Mercurial is a distributed revision-control tool for software developers. It is supported on Microsoft Windows and Unix-like systems, such as FreeBSD, macOS and Linux. Mercurial's major design goals include high performance and scalability, decentralization, fully distributed collaborative development, robust handling of both plain text and binary files, and advanced branching and merging capabilities, while remaining conceptually simple. It includes an integrated web-interface. Mercurial has also taken steps to ease the transition for users of other version control systems, particularly Subversion. Mercurial is primarily a command-line driven program, but graphical user interface extensions are available, e.g. TortoiseHg, and several IDEs offer support for version control with Mercurial. All of Mercurial's operations are invoked as arguments to its driver program hg. Matt Mackall originated Mercurial and served as its lead developer until late 2016. Mercurial is released as free software under the terms of the GNU GPL v2. It is mainly implemented using the Python programming language, but includes a binary diff implementation written in C.
History
Mackall first announced Mercurial on 19 April 2005. The impetus for this was the announcement earlier that month by Bitmover that they were withdrawing the free version of BitKeeper because of the development of SourcePuller. BitKeeper had been used for the version control requirements of the Linux kernel project. Mackall decided to write a distributed version control system as a replacement for use with the Linux kernel. This project started a few days after the now well-known Git project was initiated by Linus Torvalds with similar aims. The Linux kernel project decided to use Git rather than Mercurial, but Mercurial is now used by many other projects. "Git vs. Mercurial" has become one of the holy wars of hacker culture. In an answer on the Mercurial mailing list, Matt Mackall explained how the name "Mercurial" was chosen:
Design
Mercurial uses SHA-1 hashes to identify revisions. For repository access via a network, Mercurial uses an HTTP-based protocol that seeks to reduce round-trip requests, new connections and data transferred. Mercurial can also work over SSH where the protocol is very similar to the HTTP-based protocol. By default it uses a 3-way merge before calling external merge tools.
Usage
Figure 1 shows some of the most important operations in Mercurial and their relations to Mercurial's concepts.
Adoption
Although Mercurial was not selected to manage the Linux kernel sources, it has been adopted by several organizations, including Facebook, the W3C, and Mozilla. Facebook is using the Rust programming language to write Mononoke, a Mercurial server specifically designed to support large multi-project repositories. In 2013, Facebook adopted Mercurial and began work on scaling it to handle their large, unified code repository. Bitbucket announced that its web-based version control services would end support for Mercurial in June 2020, explaining that "less than 1% of new projects use it, and developer surveys indicated that 90% of developers use Git.", while hundreds of users openly questioned the reasoning for massively deleting historical data, asked why there was not significant warning nor a formal migration plan, and paniced about the rushed timing in their official company support thread.
