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Chapter 6 — Merging

Merging takes the work from one branch and integrates it into another.

Fast-Forward Merge

When the target branch hasn't diverged, Git simply moves the pointer forward.

Example: Fast-forward merge

git switch main
git merge feature/signup
Updating a1b2c3d..d5e6f7g
Fast-forward
 signup.js | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 signup.js

Before:

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

After:

main:           A --- B --- C --- D

🧠 What happened? No merge commit was created. Git simply moved main forward to where feature/signup was.

Three-Way Merge

When both branches have new commits, Git creates a merge commit.

Example: Merge with a merge commit

# Set up: both branches have diverged
git switch main
echo "main work" >> app.js && git add . && git commit -m "Work on main"

git switch feature/login
echo "login work" >> login.js && git add . && git commit -m "Work on login"

git switch main
git merge feature/login
Merge made by the 'ort' strategy.
 login.js | 1 +
 1 file changed, 1 insertion(+)

git log --oneline --graph
*   h8i9j0k Merge branch 'feature/login'
|\
| * f7g8h9i Work on login
* | e4f5a6b Work on main
|/
* a1b2c3d Initial commit

Handling Merge Conflicts

Conflicts happen when both branches changed the same lines.

Example: Create a conflict

# On main
echo "version = 2" > config.txt && git add . && git commit -m "Set version 2"

# On feature branch
git switch -c feature/config
echo "version = 3" > config.txt && git add . && git commit -m "Set version 3"

# Try to merge
git switch main
git merge feature/config
Auto-merging config.txt
CONFLICT (content): Merge conflict in config.txt
Automatic merge failed; fix conflicts and then commit the result.

Example: See the conflict markers

cat config.txt
<<<<<<< HEAD
version = 2
=======
version = 3
>>>>>>> feature/config

Example: Resolve the conflict

Edit the file to keep what you want:

version = 3

Then:

git add config.txt
git commit -m "Merge feature/config: use version 3"

🧠 What happened? Git couldn't automatically decide which version to keep, so it put both versions in the file with markers. You manually resolved it, then committed the result.

Example: Abort a merge

If you're in the middle of a conflict and want to start over:

git merge --abort

Everything goes back to the state before you ran git merge.

Merge Strategies

Example: Force a merge commit (no fast-forward)

git merge --no-ff feature/signup

This creates a merge commit even when fast-forward is possible. Useful for preserving the fact that work happened on a branch.

Example: Squash merge

git merge --squash feature/signup
git commit -m "Add signup feature"

🧠 What happened? All commits from the branch are combined into a single set of changes in the staging area. You then commit them as one commit. The branch's individual commits don't appear in main's history.

← Chapter 5: Branches · Next: Chapter 7 — Rebasing →