Putting a foreground process into the background from a background process

Question

Is it possible to put a process into the background from another background process with bash?

For instance, if we have the following code:

function check_if_server_running {
    while [ server_running -ne true ]
        curl localhost:8000
    done
    bg %-
}

check_if_server_running &
./startServer

# Some other code...

Could we replace bg %- with something to make the ./startServer run in the background once it's up? Normally bg %- puts the last process in the background but it is not working for me

Answer 1

No, that doesn't really work this way, since the bg and fg commands are only available in the interactive mode of the shell, which is definitely not the case when you're running a function or subshell in background already...

(Also, bg itself doesn't put a job that is already in background, it just resumes a stopped job in background, you need to stop the job first with Ctrl+Z.)

Consider reworking your code to start the server in background, then wait until it's running in foreground:

function wait_for_server_running {
    while ! server_running; do
        curl localhost:8000
    done
}

./startServer &
wait_for_server_running

# Some other code...

That looks similar to what you were trying to achieve, right?