Build Your Own Git
Target Audience: Students and Junior Engineers
Language: JavaScript (with Java & Go alternatives)
Duration: 2-3 weeks
Difficulty: ⭐⭐⭐☆☆
Why Build Git?
Git is the most ubiquitous tool in software development, yet most developers treat it as magic. By building your own Git, you’ll:
- Understand content-addressable storage — the elegant idea behind Git’s object model
- Master hashing and cryptography basics — SHA-1 in practice
- Learn tree data structures — directories as trees, commits as a DAG
- Build a real CLI tool — practical software engineering
This is NOT a tutorial on using Git. This is about understanding Git’s internals deeply enough to reimplement them.
Git’s Beautiful Architecture
┌─────────────────────────────────────────────────────────────────────────────┐
│ GIT OBJECT MODEL │
├─────────────────────────────────────────────────────────────────────────────┤
│ │
│ BLOB TREE COMMIT │
│ ───── ───── ────── │
│ File contents Directory listing Snapshot + metadata │
│ SHA-1 of content Points to blobs/trees Points to tree + parent │
│ │
│ ┌───────────┐ ┌───────────────────┐ ┌───────────────────┐ │
│ │ Hello │ │ 100644 hello.txt │ │ tree abc123 │ │
│ │ World │ │ 040000 src/ │ │ parent def456 │ │
│ └───────────┘ └───────────────────┘ │ author John │ │
│ │ │ │ message: Initial │ │
│ └────────────────────────┴───────────────────┘ │
│ │
│ ALL OBJECTS ARE CONTENT-ADDRESSED: │
│ SHA-1(type + size + content) → 40-character hex string │
│ │
└─────────────────────────────────────────────────────────────────────────────┘
What You’ll Build
Core Commands
| Command | Description | Concepts Learned |
|---|
init | Initialize repository | File structure, .git directory |
hash-object | Hash and store a file | SHA-1, zlib compression |
cat-file | Read object contents | Object types, decompression |
add | Stage files | Index file format |
commit | Create commit object | Tree building, commit linking |
log | Show commit history | DAG traversal |
status | Show working tree status | Diff against index |
branch | Manage branches | Refs, symbolic refs |
checkout | Switch branches | Updating HEAD, working tree |
Implementation: JavaScript
Project Structure
mygit/
├── src/
│ ├── commands/
│ │ ├── init.js
│ │ ├── hashObject.js
│ │ ├── catFile.js
│ │ ├── add.js
│ │ ├── commit.js
│ │ ├── log.js
│ │ ├── status.js
│ │ ├── branch.js
│ │ └── checkout.js
│ ├── objects/
│ │ ├── blob.js
│ │ ├── tree.js
│ │ └── commit.js
│ ├── utils/
│ │ ├── hash.js
│ │ ├── compression.js
│ │ ├── index.js
│ │ └── refs.js
│ └── mygit.js
├── package.json
└── README.md
Core Implementation
const crypto = require('crypto');
const zlib = require('zlib');
const fs = require('fs');
const path = require('path');
/**
* Compute SHA-1 hash of content with Git's format
* Git format: "{type} {size}\0{content}"
*/
function hashObject(content, type = 'blob') {
const header = `${type} ${content.length}\0`;
const store = Buffer.concat([Buffer.from(header), Buffer.from(content)]);
const hash = crypto.createHash('sha1').update(store).digest('hex');
return { hash, store };
}
/**
* Write object to .git/objects/{hash[0:2]}/{hash[2:40]}
*/
function writeObject(gitDir, hash, store) {
const objectDir = path.join(gitDir, 'objects', hash.slice(0, 2));
const objectPath = path.join(objectDir, hash.slice(2));
if (!fs.existsSync(objectDir)) {
fs.mkdirSync(objectDir, { recursive: true });
}
// Git stores objects compressed with zlib
const compressed = zlib.deflateSync(store);
fs.writeFileSync(objectPath, compressed);
return hash;
}
/**
* Read object from .git/objects
*/
function readObject(gitDir, hash) {
const objectPath = path.join(
gitDir, 'objects',
hash.slice(0, 2),
hash.slice(2)
);
if (!fs.existsSync(objectPath)) {
throw new Error(`Object ${hash} not found`);
}
const compressed = fs.readFileSync(objectPath);
const store = zlib.inflateSync(compressed);
// Parse header: "{type} {size}\0{content}"
const nullIndex = store.indexOf(0);
const header = store.slice(0, nullIndex).toString();
const [type, size] = header.split(' ');
const content = store.slice(nullIndex + 1);
return { type, size: parseInt(size), content };
}
module.exports = { hashObject, writeObject, readObject };
Exercises
Level 1: Basic Understanding
- Initialize a repository and create a blob manually
- Understand how SHA-1 hashing creates content-addressable storage
- Create a commit and inspect its structure with
cat-file
Level 2: Core Implementation
- Implement the
status command (compare index to working tree)
- Add support for
.gitignore patterns
- Implement
diff to show changes between commits
Level 3: Advanced Features
- Implement merge (fast-forward and three-way)
- Add remote repository support (fetch, push)
- Implement pack files for efficient storage
What You’ve Learned
Content-addressable storage using SHA-1 hashing
Tree data structures for representing directories
DAG (Directed Acyclic Graph) for commit history
Binary file formats (index file)
CLI tool development in JavaScript
Next Steps
Go Implementation
See how the same concepts translate to Go
Java Implementation
Enterprise-grade implementation with strong typing
Build Redis
Ready for the next challenge? Build a Redis clone
