nixpkgs/doc/functions/trivial-builders.xml

<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>