diff --git a/mainwin.cc b/mainwin.cc
index fcf70988..c6f2f1f6 100644
--- a/mainwin.cc
+++ b/mainwin.cc
@@ -411,7 +411,7 @@ void MainWindow::compile(bool procevents)
 	root_ctx.set_variable("$vpr", vpr);
 
 	last_compiled_doc = editor->toPlainText();
-	root_module = parse(last_compiled_doc.toAscii().data(), false);
+	root_module = parse((last_compiled_doc + commandline_commands).toAscii().data(), false);
 
 	delete highlighter;
 	highlighter = new Highlighter(editor->document());
diff --git a/openscad.cc b/openscad.cc
index fc9efc89..571ead5c 100644
--- a/openscad.cc
+++ b/openscad.cc
@@ -30,11 +30,13 @@
 
 static void help(const char *progname)
 {
-	fprintf(stderr, "Usage: %s [ -m make_command ] [ filename ]\n", progname);
-	fprintf(stderr, "       %s { -s stl_file | -o off_file } [ -d deps_file ] [ -m make_command ] filename\n", progname);
+	fprintf(stderr, "Usage: %s [ { -s stl_file | -o off_file } [ -d deps_file ] ]\\\n"
+			"%*s[ -m make_command ] [ -D var=val [..] ] filename\n",
+			progname, strlen(progname)+8, "");
 	exit(1);
 }
 
+QString commandline_commands;
 const char *make_command = NULL;
 QSet<QString> dependencies;
 
@@ -80,7 +82,7 @@ int main(int argc, char **argv)
 
 	int opt;
 
-	while ((opt = getopt(argc, argv, "s:o:d:m:")) != -1)
+	while ((opt = getopt(argc, argv, "s:o:d:m:D:")) != -1)
 	{
 		switch (opt)
 		{
@@ -104,6 +106,9 @@ int main(int argc, char **argv)
 				help(argv[0]);
 			make_command = optarg;
 			break;
+		case 'D':
+			commandline_commands += QString(optarg) + QString(";\n");
+			break;
 		default:
 			help(argv[0]);
 		}
diff --git a/openscad.h b/openscad.h
index 17a22a12..e34c4911 100644
--- a/openscad.h
+++ b/openscad.h
@@ -828,6 +828,7 @@ public slots:
 extern AbstractModule *parse(const char *text, int debug);
 extern int get_fragments_from_r(double r, double fn, double fs, double fa);
 
+extern QString commandline_commands;
 extern int parser_error_pos;
 extern QPointer<MainWindow> current_win;
 
diff --git a/parser.y b/parser.y
index 8578fdd0..7d742093 100644
--- a/parser.y
+++ b/parser.y
@@ -112,9 +112,19 @@ statement:
 		}
 	} |
 	TOK_ID '=' expr ';' {
-		module->assignments_var.append($1);
-		module->assignments_expr.append($3);
-		free($1);
+		bool add_new_assignment = true;
+		for (int i = 0; i < module->assignments_var.size(); i++) {
+			if (module->assignments_var[i] != QString($1))
+				continue;
+			delete module->assignments_expr[i];
+			module->assignments_expr[i] = $3;
+			add_new_assignment = false;
+		}
+		if (add_new_assignment) {
+			module->assignments_var.append($1);
+			module->assignments_expr.append($3);
+			free($1);
+		}
 	} |
 	TOK_MODULE TOK_ID '(' arguments_decl optional_commas ')' {
 		Module *p = module;