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Store and retrieve multiple revisions of text. r 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.

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Maintain a complete history of changes. r logs all changes automatically. Besides the text of each revision, r 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.

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

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Maintain a tree of revisions. r can maintain separate lines of development for each module. It stores a tree structure that represents the ancestral relationships among revisions.

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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, r alerts the user about the overlapping changes.

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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.

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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.

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Minimize secondary storage. r needs little extra space for the revisions (only the differences). If intermediate revisions are deleted, the corresponding deltas are compressed accordingly.

Suppose you have a file f.c that you wish to put under control of r. If you have not already done so, make an r directory with the command

mkdir RCS

Then invoke the check-in command

ci f.c

This command creates an r 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 r directory are called r 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 r 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 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 r 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 r 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 r files, create a subdirectory called RCS in your working directory, and move all your r files there. r 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 r and working files can be specified in three ways: (a) both are given, (b) only the working file is given, (c) only the r file is given. Both r and working files may have arbitrary path prefixes; r 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 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 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).

r 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. r 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. r 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.

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.