The Revision Control System (RCS) manages multiple revisions of files.
RCS automates the storing, retrieval, logging, identification, and
merging of revisions. RCS is useful for text that is revised fre-
quently, for example programs, documentation, graphics, papers, and
The basic user interface is extremely simple. The novice only needs to
learn two commands: ci(1) and co(1). ci, short for "check in",
deposits the contents of a file into an archival file called an RCS
file. An RCS file contains all revisions of a particular file. co,
short for "check out", retrieves revisions from an RCS file.
Functions of RCS
o Store and retrieve 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 Maintain 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 happened to a module, without having to compare source
listings or having to track down colleagues.
o Resolve 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 Maintain 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 Merge revisions and resolve 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 Control releases and configurations. 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 Automatically identify 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 Minimize secondary storage. RCS needs little extra space for
the revisions (only the differences). If intermediate revisions
This command creates an RCS file in the RCS directory, stores f.c into
it as revision 1.1, and deletes f.c. It also asks you for a descrip-
tion. 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 summarize the changes that you made.
Files in the RCS directory are called RCS files; the others are called
working files. To get back the working file f.c in the previous exam-
ple, use the check-out command
This command extracts the latest revision from the RCS file and writes
it into f.c. If you want to edit f.c, you must lock it as you check it
out with the command
co -l f.c
You can now edit f.c.
Suppose after some editing you want to know what changes that you have
made. The command
tells you the difference between the most recently checked-in version
and the working file. You can check the file back in by invoking
This increments the revision number properly.
If ci complains with the message
ci error: no lock set by your name
then you have tried to check in a file even though you did not lock it
when you checked it out. Of course, it is too late now to do the
check-out with locking, because another 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
still work, without any modification. (Actually, pairs of RCS and
working files can be specified in three 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 or compiling), 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 you want to continue editing, the second one if
you just want to read the file. 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 num-
ber 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, respec-
tively. co without a revision number selects the latest revision on
the trunk, i.e. the highest revision with a number consisting of two
fields. Numbers with more than two 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(5).
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
$Id: filename revision date time author state $
With such a marker on the first page of each module, you can always see
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 identi-
fication markers; see co(1) for details.
Author: Walter F. Tichy.
Manual Page Revision: 5.3; Release Date: 1993/11/03.
Copyright (C) 1982, 1988, 1989 Walter F. Tichy.
Copyright (C) 1990, 1991, 1992, 1993 Paul Eggert.
ci(1), co(1), ident(1), rcs(1), rcsdiff(1), rcsintro(1), rcsmerge(1),
Walter F. Tichy, RCS--A System for Version Control, Software--Practice
& Experience 15, 7 (July 1985), 637-654.
GNU 1993/11/03 RCSINTRO(1)
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