Send to background
Syntax bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been started with &.
bg takes a “jobspec” available from jobs, not a PID.
If jobspec is not present, the shell's notion of the current job is used.
bg jobspec returns 0 unless run when job control is disabled or, when run with job control enabled, any specified jobspec was not found or was started without job control.
Put the job with job id 0 in the background:
$ bg %0
To start a new process in the background you can do:
$ long_running_command &
“Keep your eyes on the stars, and your feet on the ground” - Theodore Roosevelt
bg man page - Apple.com
fg - Send job to foreground
wait - Wait for a process to complete