Is it allowed to add an attribute to a shortcode without giving it a value? something like:

[shortcode attr1="hasvalue" attr2]

The attribute attr1 has a value, which I can easily catch in a function like: function shortcode( $atts ) {... with $atts['attr1'].

Can I also catch the other attribute attr2? Its value doesn't exist, so $atts['attr2'] is undefined/empty. But can I maybe simply detect if this attribute has been written or not?

This would be useful for binary settings, such as a true or false indication, where it is a bit redundant to write true="true" or something like that.