1 Introduction

This document is written using the linuxdoc-sgml DTD. It contains more or less everything you need to know to write SGML docs with this DTD. See example.sgml for an example of an SGML document that you can use as a model for your own docs.

I chose SGML for this system because SGML is made specifically for translation to other formats. SGML, which stands for Standard Generalized Markup Language, allows you to specify the structure of a document---that is, what kinds of things make up the document. You specify the structure of a document with a DTD (Document Type Definition). linuxdoc-sgml is one DTD that specifies the structure for Linux HOWTOs and other docs. QWERTZ is another DTD; the SGML standard provides DTD's for books, articles, and other generic document types.

The DTD specifies the names of ``elements'' within the document. An element is just a bit of structure---like a section, a subsection, a paragraph, or even something smaller like emphasised text. Unlike LaTeX, however, these elements are not in any way intrinsic to SGML itself. The linuxdoc-sgml DTD happens to define elements that look a lot like their LaTeX counterparts---you have sections, subsections, verbatim ``environments'', and so forth. However, using SGML you can define any kind of structure for the document that you like. In a way, SGML is like low-level TeX, while the linuxdoc-sgml DTD is like LaTeX.

Don't be confused by this analogy. SGML is not a text-formatting system. There is no ``SGML formatter'' per se. SGML source is only converted to other formats for processing. Furthermore, SGML itself is used only to specify the document structure. There are no text-formatting facilities or ``macros'' intrinsic to SGML itself. All of those things are defined within the DTD. You can't use SGML without a DTD---a DTD defines what SGML does.

Here's how processing a document with SGML and the linuxdoc-sgml DTD works. First, you need a DTD. I'm using the QWERTZ DTD which was produced, originally, by a group of people who needed a LaTeX-like DTD. I've modified the QWERTZ DTD to produce the linuxdoc-sgml DTD for our purposes. The DTD simply sets up the structure of the document. A small portion of it looks like this:

<!element article - -
        (titlepag, header?, 
         toc?, lof?, lot?, p*, sect*, 
         (appendix, sect+)?, biblio?) +(footnote)>

As you can see, the DTD doesn't say anything about how the document should be formatted or what it should look like. It just defines what parts make up the document. Elsewhere in the DTD the structure of the titlepag, header, sect, and other elements are defined.

You don't need to know anything about the syntax of the DTD in order to write documents. I'm just presenting it so you know what it looks like and what it does. You do need to be familiar with the document structure that the DTD defines. If not, you might violate the structure when attempting to write a document, and be very confused about the resulting error messages. We'll describe the structure of linuxdoc-sgml documents in detail later.

The next step is to write a document using the structure defined by the DTD. Again, the linuxdoc-sgml DTD makes documents look a lot like LaTeX---it's very easy to follow. In SGML jargon a single document written using a particular DTD is known as an ``instance'' of that DTD.

In order to translate the SGML source into another format (such as LaTeX or nroff) for processing, the SGML source (the document that you wrote) is parsed along with the DTD by (you guessed it) the SGML parser. I'm using the sgmls parser by James Clark, jjc@jclark.com, who also happens to be the author of groff. We're in good hands. The parser (the executable sgmls simply picks through your document and verifies that it follows the structure set forth by the DTD. It also spits out a more explicit form of your document, with all ``macros'' and elements expanded, which is understood by sgmlsasp, the next part of the process.

sgmlsasp is responsible for converting the output of sgmls to another format (such as LaTeX). It does this using replacement files, which describe how to convert elements in the original SGML document into corresponding source in the ``target'' format (such as LaTeX or nroff).

For example, part of the replacement file for LaTeX looks like:

<itemize>       +       "\\begin{itemize}"      +
</itemize>      +       "\\end{itemize}"        +

\begin{itemize}

So, to convert the SGML to another format, all you have to do is write a new replacement file for that format that gives the appropriate analogues to the SGML elements in that new format. In practice, it's not that simple---for example, if you're trying to convert to a format that isn't structured at all like your DTD, you're going to have trouble. In any case, it's much easier to do than writing individual parsers and translators for many kinds of output formats; SGML provides a generalized system for converting one source to many formats.

Once sgmlsasp has completed its work, you have LaTeX source which corresponds to your original SGML document, which you can format using LaTeX as you normally would. Later in this document I'll give examples and show the commands used to do the translation and formatting. You can do this all on one command line.

But first, I should describe how to install and configure the software.