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authorEelco Dolstra <edolstra@gmail.com>2021-09-15 20:33:44 +0200
committerGitHub <noreply@github.com>2021-09-15 20:33:44 +0200
commit79152e307e7eef667c3de9c21571d017654a7c32 (patch)
tree67fd413bcf0b42c5ada7eddc41a04f7bd99df3a8 /doc/manual/expressions/builder-syntax.xml
parent7349f257da8278af9aae35544b15c9a204e2a57b (diff)
parent3b82c1a5fef521ebadea5df12384390c8c24100c (diff)
Merge pull request #5212 from mkenigs/auto-uid-allocation
Merge master into #3600
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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-builder-syntax'>
-
-<title>Builder Syntax</title>
-
-<example xml:id='ex-hello-builder'><title>Build script for GNU Hello
-(<filename>builder.sh</filename>)</title>
-<programlisting>
-source $stdenv/setup <co xml:id='ex-hello-builder-co-1' />
-
-PATH=$perl/bin:$PATH <co xml:id='ex-hello-builder-co-2' />
-
-tar xvfz $src <co xml:id='ex-hello-builder-co-3' />
-cd hello-*
-./configure --prefix=$out <co xml:id='ex-hello-builder-co-4' />
-make <co xml:id='ex-hello-builder-co-5' />
-make install</programlisting>
-</example>
-
-<para><xref linkend='ex-hello-builder' /> shows the builder referenced
-from Hello's Nix expression (stored in
-<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>).
-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>
-
-<calloutlist>
-
- <callout arearefs='ex-hello-builder-co-1'>
-
- <para>When Nix runs a builder, it initially completely clears the
- environment (except for the attributes declared in the
- derivation). For instance, the <envar>PATH</envar> variable is
- empty<footnote><para>Actually, it's initialised to
- <filename>/path-not-set</filename> to prevent Bash from setting it
- to a default value.</para></footnote>. This is done to prevent
- undeclared inputs from being used in the build process. If for
- example the <envar>PATH</envar> 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
- <envar>stdenv</envar> 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>
-
- </callout>
-
- <callout arearefs='ex-hello-builder-co-2'>
-
- <para>Since Hello needs Perl, we have to make sure that Perl is in
- the <envar>PATH</envar>. The <envar>perl</envar> environment
- variable points to the location of the Perl package (since it
- was passed in as an attribute to the derivation), so
- <filename><replaceable>$perl</replaceable>/bin</filename> is the
- directory containing the Perl interpreter.</para>
-
- </callout>
-
- <callout arearefs='ex-hello-builder-co-3'>
-
- <para>Now we have to unpack the sources. The
- <varname>src</varname> attribute was bound to the result of
- fetching the Hello source tarball from the network, so the
- <envar>src</envar> environment variable points to the location in
- the Nix store to which the tarball was downloaded. After
- unpacking, we <command>cd</command> to the resulting source
- directory.</para>
-
- <para>The whole build is performed in a temporary directory
- created in <varname>/tmp</varname>, by the way. This directory is
- removed after the builder finishes, so there is no need to clean
- up the sources afterwards. Also, the temporary directory is
- always newly created, so you don't have to worry about files from
- previous builds interfering with the current build.</para>
-
- </callout>
-
- <callout arearefs='ex-hello-builder-co-4'>
-
- <para>GNU Hello is a typical Autoconf-based package, so we first
- have to run its <filename>configure</filename> script. In Nix
- every package is stored in a separate location in the Nix store,
- for instance
- <filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
- Nix computes this path by cryptographically hashing all attributes
- of the derivation. The path is passed to the builder through the
- <envar>out</envar> environment variable. So here we give
- <filename>configure</filename> the parameter
- <literal>--prefix=$out</literal> to cause Hello to be installed in
- the expected location.</para>
-
- </callout>
-
- <callout arearefs='ex-hello-builder-co-5'>
-
- <para>Finally we build Hello (<literal>make</literal>) and install
- it into the location specified by <envar>out</envar>
- (<literal>make install</literal>).</para>
-
- </callout>
-
-</calloutlist>
-
-<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> \ No newline at end of file