* Document --arg.
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>nix-build</command>
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<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-common-syn.xml#xpointer(/nop/*)" />
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<arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
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<arg><option>--add-drv-link</option></arg>
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<arg><option>--drv-link </option><replaceable>drvlink</replaceable></arg>
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<arg><option>--no-out-link</option></arg>
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@ -56,6 +58,12 @@ or renamed. So don’t rename the symlink.</para></warning>
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<refsection><title>Options</title>
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<para>See also <xref linkend="sec-common-options" />. All options not
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listed here are passed to <command>nix-store --realise</command>,
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except for <option>--arg</option> and <option>--attr</option> /
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<option>-A</option> which are passed to
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<command>nix-instantiate</command>.</para>
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<variablelist>
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<varlistentry><term><option>--add-drv-link</option></term>
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<cmdsynopsis>
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<command>nix-env</command>
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<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-common-syn.xml#xpointer(/nop/*)" />
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<arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
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<arg>
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<group choice='req'>
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<arg choice='plain'><option>--file</option></arg>
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@ -24,7 +25,6 @@
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</group>
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<replaceable>path</replaceable>
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</arg>
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<arg><option>--preserve-installed</option></arg>
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<arg>
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<arg choice='plain'><option>--system-filter</option></arg>
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<replaceable>system</replaceable>
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@ -106,19 +106,6 @@ linkend="sec-common-options" />.</para>
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</varlistentry>
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<varlistentry><term><option>--preserve-installed</option></term>
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<listitem><para>By default, when you install a derivation with the
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<option>--install</option> operation, it will replace previously
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installed versions with the same derivation name (regardless of
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the version number). This option causes those previously
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installed versions to be kept in the new generation of the
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profile. Note that this will generally cause conflicts in the
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creation of the user environment (since multiple versions of a
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package typically contain the same programs).</para></listitem>
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</varlistentry>
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<varlistentry><term><option>--system-filter</option> <replaceable>system</replaceable></term>
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<listitem><para>By default, operations such as <option>--query
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<cmdsynopsis>
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<command>nix-instantiate</command>
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<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-common-syn.xml#xpointer(/nop/*)" />
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<arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
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<arg><option>--add-root</option> <replaceable>path</replaceable></arg>
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<arg><option>--indirect</option></arg>
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<group choice='opt'>
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<arg><option>--fallback</option></arg>
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<arg><option>--readonly-mode</option></arg>
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<arg><option>--log-type</option> <replaceable>type</replaceable></arg>
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<sbr />
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</nop>
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</varlistentry>
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<varlistentry><term><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>
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<listitem><para>This option is accepted by
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<command>nix-env</command>, <command>nix-instantiate</command> and
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<command>nix-build</command>. When evaluating Nix expressions, the
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expression evaluator will automatically try to call functions that
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it encounters. It can automatically call functions for which every
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argument has a default value (e.g.,
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<literal>{<replaceable>argName</replaceable> ?
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<replaceable>defaultValue</replaceable>}:
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<replaceable>...</replaceable></literal>). With
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<option>--arg</option>, you can also call functions that have
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arguments without a default value (or override a default value).
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That is, if the evaluator encounters a function with an argument
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named <replaceable>name</replaceable>, it will call it with value
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<replaceable>value</replaceable>.</para>
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<para>For instance, the file
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<literal>pkgs/top-level/all-packages.nix</literal> in Nixpkgs is
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actually a function:
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<programlisting>
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{ # The system (e.g., `i686-linux') for which to build the packages.
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system ? __currentSystem
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<replaceable>...</replaceable>
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}: <replaceable>...</replaceable></programlisting>
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So if you call this Nix expression (e.g., when you do
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<literal>nix-env -i <replaceable>pkgname</replaceable></literal>),
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the function will be called automatically using the value <link
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linkend='builtin-currentSystem'><literal>__currentSystem</literal></link>
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for the <literal>system</literal> argument. You can override this
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using <option>--arg</option>, e.g., <literal>nix-env -i
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<replaceable>pkgname</replaceable> --arg system
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\"i686-freebsd\"</literal>. (Note that since the argument is a Nix
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string literal, you have to escape the quotes.)</para></listitem>
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</varlistentry>
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</variablelist>
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@ -1387,7 +1387,8 @@ if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>
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</varlistentry>
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<varlistentry><term><varname>currentSystem</varname></term>
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<varlistentry
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xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>
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<listitem><para>The built-in value <varname>currentSystem</varname>
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evaluates to the Nix platform identifier for the Nix installation
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