Fast-forward merge in Git - Step-by-Step Execution

Process Flow - Fast-forward merge

Start on main branch

↓

Create feature branch

↓

Make commits on feature branch

↓

Switch back to main branch

↓

Check if main is behind feature

Yes↓

Fast-forward merge: move main pointer to feature

↓

Main branch updated, no merge commit

↓

End

This flow shows how a fast-forward merge moves the main branch pointer forward to the feature branch commit when no divergent commits exist.

Execution Sample

Git

git checkout main

git checkout -b feature

git commit -m "Add feature"

git checkout main

git merge feature

This sequence creates a feature branch, commits to it, then merges it back into main using fast-forward.

Process Table

Step	Command	Branch HEAD	Action	Result
1	git checkout main	main	Switch to main branch	HEAD points to main branch tip
2	git checkout -b feature	feature	Create and switch to feature branch	feature branch created at main tip, HEAD points to feature
3	git commit -m "Add feature"	feature	Commit on feature branch	feature branch moves forward by one commit
4	git checkout main	main	Switch back to main branch	HEAD points to main branch tip (behind feature)
5	git merge feature	main	Fast-forward merge	main branch pointer moves to feature commit, no merge commit created

💡 Fast-forward merge completes because main branch has no new commits since feature branch was created.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
HEAD	main	main	feature	feature	main	main
main branch pointer	commit0	commit0	commit0	commit0	commit0	commit1 (feature commit)
feature branch pointer	commit0	commit0	commit1	commit1	commit1	commit1

Key Moments - 3 Insights

Why does the merge not create a new commit?

Because main branch has no new commits after branching, git just moves main pointer forward to feature commit (see execution_table step 5).

What happens if main had new commits after feature branch was created?

Fast-forward merge would not be possible; git would create a merge commit to combine histories.

Does fast-forward merge change the commit history?

No, it only moves the branch pointer forward without creating new commits, preserving a linear history.

Visual Quiz - 3 Questions

Look at the execution table, what is the branch HEAD after step 3?

A feature

B main

C detached HEAD

D unknown

💡 Hint

Check the 'Branch HEAD' column at step 3 in the execution_table.

At which step does the main branch pointer move to the feature commit?

A Step 3

B Step 4

C Step 5

D It never moves

💡 Hint

Look at the 'Result' column in execution_table for when main branch pointer updates.

If main had new commits after branching, what would change in the execution table?

A HEAD would point to detached state

B Step 5 would create a merge commit instead of fast-forward

C Step 3 would fail

D Feature branch would be deleted

💡 Hint

Refer to key_moments about when fast-forward merge is possible.

Concept Snapshot

Fast-forward merge moves the branch pointer forward when no divergent commits exist.
Syntax: git merge <branch>
If main is behind feature, main pointer moves to feature commit.
No merge commit is created.
Preserves linear history.
Only works if main has no new commits since branching.

Full Transcript

Fast-forward merge happens when you merge a branch that is ahead of your current branch without any new commits on the current branch. You start on main, create a feature branch, make commits there, then switch back to main. When you run git merge feature, git moves the main branch pointer forward to the feature commit because main has no new commits. This merge does not create a new commit, keeping history linear. If main had new commits, git would create a merge commit instead. This process is simple and keeps your commit history clean.

Practice

1. What happens during a fast-forward merge in Git?

easy

A. The source branch is deleted automatically.

B. A new merge commit is always created.

C. The branch pointer moves forward without creating a new commit.

D. The commit history becomes non-linear.

Solution

1. Step 1: Understand fast-forward merge behavior

   A fast-forward merge moves the branch pointer forward to the latest commit of the source branch without creating a new merge commit.

2. Step 2: Compare with other merge types

   Unlike a normal merge, it does not create a new commit and keeps history linear.

3. Final Answer:

   The branch pointer moves forward without creating a new commit. -> Option C

4. Quick Check:

   Fast-forward merge = pointer moves forward [OK]

Hint: Fast-forward means no new commit, just pointer moves [OK]

Common Mistakes:

• Thinking a merge commit is always created
• Assuming source branch deletes automatically
• Believing history becomes non-linear

2. Which Git command syntax performs a fast-forward merge of branch feature into main only if possible, otherwise aborts?

easy

A. git merge --squash feature

B. git merge feature

C. git merge --no-ff feature

D. git merge --ff-only feature

Solution

1. Step 1: Identify command for fast-forward only

   The option --ff-only tells Git to merge only if it can fast-forward, otherwise it aborts.

2. Step 2: Compare other options

   --no-ff disables fast-forward, --squash creates a single commit without merging, and plain git merge feature may create a merge commit.

3. Final Answer:

   git merge --ff-only feature -> Option D

4. Quick Check:

   Fast-forward only = --ff-only [OK]

Hint: Use --ff-only to ensure only fast-forward merges happen [OK]

Common Mistakes:

• Using --no-ff disables fast-forward merges
• Assuming plain merge always fast-forwards
• Confusing --squash with fast-forward

3. Given the following Git commands, what is the output of git log --oneline main after merging?

git checkout main
# main points to commit A

git checkout -b feature
# feature branch created from A

git commit --allow-empty -m "Add feature commit"
# feature now points to commit B

git checkout main

git merge feature

medium

A. B

B. B\nA

C. A

D. Merge commit with A and B

Solution

1. Step 1: Understand branch states before merge

   Main points to commit A. Feature branch adds commit B on top of A.

2. Step 2: Analyze merge behavior

   Since main has no new commits after branching, merging feature into main will fast-forward main to B.

3. Final Answer:

   B\nA -> Option B

4. Quick Check:

   Fast-forward merge moves main to B [OK]

Hint: If main unchanged, merge moves pointer to feature commit [OK]

Common Mistakes:

• Expecting merge commit creation
• Thinking main stays at A
• Confusing commit hashes output

4. You tried to fast-forward merge branch feature into main using git merge --ff-only feature, but Git returned an error. What is the most likely cause?

medium

A. The main branch has new commits not in feature.

B. The feature branch is behind main.

C. The feature branch has no commits.

D. You forgot to commit changes on feature branch.

Solution

1. Step 1: Understand --ff-only error cause

   The --ff-only option fails if a fast-forward merge is not possible.

2. Step 2: Identify when fast-forward is impossible

   If main has new commits not in feature, Git cannot fast-forward main to feature, causing the error.

3. Final Answer:

   The main branch has new commits not in feature. -> Option A

4. Quick Check:

   Fast-forward fails if main has new commits [OK]

Hint: Fast-forward fails if main moved ahead since branching [OK]

Common Mistakes:

• Assuming feature must be behind main
• Thinking empty feature branch causes error
• Confusing uncommitted changes with merge errors

5. You have a main branch and a feature branch. Both have new commits since branching. You want to merge feature into main but keep history linear without merge commits. Which approach is best?

hard

A. Use git rebase main on feature, then fast-forward merge.

B. Use git merge --no-ff feature to force a merge commit.

C. Use git merge --ff-only feature and abort if not fast-forward.

D. Delete main and rename feature to main.

Solution

1. Step 1: Understand the problem with fast-forward

   Since both branches have new commits, a fast-forward merge is not possible directly.

2. Step 2: Use rebase to linearize history

   Rebasing feature onto main moves feature commits on top of main, enabling a fast-forward merge afterward.

3. Step 3: Perform fast-forward merge after rebase

   After rebase, merging feature into main will be a fast-forward, keeping history linear without merge commits.

4. Final Answer:

   Use git rebase main on feature, then fast-forward merge. -> Option A

5. Quick Check:

   Rebase then merge = linear history [OK]

Hint: Rebase feature on main to enable fast-forward merge [OK]

Common Mistakes:

• Trying --ff-only merge when not possible
• Forcing merge commit breaks linear history
• Deleting branches unnecessarily