Ubuntu Manpage: git-repository — Git repository format

NAME

       git-repository — Git repository format

DESCRIPTION

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 four types of objects as nodes:

Blobs The content of tracked files is stored in objects of type blob.

Trees A tree object points to any number of such blobs, and also to other trees in order to represent
a hierarchy of files and directories.

Commits A commit 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 branches, can be represented. A commit which precedes another commit is referred to as that
other commit's parent commit. A commit with multiple parents unites disparate lines of history
and is known as a merge commit.

Tags A tag 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 an hash calculated over both the object's header and the data stored in the
object. The hashing algorithm used is specified in the repository config file for newer repositories
while older ones use SHA1 implicitly.

OBJECT STORAGE

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 NUL 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 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.

REFERENCES

       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 refs/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 or gotadmin cleanup.

FILES

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

HISTORY

       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.

CAVEATS

       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.

Debian                                             $Mdocdate$                                  GIT-REPOSITORY(5)