* Document --arg.

doc/manual/opt-common.xml

@@ -217,6 +217,46 @@
 </varlistentry>
 
 
+<varlistentry><term><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>
+
+  <listitem><para>This option is accepted by
+  <command>nix-env</command>, <command>nix-instantiate</command> and
+  <command>nix-build</command>.  When evaluating Nix expressions, the
+  expression evaluator will automatically try to call functions that
+  it encounters.  It can automatically call functions for which every
+  argument has a default value (e.g.,
+  <literal>{<replaceable>argName</replaceable> ?
+  <replaceable>defaultValue</replaceable>}:
+  <replaceable>...</replaceable></literal>).  With
+  <option>--arg</option>, you can also call functions that have
+  arguments without a default value (or override a default value).
+  That is, if the evaluator encounters a function with an argument
+  named <replaceable>name</replaceable>, it will call it with value
+  <replaceable>value</replaceable>.</para>
+
+  <para>For instance, the file
+  <literal>pkgs/top-level/all-packages.nix</literal> in Nixpkgs is
+  actually a function:
+
+<programlisting>
+{ # The system (e.g., `i686-linux') for which to build the packages.
+  system ? __currentSystem
+  <replaceable>...</replaceable>
+}: <replaceable>...</replaceable></programlisting>
+
+  So if you call this Nix expression (e.g., when you do
+  <literal>nix-env -i <replaceable>pkgname</replaceable></literal>),
+  the function will be called automatically using the value <link
+  linkend='builtin-currentSystem'><literal>__currentSystem</literal></link>
+  for the <literal>system</literal> argument.  You can override this
+  using <option>--arg</option>, e.g., <literal>nix-env -i
+  <replaceable>pkgname</replaceable> --arg system
+  \"i686-freebsd\"</literal>.  (Note that since the argument is a Nix
+  string literal, you have to escape the quotes.)</para></listitem>
+
+</varlistentry>
+
+
 </variablelist>