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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"
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 <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 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 (<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>
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