9.8. Links
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9.8. Links

Note:

Links are also in-line elements. To show a URI without creating a link, see Section 9.6.9, “Uniform Resource Identifiers (URIs)”.

9.8.1. xml:id Attributes

Most DocBook elements accept an xml:id attribute to give that part of the document a unique name. The xml:id can be used as a target for a crossreference or link.

Any portion of the document that will be a link target must have an xml:id attribute. Assigning an xml:id to all chapters and sections, even if there are no current plans to link to them, is a good idea. These xml:ids can be used as unique reference points by anyone referring to the HTML version of the document.

Example 9.35. xml:id on Chapters and Sections Example
<chapter xml:id="introduction">
  <title>Introduction</title>

  <para>This is the introduction.  It contains a subsection,
    which is identified as well.</para>

  <sect1 xml:id="introduction-moredetails">
    <title>More Details</title>

    <para>This is a subsection.</para>
  </sect1>
</chapter>

Use descriptive values for xml:id names. The values must be unique within the entire document, not just in a single file. In the example, the subsection xml:id is constructed by appending text to the chapter xml:id. This ensures that the xml:ids are unique. It also helps both reader and anyone editing the document to see where the link is located within the document, similar to a directory path to a file.

9.8.2. Crossreferences with xref

xref provides the reader with a link to jump to another section of the document. The target xml:id is specified in the linkend attribute, and xref generates the link text automatically.

Example 9.36. xref Example

Assume that this fragment appears somewhere in a document that includes the xml:id example shown above:

<para>More information can be found
  in <xref linkend="introduction"/>.</para>

<para>More specific information can be found
  in <xref linkend="introduction-moredetails"/>.</para>

The link text will be generated automatically, looking like (emphasized text indicates the link text):

More information can be found in Chapter 1, Introduction.

More specific information can be found in Section 1.1, “More Details”.


The link text is generated automatically from the chapter and section number and title elements.

9.8.3. Linking to Other Documents on the Web

The link element described here allows the writer to define the link text. When link text is used, it is very important to be descriptive to give the reader an idea of where the link goes. Remember that DocBook can be rendered to multiple types of media. The reader might be looking at a printed book or other form of media where there are no links. If the link text is not descriptive enough, the reader might not be able to locate the linked section.

The xlink:href attribute is the URL of the page, and the content of the element is the text that will be displayed for the user to activate.

In many situations, it is preferable to show the actual URL rather than text. This can be done by leaving out the element text entirely.

Example 9.37. link to a FreeBSD Documentation Web Page Example

Link to the book or article URL entity. To link to a specific chapter in a book, add a slash and the chapter file name, followed by an optional anchor within the chapter. For articles, link to the article URL entity, followed by an optional anchor within the article. URL entities can be found in doc/share/xml/urls.ent.

Usage for FreeBSD book links:

<para>Read the <link
    xlink:href="&url.books.handbook;/svn.html#svn-intro">SVN
    introduction</link>, then pick the nearest mirror from
  the list of <link
    xlink:href="&url.books.handbook;/svn.html#svn-mirrors">Subversion
    mirror sites</link>.</para>

Appearance:

Read the SVN introduction, then pick the nearest mirror from the list of Subversion mirror sites.

Usage for FreeBSD article links:

<para>Read this
  <link xlink:href="&url.articles.bsdl-gpl;">article
    about the BSD license</link>, or just the
  <link xlink:href="&url.articles.bsdl-gpl;#intro">introduction</link>.</para>

Appearance:

Read this article about the BSD license, or just the introduction.


Example 9.38. link to a FreeBSD Web Page Example

Usage:

<para>Of course, you could stop reading this document and go to the
  <link xlink:href="&url.base;/index.html">FreeBSD home page</link> instead.</para>

Appearance:

Of course, you could stop reading this document and go to the FreeBSD home page instead.


Example 9.39. link to an External Web Page Example

Usage:

<para>Wikipedia has an excellent reference on
  <link
    xlink:href="http://en.wikipedia.org/wiki/GUID_Partition_Table">GUID
    Partition Tables</link>.</para>

Appearance:

Wikipedia has an excellent reference on GUID Partition Tables.

The link text can be omitted to show the actual URL:

<para>Wikipedia has an excellent reference on
  GUID Partition Tables: <link
    xlink:href="http://en.wikipedia.org/wiki/GUID_Partition_Table"></link>.</para>

The same link can be entered using shorter notation instead of a separate ending tag:

<para>Wikipedia has an excellent reference on
  GUID Partition Tables: <link
    xlink:href="http://en.wikipedia.org/wiki/GUID_Partition_Table"/>.</para>

The two methods are equivalent. Appearance:

Wikipedia has an excellent reference on GUID Partition Tables: http://en.wikipedia.org/wiki/GUID_Partition_Table.


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