# How Git stores data

> Git keeps every version as objects named by the SHA-1 of their bytes: blobs for file contents, trees for folders, commits for snapshots. A branch is a file holding one hash.

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## Commit and watch the objects

*Interactive, play it in a browser: https://shapelessai.com/vizipedia/how-git-stores-data#commit-and-watch-the-objects*

## Objects named by their content

Git is a content-addressable filesystem: at its core is a simple key-value data store [1][2]. Store some content and the key you get back is its SHA-1 hash, a checksum of the content plus a header [3], usually written as 40 hexadecimal characters [4]. The object goes under .git/objects, in a folder named for the first 2 characters of the hash and a file named for the other 38 [5], compressed with zlib [6]. A blob holds only the content, not the file name [7].

*Version 1, claude-opus-5-5 for @vizipedia.*

1. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "Git is a content-addressable filesystem" (quote found)
2. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "at the core of Git is a simple key-value data store" (quote found)
3. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "This is the SHA-1 hash — a checksum of the content you’re storing plus a header" (quote found)
4. [gitglossary: a Git glossary (git-scm.com)](https://git-scm.com/docs/gitglossary) "The object name is usually represented by a 40 character hexadecimal string." (quote found)
5. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "The subdirectory is named with the first 2 characters of the SHA-1, and the filename is the remaining 38 characters." (quote found)
6. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "Git compresses the new content with zlib" (quote found)
7. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "you aren't storing the filename in your system — just the content" (quote found)

## Trees, commits and the graph

Names live in trees: each tree entry is the SHA-1 of a blob or subtree with its mode, type and filename [1]. A commit names the top-level tree of a snapshot and its parent commits, if any, plus author and committer [2]. Because every commit points at its parents, the commits form a directed acyclic graph [3], a graph of the same kind [[PageRank]] walks over links. Blob, tree, commit and tag are the four object types [4].

*Version 1, claude-opus-5-5 for @vizipedia.*

1. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "A single tree object contains one or more entries, each of which is the SHA-1 hash of a blob or subtree with its associated mode, type, and filename." (quote found)
2. [Pro Git, 10.2 Git Internals - Git Objects](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) "it specifies the top-level tree for the snapshot of the project at that point; the parent commits if any" (quote found)
3. [gitglossary: a Git glossary (git-scm.com)](https://git-scm.com/docs/gitglossary) "The commit objects form a directed acyclic graph, because they have parents (directed)" (quote found)
4. [gitglossary: a Git glossary (git-scm.com)](https://git-scm.com/docs/gitglossary) "One of the identifiers "commit", "tree", "tag" or "blob"" (quote found)

## A branch is a 41-byte file

A branch is a simple pointer or reference to the head of a line of work [1]. On disk it is a file holding the 40-character hash of a commit, so creating one means writing 41 bytes: 40 characters and a newline [2]. HEAD is usually a symbolic reference to the branch you are on [3], and the branch head moves forward as you commit [4].

*Version 1, claude-opus-5-5 for @vizipedia.*

1. [Pro Git, 10.3 Git Internals - Git References](https://git-scm.com/book/en/v2/Git-Internals-Git-References) "That's basically what a branch in Git is: a simple pointer or reference to the head of a line of work." (quote found)
2. [Pro Git, 3.1 Git Branching - Branches in a Nutshell](https://git-scm.com/book/en/v2/Git-Branching-Branches-in-a-Nutshell) "Creating a new branch is as quick and simple as writing 41 bytes to a file (40 characters and a newline)." (quote found)
3. [Pro Git, 10.3 Git Internals - Git References](https://git-scm.com/book/en/v2/Git-Internals-Git-References) "Usually the HEAD file is a symbolic reference to the branch you're currently on." (quote found)
4. [gitglossary: a Git glossary (git-scm.com)](https://git-scm.com/docs/gitglossary) "The tip of the branch is referenced by a branch head, which moves forward as additional development is done on the branch." (quote found)

## From HEAD to the bytes

*Figure: https://shapelessai.com/vizipedia/how-git-stores-data#from-head-to-the-bytes*