unix. Send running process to background

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Using the Job Control of bash to send the process into the background:

ctrl+z to stop (pause) the program and get back to the shell
bg to run it in the background
disown -h so that the process isn't killed when the terminal closes

Suppose for some reason Ctrl+Z is also not working, go to another terminal, find the process id (using ps) and run

kill -20 PID 
kill -18 PID
kill -20 will suspend the process and kill -18 will resume the process, in your other terminal