Git

History

Git development began in April 2005, after many developers of the Linux kernel gave up access to BitKeeper, a proprietary source control management (SCM) system that they had formerly used to maintain the project. The copyright holder of BitKeeper, Larry McVoy, had withdrawn free use of the product after claiming that Andrew Tridgell had reverse-engineered the BitKeeper protocols.

Linus Torvalds wanted a distributed system that he could use like BitKeeper, but none of the available free systems met his needs, especially for performance. Torvalds 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 syncing with fellow maintainers could require 250 such actions at once. For his design criteria, he specified that patching should take no more than three seconds, and added three more points:

Take 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 then-extant version control system except Monotone. Performance considerations excluded it, too. So immediately after the 2.6.12-rc2 Linux kernel development release, Torvalds set out to write his own system.

Torvalds 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". The readme file of the source code elaborates further:

The development of Git began on 3 April 2005. Torvalds announced the project on 6 April; it became self-hosting as of 7 April. 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 the rate of 6.7 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, and remains the project's maintainer.

Design

Git's design was inspired by BitKeeper and Monotone. Git was originally designed as a low-level version control system engine on top of which others could write front ends, such as Cogito or StGIT. The core Git project has since become a complete version control system that is usable directly. While strongly influenced by BitKeeper, Torvalds deliberately avoided conventional approaches, leading to a unique design.

Characteristics

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. With its parental commits, the full branch structure can be constructed.

Distributed development

Like Darcs, BitKeeper, Mercurial, SVK, 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 extant systems and protocols

Repositories can be published via Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), rsync (until Git 2.8.0), 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 extant CVS clients and IDE plugins to access Git repositories. Subversion and svk 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 it was an order of magnitude faster than some version control systems, and 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 (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. (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. 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 --prune.

Periodic explicit object packing

Git stores each newly created object as a separate file. Although individually compressed, this takes a great deal of space and is inefficient. This is solved by the use of packs that store a large number of objects in one file (or network byte stream) called packfile, delta-compressed among themselves. Packs are compressed using the heuristic that files with the same name are probably similar, but do not depend on it for correctness. A corresponding index file is created for each packfile, telling the offset of each object in the packfile. Newly created objects (newly added history) are still stored singly, 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 work day. Git does periodic repacking automatically but manual repacking is also possible with the git gc command. For data integrity, both packfile and its index have SHA-1 checksum inside, and also the file name of packfile contains a SHA-1 checksum. To check integrity, 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 (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. Consequently, Git does not explicitly record file revision relationships at any level below the source code tree.

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 (analogous to an inode number) 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 (immutable) history. Git addresses the issue by detecting renames while browsing the history of snapshots rather than recording it when making the snapshot. (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 central processing unit (CPU) intensive work every time history is reviewed, and several options to adjust the heuristics. 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 changes separately.

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

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 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.

octopus: This is the default when merging more than two heads.

Data structures

Git's primitives are not inherently a source code management (SCM) system. Torvalds explains,

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 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, with features mostly being created as needed, then refined and extended over time.

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 connection point between the object database and the working tree.

The object database contains four types of objects:

A blob (binary large object) 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.)

A tree object is the equivalent of a directory. It contains a list of file names, 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 exactly 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 a history. It contains the name of a tree object (of the top-level source directory), a time stamp, a log message, and the names of zero or more parent commit objects.

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.

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.

Git servers typically listen on TCP port 9418.

References

Every object in the Git database which is not referred to may be cleaned up by using a garbage collection command, or automatically. An object may be referenced by another object, or an explicit reference. Git knows different types of references. The commands to create, move, and delete references vary. "git show-ref" lists all references. Some types are:

heads: refers to an object locally

remotes: refers to an object which exists in a remote repository

stash: refers to an object not yet committed

meta: e.g. a configuration in a bare repository, user rights; the refs/meta/config namespace was introduced resp gets used by Gerrit

tags: see above

Implementations

Git is primarily developed on Linux, although it also supports most major operating systems including BSD, Solaris, macOS, and Windows.

The first Microsoft Windows "port" of Git was primarily a Linux emulation framework that hosts the Linux version. Installing Git under Windows creates a similarly named Program Files directory containing the MinGW port of the GNU Compiler Collection, Perl 5, msys2.0 (itself a fork of Cygwin, a Unix-like emulation environment for Windows) and various other Windows ports or emulations of Linux utilities and libraries. Currently native Windows builds of Git are distributed as 32 and 64-bit installers.

The JGit implementation of Git is a pure Java software library, designed to be embedded in any Java application. JGit is used in the Gerrit code review tool and in EGit, a Git client for the Eclipse IDE.

The Dulwich implementation of Git is a pure Python software component for Python 2.

The libgit2 implementation of Git is an ANSI C software library with no other dependencies, which can be built on multiple platforms including Windows, Linux, macOS, and BSD. It has bindings for many programming languages, including Ruby, Python, and Haskell.

JS-Git is a JavaScript implementation of a subset of Git.

Git server

As Git is a distributed version control system, it can be used as a server out of the box. Dedicated Git server software helps, amongst other features, to add access control, display the contents of a Git repository via the web, and help managing multiple repositories. Remote file store and shell access: A Git repository can be cloned to a shared file system, and accessed by other persons. It can also be accessed via remote shell just by having the Git software installed and allowing a user to log in.

Adoption

The Eclipse Foundation reported in its annual community survey that as of May 2014, Git is now the most widely used source code management tool, with 42.9% of professional software developers reporting that they use Git as their primary source control system compared with 36.3% in 2013, 32% in 2012; or for Git responses excluding use of GitHub: 33.3% in 2014, 30.3% in 2013, 27.6% in 2012 and 12.8% in 2011. Open source directory Black Duck Open Hub reports a similar uptake among open source projects. The Stack Overflow developer survey reported in 2015 that 69.3% of developers use Git; 36.9% use Subversion; 12.2% use TFS; and 7.9% use Mercurial.

The UK IT jobs website itjobswatch.co.uk reports that as of late September 2016, 29.27% of UK permanent software development job openings have cited Git, ahead of 12.17% for Microsoft Team Foundation Server, 10.60% for Subversion, 1.30% for Mercurial, and 0.48% for Visual SourceSafe.

Security

Git does not provide access control mechanisms, but was designed for operation with other tools that specialize in access control.

On 17 December 2014, an exploit was found affecting the Windows and Mac versions of the Git client. An attacker could perform arbitrary code execution on a target computer with Git installed by creating a malicious Git tree (directory) named .git (a directory in Git repositories that stores all the data of the repository) in a different case (such as .GIT or .Git, needed because Git doesn't allow the all-lowercase version of .git to be created manually) with malicious files in the .git/hooks subdirectory (a folder with executable files that Git runs) on a repository that the attacker made or on a repository that the attacker can modify. If a Windows or Mac user pulls (downloads) a version of the repository with the malicious directory, then switches to that directory, the .git directory will be overwritten (due to the case-insensitive trait of the Windows and Mac filesystems) and the malicious executable files in .git/hooks may be run, which results in the attacker's commands being executed. An attacker could also modify the .git/config configuration file, which allows the attacker to create malicious Git aliases (aliases for Git commands or external commands) or modify extant aliases to execute malicious commands when run. The vulnerability was patched in version 2.2.1 of Git, released on 17 December 2014, and announced on the next day.

Git version 2.6.1, released on 29 September 2015, contained a patch for a security vulnerability (CVE-2015-7545) which allowed arbitrary code execution. The vulnerability was exploitable if an attacker could convince a victim to clone a specific URL, as the arbitrary commands were embedded in the URL itself. An attacker could use the exploit via a man-in-the-middle attack if the connection was unencrypted, as they could redirect the user to a URL of their choice. Recursive clones were also vulnerable, since they allowed the controller of a repository to specify arbitrary URLs via the gitmodules file.

Git uses SHA-1 hashes internally. Linus Torvalds has responded that the hash was mostly to guard against accidental corruption, and the security a cryptographically secure hash gives was just an accidental side effect, with the main security being signing elsewhere.