Introduction to SGML(Standard Generalized Markup Language)

Introduction to SGML

•       Standard Generalized Markup Language

•       Strictly descriptive

•       Contains no means to mark up presentational aspects of documents

•       Can be easily interfaced to external procedural markup systems and style sheets

•       Like HTML, SGML is a computer language rather than a data format

•       SGML files can be created manually, or through SGML editor software tools.

•       SGML is a meta-language that defines only the syntax of a standard generalised markup language, i.e. is prescribes how we should specify markup, but not what that markup is, nor what it means.

SGML Parser

•       Software that reads and analyzes an SGML document

•       Validation or transformation

•       Not much use by itself

•       Part of a bigger SGML application system or browser

SGML History

•       More than 10 years history of use and growth…

•       Widelypublishingusedand-aerospace,otherindustriesautomotive, defense, software, semiconductor, pharmaceutical,

•       ISO standard (ISO 8879) – adopted by several other standards bodies

SGML: Key Features

•       Descriptive markup

•       Document types

•       Data independence

Descriptive markup

•       Use of markup codes (names) to categorize parts of a document

•       Example: <para> to identify a paragraph

•       Advantage: Same document can be processed by different software for different purposes

Document Type

•       Notion of ‗document type‘ (hence DTD)

•       Type of a document is formally defined by its constituent parts and their structure – expressed in a tree structure

•       Example: Report

•                     Title, followed by author (optional), abstract, one or more paragraphs

•       If title is absent, it is not a report

•       If abstract follows paragraphs, it is not a report

Data Independence

•       Document portability across different HW and SW environments

•       How to handle character set differences

•       Descriptive mapping for non-portable characters

•       String substitution mechanism (entities): process time substitution of a particular string of characters by other string of characters

Defining an SGML Application

•       From SGML view, a document is a hierarchical structure of nested elements (chapters, sections, paragraphs, etc.)

•       SGML does not specify any presentational aspects of these elements

•       SGML also does not convey any meaning or role of these elements – meaning is implied by the application

•       SGML specifies the contexts and levels of document hierarchy in which an element can or must occur

•       All documents that can be marked up with the same hierarchy of elements are said to belong to a certain document type

•       SGML defines the structure of a particular type of documents via the DTD (Document Type Definition)

•       Some general features of an SGML application are specified in another component called SGML Declaration.

•       SGML Syntax:

SGML statements are enclosed in angle brackets (<>) and contain a keyword or name followed by one or more parameters separated by spacesCharacter ‗!‘ is inserted between

‗<‗ and the statement keyword

•       Example: <!ELEMENT IMG – O EMPTY – Embedded image -->

•       Comments within a <! Statement are enclosed in double hyphens (--)

Components of an SGML Document

•       SGML Declaration:

•       Character set, syntax (e.g. delimiters), optional features

•       Usually a single declaration is used for all documents under a particular system

•       Prolog:

•       Usually a single document type definition (DTD)

•       Contains rules to which any document of a given type must conform

•       Document instance:

•       The document itself marked up following the SGML usage conventions specified in the SGML declaration and the DTD.

Advantages of SGML:

•       Capable of dealing with any media type or linking protocol

•       Extremely flexible

•       Separates content from appearance

•       Style sheets make content repurposing easy

•       ISO standard - thus well supported with many tools available

Disadvantages of SGML:

•       Creating DTD's requires exacting software engineering

•       Linking tends to be complex

•       Writing SGML software is extremely hard

•       SGML tools tend to be expansive