2 .\" Copyright (c) 2018 Stefan Sperling <stsp@openbsd.org>
4 .\" Permission to use, copy, modify, and distribute this software for any
5 .\" purpose with or without fee is hereby granted, provided that the above
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21 .Nd Git repository format
23 A Git repository stores a series of versioned snapshots of a file hierarchy.
24 Conceptually, the repository's data model is a directed acyclic graph which
25 contains three types of objects as nodes:
28 The content of tracked files is stored in objects of type
33 object points to any number of such blobs, and also to other trees in
34 order to represent a hierarchy of files and directories.
38 object points to the root element of one tree, and thus records the
39 state of this entire tree as a snapshot.
40 Commit objects are chained together to form lines of version control history.
41 Most commits have just one successor commit, but commits may be succeeded by
42 an arbitrary number of subsequent commits so that diverging lines of version
43 control history, known as
46 A commit which precedes another commit is referred to as that other commit's
48 A commit with multiple parents unites disparate lines of history and is
54 object associates an user-defined label with another object, which is
55 typically a commit object.
56 Tag objects also contains a tag message, as well as author and
57 timestamp information.
60 Each object is identified by a SHA1 hash calculated over both the object's
61 header and the data stored in the object.
63 Loose objects are stored as individual files beneath the directory
65 spread across 256 sub-directories named after the 256 possible hexadecimal
66 values of the first byte of an object identifier.
67 The name of the loose object file corresponds to the remaining hexadecimal
68 byte values of the object's identifier.
70 A loose object file begins with a header which specifies the type of object
71 as an ASCII string, followed by an ASCII space character, followed by the
72 object data's size encoded as an ASCII number string.
73 The header is terminated by a
75 character, and the remainder of the file contains object data.
76 Loose objects files are compressed with
79 Multiple objects can be bundled in a
81 for better disk space efficiency and increased run-time performance.
82 The pack file format knows two additional types of objects in addition
83 to blobs, trees, and commits:
85 .It Offset Delta Objects
86 This object is represented as a delta against another object in the
88 This other object is referred to by its offset in the pack file.
89 .It Reference Delta Objects
90 This object is represented as a delta against another object in the
92 The other object is referred to by its SHA1 object identifier.
95 Pack files are self-contained and may not refer to loose objects or
96 objects stored in other pack files.
97 Deltified objects may refer to other deltified objects as their delta base,
98 forming chains of deltas.
99 The ultimate base of a delta chain must be an object of the same type as
100 the original object which is stored in deltified form.
102 Each pack file is accompanied by a corresponding
104 file, which lists the IDs and offsets of all objects contained in the
107 A reference associates a name with an object ID.
108 A prominent use of references is providing names to branches in the
109 repository by pointing at commit objects which represent the current
110 tip commit of a branch.
111 Because references may point to arbitrary object IDs their use
112 is not limited to branches.
114 The name is a UTF-8 string with the following disallowed characters:
122 [ (opening square bracket),
124 Additionally, the name may not contain the two-character sequences
127 Reference names may optionally have multiple components separated by
128 the / (slash) character, forming a hierarchy of reference namespaces.
131 reference namespace for internal use.
133 A symbolic reference associates a name with the name of another reference.
134 The most prominent example is the
136 reference which points at the name of the repository's default branch
139 References are stored either as a plain file within the repository,
144 file which contains one reference definition per line.
146 Any object which is not directly or indirectly reachable via a reference
147 is subject to deletion by Git's garbage collector.
149 .Bl -tag -width packed-refs -compact
151 A reference to the current head commit of the Git work tree.
152 In bare repositories, this files serves as a default reference.
154 Reference to original head commit.
155 Set by some Git operations.
157 Reference to a branch tip commit most recently fetched from another repository.
159 Legacy directory used by the deprecated Gogito Git interface.
161 Git configuration file.
165 A human-readable description of the repository.
167 This directory contains hook scripts to run when certain events occur.
169 The file index used by
171 This file is not used by
177 Various configuration items.
179 Directory where reflogs are stored.
181 Loose and packed objects are stored in this directory.
183 A file which stores references.
184 Corresponding on-disk references take precedence over those stored here.
186 The default directory to store references in.
189 A typical Git repository exposes a work tree which allows the user to make
190 changes to versioned files and create new commits.
191 When a Git work tree is present, the actual repository data is stored in a
193 subfolder of the repository's root directory.
194 A Git repository without a work tree is known as a
198 does not make use of Git's work tree and treats every repository as if it
206 The Git repository format was initially designed by Linus Torvalds in 2005
207 and has since been extended by various people involved in the development
208 of the Git version control system.
210 The particular set of disallowed characters in reference names is a
211 consequence of design choices made for the command-line interface of
213 The same characters are disallowed by Got for compatibility purposes.
214 Got additionally prevents users from creating reference names with
215 a leading - (dash) character, because this is rarely intended and
216 not considered useful.
