def do_show(self, cmd, args):
        """\
        Display content of a file.
            cat                 Display current working directory.
            cat <file>          Display content of a file.
        """
        if not args:
            self.stdout.write(os.getcwd())
            self.stdout.write('\n')
            return
        fname = args[0]
        with open(fname, 'r', encoding = 'utf8') as f:
            self.stdout.write(f.read())
            self.stdout.write('\n')