# Switch to main
git switch main

# Make a change on main
echo "updated readme" >> README.md
git add README.md
git commit -m "docs: update readme"

# Merge feature
git merge feature/contact-form

# Handle any conflicts...
# Push to remote
git push

Best Practices

• Always pull before merging
• Use --no-ff to preserve branch history
• Review changes before merging
• Resolve conflicts carefully — test after resolving
• Delete merged branches
• Use pull requests for team code review

Mini Practice

1. Create a branch, make commits, and merge into main
2. Create a merge conflict and resolve it
3. Use --no-ff to force a merge commit
4. Use --squash to combine multiple commits
5. View the merge graph with git log --graph

Up Next

Next: Conflicts →