private function read_from_file_or_stdin( $arg ) {
		if ( '-' !== $arg ) {
			$readfile = $arg;
			if ( ! file_exists( $readfile ) || ! is_file( $readfile ) ) {
				WP_CLI::error( "Unable to read content from '{$readfile}'." );
			}
		} else {
			$readfile = 'php://stdin';
		}
		return file_get_contents( $readfile );
	}