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Hydrogen Documentation

!!! Deprecation warning !!!

The Docbook-based toolchain will be used for the 1.2.X releases of Hydrogen and will be ported to a Markdown-based one starting with 1.3.0. In addition, the manual won't be bundled and shipped with Hydrogen anymore starting from version 1.3.0.


  1. Overview
  2. Installation and Usage
  3. Translators
  4. Documentors
  5. Developers
  6. XML and Validation
  7. Additional DocBook Authoring Restrictions

1. Overview

Hydrogen documentation is maintained in DocBook 4.5 (XML) and translated to HTML before release. Different translations are managed through GNU gettext PO files. When the documentation is generated, the PO files and the master documentation are merged to create the doc for that specific language.

This document is split up into the three types of people who may want to contribute to Hydrogen documentation: Translators, Documentors, and Developers. A section is devoted to each one, individually.

This document assumes that you don't know much, and tries to give you pointers to the stuff you need to know.

2. Installation and Usage

The following tools you must install in order to maintain and process the documentation (on Debian-based systems).

sudo apt install itstool gettext xmlto libxml2-utils make

To update the resulting HTML files after changing a master document (manual.docbook and tutorial.docbook) or a translation (*.po), you just have to enter


2.1. Detailed description

The toolchain consists of the following programs:

  • itstool - Uses the rules of the W3C Internationalization Tag Set (ITS) to create the translation templates (*.pot) based on the original DocBook source and merges the compiled *.mo files with the original to produce the translated DocBook files.

  • msgmerge, msginit, msgfmt - Are part of the gettext internationalization framework and used to update, initialize, and compile the *.po translation files.

  • xmlto - Converts DocBook files to HTML. (Note that xmlto is part of libxml2 and can convert DocBook to much more than just HTML.)

  • xmllint - Used to validate the DocBook files against the DocBook DTD.

  • make - GNU's make utility.

3. Translators

To translate documentation for Hydrogen you will need:

  • A PO-file editor. (Note that a text editor works fine, but a translation assistant like poedit is better.)
  • To be able to read and understand English.

If you don't have all the tools listed in Section 2 or do not know how to install them on your particular system, that's OK. Ask the Hydrogen Developer list and someone there can process files for you.

To make a new translation of the Hydrogen manual or tutorial, you should use the following command

msginit -l LL -i manual.pot -o manual_LL.po

Note that the _LL is added to the translation template, and LL is the IANA abbreviation code for that language. (E.g. 'ja' is for 'Japanese.') The registry for the codes is located here:

All translation files (.po) will be updated and converted to .html files as soon as one of the master documents - manual.docbook or tutorial.docbook - is touched and the make command is executed.

You can either see the changes using the commandline tool diff, the graphical program meld, or your favorite translation editor.

NOTICE: Before creating the HTML file, the document will be validated. If the document does not validate, you will have to alter your translation file so that the output is a valid DocBook document. For more information on Validation, see Section 6.


  • No new content. New content must be first added to the master (English) manual, and then translated to all the other manuals.

  • All content enclosed by angle brackets must not be translated. E.g.

    <title>Hydrogen Tutorial</title>

    will be translated to

    <title>Tutoriel de Hydrogen</title>
  • Maintain the DocBook XML structure as closely as possible. Do not add sections, divide paragraphs, or alter the markup significantly.

  • The English translation uses a little humor to try to keep the reading interesting. When this happens, the language and idioms being used are very cultural. Please do NOT translate the words literally. Instead, please translate the ideas to your culture as you see fit. We've asked the Documentors to mark when this is happening so that you don't miss the joke. This should show up in the PO file.

  • If your culture doesn't like American-style informal writing, please feel free to make a humor-less translation.

  • Submit translations to the Hydrogen Developers list (

4. Documentors

The master documents - manual.docbook and tutorial.docbook - are in English. All new content and major revisions shall be done there first. In order to work on the documentation, you will need:

  • To know and understand DocBook well.

  • To know and understand Hydrogen well enough to ask questions.

  • An XML editor (note: any text editor will do).

  • A way to create images. (e.g. Inkscape, GIMP, PhotoShop)

What you write is pretty much your own style. Please do keep the text interesting to read by using wit and a more familiar conversational style. If you are using an English pun or expression that is intended to convey humor, please mark it so that the translator gets the joke:

<!-- TRANSLATORS: "Have your squash and eat it, too." This
combines a well-known cliche ("Have your cake and eat it, too.")
and a pun on the word "squash."  In English, "squash" is a
vegetable (and not a very popular one) and "squash" is also a verb
meaning "to flatten in a destructive way."  For example: "I
squashed the bug to kill it." -->

(Ahem, don't ask me how I came up with that one.....)

Before submitting (or committing) your changes, please make sure that your documents validate (see Section 6 below). Some guidelines:

  • Use double-quotes for all attributes.

    • Good:
      <foo id="bar"/>
    • Bad:
      <foo id='bar'/>
  • Please do not indent any elements that go into the <screen> tag or at least do so with care. All white spaces will show up in the resulting HTML file as well.

  • Please make good use of <link> and <xref> tags for internal links within the document.

  • Since we're using <link> and <xref>'s -- if you change an id=".." attribute, make sure that you change all its references, too. If you create a broken link, the document won't validate.

  • Do not make reference to specific section numbers, figure numbers, or titles (e.g. "See section 2.1.3 The Menu Bar"). Instead, use <xref> tags so that this text will be generated for you.

  • For italics, you do not need to set the role="italic" attribute, since that is the default. To get boldface, you must use role="bold".

    • Example:
      <emphasis role="bold">really</emphasis>
    • Bad:
      <emphasis rold="italic">might</emphasis>
  • For web links, don't write the URL twice. The processor will do that for you, and it makes it more readable.

    • Good:
      <ulink url=""/>
    • Bad:
      <ulink url=""></ulink>
  • Don't worry about typesetting in the DocBook documents. That's what XSL and CSS stylesheets are for. Get the content done, and then worry about formatting. Don't get distracted by stuff like section indents or relative font sizes.

  • However, do worry about typesetting on pre-formatted tags like <screen> and <code> and <literallayout>. Extra spaces and indents in the source document will carry through all the way to the final document.

5. Developers

Since we don't want to add xmlto, itstool, xmllint, msgmerge, msgfmt, and the DocBook DTD's to our normal build requirements: All HTML files need to be generated and committed before releasing. After the release, the generated HTML files can (and should) be deleted. Do not commit the generated DocBook documents.

If you are preparing a Hydrogen release, you must have all the tools listed above so that you can process the documents. You may also need to understand DocBook enough to help a translator with validation issues. (See Section 6 below.)

The reason for doing it this way is that the tools are not very portable across Linux, Mac, and Windows. Nor or they even very portable across different Linux distributions. However, the tools are fairly stable on Debian/Ubuntu -- which most of the current developers are using.

6. XML and Validation

DocBook is a specific flavor of XML and in case you already have read or written HTML code in recent years - since its port to XHTML - you can get started with DocBook in no time. To read more about XML, check out the Wikipedia article:

Some basics:

  • The tags are case sensitive. <IMG> and <img> are not the same.

  • Closing tags are not optional.

  • Empty tags are like this: <br/>

If a document follows all the rules of XML, it is called "Well-Formed." For example, the following is a well-formed XML document:

<?xml version='1.0' encoding='UTF-8'?>
   <because>It's</because><ok/> to <make it="up">as</make>
   you go</ijustmadethistagup>

But the following is NOT a well-formed XML document:

<?xml version='1.0' encoding='UTF-8'?>
   <because>It's <ok/> to <make it=up>as</make>
   you go</ijustmadethistagup>

(Can you find the errors? If you get stumped, feed it to a validator.)

While it's imperative that documents be well-formed, many documents (such as DocBook) have a specific structure that must be maintained. For example, in HTML you should only have paragraphs inside of the body:

   <p>I am the very model of a modern
    major general.</p>

But, if I do this, it will still be well-formed XML:

   <body>I am the <happy>very</happy> model of a modern
    major general.</body>

It is not, however, a Valid HTML document. The W3C published a DTD (Document Type Definition) for HTML that clearly specifies that the former is OK and the latter is degenerate.

When a document is checked against the DTD, it is said to be "Valid" or "Validated" if it passes all the requirements of the DTD.

Validation is important, because our DocBook source files are going to be processed by several automatic tools. These tools know the structure of DocBook, and are able to generate output based on that. However, if we feed them an invalid document, the tools may process the data -- but they probably won't process it right.

7. Additional DocBook Authoring Restrictions

It has been found that the toolchain is assuming some things about the document structure that are not specified in any DTD. Thus, in order to get our files to work with poxml, we have to add the following restrictions:

  • <para> may not contain an <itemizedlist>

  • The 'msgid' string inside the .po file may not contain an empty element. So, if you have some text that includes

    <ulink url=""/>

    you will need to convert it to

     <ulink url=""></ulink>
  • You must use double quotes " for attributes, not single ', even though BOTH are OK in XML.

  • The manual may not contain >, it must always be &gt;.

  • Avoid embedding a lot of structured markup inside a paragraph. For example:

    <para>Install hydrogen like this:
        <prompt>$</prompt> <command>apt-get hydrogen</command>

    This makes things a little funky in the PO files. Also, it doesn't really make sense to embed a <screen> (like HTML <pre>) inside of a <para>. Instead, do it like this:

     <para>Install hydrogen like this:</para>
       <prompt>$</prompt> <command>apt-get hydrogen</command>

    However, if you really need to, go ahead and try it. Just be sure to test that it will match the strings for translation.

  • If you change indentation... in tags... be careful how that affects the .po files. For example, if you had:


    But change it to:


    You will need to add spaces in the .po files like this:

    • Before:
    • After:
      <foo> <bar>bat</bar> <baz>zap</baz> </foo>