Part of test.c in Bash source codes.

part_of_test.c

    case 't':	/* File fd is a terminal? */
      if (legal_number (arg, &r) == 0)
	  return (FALSE);
      return ((r == (int)r) && isatty ((int)r));

    case 'n':			/* True if arg has some length. */
      return (arg[0] != '\0');

    case 'z':			/* True if arg has no length. */
      return (arg[0] == '\0');

    case 'o':			/* True if option `arg' is set. */
      return (minus_o_option_value (arg) == 1);

    case 'v':
      v = find_variable (arg);