290 lines
10 KiB
XML
290 lines
10 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-advanced-attributes">
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<title>Advanced Attributes</title>
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<para>Derivations can declare some infrequently used optional
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attributes.</para>
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<variablelist>
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<varlistentry><term><varname>allowedReferences</varname></term>
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<listitem><para>The optional attribute
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<varname>allowedReferences</varname> specifies a list of legal
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references (dependencies) of the output of the builder. For
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example,
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<programlisting>
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allowedReferences = [];
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</programlisting>
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enforces that the output of a derivation cannot have any runtime
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dependencies on its inputs. To allow an output to have a runtime
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dependency on itself, use <literal>"out"</literal> as a list item.
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This is used in NixOS to check that generated files such as
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initial ramdisks for booting Linux don’t have accidental
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dependencies on other paths in the Nix store.</para></listitem>
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</varlistentry>
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<varlistentry><term><varname>allowedRequisites</varname></term>
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<listitem><para>This attribute is similar to
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<varname>allowedReferences</varname>, but it specifies the legal
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requisites of the whole closure, so all the dependencies
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recursively. For example,
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<programlisting>
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allowedRequisites = [ foobar ];
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</programlisting>
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enforces that the output of a derivation cannot have any other
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runtime dependency than <varname>foobar</varname>, and in addition
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it enforces that <varname>foobar</varname> itself doesn't
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introduce any other dependency itself.</para></listitem>
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</varlistentry>
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<varlistentry><term><varname>exportReferencesGraph</varname></term>
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<listitem><para>This attribute allows builders access to the
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references graph of their inputs. The attribute is a list of
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inputs in the Nix store whose references graph the builder needs
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to know. The value of this attribute should be a list of pairs
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<literal>[ <replaceable>name1</replaceable>
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<replaceable>path1</replaceable> <replaceable>name2</replaceable>
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<replaceable>path2</replaceable> <replaceable>...</replaceable>
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]</literal>. The references graph of each
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<replaceable>pathN</replaceable> will be stored in a text file
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<replaceable>nameN</replaceable> in the temporary build directory.
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The text files have the format used by <command>nix-store
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--register-validity</command> (with the deriver fields left
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empty). For example, when the following derivation is built:
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<programlisting>
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derivation {
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...
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exportReferencesGraph = [ "libfoo-graph" libfoo ];
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};
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</programlisting>
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the references graph of <literal>libfoo</literal> is placed in the
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file <filename>libfoo-graph</filename> in the temporary build
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directory.</para>
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<para><varname>exportReferencesGraph</varname> is useful for
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builders that want to do something with the closure of a store
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path. Examples include the builders in NixOS that generate the
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initial ramdisk for booting Linux (a <command>cpio</command>
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archive containing the closure of the boot script) and the
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ISO-9660 image for the installation CD (which is populated with a
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Nix store containing the closure of a bootable NixOS
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configuration).</para></listitem>
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</varlistentry>
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<varlistentry><term><varname>impureEnvVars</varname></term>
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<listitem><para>This attribute allows you to specify a list of
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environment variables that should be passed from the environment
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of the calling user to the builder. Usually, the environment is
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cleared completely when the builder is executed, but with this
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attribute you can allow specific environment variables to be
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passed unmodified. For example, <function>fetchurl</function> in
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Nixpkgs has the line
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<programlisting>
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impureEnvVars = [ "http_proxy" "https_proxy" <replaceable>...</replaceable> ];
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</programlisting>
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to make it use the proxy server configuration specified by the
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user in the environment variables <envar>http_proxy</envar> and
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friends.</para>
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<para>This attribute is only allowed in <link
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linkend="fixed-output-drvs">fixed-output derivations</link>, where
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impurities such as these are okay since (the hash of) the output
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is known in advance. It is ignored for all other
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derivations.</para></listitem>
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</varlistentry>
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<varlistentry xml:id="fixed-output-drvs">
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<term><varname>outputHash</varname></term>
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<term><varname>outputHashAlgo</varname></term>
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<term><varname>outputHashMode</varname></term>
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<listitem><para>These attributes declare that the derivation is a
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so-called <emphasis>fixed-output derivation</emphasis>, which
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means that a cryptographic hash of the output is already known in
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advance. When the build of a fixed-output derivation finishes,
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Nix computes the cryptographic hash of the output and compares it
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to the hash declared with these attributes. If there is a
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mismatch, the build fails.</para>
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<para>The rationale for fixed-output derivations is derivations
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such as those produced by the <function>fetchurl</function>
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function. This function downloads a file from a given URL. To
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ensure that the downloaded file has not been modified, the caller
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must also specify a cryptographic hash of the file. For example,
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<programlisting>
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fetchurl {
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url = http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz;
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md5 = "70c9ccf9fac07f762c24f2df2290784d";
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}
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</programlisting>
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It sometimes happens that the URL of the file changes, e.g.,
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because servers are reorganised or no longer available. We then
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must update the call to <function>fetchurl</function>, e.g.,
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<programlisting>
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fetchurl {
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url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
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md5 = "70c9ccf9fac07f762c24f2df2290784d";
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}
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</programlisting>
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If a <function>fetchurl</function> derivation was treated like a
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normal derivation, the output paths of the derivation and
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<emphasis>all derivations depending on it</emphasis> would change.
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For instance, if we were to change the URL of the Glibc source
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distribution in Nixpkgs (a package on which almost all other
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packages depend) massive rebuilds would be needed. This is
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unfortunate for a change which we know cannot have a real effect
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as it propagates upwards through the dependency graph.</para>
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<para>For fixed-output derivations, on the other hand, the name of
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the output path only depends on the <varname>outputHash*</varname>
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and <varname>name</varname> attributes, while all other attributes
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are ignored for the purpose of computing the output path. (The
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<varname>name</varname> attribute is included because it is part
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of the path.)</para>
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<para>As an example, here is the (simplified) Nix expression for
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<varname>fetchurl</varname>:
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<programlisting>
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{ stdenv, curl }: # The <command>curl</command> program is used for downloading.
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{ url, md5 }:
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stdenv.mkDerivation {
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name = baseNameOf (toString url);
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builder = ./builder.sh;
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buildInputs = [ curl ];
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# This is a fixed-output derivation; the output must be a regular
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# file with MD5 hash <varname>md5</varname>.
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outputHashMode = "flat";
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outputHashAlgo = "md5";
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outputHash = md5;
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inherit url;
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}
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</programlisting>
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</para>
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<para>The <varname>outputHashAlgo</varname> attribute specifies
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the hash algorithm used to compute the hash. It can currently be
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<literal>"md5"</literal>, <literal>"sha1"</literal> or
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<literal>"sha256"</literal>.</para>
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<para>The <varname>outputHashMode</varname> attribute determines
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how the hash is computed. It must be one of the following two
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values:
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<variablelist>
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<varlistentry><term><literal>"flat"</literal></term>
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<listitem><para>The output must be a non-executable regular
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file. If it isn’t, the build fails. The hash is simply
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computed over the contents of that file (so it’s equal to what
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Unix commands like <command>md5sum</command> or
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<command>sha1sum</command> produce).</para>
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<para>This is the default.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>"recursive"</literal></term>
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<listitem><para>The hash is computed over the NAR archive dump
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of the output (i.e., the result of <link
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linkend="refsec-nix-store-dump"><command>nix-store
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--dump</command></link>). In this case, the output can be
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anything, including a directory tree.</para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>The <varname>outputHash</varname> attribute, finally, must
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be a string containing the hash in either hexadecimal or base-32
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notation. (See the <link
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linkend="sec-nix-hash"><command>nix-hash</command> command</link>
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for information about converting to and from base-32
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notation.)</para></listitem>
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</varlistentry>
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<varlistentry><term><varname>passAsFile</varname></term>
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<listitem><para>A list of names of attributes that should be
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passed via files rather than environment variables. For example,
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if you have
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<programlisting>
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passAsFile = ["big"];
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big = "a very long string";
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</programlisting>
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then when the builder runs, the environment variable
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<envar>bigPath</envar> will contain the absolute path to a
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temporary file containing <literal>a very long
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string</literal>. That is, for any attribute
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<replaceable>x</replaceable> listed in
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<varname>passAsFile</varname>, Nix will pass an environment
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variable <envar><replaceable>x</replaceable>Path</envar> holding
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the path of the file containing the value of attribute
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<replaceable>x</replaceable>. This is useful when you need to pass
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large strings to a builder, since most operating systems impose a
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limit on the size of the environment (typically, a few hundred
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kilobyte).</para></listitem>
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</varlistentry>
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<varlistentry><term><varname>preferLocalBuild</varname></term>
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<listitem><para>If this attribute is set to
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<literal>true</literal>, it has two effects. First, the
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derivation will always be built, not substituted, even if a
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substitute is available. Second, if <link
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linkend="chap-distributed-builds">distributed building is
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enabled</link>, then, if possible, the derivaton will be built
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locally instead of forwarded to a remote machine. This is
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appropriate for trivial builders where the cost of doing a
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download or remote build would exceed the cost of building
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locally.</para></listitem>
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</varlistentry>
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</variablelist>
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</section>
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