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The object model
At its core Git is a content-addressable store: a key–value database in which the key of every object is the hash of its content. Four object types build everything else.
- Blob — the content of a file, with no name or metadata of its own. Identical content anywhere in history is one blob.
- Tree — a directory: a list of entries, each a name, a mode (permissions), and the hash of a blob (a file) or another tree (a subdirectory). A tree is a snapshot of a directory’s shape.
- Commit — a snapshot of the whole project at a moment: it points to one top-level tree, to its parent commit(s), and carries author and committer identity, timestamps, and a message. The chain of parent links is the history.
- Tag — a named, optionally signed pointer to another object (usually a commit), carrying its own message and tagger.
Because an object’s name is the hash of its content, objects are immutable — any change produces a different object with a different name — and identical content is automatically de-duplicated. The same property makes integrity intrinsic: a commit’s hash depends on its tree and parents, so it transitively fixes the whole history behind it. Objects are stored zlib-compressed under .git/objects/, and bundled into packfiles for transport and storage.
The index
Between the working files and the object store sits the index (the staging area): a mutable description of what the next commit will contain. git add records content into the index; committing turns the index into a tree and a commit. The index is what lets a commit be composed deliberately rather than capturing the working directory wholesale.
Hashing: SHA-1 to SHA-256
Git originally named objects by SHA-1 — chosen, in Torvalds’ account, mainly to guard against accidental corruption rather than as a security measure. After the 2017 SHAttered demonstration of a practical SHA-1 collision, Git adopted a collision-detecting SHA-1 variant, and a transition to SHA-256 has been specified; as of 2026 it is available but not the default, and most repositories remain on SHA-1.
For the byte-level details, see the Git Internals chapter of Pro Git.
Sources
- Pro Git, Git Internals — the object database in full.
- The SHAttered attack — the 2017 SHA-1 collision.