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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"
version="5.0"
xml:id='sec-build-script'>
<title>Build Script</title>
<para>Here is the builder referenced
from Hello's Nix expression (stored in
<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>):</para>
<programlisting>
source $stdenv/setup ①
PATH=$perl/bin:$PATH ②
tar xvfz $src ③
cd hello-*
./configure --prefix=$out ④
make ⑤
make install</programlisting>
<para>The builder can actually be made a lot shorter by using the
<emphasis>generic builder</emphasis> functions provided by
<varname>stdenv</varname>, but here we write out the build steps to
elucidate what a builder does. It performs the following
steps:</para>
<orderedlist>
<listitem>
<para>When Nix runs a builder, it initially completely clears the
environment (except for the attributes declared in the
derivation). This is done to prevent undeclared inputs from being
used in the build process. If for example the
<literal>PATH</literal> contained <filename>/usr/bin</filename>,
then you might accidentally use
<filename>/usr/bin/gcc</filename>.</para>
<para>So the first step is to set up the environment. This is
done by calling the <filename>setup</filename> script of the
standard environment. The environment variable
<literal>stdenv</literal> points to the location of the standard
environment being used. (It wasn't specified explicitly as an
attribute in <xref linkend='ex-hello-nix' />, but
<varname>mkDerivation</varname> adds it automatically.)</para>
</listitem>
<listitem>
<para>Since Hello needs Perl, we have to make sure that Perl is in
the <literal>PATH</literal>. The <literal>perl</literal> environment
variable points to the location of the Perl package (since it
was passed in as an attribute to the derivation), so
<filename><emphasis>$perl</emphasis>/bin</filename> is the
directory containing the Perl interpreter.</para>
</listitem>
<listitem>
<para>Now we have to unpack the sources. The
<varname>src</varname> attribute was bound to the result of
fetching the Hello source tarball from the network, so the
<literal>src</literal> environment variable points to the location in
the Nix store to which the tarball was downloaded. After
unpacking, we <command>cd</command> to the resulting source
directory.</para>
<para>The whole build is performed in a temporary directory
created in <varname>/tmp</varname>, by the way. This directory is
removed after the builder finishes, so there is no need to clean
up the sources afterwards. Also, the temporary directory is
always newly created, so you don't have to worry about files from
previous builds interfering with the current build.</para>
</listitem>
<listitem>
<para>GNU Hello is a typical Autoconf-based package, so we first
have to run its <filename>configure</filename> script. In Nix
every package is stored in a separate location in the Nix store,
for instance
<filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
Nix computes this path by cryptographically hashing all attributes
of the derivation. The path is passed to the builder through the
<literal>out</literal> environment variable. So here we give
<filename>configure</filename> the parameter
<literal>--prefix=$out</literal> to cause Hello to be installed in
the expected location.</para>
</listitem>
<listitem>
<para>Finally we build Hello (<literal>make</literal>) and install
it into the location specified by <literal>out</literal>
(<literal>make install</literal>).</para>
</listitem>
</orderedlist>
<para>If you are wondering about the absence of error checking on the
result of various commands called in the builder: this is because the
shell script is evaluated with Bash's <option>-e</option> option,
which causes the script to be aborted if any command fails without an
error check.</para>
</section>
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