notabot

Tmux is a very helpful terminal multiplexer, meaning it can split your terminal into multiple panes. So, create two side by side panes, then one way of doing it is:

• on the left, run your cmd | tee >(grep 'denied' > error.log)
• on the right, run tail -f error.log

The tee process takes it's standard in, and writes itbto both standard out, so you see all the lines, and the path it's been given. The >(...) operator runs the grep in a subprocess, and returns the path to it's standard input pipe, so grep receives every line, and writes the denied lines to a log file which you display with tail in the other pane.

Rather than using a file for error.log you could also use a named pipe in much the same way.

What do we call it if it is also cut and filled in the conventional bagel plane?

You really shouldn't have something kike SSHD open to the world, that's just an unnecessary atrack surface. Instead, run a VPN on the server (or even one for a network if you have several servers on one subnet), connect to that then ssh to your server. The advantage is that a well setup VPN simply won't respond to an invalid connection, and to an attacker, looks just like the firewall dropping the packet. Wireguard is good for this, and easy to configure. OpenVPN is pretty solid too.