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Re: bash process substitution process not awaited before next statement


From: Greg Wooledge
Subject: Re: bash process substitution process not awaited before next statement started?
Date: Wed, 23 Nov 2011 16:34:29 -0500
User-agent: Mutt/1.4.2.3i

On Wed, Nov 23, 2011 at 08:41:56AM +0100, address@hidden wrote:
> Description:
>       It looks as if bash is not waiting for a process-substition process
>       which is reading stdin to complete before bash moves on to executing
>       the next statement.

Process substitutions are background processes.  They are exactly the same
as doing:

mkfifo tmp-fifo
your job <tmp-fifo & disown
some other job >tmp-fifo

>       spew_and_slurp_with_lock()
>       {
>               local I
>       
>               for ((I=0; I<1000; I++)); do
>                       echo "some junk"
>               done > >(mkdir $LOCK_DIR; cat > /dev/null; rmdir $LOCK_DIR)
>       }

In this case, the entire mkdir;cat;rmdir job is launched in the background,
with a file descriptor pointing to it.  The for loop is being redirected to
that.  The for loop should block until the cat command reads from the
pipe, at which point the main shell may move on.  Meanwhile, the cat command
which is in the background does its processing, and terminates; and then
finally rmdir gets called.

So yes, the main shell and the rmdir could be doing things simultaneously.
That's how process substitutions are defined.

If you want to ensure that the main shell waits for the rmdir, then
you must abandon the process substitution syntax and use your own explicit
FIFOs:

mkfifo tmp-fifo
{ mkdir "$LOCK_DIR"; cat >/dev/null; rmdir "$LOCK_DIR"; } < tmp-fifo & pid=$!
for ... done > tmp-fifo
wait $pid
rm tmp-fifo

Only then will you be assured that things will run the way you need them to.



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