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<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 <nixpkgs> {}).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>
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