Concurrent Versions System

CVS uses a client–server architecture: a server stores the current version(s) of a 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, the client and server connect over a LAN or 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 (although at least the CVSNT server also supports various flavours of Microsoft Windows), while CVS clients may run on any major operating-system platform.

Several developers may work on the same project concurrently, each one editing files within their own "working copy" of the project, and sending (or checking in) their modifications to the server. To avoid conflicts, the server only accepts changes made to the most recent version of a file. Developers are therefore expected to keep their working copy up-to-date by incorporating other people's changes on a regular basis. This task is mostly handled automatically by the CVS client, requiring manual intervention only when an edit conflict arises between a checked-in modification and the yet-unchecked local version of a file.

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. CVS can also run external, user-specified log processing scripts following each commit. These scripts are installed by an entry in CVS's loginfo file, which can trigger email notification or convert the log data into a Web-based format.

Clients can also compare versions, request a complete history of changes, or check out a historical snapshot of the project (e.g.: based on a given date).

CVS servers can allow "anonymous read access",[5] wherein clients may check out and compare versions with either a blank or simple published password (e.g., "anoncvs"); only the check-in of changes requires a personal account and password in these scenarios.

Clients can also use the "update" command 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, can form a separate branch.

CVS uses delta compression for efficient storage of different versions of the same file. This works well with large text files with few changes from one version to the next. This is usually the case for source code files. On the other hand, when CVS is told to store a file as binary, it will keep each individual version on the server. Storing files as binary is important in order to avoid corruption of binary files.

In the world of open source software, the Concurrent Version System (CVS) has long been the tool of choice for version control. And rightly so. CVS itself is free software, and its non-restrictive modus operandi and support for networked operation – which allow dozens of geographically dispersed programmers to share their work – fits the collaborative nature of the open-source world very well. CVS and its semi-chaotic development model have become cornerstones of open-source.

— Collins-Sussman, Version Control with Subversion For Subversion 1.1, 2005

CVS labels a single project (set of related files) that it manages as a module. A CVS server stores the modules it manages in its repository. Programmers acquire copies of modules by checking out. The checked-out files serve as a working copy, sandbox or workspace. Changes to the working copy are reflected in the repository by committing them. To update is to acquire or merge the changes in the repository with the working copy.

CVS was designed:

• To exclude symbolic links because when they are stored in a version control system they can pose a security risk. For instance, a symbolic link to a sensitive file can be stored in the repository, making the sensitive file accessible even when it is not checked in. In place of symbolic links, scripts that require certain privileges and conscious intervention to execute may be checked into CVS.
• Use the native ASCII character set in filenames, precluding the use of multi-lingual filenames. Now many Unix systems run in UTF-8,[6] and CVS on such systems handles UTF-8 multi-lingual filenames natively.
• Create revisions per file, not an identifier for all revisions created during a commit ( this is sometimes referred to as atomic but not in the transactional database sense, where a commit automatically rolls back if it fails for any reason, but in the sense that each commit can be uniquely identified). This design choice was made even when servers could easily have had insufficient resilience to complete a commit without crashing. The lack of atomicity is mitigated by the fact that in many code management processes, development work is performed on branches and then merged into the trunk after code review. That final merge is atomic, and performed in the data center by QA. Tracking each commit can be accomplished by modifying the correct trigger.
• To assume that the majority of work takes place on the trunk, and that branches should generally be short-lived or historical. When used as designed, branches are easily managed and branch operations are efficient and fast.[7][8]
• That text files are expected to be the primary file type stored in the CVS repository. However, binary files are also supported, and files with a particular file extension can automatically be recognized as being binary.
• That changes would be frequently committed to the centrally checked-in copies of files in order to aid merging and foster rapid distribution of changes to all users, so there is no support for distributed revision control or unpublished changes.

Dick Grune developed CVS as a front end for the Revision Control System (RCS), an older version control system that manages individual files but not whole projects.

I created CVS to be able to cooperate with my students, Erik Baalbergen and Maarten Waage, on the ACK (Amsterdam Compiler Kit) C compiler. The three of us had vastly different schedules (one student was a steady 9-5 worker, the other was irregular, and I could work on the project only in the evenings). Their project ran from July 1984 to August 1985. CVS was initially called cmt, for the obvious reason that it allowed us to commit versions independently.

— http://dickgrune.com/Programs/CVS.orig/#History

Grune publicly released the code on June 23, 1986.[9]

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. Brian Berliner wrote a paper introducing his improvements to the CVS program—which describes how the tool was extended and used internally by Prisma, a third-party developer working on the SunOS kernel, and was released for the benefit of the community under the GPL. On November 19, 1990, CVS version 1.0 was submitted to the Free Software Foundation for development and distribution.[10]

CVS supports distributed, multi-site and offline operations due to the unreliability of the few computer networks that existed at the time CVS evolved.

• StatCVS, a program that generates graphical reports about CVS repositories.
• TortoiseCVS, a shell-integrated CVSNT client for Microsoft Windows (commonly used with CVS servers)[23][24]
• Bonsai CVS code management system, a tool to report on files, directories, and branches under CVS management.
• Cervisia, a KDE GUI frontend for CVS
• ViewVC, a browser interface for CVS repositories
• OpenGrok, a browser interface for CVS repositories
• CVSup, a utility and protocol to check out and mirror CVS repositories efficiently

• Official website
• Version Management with CVS – manual for CVS 1.12.13, by Per Cederqvist et al.