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Re: output redirection with process substitution asynchronous?


From: DennisW
Subject: Re: output redirection with process substitution asynchronous?
Date: Sat, 5 Dec 2009 06:51:57 -0800 (PST)
User-agent: G2/1.0

On Dec 5, 3:14 am, pjodrr <address@hidden> wrote:
> Hello,
>
> On Dec 4, 8:18 pm, DennisW <address@hidden> wrote:
>
> > It works for me. Does it not for you? If you're asking why not do it,
> > then the answer is "why call an external program unnecessarily?".
>
> > Sorry, by the way, I missed what you were doing with the file
> > descriptor on my first read. What is it that you're trying to
> > accomplish? Are you doing this only to number the lines or is either
> > seq or the while loop a stand-in for something else?
>
> the seq was only an example for demonstration.
> here is another example that shows what I mean:
>
> $ exec 3> >(while read line; do echo "tag: $line"; done)
> $ seq 4 >&3
> tag: 1
> tag: 2
> tag: 3
> tag: 4
>
> $ exec 3> >(while read line; do echo "$(date): $line"; done)
> $ seq 4 >&3
> $ Sat Dec  5 10:11:25 CET 2009: 1
> Sat Dec  5 10:11:25 CET 2009: 2
> Sat Dec  5 10:11:25 CET 2009: 3
> Sat Dec  5 10:11:25 CET 2009: 4
>
> while in the first example the prompt returns after the
> command completes, the prompt returns immediately
> in the second example.
>
> thanks for your attention,
>
>   Peter

Your example here:

$ exec 3> >(while read line; do echo "tag: $line"; done)
$ seq 4 >&3

just executes too quickly to exhibit this behavior. Try this and it
will do it, too:

$ exec 3> >(while read line; do for i in {1..10000}; do :; done; echo
'.'; done)
$ seq 4 >&3

I think the thing to remember is that doing this is like running
something in the background with "&". So, yes, it's going to be
asynchronous.


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