is_single()

function is_single( $post = '' ) {
	global $gc_query;

	if ( ! isset( $gc_query ) ) {
		_doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1.0' );
		return false;
	}

	return $gc_query->is_single( $post );
}

is_single();
// When any single Post page is being displayed.

is_single('17');
// When Post 17 (ID) is being displayed.

is_single(17);
// When Post 17 (ID) is being displayed. Integer parameter also works
is_single('Irish Stew');
// When the Post with post_title of "Irish Stew" is being displayed.

is_single('beef-stew');
// When the Post with post_name (slug) of "beef-stew" is being displayed.

is_single(array(17,'beef-stew','Irish Stew'));
// 响应s true when the single post being displayed is either post ID 17,
// or the post_name is "beef-stew", or the post_title is "Irish Stew".
// Note: the array ability was added in version 2.5.