diff --git a/lib/spack/spack/util/executable.py b/lib/spack/spack/util/executable.py
index 6848a7e2a5..6088d4514e 100644
--- a/lib/spack/spack/util/executable.py
+++ b/lib/spack/spack/util/executable.py
@@ -100,7 +100,8 @@ def streamify(arg, mode):
             out, err = proc.communicate()
             self.returncode = proc.returncode
 
-            output.write(out)
+            if not return_output:
+                output.write(out)
             error.write(err)
 
             rc = proc.returncode