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
Adding keyword anchor text alone 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 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
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
Here is the latest report from the front lines. $LastChangedDate$ $Rev$ Cumulus clouds are appearing more frequently as summer approaches.
svn:keywords property set on
that file, Subversion will do nothing special. Now, let's
enable substitution of the
$ 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
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
Date Author, yet the keyword
anchor used the alias
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
file will be resubstituted with information that
reflects the most recent known commit to that file.
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—for the sake of backward
compatibility, 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 keywords, some of which have aliases that you can also use:
This 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
Idkeyword, which uses UTC, the
Datekeyword displays dates using the local time zone.
This 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
This 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
This keyword describes the full URL to the latest version of the file in the repository, and looks something like
$HeadURL: http://svn.example.com/repos/trunk/calc.c $. It may be abbreviated as
This 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.cwas 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
Datekeyword (which uses the local time zone).
This keyword is similar to the
Idkeyword but contains the full URL of the latest revision of the item, identical to
HeadURL. Its substitution looks something like
$Header: http://svn.example.com/repos/trunk/calc.c 148 2006-07-28 21:30:43Z sally $.
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.
In addition to previous set of stock keyword definitions and
aliases, Subversion 1.8 allows you the freedom to define and use
custom keywords. To define a custom keyword, add a token to the
value of the
svn:keywords property which is
of the form
MyKeyword is the keyword name
(which you'll use in the keyword anchor)
FORMAT is a format string into
which information will be substituted when your keyword is
expanded inside your file.
The format string syntax used for custom keywords supports the following format codes:
The author of the revision given by
The basename of the URL of the file.
Short format of the date of the revision given by
Long format of the date of the revision given by
The file's path, relative to the repository root.
The last known revision in which this file changed in the repository. (This is the same revision which would be substituted for the
The URL to the root of the repository.
The URL of the file.
A space character. (Keyword definitions cannot contain a literal space character.)
A literal percent sign ('
As you can see, many of the individual format codes serve as placeholders for the same information available through the stock keywords. But of course, the custom keyword format allows you to more flexibly string together multiple bits of information. For example, you might wish to have a single keyword in your files which reports the repository relative path of the file and last-changed revision, formatted in a pleasant, human-readable way. To do so, you'd first define your custom keyword:
$ svn pset svn:keywords "PathRev=%P,%_r%r" calc/button.c property 'svn:keywords' set on 'button.c' $
Next, you'd edit the file's contents to add the keyword
anchor for your custom keyword, which in this case
$PathRev$. After committing these
changes, an examination of your file's contents will show that
your custom keyword was substituted as you would
expect—where previously the file
$PathRev$, it now reads
$PathRev: trunk/calc/button.c, r23 $.
Subversion will automatically truncate any keyword expansions which exceed 255 bytes in length. Also custom keywords defined with names that exceed 255 bytes will be ignored altogether.
You can also instruct Subversion to maintain a fixed length
(in terms of the number of bytes consumed) for the substituted
keyword. By using a double colon (
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
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
padded with spaces, and the long
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. Of course, where binary file formats are concerned, you must always take great care that any keyword substitution you introduce—fixed-length or otherwise—does not violate the integrity of that format. While it might sound easy enough, this can be an astonishingly difficult task for most of the popular binary file formats in use today, and not something to be undertaken by the faint of heart!
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.
 … or maybe even a section of a book …