12.2 Using keywords

To include a keyword string you simply include the relevant text string, such as $Id$, inside the file, and commit the file. CVS will automatically (Or, more accurately, as part of the update run that automatically happens after a commit.) expand the string as part of the commit operation.

It is common to embed the $Id$ string in the source files so that it gets passed through to generated files. For example, if you are managing computer program source code, you might include a variable which is initialized to contain that string. Or some C compilers may provide a #pragma ident directive. Or a document management system might provide a way to pass a string through to generated files.

The ident command (which is part of the RCS package) can be used to extract keywords and their values from a file. This can be handy for text files, but it is even more useful for extracting keywords from binary files.

$ ident samp.c
samp.c:
     $Id: samp.c,v 1.5 1993/10/19 14:57:32 ceder Exp $
$ gcc samp.c
$ ident a.out
a.out:
     $Id: samp.c,v 1.5 1993/10/19 14:57:32 ceder Exp $

SCCS is another popular revision control system. It has a command, what, which is very similar to ident and used for the same purpose. Many sites without RCS have SCCS. Since what looks for the character sequence @(#) it is easy to include keywords that are detected by either command. Simply prefix the keyword with the magic SCCS phrase, like this:

static char *id="@(#) $Id: ab.c,v 1.5 1993/10/19 14:57:32 ceder Exp $";