Update the command's dependencies based on the evaluated input and
        output of previous commands.

def update_dependent_files(self, prev_commands=[]):
        """ Update the command's dependencies based on the evaluated input and
        output of previous commands.
        """
        for command in prev_commands:
            for my_input in self.input_parts:
                for their_output in command.output_parts:
                    if their_output == my_input:
                        my_input.filename = their_output.eval()