1 files changed, 28 insertions, 1 deletions
diff --git a/doc/manual/nix-build.xml b/doc/manual/nix-build.xml
index 6640898b44cd..61b59c1e0adc 100644
--- a/doc/manual/nix-build.xml
+++ b/doc/manual/nix-build.xml
@@ -42,6 +42,7 @@
       <option>--run-env</option>
       <arg><option>--command</option> <replaceable>cmd</replaceable></arg>
       <arg><option>--exclude</option> <replaceable>regexp</replaceable></arg>
+      <arg><option>--pure</option></arg>
     </arg>
     <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
   </cmdsynopsis>
@@ -147,7 +148,10 @@ also <xref linkend="sec-common-options" />.</phrase></para>
 
     <listitem><para>In the environment of the derivation, run the
     shell command <replaceable>cmd</replaceable> instead of starting
-    an interactive shell.</para></listitem>
+    an interactive shell.  However, if you end the shell command with
+    <literal>return</literal>, you still get an interactive shell.
+    This can be useful for doing any additional
+    initialisation.</para></listitem>
 
   </varlistentry>
 
@@ -159,6 +163,21 @@ also <xref linkend="sec-common-options" />.</phrase></para>
 
   </varlistentry>
 
+  <varlistentry><term><option>--pure</option></term>
+
+    <listitem><para>If this flag is specified, the environment is
+    almost entirely cleared before the interactive shell is started,
+    so you get an environment that more closely corresponds to the
+    “real” Nix build.  A few variables, in particular
+    <envar>HOME</envar>, <envar>USER</envar> and
+    <envar>DISPLAY</envar>, are retained.  Note that
+    <filename>~/.bashrc</filename> and (depending on your Bash
+    installation) <filename>/etc/bashrc</filename> are still sourced,
+    so any variables set there will affect the interactive
+    shell.</para></listitem>
+
+  </varlistentry>
+
 </variablelist>
 
 </refsection>
@@ -189,6 +208,14 @@ $ buildPhase
 $ ./pan/gui/pan
 </screen>
 
+To clear the environment first, and do some additional automatic
+initialisation of the interactive shell:
+
+<screen>
+$ nix-build '&lt;nixpkgs>' --run-env -A pan --pure \
+    --command 'export NIX_DEBUG=1; export NIX_CORES=8; return'
+</screen>
+
 </para>
 
 <para>If a derivation has multiple outputs,