On Friday 04 May 2012 15:25:25 John Kearney wrote:
> Am 04.05.2012 21:13, schrieb Mike Frysinger:
> > On Friday 04 May 2012 15:02:27 John Kearney wrote:
> >> Am 04.05.2012 20:53, schrieb Mike Frysinger:
> >>> On Friday 04 May 2012 13:46:32 Andreas Schwab wrote:
> >>>> Mike Frysinger <vap...@gentoo.org> writes:
> >>>>> i wish there was a way to use `wait` that didn't block until all the
> >>>>> pids returned. maybe a dedicated option, or a shopt to enable this,
> >>>>> or a new command.
> >>>>>
> >>>>> for example, if i launched 10 jobs in the background, i usually want
> >>>>> to wait for the first one to exit so i can queue up another one, not
> >>>>> wait for all of them.
> >>>>
> >>>> If you set -m you can trap on SIGCHLD while waiting.
> >>>
> >>> awesome, that's a good mitigation
> >>>
> >>> #!/bin/bash
> >>> set -m
> >>> cnt=0
> >>> trap ': $(( --cnt ))' SIGCHLD
> >>> for n in {0..20} ; do
> >>> (
> >>> d=$(( RANDOM % 10 ))
> >>> echo $n sleeping $d
> >>> sleep $d
> >>> ) &
> >>> : $(( ++cnt ))
> >>> if [[ ${cnt} -ge 10 ]] ; then
> >>> echo going to wait
> >>> wait
> >>> fi
> >>> done
> >>> trap - SIGCHLD
> >>> wait
> >>>
> >>> it might be a little racy (wrt checking cnt >= 10 and then doing a
> >>> wait), but this is good enough for some things. it does lose
> >>> visibility into which pids are live vs reaped, and their exit status,
> >>> but i more often don't care about that ...
> >>
> >> That won't work I don't think.
> >
> > seemed to work fine for me
> >
> >> I think you meant something more like this?
> >
> > no. i want to sleep the parent indefinitely and fork a child asap (hence
> > the `wait`), not busy wait with a one second delay. the `set -m` +
> > SIGCHLD interrupted the `wait` and allowed it to return.
>
> The functionality of the code doesn't need SIGCHLD, it still waits till
> all the 10 processes are finished before starting the next lot.not on my system it doesn't. maybe a difference in bash versions. as soon as one process quits, the `wait` is interrupted, a new one is forked, and the parent goes back to sleep until another child exits. if i don't `set -m`, then i see what you describe -- the wait doesn't return until all 10 children exit. -mike
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