In my command-line, why does echo $0 return "-"?

Question

When I type echo $0 I see -

I expect to see bash or some filename, what does it mean if I just get a "-"?

Answer 1

A hyphen in front of $0 means that this program is a login shell.

note: $0 does not always contain accurate path to the running executable as there is a way to override it when calling execve(2).