rcsintro - introduction to RCS commands
The Revision Control System (RCS) manages multiple revisions of text files. RCS automates the storing, retrieval, logging, identification,
and merging of revisions. RCS is useful for text that is revised frequently, for example programs, documentation, graphics, papers, form
The basic user interface is extremely simple. The novice only needs to learn two commands: ci(1L) and co(1L). Ci, short for "check in",
deposits the contents of a text file into an archival file called an RCS file. An RCS file contains all revisions of a particular text
file. Co, short for "check out", retrieves revisions from an RCS file.
Functions of RCS
o Storage and retrieval of multiple revisions of text. RCS saves all old revisions in a space efficient way. Changes no longer
destroy the original, because the previous revisions remain accessible. Revisions can be retrieved according to ranges of revision
numbers, symbolic names, dates, authors, and states.
o Maintenance of a complete history of changes. RCS logs all changes automatically. Besides the text of each revision, RCS stores the
author, the date and time of check-in, and a log message summarizing the change. The logging makes it easy to find out what hap-
pened to a module, without having to compare source listings or having to track down colleagues.
o Resolution of access conflicts. When two or more programmers wish to modify the same revision, RCS alerts the programmers and pre-
vents one modification from corrupting the other.
o Maintenance of a tree of Revisions. RCS can maintain separate lines of development for each module. It stores a tree structure that
represents the ancestral relationships among revisions.
o Merging of revisions and resolution of conflicts. Two separate lines of development of a module can be coalesced by merging. If
the revisions to be merged affect the same sections of code, RCS alerts the user about the overlapping changes.
o Release and configuration control. Revisions can be assigned symbolic names and marked as released, stable, experimental, etc. With
these facilities, configurations of modules can be described simply and directly.
o Automatic identification of each revision with name, revision number, creation time, author, etc. The identification is like a
stamp that can be embedded at an appropriate place in the text of a revision. The identification makes it simple to determine which
revisions of which modules make up a given configuration.
o Minimization of secondary storage. RCS needs little extra space for the revisions (only the differences). If intermediate revisions
are deleted, the corresponding deltas are compressed accordingly.
Getting Started with RCS
Suppose you have a file f.c that you wish to put under control of RCS. Invoke the check-in command
This command creates the RCS file f.c,v, stores f.c into it as revision 1.1, and deletes f.c. It also asks you for a description. The
description should be a synopsis of the contents of the file. All later check-in commands will ask you for a log entry, which should summa-
rize the changes that you made.
Files ending in ,v are called RCS files (`v' stands for `versions'), the others are called working files. To get back the working file f.c
in the previous example, use the check-out command
This command extracts the latest revision from f.c,v and writes it into f.c. You can now edit f.c and check it back in by invoking
Ci increments the revision number properly. If ci complains with the message
ci error: no lock set by <your login>
then your system administrator has decided to create all RCS files with the locking attribute set to `strict'. In this case, you should
have locked the revision during the previous check-out. Your last check-out should have been
co -l f.c
Of course, it is too late now to do the check-out with locking, because you probably modified f.c already, and a second check-out would
overwrite your modifications. Instead, invoke
rcs -l f.c
This command will lock the latest revision for you, unless somebody else got ahead of you already. In this case, you'll have to negotiate
with that person.
Locking assures that you, and only you, can check in the next update, and avoids nasty problems if several people work on the same file.
Even if a revision is locked, it can still be checked out for reading, compiling, etc. All that locking prevents is a CHECK-IN by anybody
but the locker.
If your RCS file is private, i.e., if you are the only person who is going to deposit revisions into it, strict locking is not needed and
you can turn it off. If strict locking is turned off, the owner of the RCS file need not have a lock for check-in; all others still do.
Turning strict locking off and on is done with the commands
rcs -U f.c and rcs -L f.c
If you don't want to clutter your working directory with RCS files, create a subdirectory called RCS in your working directory, and move
all your RCS files there. RCS commands will look first into that directory to find needed files. All the commands discussed above will
still work, without any modification. (Actually, pairs of RCS and working files can be specified in 3 ways: (a) both are given, (b) only
the working file is given, (c) only the RCS file is given. Both RCS and working files may have arbitrary path prefixes; RCS commands pair
them up intelligently).
To avoid the deletion of the working file during check-in (in case you want to continue editing), invoke
ci -l f.c or ci -u f.c
These commands check in f.c as usual, but perform an implicit check-out. The first form also locks the checked in revision, the second one
doesn't. Thus, these options save you one check-out operation. The first form is useful if locking is strict, the second one if not
strict. Both update the identification markers in your working file (see below).
You can give ci the number you want assigned to a checked in revision. Assume all your revisions were numbered 1.1, 1.2, 1.3, etc., and you
would like to start release 2. The command
ci -r2 f.c or ci -r2.1 f.c
assigns the number 2.1 to the new revision. From then on, ci will number the subsequent revisions with 2.2, 2.3, etc. The corresponding co
co -r2 f.c and co -r2.1 f.c
retrieve the latest revision numbered 2.x and the revision 2.1, respectively. Co without a revision number selects the latest revision on
the "trunk", i.e., the highest revision with a number consisting of 2 fields. Numbers with more than 2 fields are needed for branches. For
example, to start a branch at revision 1.3, invoke
ci -r1.3.1 f.c
This command starts a branch numbered 1 at revision 1.3, and assigns the number 126.96.36.199 to the new revision. For more information about
branches, see rcsfile(5L).
RCS can put special strings for identification into your source and object code. To obtain such identification, place the marker
into your text, for instance inside a comment. RCS will replace this marker with a string of the form
$Header: filename revision_number date time author state $
With such a marker on the first page of each module, you can always see with which revision you are working. RCS keeps the markers up to
date automatically. To propagate the markers into your object code, simply put them into literal character strings. In C, this is done as
static char rcsid = "$Header$";
The command ident extracts such markers from any file, even object code and dumps. Thus, ident lets you find out which revisions of which
modules were used in a given program.
You may also find it useful to put the marker $Log$ into your text, inside a comment. This marker accumulates the log messages that are
requested during check-in. Thus, you can maintain the complete history of your file directly inside it. There are several additional
identification markers; see co(1L) for details.
Author: Walter F. Tichy, Purdue University, West Lafayette, IN, 47907.
Revision Number: 1.2 ; Release Date: 87/02/27 .
Copyright (C) 1982 by Walter F. Tichy.
ci(1L), co(1L), ident(1L), merge(1L), rcs(1L), rcsdiff(1L), rcsmerge(1L), rlog(1L)
Walter F. Tichy, "Design, Implementation, and Evaluation of a Revision Control System," in Proceedings of the 6th International Conference
on Software Engineering, IEEE, Tokyo, Sept. 1982.
Purdue University May 11, 1983 RCSINTRO(1L)