<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         xml:id="sec-trivial-builders">
 <title>Trivial builders</title>

 <para>
   Nixpkgs provides a couple of functions that help with building
   derivations. The most important one,
   <function>stdenv.mkDerivation</function>, has already been
   documented above. The following functions wrap
   <function>stdenv.mkDerivation</function>, making it easier to use
   in certain cases.
 </para>

 <variablelist>
  <varlistentry>
   <term>
    <literal>runCommand</literal>
   </term>
   <listitem>
     <para>
       This takes three arguments, <literal>name</literal>,
       <literal>env</literal>, and <literal>buildCommand</literal>.
       <literal>name</literal> is just the name that Nix will append
       to the store path in the same way that
       <literal>stdenv.mkDerivation</literal> uses its
       <literal>name</literal> attribute. <literal>env</literal> is an
       attribute set specifying environment variables that will be set
       for this derivation. These attributes are then passed to the
       wrapped <literal>stdenv.mkDerivation</literal>.
       <literal>buildCommand</literal> specifies the commands that
       will be run to create this derivation. Note that you will need
       to create <literal>$out</literal> for Nix to register the
       command as successful.
     </para>
     <para>
       An example of using <literal>runCommand</literal> is provided
       below.
     </para>
     <programlisting>
       (import &lt;nixpkgs&gt; {}).runCommand "my-example" {} ''
         echo My example command is running

         mkdir $out

         echo I can write data to the Nix store > $out/message

         echo I can also run basic commands like:

         echo ls
         ls

         echo whoami
         whoami

         echo date
         date
       ''
     </programlisting>
   </listitem>
  </varlistentry>
  <varlistentry>
   <term>
    <literal>runCommandCC</literal>
   </term>
   <listitem>
     <para>
       This works just like <literal>runCommand</literal>. The only
       difference is that it also provides a C compiler in
       <literal>buildCommand</literal>’s environment. To minimize your
       dependencies, you should only use this if you are sure you will
       need a C compiler as part of running your command.
    </para>
   </listitem>
  </varlistentry>
  <varlistentry>
   <term>
    <literal>writeTextFile</literal>, <literal>writeText</literal>,
    <literal>writeTextDir</literal>, <literal>writeScript</literal>,
    <literal>writeScriptBin</literal>
   </term>
   <listitem>
     <para>
       These functions write <literal>text</literal> to the Nix store.
       This is useful for creating scripts from Nix expressions.
       <literal>writeTextFile</literal> takes an attribute set and
       expects two arguments, <literal>name</literal> and
       <literal>text</literal>. <literal>name</literal> corresponds to
       the name used in the Nix store path. <literal>text</literal>
       will be the contents of the file. You can also set
       <literal>executable</literal> to true to make this file have
       the executable bit set.
     </para>
     <para>
       Many more commands wrap <literal>writeTextFile</literal>
       including <literal>writeText</literal>,
       <literal>writeTextDir</literal>,
       <literal>writeScript</literal>, and
       <literal>writeScriptBin</literal>. These are convenience
       functions over <literal>writeTextFile</literal>.
     </para>
   </listitem>
  </varlistentry>
  <varlistentry>
   <term>
    <literal>symlinkJoin</literal>
   </term>
   <listitem>
    <para>
     This can be used to put many derivations into the same directory
     structure. It works by creating a new derivation and adding
     symlinks to each of the paths listed. It expects two arguments,
     <literal>name</literal>, and <literal>paths</literal>.
     <literal>name</literal> is the name used in the Nix store path
     for the created derivation. <literal>paths</literal> is a list of
     paths that will be symlinked. These paths can be to Nix store
     derivations or any other subdirectory contained within.
    </para>
   </listitem>
  </varlistentry>
 </variablelist>

</section>
