diff --git a/tools/testing/selftests/net/mptcp/mptcp_lib.sh b/tools/testing/selftests/net/mptcp/mptcp_lib.sh
index aca35376006b..556a7d9784d7 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_lib.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_lib.sh
@@ -352,20 +352,24 @@ mptcp_lib_check_tools() {
 }
 
 mptcp_lib_check_output() {
-	local err="$1"
-	local cmd="$2"
-	local expected="$3"
+	local err="${1}"
+	local cmd="${2}"
+	local expected="${3}"
 	local cmd_ret=0
-	local out=$(${cmd} 2>${err}) || cmd_ret=${?}
+	local out
 
-	if [ $cmd_ret -ne 0 ]; then
-		mptcp_lib_print_err "[FAIL] command execution '$cmd' stderr"
+	if ! out=$(${cmd} 2>"${err}"); then
+		cmd_ret=${?}
+	fi
+
+	if [ ${cmd_ret} -ne 0 ]; then
+		mptcp_lib_print_err "[FAIL] command execution '${cmd}' stderr"
 		cat "${err}"
 		return 2
-	elif [ "$out" = "$expected" ]; then
+	elif [ "${out}" = "${expected}" ]; then
 		return 0
 	else
-		mptcp_lib_print_err "[FAIL] expected '$expected' got '$out'"
+		mptcp_lib_print_err "[FAIL] expected '${expected}' got '${out}'"
 		return 1
 	fi
 }