job control in System V

[Charles Root]

As of System V release 2 version 2 there is a facility that is very
close to job control.  I happen to prefer real Berkeley job control,
but certainly many of the jibes at ATT for doing nothing in this area
are undeserved.  The facility is shl (shell layers).  It has been
claimed that this is useful mostly for terminals such as the BLT.  I
only have conventional terminals, so I will just describe what it does
there.  shl is a sort of top level.  You run shells (typically) under
its control.  These subsidiary shells are called "layers".  To get
from one of the layers back to shl, you just type ^Z (sound
familiar?).  To continue a layer, you just type its name (not very
different from %name in the csh to continue a job).  The one thing I
don't like about it is that you must explicitly create a new layer
using the create command, rather than just ^Zing one process and
starting another one, or starting several processing with &.  But it
isn't *that* bad.  I am not sure how all of this is implemented.  At
first I was afraid that the top level put the layers on the equivalent
of pty's, and had to process every characater.  But this does not seem
to be true.  A devices called an "sxt" is used to coordinate the
layers.  Each layer gets its own sxt (actually its own subchannel on
the same sxt, but it shows up as a separate device).  But it appears
that the currently active sxt is somehow connected directly to the
controlling tty, and thus its I/O does not go through the top-level
process.  

It is not yet clear to me whether programs can interact with this
system the way they can with job control.  E.g.  under 4.2, Emacs has
its own ^Z command that does some cleaning up and then triggers job
control to suspend the process.  When you resume it, Emacs knows what
is going on, and redraws the screen.  I don't know whether this is
possible under shell layers.

Again, let me be clear that I would have preferred to see ATT adopt
some redesign of Berkeley's job control.  But fairness requires that
you know what they actually have done.