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Concurrent Versions System

The Concurrent Versions System (CVS), also known as the Concurrent Version System and the Concurrent Versioning System, implements a version control system: it keeps track of all work and all changes in the implementation of a (software) project and allows several (potentially widely separated) developers to collaborate. CVS has become popular in the free software world. Its developers release the system under the GNU General Public License.


CVS utilises a client-server architecture: a server stores the current version(s) of the project and its history, and clients connect to the server in order to check-out a complete copy of the project, work on this copy and then later check-in their changes. Typically, client and server connect over the internet, but client and server may both run on the same machine if CVS has the task of keeping track of the version history of a project with only local developers. The server software normally runs on Unix, while CVS clients may run on any major operating-system platform.

Several clients may check-out copies of the project concurrently. When they later check-in their changes, the server attempts to merge them. If this fails, for instance because two clients attempted to change the same line in a certain file, then the server denies the second check-in operation and informs the client about the conflict, which the user will need to resolve by hand. If the check-in operation succeeds, then the version numbers of all files involved automatically increment, and the CVS server writes a user-supplied description line, the date and the author's name to its log files.

Clients can also compare different versions of files, request a complete history of changes, or check-out a historical snapshot of the project as of a given date or as of a revision number. Many open-source projects allow "anonymous read access", meaning that the clients may check-out and compare versions without a password; only the check-in of changes requires a password in these scenarios.

Clients can also use the "update" command in order to bring their local copies up-to-date with the newest version on the server. This eliminates the need for repeated downloading of the whole project.

CVS can also maintain different "branches" of a project. For instance, a released version of the software project may form one branch, used for bug fixes, while a version under current development, with major changes and new features, forms a separate branch.


CVS terminology dubs a single project (set of related files) managed by CVS as a module. A CVS server can manage several modules; it stores all the modules it manages in its repository. The copy of a module that has been downloaded by a client serves as a working copy.

History and Status

CVS developed from an earlier versioning system called RCS, still in use, which manages individual files but not whole projects. Dick Grune first conceived the project-based approach and the conflict-resolution algorithms of CVS in December 1986. The code that eventually evolved into the current version of CVS started with Brian Berliner in April 1989, with later input from Jeff Polk and many other contributors. Nowadays, a group of volunteers maintains the CVS code.

The relationship between CVS and the GNU project can appear somewhat ambiguous: the GNU website distributes the program, labelling it "GNU package" on one page and "other GPL-licensed project" on another. On the FTP site, the program resides in the /non-gnu/ directory.

See also version control system.

References and external links
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