docs: 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')