As long as you edit source files inside a working directory you can always find out the state of your files via `cvs status' and `cvs log'. But as soon as you export the files from your development environment it becomes harder to identify which revisions they are.
CVS can use a mechanism known as keyword
substitution (or keyword expansion) to help
identifying the files. Embedded strings of the form
$keyword$
and
$keyword:...$
in a file are replaced
with strings of the form
$keyword:value$
whenever you obtain
a new revision of the file.
This is a list of the keywords:
$Author$
$Date$
$Header$
$Id$
$Header$
, except that the RCS
filename is without a path.
$Name$
cvs
co -r first
, the keyword expands to `Name: first'.
$Locker$
cvs admin -l
is in use).
$Log$
$Log:...$
.
Each new line is prefixed with the same string which
precedes the $Log
keyword. For example, if the
file contains
/* Here is what people have been up to: * * $Log: frob.c,v $ * Revision 1.1 1997/01/03 14:23:51 joe * Add the superfrobnicate option * */then additional lines which are added when expanding the
$Log
keyword will be preceded by ` * '.
Unlike previous versions of CVS and RCS, the
comment leader from the RCS file is not used.
The $Log
keyword is useful for
accumulating a complete change log in a source file,
but for several reasons it can be problematic.
See section Problems with the $Log$ keyword..
$RCSfile$
$Revision$
$Source$
$State$
cvs admin -s
---see section admin options.
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
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 $";
Keyword substitution has its disadvantages. Sometimes you might want the literal text string `$'Author$ to appear inside a file without CVS interpreting it as a keyword and expanding it into something like `$'Author: ceder $.
There is unfortunately no way to selectively turn off keyword substitution. You can use `-ko' (see section Substitution modes) to turn off keyword substitution entirely.
In many cases you can avoid using keywords in
the source, even though they appear in the final
product. For example, the source for this manual
contains `$@asis{}Author$' whenever the text
`$'Author$ should appear. In nroff
and troff
you can embed the null-character
\&
inside the keyword for a similar effect.
Each file has a stored default substitution mode, and
each working directory copy of a file also has a
substitution mode. The former is set by the `-k'
option to cvs add
and cvs admin
; the
latter is set by the `-k' or `-A' options to cvs
checkout
or cvs update
. cvs diff
also
has a `-k' option. For some examples,
see section Handling binary files, and section Merging and keywords.
The modes available are:
$
Revision: 5.7 $ for the Revision
keyword.
cvs admin
-l
is in use.
Revision
keyword, generate the string $
Revision$
instead of $
Revision: 5.7 $. This option
is useful to ignore differences due to keyword
substitution when comparing different revisions of a
file (see section Merging and keywords).
Revision
keyword, generate the string
$
Revision: 1.1 $ instead of
$
Revision: 5.7 $ if that is how the
string appeared when the file was checked in.
Revision
keyword, generate the string
5.7
instead of $
Revision: 5.7 $.
This can help generate files in programming languages
where it is hard to strip keyword delimiters like
$
Revision: $ from a string. However,
further keyword substitution cannot be performed once
the keyword names are removed, so this option should be
used with care.
One often would like to use `-kv' with cvs
export
---see section export--Export sources from CVS, similar to checkout. But be aware that doesn't
handle an export containing binary files correctly.
The $
Log$ keyword is somewhat
controversial. As long as you are working on your
development system the information is easily accessible
even if you do not use the $
Log$
keyword--just do a cvs log
. Once you export
the file the history information might be useless
anyhow.
A more serious concern is that CVS is not good at
handling $
Log$ entries when a branch is
merged onto the main trunk. Conflicts often result
from the merging operation.
People also tend to "fix" the log entries in the file
(correcting spelling mistakes and maybe even factual
errors). If that is done the information from
cvs log
will not be consistent with the
information inside the file. This may or may not be a
problem in real life.
It has been suggested that the $
Log$
keyword should be inserted last in the file, and
not in the files header, if it is to be used at all.
That way the long list of change messages will not
interfere with everyday source file browsing.
Go to the first, previous, next, last section, table of contents.