Is there an easy way by which I can tell which type of Unix shell I am in?

A command that shows whether I am in a Bash, [C shell](https://en.wikipedia.org/wiki/C_shell), [KornShell](https://en.wikipedia.org/wiki/KornShell) (`ksh`), [tcsh](https://en.wikipedia.org/wiki/Tcsh), or [Z shell](https://en.wikipedia.org/wiki/Z_shell) (`zsh`)?

E.g.,

`whatshellisthis`