diff --git a/cmd/nixtest/main.go b/cmd/nixtest/main.go
index 4e11cac..e092788 100644
--- a/cmd/nixtest/main.go
+++ b/cmd/nixtest/main.go
@@ -106,7 +106,11 @@ func buildAndRun(derivation string) (exitCode int, stdout bytes.Buffer, stderr b
 		return
 	}
 
-	cmd := exec.Command("bash", path)
+	args := []string{"bash", path}
+	if *pure {
+		args = append([]string{"env", "-i"}, args...)
+	}
+	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr
 
@@ -296,6 +300,7 @@ var (
 	)
 	updateSnapshots *bool   = flag.Bool("update-snapshots", false, "Update all snapshots")
 	skipPattern     *string = flag.String("skip", "", "Regular expression to skip (e.g., 'test-.*|.*-b')")
+	pure            *bool   = flag.Bool("pure", false, "Unset all env vars before running script tests")
 )
 
 func main() {
diff --git a/flake.nix b/flake.nix
index 1f81625..5eb6c64 100644
--- a/flake.nix
+++ b/flake.nix
@@ -83,7 +83,9 @@
                   type = "script";
                   script = ''
                     echo Test something here
-                    exit 1
+                    # required in pure mode:
+                    export PATH="${lib.makeBinPath [pkgs.gnugrep]}"
+                    grep -q "test" ${builtins.toFile "test" "test"}
                   '';
                 }
               ];