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<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">
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<title>A Simple Nix Expression</title>
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<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" />
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</chapter>