Recently, I started using `tmux`. I find it nice, but I'm still having issues understanding this application. One of the basic questions I have is: How do I know (from the command line) what is the name of the `tmux` session I'm logged to? If I'm logged to some `tmux` session, it will tell me its name. But if I'm not logged to a `tmux` session, it will print either nothing or some sort of an error.