git-repositoryGit repository format

A Git repository stores a series of versioned snapshots of a file hierarchy. Conceptually, the repository's data model is a directed acyclic graph which contains three types of objects as nodes:

The content of tracked files is stored in objects of type .
A object points to any number of such blobs, and also to other trees in order to represent a hierarchy of files and directories.
A object points to the root element of one tree, and thus records the state of this entire tree as a snapshot. Commit objects are chained together to form lines of version control history. Most commits have just one successor commit, but commits may be succeeded by an arbitrary number of subsequent commits so that diverging lines of version control history, known as , can be represented. A commit which precedes another commit is referred to as that other commit's . A commit with multiple parents unites disparate lines of history and is known as a .
A object associates a user-defined label with another object, which is typically a commit object. Tag objects also contain a tag message, as well as author and timestamp information.

Each object is identified by a SHA1 hash calculated over both the object's header and the data stored in the object.

Loose objects are stored as individual files beneath the directory objects, spread across 256 sub-directories named after the 256 possible hexadecimal values of the first byte of an object identifier. The name of the loose object file corresponds to the remaining hexadecimal byte values of the object's identifier.

A loose object file begins with a header which specifies the type of object as an ASCII string, followed by an ASCII space character, followed by the object data's size encoded as an ASCII number string. The header is terminated by a character, and the remainder of the file contains object data. Loose objects files are compressed with deflate(3).

Multiple objects can be bundled in a pack file for better disk space efficiency and increased run-time performance. The pack file format introduces two additional types of objects:

Offset Delta Objects
This object is represented as a delta against another object in the same pack file. This other object is referred to by its offset in the pack file.
Reference Delta Objects
This object is represented as a delta against another object in the same pack file. The other object is referred to by its SHA1 object identifier.

Pack files are self-contained and may not refer to loose objects or objects stored in other pack files. Deltified objects may refer to other deltified objects as their delta base, forming chains of deltas. The ultimate base of a delta chain must be an object of the same type as the original object which is stored in deltified form.

Each pack file is accompanied by a corresponding pack index file, which lists the IDs and offsets of all objects contained in the pack file.

A reference associates a name with an object ID. A prominent use of references is providing names to branches in the repository by pointing at commit objects which represent the current tip commit of a branch. Because references may point to arbitrary object IDs their use is not limited to branches.

The name is a UTF-8 string with the following disallowed characters: ‘ ’ (space), ~ (tilde), ^ (caret), : (colon), ? (question mark), * (asterisk), [ (opening square bracket), \ (backslash). Additionally, the name may not contain the two-character sequences //, .. , and @{.

Reference names may optionally have multiple components separated by the / (slash) character, forming a hierarchy of reference namespaces. Got reserves the got/ reference namespace for internal use.

A symbolic reference associates a name with the name of another reference. The most prominent example is the HEAD reference which points at the name of the repository's default branch reference.

References are stored either as a plain file within the repository, typically under the refs/ directory, or in the packed-refs file which contains one reference definition per line.

Any object which is not directly or indirectly reachable via a reference is subject to deletion by Git's garbage collector.

A reference to the current head commit of the Git work tree. In bare repositories, this files serves as a default reference.
Reference to original head commit. Set by some Git operations.
Reference to a branch tip commit most recently fetched from another repository.
Legacy directory used by the deprecated Gogito Git interface.
Git configuration file. See git-config(1).
A human-readable description of the repository.
This directory contains hook scripts to run when certain events occur.
The file index used by git(1). This file is not used by got(1), which uses the got-worktree(5) file index instead.
Various configuration items.
Directory where reflogs are stored.
Loose and packed objects are stored in this directory.
A file which stores references. Corresponding on-disk references take precedence over those stored here.
The default directory to store references in.

A typical Git repository exposes a work tree which allows the user to make changes to versioned files and create new commits. When a Git work tree is present, the actual repository data is stored in a .git subfolder of the repository's root directory. A Git repository without a work tree is known as a “bare” repository. got(1) does not make use of Git's work tree and treats every repository as if it was bare.

got(1), deflate(3), SHA1(3), got-worktree(5)

The Git repository format was initially designed by Linus Torvalds in 2005 and has since been extended by various people involved in the development of the Git version control system.

The particular set of disallowed characters in reference names is a consequence of design choices made for the command-line interface of git(1). The same characters are disallowed by Got for compatibility purposes. Got additionally prevents users from creating reference names with a leading - (dash) character, because this is rarely intended and not considered useful.

August 2, 2020 OpenBSD 6.7