On: Reptyr – Reparent a running program to a new termi...

[pronoiac]

This may be a dumb question, but wouldn't this work?

* press ctrl-z to put the program in the background

* start screen

* run fg within screen

[caipre → pronoiac]

screen (and tmux) start a new instance of the shell, which would have no knowledge of the backgrounded process. You'd just see the message "fg: no current job"

[emmelaich → caipre]

And even if you could(1), the background process typically has the terminal open - and screen uses a separate pseudo terminal aka pty -- As suggested by the name - rePTYr

(1) ^Z sends SIGSTOP; to continue you can signal SIGCONT. But then what?!