I have been there... The problem is that this happens to just rarely enough I don't remember ^Q and randomly poke at the keyboard for a while until I hit the right combination.

Remember that holding control and any alphabet letter is supposed to have a function, and then remember "Quick" and "Stop".

Probably you've used Control-C a few times, maybe Control-Z too .. well, these basic characters have a control function over your process' terminal input/output, just like ^M/^N (carriage return/line-feed), ^L (clear screen) and so on .. Just like Control-C is a shortcut for "Cancel" process.. C-S-Q is sort of the 'gearing' over process output, where Z, which is 'detach process i/o from console', otherwise known as "background the process", ( * ) might be considered the 'clutch'.

Anyway, C-S-Q .. those 4 keys control the process. Cancel, Stop, Quick.

Practice it a few times with some cat /somelargefile.txt paging and you'll see there's a use for it beyond perplexity. When 'less' and even 'more' are not around, Control-S/-Q are your trusty pals.

(* - Another "oh, fudge, dunno what happened, guess I'll just reboot.." moment can occur with errant Control-Z usage over a process, where you accidentally put the process in the background and don't know how to re-attach it. In that case, the command 'fg' is your friend .. I've see far too many experienced people not know they can put their processes back in place during debug/admin sessions, opting indeed to reboot, or at least restart the term session, that I can't help but feel that, like Stop/Quick, Control-Z+'fg' is somewhat of a lost lore..)

I love ctrl-Z, but I think of it more as "suspend" than "background". After all, a ctrl-Z'd process isn't really doing anything. If you want to run such in the backround, just use bg, which is precisely analogous to fg.

Yes, you're probably right. It disconnects the console i/o from the process fd's, and so the process sits there blocking - and when you fg, the fd's get re-attached and things kick back into place .. while bg'ing the process, I think, re-attaches it to a new thread completely? Ah well, the details are out there somewhere ..