Subversion has the ability to substitute keywords—pieces of useful, dynamic information about a versioned file—into the contents of the file itself. Keywords generally provide information about the last modification made to the file. Because this information changes each time the file changes, and more importantly, just after the file changes, it is a hassle for any process except the version control system to keep the data completely up to date. Left to human authors, the information would inevitably grow stale.
For example, say you have a document in which you would
      like to display the last date on which it was modified.  You
      could burden every author of that document to, just before
      committing their changes, also tweak the part of the
      document that describes when it was last changed.  But
      sooner or later, someone would forget to do that.  Instead,
      simply ask Subversion to perform keyword substitution on the
      LastChangedDate keyword.  You control
      where the keyword is inserted into your document by placing
      a keyword anchor at the desired
      location in the file.  This anchor is just a string of text
      formatted as
      $KeywordName$.
All keywords are case-sensitive where they appear as
      anchors in files: you must use the correct capitalization
      for the keyword to be expanded.  You should consider the
      value of the svn:keywords property to be
      case-sensitive, too—certain keyword names will be recognized
      regardless of case, but this behavior is deprecated.
Subversion defines the list of keywords available for substitution. That list contains the following five keywords, some of which have aliases that you can also use:
DateThis keyword describes the last time the file was
            known to have been changed in the repository, and is of
            the form $Date: 2006-07-22 21:42:37 -0700 (Sat,
            22 Jul 2006) $.  It may also be specified as
            LastChangedDate.  Unlike the
            Id keyword, which uses UTC, the
            Date keyword displays dates using the
            local time zone.
RevisionThis keyword describes the last known revision in
            which this file changed in the repository, and looks
            something like $Revision: 144 $.  
            It may also be specified as
            LastChangedRevision or
            Rev.
AuthorThis keyword describes the last known user to
            change this file in the repository, and looks
            something like $Author: harry $.  
            It may also be specified as 
            LastChangedBy.
HeadURLThis keyword describes the full URL to the latest
            version of the file in the repository, and looks
            something like $HeadURL:
            http://svn.collab.net/repos/trunk/README $.
            It may be abbreviated as
            URL.
IdThis keyword is a compressed combination of the other
            keywords.  Its substitution looks something like
            $Id: calc.c 148 2006-07-28 21:30:43Z sally
            $, and is interpreted to mean that the file
            calc.c was last changed in revision
            148 on the evening of July 28, 2006 by the user
            sally.  The date displayed by this
            keyword is in UTC, unlike that of the
            Date keyword (which uses the local time
            zone).
Several of the preceding descriptions use the phrase “last known” or similar wording. Keep in mind that keyword expansion is a client-side operation, and your client “knows” only about changes that have occurred in the repository when you update your working copy to include those changes. If you never update your working copy, your keywords will never expand to different values even if those versioned files are being changed regularly in the repository.
Simply adding keyword anchor text to your file does nothing special. Subversion will never attempt to perform textual substitutions on your file contents unless explicitly asked to do so. After all, you might be writing a document [13] about how to use keywords, and you don't want Subversion to substitute your beautiful examples of unsubstituted keyword anchors!
To tell Subversion whether to substitute keywords
      on a particular file, we again turn to the property-related
      subcommands.  The svn:keywords property,
      when set on a versioned file, controls which keywords will
      be substituted on that file.  The value is a space-delimited
      list of keyword names or aliases.
For example, say you have a versioned file named
      weather.txt that looks like
      this:
Here is the latest report from the front lines. $LastChangedDate$ $Rev$ Cumulus clouds are appearing more frequently as summer approaches.
With no svn:keywords property set on
      that file, Subversion will do nothing special.  Now, let's
      enable substitution of the
      LastChangedDate keyword.
$ svn propset svn:keywords "Date Author" weather.txt property 'svn:keywords' set on 'weather.txt' $
Now you have made a local property modification on the
      weather.txt file.  You will see no
      changes to the file's contents (unless you made some of your
      own prior to setting the property).  Notice that the file
      contained a keyword anchor for the Rev
      keyword, yet we did not include that keyword in the property
      value we set.  Subversion will happily ignore requests to
      substitute keywords that are not present in the file and
      will not substitute keywords that are not present in the
      svn:keywords property value.
Immediately after you commit this property change,
      Subversion will update your working file with the new
      substitute text.  Instead of seeing your keyword anchor
      $LastChangedDate$, you'll see its
      substituted result.  That result also contains the name of
      the keyword and continues to be delimited by the dollar sign
      ($) characters.  And as we predicted, the
      Rev keyword was not substituted because
      we didn't ask for it to be.
Note also that we set the svn:keywords
      property to Date Author, yet the keyword
      anchor used the alias $LastChangedDate$
      and still expanded correctly:
Here is the latest report from the front lines. $LastChangedDate: 2006-07-22 21:42:37 -0700 (Sat, 22 Jul 2006) $ $Rev$ Cumulus clouds are appearing more frequently as summer approaches.
If someone else now commits a change to
      weather.txt, your copy of that file
      will continue to display the same substituted keyword value
      as before—until you update your working copy.  At that
      time, the keywords in your weather.txt
      file will be resubstituted with information that
      reflects the most recent known commit to that file.
Subversion 1.2 introduced a new variant of the keyword
      syntax, which brought additional, useful—though perhaps
      atypical—functionality.  You can now tell Subversion
      to maintain a fixed length (in terms of the number of bytes
      consumed) for the substituted keyword.  By using a
      double colon (::) after the keyword name,
      followed by a number of space characters, you define that
      fixed width.  When Subversion goes to substitute your
      keyword for the keyword and its value, it will essentially
      replace only those space characters, leaving the overall
      width of the keyword field unchanged.  If the substituted
      value is shorter than the defined field width, there will be
      extra padding characters (spaces) at the end of the
      substituted field; if it is too long, it is truncated with a
      special hash (#) character just before
      the final dollar sign terminator.
For example, say you have a document in which you have some section of tabular data reflecting the document's Subversion keywords. Using the original Subversion keyword substitution syntax, your file might look something like:
$Rev$: Revision of last commit $Author$: Author of last commit $Date$: Date of last commit
Now, that looks nice and tabular at the start of things. But when you then commit that file (with keyword substitution enabled, of course), you see:
$Rev: 12 $: Revision of last commit $Author: harry $: Author of last commit $Date: 2006-03-15 02:33:03 -0500 (Wed, 15 Mar 2006) $: Date of last commit
The result is not so beautiful. And you might be tempted to then adjust the file after the substitution so that it again looks tabular. But that holds only as long as the keyword values are the same width. If the last committed revision rolls into a new place value (say, from 99 to 100), or if another person with a longer username commits the file, stuff gets all crooked again. However, if you are using Subversion 1.2 or later, you can use the new fixed-length keyword syntax and define some field widths that seem sane, so your file might look like this:
$Rev:: $: Revision of last commit $Author:: $: Author of last commit $Date:: $: Date of last commit
You commit this change to your file.  This time,
      Subversion notices the new fixed-length keyword syntax and
      maintains the width of the fields as defined by the padding
      you placed between the double colon and the trailing dollar
      sign.  After substitution, the width of the fields is
      completely unchanged—the short values for
      Rev and Author are
      padded with spaces, and the long Date
      field is truncated by a hash character:
$Rev:: 13 $: Revision of last commit $Author:: harry $: Author of last commit $Date:: 2006-03-15 0#$: Date of last commit
The use of fixed-length keywords is especially handy when performing substitutions into complex file formats that themselves use fixed-length fields for data, or for which the stored size of a given data field is overbearingly difficult to modify from outside the format's native application (such as for Microsoft Office documents).
Be aware that because the width of a keyword field is measured in bytes, the potential for corruption of multibyte values exists. For example, a username that contains some multibyte UTF-8 characters might suffer truncation in the middle of the string of bytes that make up one of those characters. The result will be a mere truncation when viewed at the byte level, but will likely appear as a string with an incorrect or garbled final character when viewed as UTF-8 text. It is conceivable that certain applications, when asked to load the file, would notice the broken UTF-8 text and deem the entire file corrupt, refusing to operate on the file altogether. So, when limiting keywords to a fixed size, choose a size that allows for this type of byte-wise expansion.