NAME
rcsintro - introduction to RCS commands

DESCRIPTION
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 letters, etc.

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 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

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.

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 happened to a module, without having to compare
source listings or having to track down colleagues.

Resolution of access conflicts. When two or more programmers
wish to modify the same revision, RCS alerts the programmers
and prevents one modification from corrupting the other.

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.

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.

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.

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.

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

ci f.c

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 summarize 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

co f.c

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 f.c

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 commands

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 1.3.1.1 to the new revision. For more
information about branches, see rcsfile(5).

Automatic Identification

RCS can put special strings for identification into your source and
object code. To obtain such identification, place the marker

$Header$

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 follows:

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(1) for details.

IDENTIFICATION
Author: Walter F. Tichy, Purdue University, West Lafayette, IN,
47907.
Revision Number: 1.3 ; Release Date: 91/08/21 .
Copyright (c) 1982, 1988, 1989 by Walter F. Tichy.

SEE ALSO
ci(1), co(1), ident(1), merge(1), rcs(1), rcsdiff(1), rcsmerge(1),
rlog(1), rcsfile(5),
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.