<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         xml:id="chap-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 xml:id="trivial-builder-runCommand">
   <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 xml:id="trivial-builder-runCommandCC">
   <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 xml:id="trivial-builder-runCommandLocal">
   <term>
    <literal>runCommandLocal</literal>
   </term>
   <listitem>
    <para>
     Variant of <literal>runCommand</literal> that forces the derivation to be built locally, it is not substituted. This is intended for very cheap commands (&lt;1s execution time). It saves on the network roundrip and can speed up a build.
    </para>
    <note><para>
      This sets <link xlink:href="https://nixos.org/nix/manual/#adv-attr-allowSubstitutes"><literal>allowSubstitutes</literal> to <literal>false</literal></link>, so only use <literal>runCommandLocal</literal> if you are certain the user will always have a builder for the <literal>system</literal> of the derivation. This should be true for most trivial use cases (e.g. just copying some files to a different location or adding symlinks), because there the <literal>system</literal> is usually the same as <literal>builtins.currentSystem</literal>.
    </para></note>
   </listitem>
  </varlistentry>
  <varlistentry xml:id="trivial-builder-writeText">
   <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 xml:id="trivial-builder-symlinkJoin">
   <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>
</chapter>
