C shell question

[Guy Harris]

>Background and foreground are concepts related to the shell, not the
>process, and not intrinsically to the unix OS.

Definitely true on systems that don't support job control or with shells
that don't on systems that due.  Not really true on systems supporting
job control, but....

>Not complete enough:  It won't be able to handle processes backgrounded
>by the ampersand (&) on the command-line.

Well, in the job control shells with which I'm familiar, jobs
backgrounded with "&" are like jobs backgrounded by stopping them and
doing "bg".

>Maybe ksh can do it better.

Probably not.  While there are ways a program on a system with job
control can find out if it's in the background in the job-control sense,
the problem is that the answer to the question "am I in the background"
is subject to change over time in a system with job control! I.e., just
because the process was in the foreground or background at the time the
it checked whether it was in the foreground or in the background doesn't
mean it's still there at the time the program actually uses this
information. 

In other words, if you think you want to make your program check whether
it's in the foreground or background, and have it act differently in
those circumstances, think harder - your scheme may not work if somebody
moves the job running your program between the foreground and background.

By and large, most UNIX programs get along quite well without bothering
to find out whether they're running in the foreground or in the
background.  Some programs *might* want to check e.g. whether they're
getting their command input from a terminal or not, as a way to see if
they're being run "interactively" or in "batch" mode, but that's a
different check entirely - yes, in a system with job control, a job in
the background can be getting its input not only from *a* terminal, but
from the user's terminal - if they try to read from that terminal,
they'll be stopped, but the user can then move them to the foreground
and type at them.  If you *really* are trying to find out if your
program is being run "interactively" or in "batch" mode, consider
checking whether the standard input is a terminal, or using a
command-line flag, or something like that.

A question for Steve Hayman: is this another candidate for the
"frequently asked questions" list?