Merging Branches

Introduction

Merging combines changes from one branch into another. This lesson covers the merge process and common scenarios.

Basic Merge

Standard Merge

To merge a branch into your current branch:

git checkout main
git merge feature-branch

This merges feature-branch into main.

Merge with Message

git merge feature-branch -m "Merge feature-branch into main"

Understanding Merge Commits

When merging branches that have diverged, Git creates a merge commit:

main:     A---B---M
             \   /
feature:  C---D---

• A, B - commits on main
• C, D - commits on feature
• M - merge commit combining both

Fast-Forward Merge

When the target branch has not been modified since creating the feature branch:

Before:   main: A---B
                    \
feature:          C---D
After (fast-forward):
feature: A---B---C---D

To enable:

git merge feature-branch

Git automatically does fast-forward if possible.

Disable Fast-Forward

To always create a merge commit:

git merge --no-ff feature-branch

This preserves branch history even with fast-forward possible.

Types of Merges

Auto-Merge

Git automatically combines changes when no conflicts exist.

Manual Merge

When conflicts occur, you must resolve them manually (covered in next lesson).

Merge Options

Squash Merge

Combines all commits into one:

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

Useful for keeping main history clean.

Rebase Merge

Equivalent to rebasing before merge:

git merge --rebase feature-branch

Handling Multiple Branches

Merge Multiple Branches

git checkout main
git merge feature-a
git merge feature-b
git merge hotfix

Merge with Strategy

git merge -s recursive feature-branch
git merge -s ours feature-branch    # Keep current, ignore other
git merge -s subtree branch-name   # For subtrees

Aborting a Merge

During Conflict

git merge --abort

Returns to state before merge attempt.

After Partial Commit

git reset --hard ORIG_HEAD

Viewing Merges

Show Merged Branches

git branch --merged

Show Unmerged Branches

git branch --no-merged

Show Merge Commit Details

git show HEAD

Or:

git log --merges

Practical Examples

Example 1: Feature Completion

# Ensure main is up to date
git checkout main
git pull origin main
Merge feature branch
git merge feature-user-login
Delete feature branch
git branch -d feature-user-login
Push changes
git push origin main

Example 2: Squash and Merge

git checkout main
git merge --squash feature-branch
git commit -m "Complete user login feature"
git push origin main

Example 3: Hotfix Merge

# Merge hotfix into main
git checkout main
git merge hotfix-critical-fix
Also merge into current feature
git checkout feature-branch
git merge hotfix-critical-fix
Push all
git push origin main feature-branch

Merge Best Practices

1. Always update main before merging 2. Review changes before merging 3. Use --no-ff for important features 4. Delete branches after merging 5. Test after merging

Summary

Key merge commands:

• git merge branch - Merge branch into current
• git merge --no-ff - Always create merge commit
• git merge --squash - Squash commits
• git merge --abort - Cancel merge
• git branch --merged - Show merged branches

Next Lesson

Learn how to resolve merge conflicts when they occur.