115 lines
4.1 KiB
XML
115 lines
4.1 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id='sec-build-script'>
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<title>Build Script</title>
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<para>Here is the builder referenced
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from Hello's Nix expression (stored in
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<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>):</para>
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<programlisting>
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source $stdenv/setup ①
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PATH=$perl/bin:$PATH ②
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tar xvfz $src ③
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cd hello-*
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./configure --prefix=$out ④
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make ⑤
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make install</programlisting>
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<para>The builder can actually be made a lot shorter by using the
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<emphasis>generic builder</emphasis> functions provided by
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<varname>stdenv</varname>, but here we write out the build steps to
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elucidate what a builder does. It performs the following
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steps:</para>
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<orderedlist>
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<listitem>
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<para>When Nix runs a builder, it initially completely clears the
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environment (except for the attributes declared in the
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derivation). This is done to prevent undeclared inputs from being
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used in the build process. If for example the
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<literal>PATH</literal> contained <filename>/usr/bin</filename>,
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then you might accidentally use
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<filename>/usr/bin/gcc</filename>.</para>
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<para>So the first step is to set up the environment. This is
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done by calling the <filename>setup</filename> script of the
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standard environment. The environment variable
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<literal>stdenv</literal> points to the location of the standard
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environment being used. (It wasn't specified explicitly as an
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attribute in <xref linkend='ex-hello-nix' />, but
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<varname>mkDerivation</varname> adds it automatically.)</para>
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</listitem>
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<listitem>
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<para>Since Hello needs Perl, we have to make sure that Perl is in
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the <literal>PATH</literal>. The <literal>perl</literal> environment
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variable points to the location of the Perl package (since it
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was passed in as an attribute to the derivation), so
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<filename><emphasis>$perl</emphasis>/bin</filename> is the
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directory containing the Perl interpreter.</para>
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</listitem>
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<listitem>
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<para>Now we have to unpack the sources. The
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<varname>src</varname> attribute was bound to the result of
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fetching the Hello source tarball from the network, so the
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<literal>src</literal> environment variable points to the location in
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the Nix store to which the tarball was downloaded. After
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unpacking, we <command>cd</command> to the resulting source
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directory.</para>
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<para>The whole build is performed in a temporary directory
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created in <varname>/tmp</varname>, by the way. This directory is
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removed after the builder finishes, so there is no need to clean
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up the sources afterwards. Also, the temporary directory is
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always newly created, so you don't have to worry about files from
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previous builds interfering with the current build.</para>
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</listitem>
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<listitem>
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<para>GNU Hello is a typical Autoconf-based package, so we first
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have to run its <filename>configure</filename> script. In Nix
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every package is stored in a separate location in the Nix store,
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for instance
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<filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
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Nix computes this path by cryptographically hashing all attributes
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of the derivation. The path is passed to the builder through the
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<literal>out</literal> environment variable. So here we give
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<filename>configure</filename> the parameter
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<literal>--prefix=$out</literal> to cause Hello to be installed in
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the expected location.</para>
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</listitem>
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<listitem>
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<para>Finally we build Hello (<literal>make</literal>) and install
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it into the location specified by <literal>out</literal>
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(<literal>make install</literal>).</para>
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</listitem>
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</orderedlist>
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<para>If you are wondering about the absence of error checking on the
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result of various commands called in the builder: this is because the
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shell script is evaluated with Bash's <option>-e</option> option,
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which causes the script to be aborted if any command fails without an
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error check.</para>
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</section>
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