diff --git a/llvm/test/FileCheck/lit.local.cfg b/llvm/test/FileCheck/lit.local.cfg
index 307da456c3815..1191450c6553e 100644
--- a/llvm/test/FileCheck/lit.local.cfg
+++ b/llvm/test/FileCheck/lit.local.cfg
@@ -1,3 +1,8 @@
+import lit
+
+# Use lit's internal shell to help guarantee test portability.
+config.test_format = lit.formats.ShTest(execute_external=False)
+
 # Unset environment variables that the FileCheck tests
 # expect not to be set.
 file_check_expected_unset_vars = [