Understanding Git Internals: Objects, Trees, Commits, Tags, and Packfiles

Git is a powerful tool used daily in software development, and this article explores its internal mechanisms.

When a file is modified and committed, Git does not store a simple diff; it records the complete file as a blob object. The workflow starts with the Working Directory, then git add moves changes to the index (staging area), and git commit creates a commit that records the tree, author, and message.

Running git init creates a .git directory containing configuration, hooks, and, most importantly, the objects , refs , HEAD , and index files. The objects directory stores data using a two‑level hash‑based path, e.g. .git/objects/ab/cdef... , to avoid filesystem limits on the number of files per directory.

Git defines four object types:

Blob : stores file contents only. Example: $ echo 'version 1' | git hash-object -w --stdin 83baae61804e65cc73a7201a7252750c76066a30

Tree : records directory structure, file names, modes, and links to blob objects. Created with git write-tree after staging files.

Commit : points to a tree and includes author, timestamp, and message. Created with git commit-tree or the higher‑level git commit .

Tag : a named reference to a commit, created with git tag -a v1.0.0 -m "test tag" .

Each object can be inspected with git cat-file (e.g., git cat-file -p <hash> to view contents, git cat-file -t <hash> to view type).

Git initially stores objects as loose files. When many objects accumulate, or when git gc is run, Git packs them into a binary packfile ( .git/objects/pack/pack‑*.pack ) along with an index file. Packing reduces disk usage by delta‑compressing similar objects; the newest version is stored fully for fast access, while older versions are stored as deltas.

Running git verify-pack -v .git/objects/pack/pack‑*.idx shows the size of each packed object and its delta relationships.

In summary, a second commit of a modified file stores the complete new file as a new blob; Git may later pack this blob together with others, keeping the original version as a delta to save space.