DATA_DIR override with environment variable (#333)

src/common.cpp

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: Apache-2.0
+/*
+ * Copyright (C) 2023 Gwenhael Goavec-Merou <[email protected]>
+ */
+
+
+#include "common.hpp"
+
+#include <string>
+#include <cstdlib>
+
+/*!
+ * \brief return shell environment variable value
+ * \param[in] key: variable name
+ * \return variable value or ""
+ */
+const std::string get_shell_env_var(const char* key,
+		const char *def_val) noexcept {
+	const char* ret = std::getenv(key);
+	return std::string(ret ? ret : def_val);
+}