You can type it at a terminal the same way you can write other low-ascii codes. For example record separator, 30, is ctrl-^ - you can type it in the terminal by the usual literal control char way, ctrl-v[1].
So for example at a Python prompt:
>>> ord('^^') # typed ctrl-v ctrl-^ here
30
(the low unprintable ascii codes correspond to certain ctrl combinations because ctrl-x is just the ascii code of x with the high bits masked off (i.e. & 0x1f) )
[1] custimizable in your terminal settings, see stty lnext ("literal next")
Can't be found, but you can enter them by hand by holding down alt while entering the ASCII code on the numeric keypad. I wonder if they'll survive posting a comment:
28∟
29↔
30▲
31▼
Edit: looks like they do survive. Hmmm... Now I wonder about extended ASCII.
└┐│└└╚╦╝
Also, you can leverage the ISO/IEC 2022 extension and represent them with Caret Notation. Note: recommend an escaping mechanism depending on data. REF: https://en.wikipedia.org/wiki/C0_and_C1_control_codes#C0_(AS...
An editor such as vim or pager like 'less' will display them. You can also 'cat -v' a file.
