rcsintro(1)rcsintro(1)NAMErcsintro - introduction to RCS commands
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
Store and retrieve multiple revisions of text. RCS saves all old revi‐
sions 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. 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 hap‐
pened to a module, without having to compare source listings or having
to track down colleagues. Resolve access conflicts. When two or more
programmers wish to modify the same revision, RCS alerts the program‐
mers and prevents one modification from corrupting the other. Maintain
a tree of revisions. RCS can maintain separate lines of development
for each module. It stores a tree structure that represents the ances‐
tral relationships among revisions. Merge revisions and resolve con‐
flicts. 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. Control
releases and configurations. Revisions can be assigned symbolic names
and marked as released, stable, experimental, etc. With these facili‐
ties, configurations of modules can be described simply and directly.
Automatically identify each revision with name, revision number, cre‐
ation time, author, etc. The identification is like a stamp that can be
embedded at an appropriate place in the text of a revision. The identi‐
fication makes it simple to determine which revisions of which modules
make up a given configuration. Minimize secondary storage. RCS needs
little extra space for the revisions (only the differences). If inter‐
mediate revisions are deleted, the corresponding deltas are compressed
Getting Started with RCS
Suppose you have a file f.c that you wish to put under control of RCS.
If you have not already done so, make an RCS directory with the command
Then invoke the check-in command ci f.c
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 co f.c
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 rcsdiff f.c
tells you the difference between the most recently checked-in version
and the working file. You can check the file back in by invoking ci f.c
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 modi‐
fications. 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, com‐
piling, etc. All that locking prevents is a check-in by anybody but
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. Turn‐
ing 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 work‐
ing 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
commands 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 188.8.131.52 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 $Id$
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
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 fol‐
lows: static char rcsid = "$Id$";
The command ident extracts such markers from any file, even object code
and dumps. Thus, ident lets you find out which revisions of which mod‐
ules 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 identifi‐
cation markers; see co(1) for details.
Author: Walter F. Tichy.
Revision Number: 184.108.40.206; Release Date: 1993/10/07.
Copyright � 1982, 1988, 1989 by Walter F. Tichy.
Copyright � 1990, 1991 by Paul Eggert.
SEE ALSOci(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.