-`<input_file>`*must* exist (if it doesn't the script will exit with an error)
-`<output_file>`/`<output_directory>`*must not* exist (if it does, the script will exit with an error)
Debug output (a lot of it!) can be enabled by setting the `NGINX_CONF_FLATTEN_DEBUG` environment variable to any non-empty string before running `nginx-conf-flatten`. For example: