📦 Git Objects - Blobs, Trees & Commits

🧱 The Building Blocks of Git

Everything in Git is built from three simple objects: Blobs, Trees, and Commits

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📺 Video Reference

Resource	Link
🎬 Video	Git Objects
📄 Transcript	02-git-objects.txt

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🧠 Mental Model: Git as a File System

"Forget about code repositories for a moment. Think of Git as a system for maintaining snapshots of a file system."

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📦 Object 1: Blob (Binary Large Object)

Definition

Blob = The contents of a file, stored as a binary stream of data.

Blob vs File: Key Differences

Aspect	File	Blob
Contains	Contents + metadata	Contents only
Has name?	Yes (filename)	No
Has date?	Yes (created, modified)	No
Has permissions?	Yes	No
Identified by	Path + name	SHA-1 hash

Visual Representation

┌─────────────────────────────────────────┐
│                  FILE                    │
│  ┌────────────────────────────────────┐ │
│  │ Name: hello.txt                    │ │
│  │ Created: 2024-01-26                │ │
│  │ Modified: 2024-01-26               │ │
│  │ Permissions: 644                   │ │
│  │ ──────────────────────────────     │ │
│  │ Contents: "Hello, World!"          │ │
│  └────────────────────────────────────┘ │
└─────────────────────────────────────────┘

┌─────────────────────────────────────────┐
│                  BLOB                    │
│  ┌────────────────────────────────────┐ │
│  │ Contents: "Hello, World!"          │ │
│  └────────────────────────────────────┘ │
│  SHA-1: aaf4c61ddcc5e8a2...            │
└─────────────────────────────────────────┘

SHA-1 Hash Explained

🔑 Key Insight: Same content = Same hash, ALWAYS!

If you and I both create a blob with "Hello World", we'll get the exact same SHA-1 hash. This is how Git deduplicates data!

SHA-1 Format

SHA-1 hash: 20 bytes = 40 hexadecimal characters

Example: aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d
         ↑                                      ↑
         First character                   Last character
         
Git often shows short version: aaf4c61 (first 7 chars)

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🌳 Object 2: Tree

Definition

Tree = A directory listing that refers to blobs (files) and other trees (subdirectories).

Tree Structure

Tree: caf7e8a...
├── 100644 blob f92a0b1... peak.png
├── 100644 blob 73d8ab2... 1.txt
└── 040000 tree 24601ac... subdirectory

Visual Representation

File Modes in Trees

Mode	Type	Description
100644	blob	Regular file
100755	blob	Executable file
120000	blob	Symbolic link
040000	tree	Directory
160000	commit	Submodule

Important: Names Are in Trees, Not Blobs!

💡 Same blob, different names! The blob doesn't know its own name. Different trees can refer to the same blob with different names.

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📸 Object 3: Commit

Definition

Commit = A snapshot of the working tree at a certain point in time, with metadata.

What a Commit Contains

commit a1337e5...
├── tree: caf7e8a...        ← Points to root tree (snapshot)
├── parent: 8b2f9c1...      ← Previous commit (optional)
├── author: John <j@x.com>  ← Who wrote the code
├── committer: Jane <j@x.com> ← Who committed
├── date: Mon Jan 26 2026   ← When committed
└── message: "Initial commit" ← Commit message

Visual Representation

Important: Commits Store FULL Snapshots!

🔑 Key Insight: Each commit has the ENTIRE snapshot, but Git is smart!

• Changed files → New blob created
• Unchanged files → Just reference existing blob (no duplication!)

This is why Git is so efficient even with full snapshots.

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🔄 How Changes Propagate

When you change a file, watch how hashes ripple upward:

Change Propagation Rules

What Changed	What Gets New Hash
File contents	Blob + all parent Trees + Commit
Directory structure	Tree + all parent Trees + Commit
Only commit message	Just the Commit

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🧪 Hands-On Lab: Exploring Objects

Setup

# Create a test repository
mkdir git-objects-lab && cd git-objects-lab
git init

Create and Inspect a Blob

# Create a file
echo "Hello, Git Internals!" > hello.txt

# Stage it (this creates the blob)
git add hello.txt

# Find the blob
find .git/objects -type f
# Output: .git/objects/5e/1c309dae7f45e0f39b1bf3ac3cd9db12e7d689

# Inspect the blob
git cat-file -t 5e1c309dae7f45e0f39b1bf3ac3cd9db12e7d689
# Output: blob

git cat-file -p 5e1c309dae7f45e0f39b1bf3ac3cd9db12e7d689
# Output: Hello, Git Internals!

Create and Inspect a Tree

# Commit to create a tree
git commit -m "Initial commit"

# Find the tree from the commit
git cat-file -p HEAD
# Output:
# tree 8b137891791fe96927ad78e64b0aad7bded08bdc
# author ...
# committer ...
# Initial commit

# Inspect the tree
git cat-file -p 8b137891791fe96927ad78e64b0aad7bded08bdc
# Output: 100644 blob 5e1c309... hello.txt

Visualize the Object Graph

# See all objects
find .git/objects -type f | while read f; do
    hash=$(echo $f | sed 's:.git/objects/::' | tr -d '/')
    type=$(git cat-file -t $hash)
    echo "$hash ($type)"
done

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🎯 Quick Reference: Git Object Commands

Command	Purpose	Example
git cat-file -t <sha>	Show object type	git cat-file -t abc123
git cat-file -p <sha>	Pretty-print object	git cat-file -p abc123
git cat-file -s <sha>	Show object size	git cat-file -s abc123
git hash-object <file>	Compute hash	git hash-object hello.txt
git hash-object -w <file>	Compute hash & write blob	git hash-object -w hello.txt

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🧠 Quiz: Test Your Understanding

1. If I create a blob with "Git is awesome" and you create the same on your computer, will we have the same hash?

✅ YES! Same content always produces the same SHA-1 hash. This is content-addressed storage.

2. If we both create a tree pointing to that blob with the same name, same hash?

✅ YES! Trees are also identified by their content (which includes the blobs they point to).

3. If we both create commits of that tree with message "hello", same hash?

❌ NO! Commits include metadata like author, timestamp, and committer - these will differ!

4. Does Git store diffs between commits?

❌ NO! Git stores full snapshots. But it's efficient because unchanged blobs are reused (not duplicated).

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📊 Object Relationship Summary

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� What's Next?

🌿 Next: Branches (Lesson 03)

Now that you understand objects, let's learn how branches work. Spoiler: they're much simpler than you think - just a file containing a commit SHA!

Continue to Lesson 03 →

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📝 Summary

Object	What It Is	Identified By	Contains
Blob	File contents	SHA-1 of content	Raw bytes
Tree	Directory	SHA-1 of listing	Blobs + Trees
Commit	Snapshot	SHA-1 of metadata + tree	Tree + parent + metadata

Remember: Everything in Git is an object, and every object has a SHA-1 hash. That's it! 🎉