Notes

git_internals

2 min read

Git Internals

Purpose

Understand Git’s object model — the low-level data structures that underpin every commit, branch, and file. This knowledge is rarely needed day-to-day but is essential for debugging complex Git situations and reasoning about what commands actually do.

Architecture

Git stores everything in its object database (.git/objects). There are four object types, all content-addressed by their SHA-1 hash:

ObjectDescription
blobContents of a single file
treeA directory listing (maps names → blobs or subtrees)
commitA snapshot: points to a tree + parent commit(s) + metadata
tagA named pointer to another object (usually a commit)

A commit object contains:

  • The SHA of the root tree (full snapshot of all tracked files)
  • The SHA(s) of parent commit(s) (none for the initial commit; two for a merge commit)
  • Author, committer, timestamps
  • Commit message

Because every object is identified by the SHA-1 hash of its content, Git history is tamper-evident: changing anything changes the hash, which cascades up through all descendant objects.

Implementation Notes

Inspect any object by its hash:

git cat-file -t <hash>   # print object type (blob, tree, commit, tag)
git cat-file -p <hash>   # pretty-print object contents

Example — inspecting a commit:

git cat-file -p 5ba786f
# tree 4e507fd...
# author  ...
# committer ...
#
# commit message

The tree listed in a commit can itself be inspected to see which blobs (files) it references.

Short hashes: Git accepts the first 7 characters of a SHA as a unique identifier, e.g. 5ba786f instead of 5ba786fcc93e8092831c01e71444b9baa2228a4f.

Refs are human-readable pointers to commits: branches, tags, and HEAD are all refs stored as files in .git/refs/ or .git/HEAD.

Trade-offs

  • SHA-1 is cryptographically weak; Git is transitioning to SHA-256 (git init --object-format=sha256) but SHA-1 remains the default.
  • The content-addressable store means identical file content is stored only once (deduplication), but large binary files can bloat .git/objects.
  • Never modify .git/objects by hand — always use Git commands.

References

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