Reimplementing Git From Scratch to Understand How It Actually Works

Why Rebuild Git

I use Git every day, but "knowing how to use Git" and "knowing how Git works" are two different things. I wanted to close that gap — not by reading about Git's internals, but by actually implementing them.

That's what gitbean is: a from-scratch clone of Git's core plumbing commands in Java, built to match Git's real object format and .git-style repository layout as closely as possible, rather than just approximating the behavior.

Starting With the Object Model

Git's foundation is its object store — everything (files, trees, commits) is stored as a compressed, content-addressed object identified by a SHA-1 hash of its contents. Before writing any command logic, I had to get this part right, since every other command depends on it.

gitbean hash-object -w test.txt
gitbean cat-file -p <hash>

hash-object reads a file, prepends a Git-style header (blob <size>\0), hashes the result, and optionally writes it to .gitbean/objects/ — compressed, in the same two-character-directory layout Git itself uses. cat-file does the reverse: given a hash, it locates, decompresses, and prints the object.

Getting the byte-for-byte format right — the exact header format, the compression, the directory structure — was the actual point. An approximation would've been faster to build, but it wouldn't have taught me anything about how Git actually stores data.

Building Up From Blobs

With blob storage working, the next step was staging — the add command, which hashes a file, stores it as a blob, and records it in an index (Git's staging area):

gitbean add test.txt
cat .gitbean/index

This is where the project currently stands: init, hash-object, cat-file (with -p, -t, and -s flags), and add are implemented. Trees and commits — the next layer up, where blobs get organized into a file structure and given history — are next.

Testing Without Wrecking My Own Repo

One early lesson: never run gitbean commands inside the gitbean project folder itself — it happily creates a .gitbean/ directory and test files right in your source tree. I keep a separate sandbox folder as a sibling directory specifically for testing each command as I build it, and reset it clean between runs.

What This Is Actually Teaching Me

A few things that don't come across from just using Git day-to-day:

• Content-addressable storage is a simple idea — hash the content, use the hash as the filename — but it's the reason Git can dedupe identical file content across your entire history for free.
• The staging area is just a file — the index isn't some abstract concept, it's a real file on disk that add writes to and commit would read from.
• Compression and headers matter — matching Git's exact object format (not just "close enough") is what makes gitbean's objects byte-compatible with real Git, and forced me to actually understand the format instead of skimming past it.

This is very much a work in progress — trees, commits, and eventually a basic log or checkout are the next milestones. But the goal was never to replace Git; it's to understand it well enough that I could rebuild it, one plumbing command at a time.