Retrieve the current shell.

def get(cls):  # type: () -> Shell
        """
        Retrieve the current shell.
        """
        if cls._shell is not None:
            return cls._shell

        try:
            name, path = detect_shell(os.getpid())
        except (RuntimeError, ShellDetectionFailure):
            raise RuntimeError("Unable to detect the current shell.")

        cls._shell = cls(name, path)

        return cls._shell