Comparing Branches

The Hook (The "Byte-Sized" Intro)

Never merge blind. Before combining two branches, you should know exactly what's different — which files changed, which commits are new, and how many lines were added or removed. git diff and git log with the .. operator give you that x-ray vision. Merging without comparing first is like signing a contract without reading it.

📖 What is Comparing Branches?

Comparing branches means seeing the differences between two branches before merging, reviewing, or deploying. Git provides two main tools: git diff (file-level changes) and git log (commit-level differences).

Conceptual Clarity

Two key operators:

Syntax	Meaning	Use Case
`A..B`	Commits in B that are NOT in A	"What's new in the feature branch?"
`A...B`	Commits in either A or B but not both	"What diverged since they split?"

Two comparison tools:

git diff main..feature — shows file-level changes (line-by-line diff)
git log main..feature — shows commit-level differences (list of commits)

Real-Life Analogy

Comparing branches is like looking at two versions of a report:

git diff = Showing tracked changes (red/green highlighting of what's different)
git log = Showing the revision history (who changed what, when, and why)

Visual Architecture

flowchart LR subgraph MAIN["main"] A["A"] --> B["B"] --> C["C"] end subgraph FEAT["feature"] B --> D["D"] --> E["E"] end C -->|"git diff main..feature"| DIFF["📝 Line differences"] E -->|"git log main..feature"| LOG["📋 Commits D, E"] style MAIN fill:#1a1a2e,stroke:#e94560,color:#e94560 style FEAT fill:#0f3460,stroke:#53d8fb,color:#53d8fb style DIFF fill:#1a1a2e,stroke:#ffd700,color:#ffd700 style LOG fill:#1a1a2e,stroke:#ffd700,color:#ffd700

Why It Matters

Pre-merge review: See exactly what a merge will bring in before you run it.
PR preparation: Understand the scope of your changes before opening a pull request.
Conflict prediction: Large diffs to shared files hint at potential conflicts.
Deployment safety: Compare the release branch to main before deploying.

Code

bash

# ─── See what commits are in feature but NOT in main ───
git log main..feature/login --oneline
# Output:
# e4f5g6h Add login validation
# d3c2b1a Create login form

# ─── See what commits are in main but NOT in feature ───
git log feature/login..main --oneline
# Output:
# a1b2c3d Fix homepage layout

# ─── See file-level diff between branches ───
git diff main..feature/login
# Shows all line-by-line changes

# ─── Just the file names that differ ───
git diff main..feature/login --name-only
# Output:
# src/login.js
# src/auth.js
# tests/login.test.js

# ─── Stats summary (insertions/deletions per file) ───
git diff main..feature/login --stat
# Output:
# src/login.js      | 45 +++++++++++++++
# src/auth.js        | 12 +++++
# tests/login.test.js | 30 ++++++++++
# 3 files changed, 87 insertions(+)

# ─── Count the number of commits ahead/behind ───
git rev-list --left-right --count main...feature/login
# Output: 1    2
# Meaning: main has 1 commit ahead, feature has 2 commits ahead

# ─── Compare with a specific commit (not just branch tips) ───
git diff a1b2c3d..e4f5g6h

Comparison Cheat Sheet

Goal	Command
Commits in feature not in main	`git log main..feature --oneline`
File differences between branches	`git diff main..feature`
Just filenames	`git diff main..feature --name-only`
Summary stats	`git diff main..feature --stat`
How far ahead/behind	`git rev-list --left-right --count main...feature`
Visual graph	`git log --oneline --graph --all`

Key Takeaways

Use git log A..B to see commits in B not in A.
Use git diff A..B to see file changes between branches.
Always compare before merging — it prevents surprises.
--name-only and --stat give quick overviews without full diffs.

Interview Prep

Q: What does git log main..feature show? A: It shows commits that exist in feature but not in main — essentially, the new work in the feature branch. Reversing the order (feature..main) shows commits in main that feature doesn't have.
Q: How do you quickly see how many commits a branch is ahead or behind? A: Use git rev-list --left-right --count main...feature. The output shows two numbers: how many commits main is ahead and how many feature is ahead.
Q: What is the difference between .. and ... in Git comparisons? A: A..B means "commits reachable from B but not from A" (one-directional). A...B means "commits reachable from either A or B but not from both" (symmetric difference — useful for seeing all divergence).