Interact with background jobs (scripted job control)

[Klaus Frank]

I have sent a large upload task into the background and would like to interact with the task and job from a script.
For e.g. lftp -c "attach $PID; jobs; exit bg; exit" doesn't work. After the attach is executed, it doesn't execute further commands and gets stuck in an interactive shell.

Am I doing something wrong or is it currently not possible to interact with an background lftp process (except maybe by manually attaching it inside screen/tmux and sending the additional commands via it instead of lftp itself, or other ugly workarounds).