diff --git a/doc/manual/command-ref/nix-shell.xml b/doc/manual/command-ref/nix-shell.xml
index a13d9d7c1..6f00e28ca 100644
--- a/doc/manual/command-ref/nix-shell.xml
+++ b/doc/manual/command-ref/nix-shell.xml
@@ -176,6 +176,22 @@ also <xref linkend="sec-common-options" />.</phrase></para>
 </refsection>
 
 
+<refsection><title>Environment variables</title>
+
+<variablelist>
+
+  <varlistentry><term><envar>NIX_BUILD_SHELL</envar></term>
+    
+    <listitem><para>Shell used to start the interactive environment. 
+    Defaults to the <command>bash</command> found in <envar>PATH</envar>.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+</refsection>
+
+
 <refsection><title>Examples</title>
 
 <para>To build the dependencies of the package Pan, and start an
diff --git a/scripts/nix-build.in b/scripts/nix-build.in
index ea099532b..b4bb2fc3f 100755
--- a/scripts/nix-build.in
+++ b/scripts/nix-build.in
@@ -270,7 +270,7 @@ foreach my $expr (@exprs) {
         my $tmp = $ENV{"TMPDIR"} // $ENV{"XDG_RUNTIME_DIR"} // "/tmp";
         if ($pure) {
             foreach my $name (keys %ENV) {
-                next if grep { $_ eq $name } ("HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL", "TZ", "PAGER");
+                next if grep { $_ eq $name } ("HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL", "TZ", "PAGER", "NIX_BUILD_SHELL");
                 delete $ENV{$name};
             }
             # NixOS hack: prevent /etc/bashrc from sourcing /etc/profile.