RCS—A System for Version Control

Walter F. Tichy

Purdue University
Department of Computer Sciences

West LafayetteIndiana 47907

July 1985

Revision History
Revision 1.0 July 1985 wft
Revision 1.1 1 June 1995  
Included with free software distribution.
Revision 1.2 July 2004 ashawley
Converted to XML Docbook 4.2 with the help of Groff

Table of Contents

Getting started with RCS
Automatic Identification
The RCS Revision Tree
When are branches needed?
Revisions are represented as deltas
Locking: A Controversial Issue
Configuration Management
RCS Selection Functions
Combining MAKE and RCS
Usage Statistics
Survey of Version Control Tools
A. Synopsis of RCS Operations


An important problem in program development and maintenance is version control, i.e., the task of keeping a software system consisting of many versions and configurations well organized. The Revision Control System (RCS) is a software tool that assists with that task. RCS manages revisions of text documents, in particular source programs, documentation, and test data. It automates the storing, retrieval, logging and identification of revisions, and it provides selection mechanisms for composing configurations. This paper introduces basic version control concepts and discusses the practice of version control using RCS. For conserving space, RCS stores deltas, i.e., differences between successive revisions. Several delta storage methods are discussed. Usage statistics show that RCS’s delta storage method is space and time efficient. The paper concludes with a detailed survey of version control tools.


Version control is the task of keeping software systems consisting of many versions and configurations well organized. The Revision Control System (RCS) is a set of UNIX commands that assist with that task.

RCS’ primary function is to manage revision groups. A revision group is a set of text documents, called revisions, that evolved from each other. A new revision is created by manually editing an existing one. RCS organizes the revisions into an ancestral tree. The initial revision is the root of the tree, and the tree edges indicate from which revision a given one evolved. Besides managing individual revision groups, RCS provides flexible selection functions for composing configurations. RCS may be combined with MAKE 1, resulting in a powerful package for version control.

RCS also offers facilities for merging updates with customer modifications, for distributed software development, and for automatic identification. Identification is the ‘stamping’ of revisions and configurations with unique markers. These markers are akin to serial numbers, telling software maintainers unambiguously which configuration is before them.

RCS is designed for both production and experimental environments. In production environments, access controls detect update conflicts and prevent overlapping changes. In experimental environments, where strong controls are counterproductive, it is possible to loosen the controls.

Although RCS was originally intended for programs, it is useful for any text that is revised frequently and whose previous revisions must be preserved. RCS has been applied successfully to store the source text for drawings, VLSI layouts, documentation, specifications, test data, form letters and articles.

This paper discusses the practice of version control using RCS. It also introduces basic version control concepts, useful for clarifying current practice and designing similar systems. Revision groups of individual components are treated in the next three sections, and the extensions to configurations follow. Because of its size, a survey of version control tools appears at the end of the paper.

Acknowledgements: Many people have helped make RCS a success by contributed criticisms, suggestions, corrections, and even whole new commands (including manual pages). The list of people is too long to be reproduced here, but my sincere thanks for their help and goodwill goes to all of them.