As far as I can see this is the first time we are passing arguments to a programme. From my experience when learning the shell, the rules you need to follow for passing arguments can be quite difficult to comprehend. Most important here is that you need at least one space between "ls" and "-F". When I showed people arguments for the first time, I made this explicit by showing them which error they get when trying to run "ls-F". With arguments that can have arguments (--max-count=4) and escaping it quickly gets way more complex, so it is important to make sure that people understand this very basic requirement for passing simple arguments to a programme.