Success message is written to STDOUT.
Typically recommended to inform user of successful script conclusion.
# wp rewrite flush expects 'rewrite_rules' option to be set after flush.
flush_rewrite_rules( \WP_CLI\Utils\get_flag_value( $assoc_args, 'hard' ) );
if ( ! get_option( 'rewrite_rules' ) ) {
WP_CLI::warning( "Rewrite rules are empty." );
} else {
WP_CLI::success( 'Rewrite rules flushed.' );
}