diff --git a/tests/cp/cp-mv-backup b/tests/cp/cp-mv-backup
index 1bad6de20..aaddaa25c 100755
--- a/tests/cp/cp-mv-backup
+++ b/tests/cp/cp-mv-backup
@@ -91,9 +91,9 @@ EOF
 
 sed 's/: x/:/' $expected-tmp |cat $expected-tmp - > $expected
 
-# Uncomment this if you see a failure and want to try to diagnose it.
-#diff -u $expected $actual 1>&2
-
 cmp $expected $actual || fail=1
 
+# Uncomment this if you see a failure and want to try to diagnose it.
+test $fail = 1 && diff -u $expected $actual 1>&2
+
 exit $fail