fg [jobspec]
Resume jobspec in the foreground, and make it the current job. If jobspec is not present, the
shell's notion of the current job is used. The return value is that of the command placed into
the foreground, or failure if run when job control is disabled or, when run with job control
enabled, if jobspec does not specify a valid job or jobspec specifies a job that was started with-
out job control.

bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been started with &. 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.