Git
Git is a distributed version control software system that is capable of managing versions of source code or data. It is often used to control source code by programmers who are developing software collaboratively.
Design goals of Git include speed, data integrity, and support for distributed, non-linear workflows—thousands of parallel branches running on different computers.
As with most other distributed version control systems, and unlike most client–server systems, Git maintains a local copy of the entire repository, also known as the "repo", with history and version-tracking abilities, independent of network access or a central server. A repository is stored on each computer in a standard directory with additional, hidden files to provide version control capabilities. Git provides features to synchronize changes between repositories that share history; for asynchronous collaboration, this extends to repositories on remote machines. Although all repositories are peers, developers often use a central server to host a repository to hold an integrated copy.
Git is free and open-source software shared under the GPL-2.0-only license.
Git was originally created by Linus Torvalds for version control in the development of the Linux kernel. The trademark "Git" is registered by the Software Freedom Conservancy.
Today, Git is the version control system most commonly used by software developers. It is 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
Torvalds started developing Git in April 2005 after the free license for BitKeeper, the proprietary source-control management system used for Linux kernel development since 2002, was revoked for Linux. 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 Mercurial, another version-control system.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 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.
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
Torvalds quipped about the name git : "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".The read-me file of the source code elaborates further:
The source code for Git refers to the program as "the information manager from hell".
Characteristics
Design
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:; 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.
; Compatibility with existing systems and protocols: Repositories can be published via Hypertext Transfer Protocol Secure, Hypertext Transfer Protocol, File Transfer Protocol, or a Git protocol over either a plain socket or Secure Shell. 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.
; Efficient handling of large projects: Torvalds has described Git as being very fast and scalable, and performance tests done by Mozilla 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.
; Cryptographic authentication of history: The Git history is stored in such a way that the ID of a particular version 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.
; Toolkit-based design: Git was designed as a set of programs written in C and several shell scripts that provide wrappers around those programs. 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.
; 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.
; 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.; 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 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, recording the offset of each object in the packfile. Newly created objects 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. For data integrity, both the packfile and its index have an SHA-1 checksum 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.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 and Revision Control System, worked on individual files and emphasized the space savings to be gained from interleaved deltas or delta encoding the 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. Consequently, Git does not explicitly record file revision relationships at any level below the source-code tree.
Downsides
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. 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 that survives renaming. Git does not record such an identifier, and this is claimed as an advantage. Source code files are sometimes split or merged, or simply renamed, and recording this as a simple rename would freeze an inaccurate description of what happened in the history. Git addresses the issue by detecting renames while browsing the history of snapshots rather than recording it when making the snapshot. 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.