> Would you consider `ls` an API for exposing your filesystem?

I don't see why not.

It has an interface for input and output, conforms to well known specifications and is publicly documented.

There's also multiple implementations behind the API.

I would consider "ls" a presentation tool that uses an API to present information about a file system. I would consider stat/lstat/opendir/readdir/closedir the API that "ls" use to gather the information.

When you combine it with shell scripts, I'd say that ls is an API to itself.