Git Internals

Also Known As

TL;DR

Explanation

Git's object model: blobs store file content, trees store directory listings, commits point to a tree plus parent commits and metadata, tags point to commits. Every object is named by the SHA-1 hash of its content. Branches and tags are simply pointers (refs) to commit hashes. HEAD points to the current branch or commit. This model explains why branches are cheap (just a file with a hash), why git is fast (content addressing), and why history rewriting changes hashes.

Diagram

flowchart TD
    subgraph Git Objects
        BLOB[Blob<br/>file content]
        TREE[Tree<br/>directory listing]
        COMMIT[Commit<br/>tree + parent + message]
        TAG[Tag<br/>points to commit]
    end
    COMMIT -->|points to| TREE
    TREE -->|contains| BLOB
    TREE -->|contains| TREE2[Subtree]
    COMMIT -->|parent| PREV[Previous commit]
    subgraph Refs
        HEAD[HEAD] -->|points to| BRANCH[branch name]
        BRANCH -->|points to| COMMIT
    end
    INFO[Everything is content-addressed<br/>SHA-1 hash of contents<br/>immutable once written]
style COMMIT fill:#6e40c9,color:#fff
style TREE fill:#1f6feb,color:#fff
style BLOB fill:#238636,color:#fff
style INFO fill:#d29922,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Thinking git stores diffs — it stores full snapshots; packfiles compress them later.
• Not understanding that branch deletion does not delete commits — unreferenced commits are garbage collected eventually.
• Not using git cat-file to inspect objects when debugging — the plumbing commands reveal what high-level commands hide.
• Not understanding that a merge commit has two parents — this is what makes merge vs rebase history different.

Code Examples

# Losing work by assuming branch deletion removes commits:
git branch -D feature/experiment
# The commits still exist! They are just unreferenced.
# Recover with: git reflog | grep 'feature/experiment'
# git checkout -b recovered <hash>

# Inspect git objects directly:
git cat-file -t HEAD        # Shows type: commit
git cat-file -p HEAD        # Shows: tree, parent, author, message
git cat-file -p HEAD^{tree} # Shows directory listing
git log --graph --oneline --all  # Visualise the DAG

References

• ↗ https://git-scm.com/book/en/v2/Git-Internals-Git-Objects

Tags

Related Terms