feature request: "wait --free-slot" for poor man's parallelization

[Mike Coleman]

Here's a bash feature I'd love to see, but don't have time to
implement myself: a "--free-slot" flag to 'wait' that will wait until
there is at least one free "slot" available, where a slot is basically
a CPU core.

Example usage:

$ for ((n=0; n<100; n++)); do
      my_experiment $n > $n.out &
      wait --free-slot 4
 done

which I would run on a quad-CPU host.  The meaning of the wait is: if
I already have four background invocations of "my_experiment" running,
wait until one finishes, then proceed.  (If there are fewer than four
running, proceed immediately.)

This provides a nice sloppy way of letting me parallelize in a script
without having to complicate things.  "make -j" and "xargs -P" provide
some of this capability, but are a lot more work to set up--I'd really
like to have this at my fingertips for interactive stuff.

Thanks,
Mike