Entities (both general and parameter) are particularly useful when used to include one file inside another.
Suppose you have some content for an SGML book organized into files, one file per chapter, called chapter1.sgml, chapter2.sgml, and so forth, with a book.sgml file that will contain these chapters.
In order to use the contents of these files as the values for your entities, you declare them with the SYSTEM keyword. This directs the SGML parser to use the contents of the named file as the value of the entity.
Example 3-12. Using General Entities to Include Files
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN" [ <!ENTITY chapter.1 SYSTEM "chapter1.sgml"> <!ENTITY chapter.2 SYSTEM "chapter2.sgml"> <!ENTITY chapter.3 SYSTEM "chapter3.sgml"> <!-- And so forth --> ]> <html> <!-- Use the entities to load in the chapters --> &chapter.1; &chapter.2; &chapter.3; </html>
Warning: When using general entities to include other files within a document, the files being included (chapter1.sgml, chapter2.sgml, and so on) must not start with a DOCTYPE declaration. This is a syntax error.
Recall that parameter entities can only be used inside an SGML context. Why then would you want to include a file within an SGML context?
You can use this to ensure that you can reuse your general entities.
Suppose that you had many chapters in your document, and you reused these chapters in two different books, each book organizing the chapters in a different fashion.
You could list the entities at the top of each book, but this quickly becomes cumbersome to manage.
Instead, place the general entity definitions inside one file, and use a parameter entity to include that file within your document.
Example 3-13. Using Parameter Entities to Include Files
First, place your entity definitions in a separate file, called chapters.ent. This file contains the following:
<!ENTITY chapter.1 SYSTEM "chapter1.sgml"> <!ENTITY chapter.2 SYSTEM "chapter2.sgml"> <!ENTITY chapter.3 SYSTEM "chapter3.sgml">
Now create a parameter entity to refer to the contents of the file. Then use the parameter entity to load the file into the document, which will then make all the general entities available for use. Then use the general entities as before:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN" [ <!-- Define a parameter entity to load in the chapter general entities --> <!ENTITY % chapters SYSTEM "chapters.ent"> <!-- Now use the parameter entity to load in this file --> %chapters; ]> <html> &chapter.1; &chapter.2; &chapter.3; </html>
Create three files, para1.sgml, para2.sgml, and para3.sgml.
Put content similar to the following in each file:
<p>This is the first paragraph.</p>
Edit example.sgml so that it looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN" [
<!ENTITY version "1.1">
<!ENTITY para1 SYSTEM "para1.sgml">
<!ENTITY para2 SYSTEM "para2.sgml">
<!ENTITY para3 SYSTEM "para3.sgml">
]>
<html>
<head>
<title>An Example HTML File</title>
</head>
<body>
<p>The current version of this document is: &version;</p>
¶1;
¶2;
¶3;
</body>
</html>
Produce example.html by normalizing example.sgml.
% osgmlnorm -d example.sgml > example.html
Load example.html into your web browser, and confirm that the paran.sgml files have been included in example.html.
Note: You must have taken the previous steps first.
Edit example.sgml so that it looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN" [
<!ENTITY % entities SYSTEM "entities.sgml"> %entities;
]>
<html>
<head>
<title>An Example HTML File</title>
</head>
<body>
<p>The current version of this document is: &version;</p>
¶1;
¶2;
¶3;
</body>
</html>
Create a new file, entities.sgml, with this content:
<!ENTITY version "1.1"> <!ENTITY para1 SYSTEM "para1.sgml"> <!ENTITY para2 SYSTEM "para2.sgml"> <!ENTITY para3 SYSTEM "para3.sgml">
Produce example.html by normalizing example.sgml.
% osgmlnorm -d example.sgml > example.html
Load example.html into your web browser, and confirm that the paran.sgml files have been included in example.html.