rcsintro

NAME

rcsintro - introduction to Revision Control System (RCS) commands

DESCRIPTION

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 frequently, for example programs, documentation, graphics, papers, and form letters. The basic user interface is extremely simple. The novice only needs to learn two commands: ci(1) and co(1). The ci(1) command (ci being an abbreviation 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. The co(1) command (co being an abbreviation for check out) retrieves revisions from an RCS file.

Functions of RCS

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.
Maintain a complete history of changes. RCS logs all changes automatically. In addition to storing the text of each revision, RCS also 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.
Resolve access conflicts. When two or more programmers want to modify the same revision, RCS alerts the programmers 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 ancestral relationships among revisions.
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.
Control releases and configurations. Revisions can be assigned symbolic names and marked as released, stable, experimental, and so on. With these facilities, configurations of modules can be described simply and directly.
Automatically identify each revision with name, revision number, creation time, author, and so on. 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.
Minimize 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

For the purposes of illustration, suppose you have a file f.c that you want to put under control of RCS. If you have not already done so, make an RCS directory with the following command:

mkdir RCS

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 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 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 example, 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.

If, after some editing, you want to see the differences between the changes you have just made and the previously checked-in version, use the command:

rcsdiff f.c

You can check the file back in by invoking:

ci f.c

This increments the revision number properly.

If ci gives you the message

ci error: no lock set by your_name,

it means that you have tried to check in a file even though you did not lock it when you checked it out. At this point, it is too late 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 another user has already done so, in which case you would have to negotiate with that person.

Locking ensures that only you can check in the next update and prevents problems that can arise if several people work on the same file. Even if a revision is locked, it can still be checked out for reading, compiling, and so on. Locking only prevents a check-in by anybody but the locker.

If your RCS file is private (that is, 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. You turn strict locking off and on with the commands:

rcs -U f.c

and

rcs -L f.c

If you do not 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 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 can 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

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 does not. Thus, these options save you one check-out operation. The first form is useful if you want to continue editing, and the second one is useful if you just want to read the file. Both update the identification markers in your working file (see later in this topic).

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, and so on, and you would like to start release 2. The following command:

ci -r2 f.c

ci -r2.1 f.c

assigns the number 2.1 to the new revision. Thereafter, ci will number the subsequent revisions with 2.2, 2.3, and so on. 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. Using co without a revision number selects the latest revision on the trunk; that is, 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 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:

$Id$

into your text; for instance, inside a comment. RCS will replace this marker with a string of the form:

$Id: file-name 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 follows:

 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 modules were used in a given program.

It can also be 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.

Manual Page Revision: 1.1; Release Date: 1996/08/12.