check_files example

$res = $this->fs_connect( array( ABSPATH, WP_CONTENT_DIR ), $parsed_args['allow_relaxed_file_ownership'] );
        if ( ! $res || is_wp_error( $res ) ) {
            return $res;
        }

        $wp_dir = trailingslashit( $wp_filesystem->abspath() );

        $partial = true;
        if ( $parsed_args['do_rollback'] ) {
            $partial = false;
        } elseif ( $parsed_args['pre_check_md5'] && ! $this->check_files() ) {
            $partial = false;
        }

        /* * If partial update is returned from the API, use that, unless we're doing * a reinstallation. If we cross the new_bundled version number, then use * the new_bundled zip. Don't though if the constant is set to skip bundled items. * If the API returns a no_content zip, go with it. Finally, default to the full zip. */
        if ( $parsed_args['do_rollback'] && $current->packages->rollback ) {
            $to_download = 'rollback';
        }