If you've already got some other SGML DTDs installed, you probably already have a catalog file. In this case, the easiest thing to do is append the DocBook catalog entries to the end of your existing catalog and then change them to point to the files on your system.

If DocBook is the first DTD that you're installing, make a copy of docbook.cat and call it catalog. Put this file in a higher-level directory and edit the relative pathnames that it contains to point to the actual locations of the files on your system. For example, if you installed DocBook in /share/sgml/docbk30/, put the catalog in /share/sgml/.

In order for applications to find your catalog file(s), you may have to change the application preferences or set an environment variable. For SP and Jade, set the environment variable SGML_CATALOG_FILES to the delimited list of catalog filenames. On my system, this looks like:

SGML_CATALOG_FILES=./catalog;n:/share/sgml/catalog;n:/adept80/doctypes/catalog.jade;j:/jade/catalog

(On a UNIX machine, use colons instead of semicolons to delimit the filenames.)

If you don't wish to set the environment variable, you can explicitly pass the name of each catalog to the SP application with the -c option, like this:

nsgmls -c ./catalog -c n:/share/sgml/catalog -c othercatalogs
 ...

The basic format of each entry in the DocBook catalog is:

PUBLIC "some public id" "some filename"

What you have to do is change each of the “some filenames” to point to the actual name of the file on your system.

To continue with the example above, let's say that you've got:

Then you would change the catalog entry for the DTD to be:

PUBLIC "-//OASIS//DTD DocBook V3.1//EN" "docbk30/docbook.dtd"

You would change the catalog entry for the general technical character entities to:

PUBLIC "ISO 8879:1986//ENTITIES General Technical//EN" "entities/8879/iso-tech.gml"

And similarly for the other public identifiers used by DocBook. In each case, the filename specified for the public identifier should be the name of the file on your system, specified as an absolute filename, or relative to the location of the catalog in which it occurs.

Since XML documents are required to have system identifiers, but are not required to have public identifiers, it's likely that some of the documents you want to process will only have system identifiers.

It turns out that you can still take advantage of the catalog in this case. The SYSTEM directive allows you to map the system identifier used in the document to the actual location on your system.

Suppose that you work with a colleague who uses the system identifier “file:///c:/sgml/db3xml/db3xml.dtd” to identify the XML version of DocBook on her system. On your system, you want to map that to “/share/sgml/db3xml/db3xml.dtd”. The following entry in your catalog will do the trick:

SYSTEM "http://docbook.org/docbook/xml/1.4/db3xml.dtd" "/share/sgml/db3xml/db3xml.dtd"

Unfortunately, this technique only works with applications that read and understand catalog files.