How does a shell know if it's in the foreground or the background?

[Guy Harris]

>In shells that support job control (like csh), jobs are put into different 
>process group than the parent shell to facilitate job control.

Note also that, in shells that support job control, "foreground" and
"background" are temporary conditions, not ways of life; a job could be
in the foreground one minute, and in the background a bit later, and
then back in the foreground again a bit later....