I don’t understand terminals, shells and SSH

Confession time: I don’t fully understand how terminals, shells and SSH really work (and my guess is you don’t either). And I don’t mean the cryptography behind SSH. I mean how SSH and the terminal — and the shell for that matter — interact with one another.

I recently realized that even though I’ve been daily remotely logging into Linux systems for all of my adult life (and type in the shell and Vim) I didn’t really grasp how these things actually work.

Of course I conceptually know what a (virtual) terminal is (entering input and displaying output) and what the shell is for (the interpreter). And SSH is the remote login protocol, right? (Or is SSH a pseudoterminal inside another pseudoterminal, who’s to say)?

The distinction between these three elements is a bit fuzzy and I do not have a clear concept of it in my head. The test being: could I draw it on a whiteboard? Or: could I explain it to a novice? The answer is probably: not really.

So I went on a bender and found these four (well-known) links that explain things like tty, rawmode, ptms/ptx, pseudoterminals and more.

• A Guide to the Terminal, Console, and Shell
  Historical background and good explanation of how the shell and terminal work.
• Terminals and pseudoterminals
  Useful visuals and explanation of pseudoterminals.
• What exactly is TTY?
  More on the TTY: historical background and flowcharts.
• The TTY demystified
  The previous link borrows heavily from this one.
• What happens when you open a terminal and enter ‘ls’
  Historical background and good visuals.

This post functions as a bookmark placeholder. I will add more links when I find them.

There’s lots of information here if you’re interested. And of course: you mostly don’t actually need to know any of these things to do your work — we are all forever standing on the shoulders of giants. But I *want* to understand these things. And I think I understand them a little bit better now. Maybe you will as well.