* Documented some of the more obscure derivation attributes (including
fixed-output derivations).
This commit is contained in:
Eelco Dolstra
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1511aa11ce
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@ -17,7 +17,7 @@ functions and values. For instance, <function>derivation</function>
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is also available as <function>builtins.derivation</function>.</para>
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<variablelist >
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<variablelist>
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<varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>
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@ -38,9 +38,13 @@
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paths.</para></listitem>
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<listitem><para>TODO: <varname>allowedReferences</varname> for
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checking the set of references in the output of a
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derivation.</para></listitem>
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<listitem><para>Derivations can specify the new special attribute
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<varname>allowedReferences</varname> to enforce that the references
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in the output of a derivation are a subset of a declared set of
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paths. For example, if <varname>allowedReferences</varname> is an
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empty list, then the output must not have any references. This is
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used in NixOS to check that generated files such as initial ramdisks
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for booting Linux don’t have any dependencies.</para></listitem>
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<listitem><para>TODO: semantic cleanups of string concatenation
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@ -54,8 +58,11 @@
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<command>nix-store --register-validity</command>.</para></listitem>
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<listitem><para>TODO: magic <varname>exportReferencesGraph</varname>
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attribute.</para></listitem>
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<listitem><para>The new attribute
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<varname>exportReferencesGraph</varname> allows builders access to
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the references graph of their inputs. This is used in NixOS for
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tasks such as generating ISO-9660 images that contain a Nix store
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populated with the closure of certain paths.</para></listitem>
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<listitem><para>TODO: option <option>--max-silent-time</option>,
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@ -109,8 +116,10 @@
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disambiguation (<command>nix-env -qaA</command>).</para></listitem>
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<listitem><para>TODO: substitutes table is gone, registering
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substitutes is now much faster.</para></listitem>
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<listitem><para>The substitutes table has been removed from the
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database. This makes operations such as <command>nix-pull</command>
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and <command>nix-channel --update</command>
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<emphasis>much</emphasis> faster.</para></listitem>
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<listitem><para><command>nix-prefetch-url</command> now has a
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@ -607,7 +607,7 @@ language.</para>
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<simplesect><title>Simple values</title>
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<para>Nix has the following basic datatypes:
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<para>Nix has the following basic data types:
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<itemizedlist>
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@ -679,7 +679,7 @@ configureFlags = "
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instance, <filename>builder.sh</filename> is not a
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path<footnote><para>It's parsed as an expression that selects the
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attribute <varname>sh</varname> from the variable
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<varname>builder</varname>.</para></footnote>. If the filename is
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<varname>builder</varname>.</para></footnote>. If the file name is
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relative, i.e., if it does not begin with a slash, it is made
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absolute at parse time relative to the directory of the Nix
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expression that contained it. For instance, if a Nix expression in
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@ -701,7 +701,7 @@ configureFlags = "
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<simplesect><title>Lists</title>
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<para>Lists are formed by enclosing a whitespace-separated list of
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values between square bracktes. For example,
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values between square brackets. For example,
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<programlisting>
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[ 123 ./foo.nix "abc" (f {x=y;}) ]</programlisting>
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@ -927,7 +927,7 @@ evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
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if <replaceable>e1</replaceable> then <replaceable>e2</replaceable> else <replaceable>e3</replaceable></programlisting>
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where <replaceable>e1</replaceable> is an expression that should
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evaluate to a boolean value (<literal>true</literal> or
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evaluate to a Boolean value (<literal>true</literal> or
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<literal>false</literal>).</para>
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</simplesect>
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@ -942,7 +942,7 @@ on or between features and dependencies hold. They look like this:
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assert <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting>
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where <replaceable>e1</replaceable> is an expression that should
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evaluate to a boolean value. If it evaluates to
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evaluate to a Boolean value. If it evaluates to
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<literal>true</literal>, <replaceable>e2</replaceable> is returned;
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otherwise expression evaluation is aborted and a backtrace is printed.</para>
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@ -1234,7 +1234,7 @@ set, the attributes of which specify the inputs of the build.</para>
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</para></listitem>
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<listitem><para>The optional argument <varname>args</varname>
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<listitem><para>The optional attribute <varname>args</varname>
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specifies command-line arguments to be passed to the builder. It
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should be a list.</para></listitem>
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@ -1337,6 +1337,233 @@ command-line argument. See <xref linkend='sec-standard-environment'
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</para>
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<section><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. This is used in NixOS to check that
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generated files such as initial ramdisks for booting Linux don’t
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have accidental dependencies on other paths in the Nix
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store.</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>
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<replaceable>...</replaceable>]</literal>. The references graph
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of each <replaceable>pathN</replaceable> will be stored in a text
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file <replaceable>nameN</replaceable> in the temporary build
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directory. The text files have the format used by
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<command>nix-store --register-validity</command> (with the deriver
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fields left empty). For example, when the following derivation is
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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 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>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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</variablelist>
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</section>
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</section>
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