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Git (/ɡɪt/) is a distributed version control system that tracks changes in any set of computer files, usually used for coordinating work among programmers who are collaboratively developing source code during software development.

Git
Original author(s)Linus Torvalds
Developer(s)Junio Hamano and others
Initial release7 April 2005; 18 years ago (2005-04-07)
Stable release
2.44.0 Edit this on Wikidata / 23 February 2024
Repository
  • git.kernel.org/pub/scm/git/git.git Edit this at Wikidata
Written inPrimarily in C, with GUI and programming scripts written in Shell script, Perl, Tcl and Python
Operating systemPOSIX (Linux, macOS, Solaris, AIX), Windows
Available inEnglish
TypeVersion control
LicenseGPL-2.0-only
Websitegit-scm.com Edit this on Wikidata

Git's goals include speed, data integrity, and support for distributed, non-linear workflows (thousands of parallel branches running on different computers). Git was originally authored by Linus Torvalds in 2005 for development of the Linux kernel, with other kernel developers contributing to its initial development. It was prompted by the revocation of the free license of BitKeeper, the proprietary source-control management system used for Linux kernel development since 2002. Since 2005, Junio Hamano has been the core maintainer of Git. As with most other distributed version control systems, and unlike most client–server systems, every Git directory on every computer is a full-fledged repository with complete history and full version-tracking abilities, independent of network access or a central server. Git is a free and open-source software shared under the GPL-2.0-only license.

Git's design benefits from Torvalds' experience with Linux and file-system performance, leading to features such as support for non-linear development, efficient handling of large projects, and cryptographic authentication of history. Its toolkit-based design allows for pluggable merge strategies and flexibility in managing version control tasks. Despite its comprehensive feature set, Git has faced security challenges, leading to updates and patches that address vulnerabilities. The trademark "Git" is registered by the Software Freedom Conservancy, marking its official recognition and continued evolution in the open-source community.

Git's adoption has grown rapidly, becoming the most popular distributed version control system, with nearly 95% of developers reporting it as their primary version control system as of 2022. It is the most widely used source-code management tool among professional developers. There are offerings of Git repository services, including GitHub, SourceForge, Bitbucket and GitLab.

History edit

Git development was started by Torvalds in April 2005 when the proprietary source-control management (SCM) system used for Linux kernel development since 2002, BitKeeper, revoked its free license for Linux development. The copyright holder of BitKeeper, Larry McVoy, claimed that Andrew Tridgell had created SourcePuller by reverse engineering the BitKeeper protocols. The same incident also spurred the creation of another version-control system, Mercurial.

Torvalds wanted a distributed system that he could use like BitKeeper, but none of the available free systems met his needs. He cited an example of a source-control management system needing 30 seconds to apply a patch and update all associated metadata, and noted that this would not scale to the needs of Linux kernel development, where synchronizing with fellow maintainers could require 250 such actions at once. For his design criterion, he specified that patching should take no more than three seconds, and added three more goals:

  • Take the Concurrent Versions System (CVS) as an example of what not to do; if in doubt, make the exact opposite decision.
  • Support a distributed, BitKeeper-like workflow.
  • Include very strong safeguards against corruption, either accidental or malicious.

These criteria eliminated every version-control system in use at the time, so immediately after the 2.6.12-rc2 Linux kernel development release, Torvalds set out to write his own.

The development of Git began on 3 April 2005. Torvalds announced the project on 6 April and became self-hosting the next day. The first merge of multiple branches took place on 18 April. Torvalds achieved his performance goals; on 29 April, the nascent Git was benchmarked recording patches to the Linux kernel tree at a rate of 6.7 patches per second. On 16 June, Git managed the kernel 2.6.12 release.

Torvalds turned over maintenance on 26 July 2005 to Junio Hamano, a major contributor to the project. Hamano was responsible for the 1.0 release on 21 December 2005.

Naming edit

Torvalds sarcastically quipped about the name git (which means "unpleasant person" in British English slang): "I'm an egotistical bastard, and I name all my projects after myself. First 'Linux', now 'git'." The man page describes Git as "the stupid content tracker".[33]

The read-me file of the source code elaborates further:[34]

"git" can mean anything, depending on your mood.

  • Random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
  • Stupid. Contemptible and despicable. Simple. Take your pick from the dictionary of slang.
  • "Global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
  • "Goddamn idiotic truckload of sh*t": when it breaks.

The source code for Git refers to the program as "the information manager from hell".

Characteristics edit

Git's design is a synthesis of Torvalds's experience with Linux in maintaining a large distributed development project, along with his intimate knowledge of file-system performance gained from the same project and the urgent need to produce a working system in short order. These influences led to the following implementation choices:[13]

Strong support for non-linear development
Git supports rapid branching and merging, and includes specific tools for visualizing and navigating a non-linear development history. In Git, a core assumption is that a change will be merged more often than it is written, as it is passed around to various reviewers. In Git, branches are very lightweight: a branch is only a reference to one commit.
Distributed development
Like Darcs, BitKeeper, Mercurial, Bazaar, and Monotone, Git gives each developer a local copy of the full development history, and changes are copied from one such repository to another. These changes are imported as added development branches and can be merged in the same way as a locally developed branch.[35]
Compatibility with existing systems and protocols
Repositories can be published via Hypertext Transfer Protocol Secure (HTTPS), Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), or a Git protocol over either a plain socket or Secure Shell (ssh). Git also has a CVS server emulation, which enables the use of existing CVS clients and IDE plugins to access Git repositories. Subversion repositories can be used directly with git-svn.[36]
Efficient handling of large projects
Torvalds has described Git as being very fast and scalable,[37] and performance tests done by Mozilla[38] showed that it was an order of magnitude faster diffing large repositories than Mercurial and GNU Bazaar; fetching version history from a locally stored repository can be one hundred times faster than fetching it from the remote server.[39]
Cryptographic authentication of history
The Git history is stored in such a way that the ID of a particular version (a commit in Git terms) depends upon the complete development history leading up to that commit. Once it is published, it is not possible to change the old versions without it being noticed. The structure is similar to a Merkle tree, but with added data at the nodes and leaves.[40] (Mercurial and Monotone also have this property.)
Toolkit-based design
Git was designed as a set of programs written in C and several shell scripts that provide wrappers around those programs.[41] Although most of those scripts have since been rewritten in C for speed and portability, the design remains, and it is easy to chain the components together.[42]
Pluggable merge strategies
As part of its toolkit design, Git has a well-defined model of an incomplete merge, and it has multiple algorithms for completing it, culminating in telling the user that it is unable to complete the merge automatically and that manual editing is needed.[43]
Garbage accumulates until collected
Aborting operations or backing out changes will leave useless dangling objects in the database. These are generally a small fraction of the continuously growing history of wanted objects. Git will automatically perform garbage collection when enough loose objects have been created in the repository. Garbage collection can be called explicitly using git gc.[44][45]
Periodic explicit object packing
Git stores each newly created object as a separate file. Although individually compressed, this takes up a great deal of space and is inefficient. This is solved by the use of packs that store a large number of objects delta-compressed among themselves in one file (or network byte stream) called a packfile. Packs are compressed using the heuristic that files with the same name are probably similar, without depending on this for correctness. A corresponding index file is created for each packfile, telling the offset of each object in the packfile. Newly created objects (with newly added history) are still stored as single objects, and periodic repacking is needed to maintain space efficiency. The process of packing the repository can be very computationally costly. By allowing objects to exist in the repository in a loose but quickly generated format, Git allows the costly pack operation to be deferred until later, when time matters less, e.g., the end of a workday. Git does periodic repacking automatically, but manual repacking is also possible with the git gc command.[46] For data integrity, both the packfile and its index have an SHA-1 checksum[47] inside, and the file name of the packfile also contains an SHA-1 checksum. To check the integrity of a repository, run the git fsck command.[48][49]

Another property of Git is that it snapshots directory trees of files. The earliest systems for tracking versions of source code, Source Code Control System (SCCS) and Revision Control System (RCS), worked on individual files and emphasized the space savings to be gained from interleaved deltas (SCCS) or delta encoding (RCS) the (mostly similar) versions. Later revision-control systems maintained this notion of a file having an identity across multiple revisions of a project. However, Torvalds rejected this concept.[50] Consequently, Git does not explicitly record file revision relationships at any level below the source-code tree.

These implicit revision relationships have some significant consequences:

  • It is slightly more costly to examine the change history of one file than the whole project.[51] To obtain a history of changes affecting a given file, Git must walk the global history and then determine whether each change modified that file. This method of examining history does, however, let Git produce with equal efficiency a single history showing the changes to an arbitrary set of files. For example, a subdirectory of the source tree plus an associated global header file is a very common case.
  • Renames are handled implicitly rather than explicitly. A common complaint with CVS is that it uses the name of a file to identify its revision history, so moving or renaming a file is not possible without either interrupting its history or renaming the history and thereby making the history inaccurate. Most post-CVS revision-control systems solve this by giving a file a unique long-lived name (analogous to an inode number) that survives renaming. Git does not record such an identifier, and this is claimed as an advantage.[52][53] Source code files are sometimes split or merged, or simply renamed,[54] and recording this as a simple rename would freeze an inaccurate description of what happened in the (immutable) history. Git addresses the issue by detecting renames while browsing the history of snapshots rather than recording it when making the snapshot.[55] (Briefly, given a file in revision N, a file of the same name in revision N − 1 is its default ancestor. However, when there is no like-named file in revision N − 1, Git searches for a file that existed only in revision N − 1 and is very similar to the new file.) However, it does require more CPU-intensive work every time the history is reviewed, and several options to adjust the heuristics are available. This mechanism does not always work; sometimes a file that is renamed with changes in the same commit is read as a deletion of the old file and the creation of a new file. Developers can work around this limitation by committing the rename and the changes separately.

Git implements several merging strategies; a non-default strategy can be selected at merge time:[56]

  • resolve: the traditional three-way merge algorithm.
  • recursive: This is the default when pulling or merging one branch, and is a variant of the three-way merge algorithm.

    When there are more than one common ancestors that can be used for a three-way merge, it creates a merged tree of the common ancestors and uses that as the reference tree for the three-way merge. This has been reported to result in fewer merge conflicts without causing mis-merges by tests done on prior merge commits taken from Linux 2.6 kernel development history. Also, this can detect and handle merges involving renames.

    — Linus Torvalds[57]
  • octopus: This is the default when merging more than two heads.

Data structures edit

Git's primitives are not inherently a source-code management system. Torvalds explains:[58]

In many ways you can just see git as a filesystem—it's content-addressable, and it has a notion of versioning, but I really designed it coming at the problem from the viewpoint of a filesystem person (hey, kernels is what I do), and I actually have absolutely zero interest in creating a traditional SCM system.

From this initial design approach, Git has developed the full set of features expected of a traditional SCM,[59] with features mostly being created as needed, then refined and extended over time.

 
Some data flows and storage levels in the Git revision control system

Git has two data structures: a mutable index (also called stage or cache) that caches information about the working directory and the next revision to be committed; and an immutable, append-only object database.

The index serves as a connection point between the object database and the working tree.

The object store contains five types of objects:[60][48]

  • A blob is the content of a file. Blobs have no proper file name, time stamps, or other metadata (a blob's name internally is a hash of its content[61]). In git, each blob is a version of a file, in which is the file's data.[62]
  • A tree object is the equivalent of a directory. It contains a list of file names,[63] each with some type bits and a reference to a blob or tree object that is that file, symbolic link, or directory's contents. These objects are a snapshot of the source tree. (In whole, this comprises a Merkle tree, meaning that only a single hash for the root tree is sufficient and actually used in commits to precisely pinpoint to the exact state of whole tree structures of any number of sub-directories and files.)
  • A commit object links tree objects together into history. It contains the name of a tree object (of the top-level source directory), a timestamp, a log message, and the names of zero or more parent commit objects.[64]
  • A tag object is a container that contains a reference to another object and can hold added meta-data related to another object. Most commonly, it is used to store a digital signature of a commit object corresponding to a particular release of the data being tracked by Git.[65]
  • A packfile object collects various other objects into a zlib-compressed bundle for compactness and ease of transport over network protocols.[66]

Each object is identified by a SHA-1 hash of its contents. Git computes the hash and uses this value for the object's name. The object is put into a directory matching the first two characters of its hash. The rest of the hash is used as the file name for that object.

Git stores each revision of a file as a unique blob. The relationships between the blobs can be found through examining the tree and commit objects. Newly added objects are stored in their entirety using zlib compression. This can consume a large amount of disk space quickly, so objects can be combined into packs, which use delta compression to save space, storing blobs as their changes relative to other blobs.

Additionally, git stores labels called refs (short for references) to indicate the locations of various commits. They are stored in the reference database and are respectively:[67]

  • Heads (branches): Named references that are advanced automatically to the new commit when a commit is made on top of them.
  • HEAD: A reserved head that will be compared against the working tree to create a commit.
  • Tags: Like branch references but fixed to a particular commit. Used to label important points in history.

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