<chapter xmlns="http://docbook.org/ns/docbook"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xi="http://www.w3.org/2001/XInclude"
      version="5.0"
      xml:id="ch-simple-expression">

<title>A Simple Nix Expression</title>

<para>This section shows how to add and test the <link
xlink:href='http://www.gnu.org/software/hello/hello.html'>GNU Hello
package</link> to the Nix Packages collection.  Hello is a program
that prints out the text <quote>Hello, world!</quote>.</para>

<para>To add a package to the Nix Packages collection, you generally
need to do three things:

<orderedlist>

  <listitem><para>Write a Nix expression for the package.  This is a
  file that describes all the inputs involved in building the package,
  such as dependencies, sources, and so on.</para></listitem>

  <listitem><para>Write a <emphasis>builder</emphasis>.  This is a
  shell script<footnote><para>In fact, it can be written in any
  language, but typically it's a <command>bash</command> shell
  script.</para></footnote> that actually builds the package from
  the inputs.</para></listitem>

  <listitem><para>Add the package to the file
  <filename>pkgs/top-level/all-packages.nix</filename>.  The Nix
  expression written in the first step is a
  <emphasis>function</emphasis>; it requires other packages in order
  to build it.  In this step you put it all together, i.e., you call
  the function with the right arguments to build the actual
  package.</para></listitem>

</orderedlist>

</para>

<xi:include href="expression-syntax.xml" />
<xi:include href="build-script.xml" />
<xi:include href="arguments-variables.xml" />
<xi:include href="simple-building-testing.xml" />
<xi:include href="generic-builder.xml" />

</chapter>
