Re: wait for the next process to finish

[Rory Mulvaney <rorymulv@gmail.com>]

On Mon, 12 Dec 2011, Wayne Davison wrote:

> On Mon, Dec 12, 2011 at 7:46 AM, Anthony R Fletcher <arif@mail.nih.gov>wrote:
> 
> > How can I wait for just the next job to finish?
> >
> 
> One thing that may help you is TRAPCHLD.  Sadly, the signal handler doesn't
> tell you what pid it is reacting to, nor the exit code.
> 
> TRAPCHLD() {
>     echo here
>     oldpids=($pids)
>     pids=( )
>     for p in $oldpids; do
>         if kill -0 $p 2>/dev/null; then
>             pids+=$p
>         else
>             #wait $p # Sadly, this doesn't work
>             echo $p exited
>         fi
>     done
> }
> pids=( )
> sleep 10 &
> pids+=$!
> sleep 20 &
> pids+=$!
> (sleep 15; false) &
> pids+=$!
> echo $pids
> wait
> echo done
> 
> It might be nice to set an environment parameter with the pid and status
> info right before the dotrap(SIGCHLD) call in jobs.c.
> 
> ..wayne..
> 

To clarify (I think this is fairly simple), you can supply the process id 
as a parameter to 'wait', and though the $! method seems rather clumsy to 
retrieve the pid (since you have to retrieve it somehow in the next 
command after spawning the background process), it seems to work mostly in 
general.

So you could do:

sleep 20000 &
sleep 20 &
pid=$!
wait $pid

That will just wait for the sleep 20 process to complete while the sleep 
20000 process still runs in the background.

For further reference, I see that $! is documented in sec. 15.5 
(Parameters set by the shell) in the zsh texinfo manual.

-Rory