This is useful for those cases where you didn't start screen first, such as when you either forgot or didn't think you'd need it but it turned out you did.

I just alias my terminal shortcut/command to start screen/tmux every time

Doesn't using screen have some drawbacks? For example it breaks native scrolling in the OS X terminal app, so I wouldn't want to run screen unless I really needed it.

Yeah I don't know. I only mentioned screen to be fair. I use tmux and I don't usually have any problems, although I get problems when I use it with mosh sometimes (particularly weechat mouse support doesn't seem to work well).

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

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"

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?!