In an interactive shell there is something that is called "job control".
If you put something in the background with an & then it becomes a "job".
For example
sleep 60 &
You can display the jobs with
jobs
The first job gets the jobnr 1.
You can kill any of them with
kill %jobnr
e.g. kill %1
Or you can pull one of them to the foreground by
fg %jobnr
Just
fg
pulls the most recent job.
How can you do the opposite, place the foreground task into the background?
For the example
sleep 60
First you must stop it. This is done with
Ctrl-Z
It sends a SIGSTOP to the foreground task, and it remains in a stopped (paused) state.
(Compare with Ctrl-C that sends a SIGINT.)
Now you have the command prompt back.
And you can list it with
jobs
And now you can put it to the background with
bg
(Or bg %jobnr if you have several running or stopped jobs.)
Of course you can put a stopped job into the foreground with
fg
Say you started a vi and would like to run some shell commands. Well, in some cases you can start another shell in another terminal. But also you can stop the vi with
Ctrl-Z
run your commands, and continue the editing with
fg
What happens if you continue the vi with
bg
?
Well, vi is not made for running in the background.
You can try and find out what happens...
Remember, any time you can pull it to the foreground.