Check if the dest_file already exists on the file system (return boolean).

def _check_file_exists_unix(self, remote_cmd=""):
        """Check if the dest_file already exists on the file system (return boolean)."""
        if self.direction == "put":
            self.ssh_ctl_chan._enter_shell()
            remote_cmd = "ls {}".format(self.file_system)
            remote_out = self.ssh_ctl_chan.send_command(
                remote_cmd, expect_string=r"[\$#]"
            )
            self.ssh_ctl_chan._return_cli()
            return self.dest_file in remote_out
        elif self.direction == "get":
            return os.path.exists(self.dest_file)