From owner-freebsd-questions  Thu Apr 18 16:37: 5 2002
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	 Thu, 18 Apr 2002 18:34:33 -0400
Subject: Re: More of a scripting question I guess.
From: "Jim C." <jconner@enterit.com>
Reply-To: jconner@enterit.com
To: Dan Nelson <dnelson@allantgroup.com>
Cc: "Jim C." <notjames@concon.homeip.net>,
	Michael E Mercer <mmercer@nc.rr.com>,
	Taylor Dondich <thexder@lvcm.com>, questions@FreeBSD.ORG
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On Thu, 2002-04-18 at 12:11, Dan Nelson wrote:
> In the last episode (Apr 18), Jim C. said:
> > On Thu, 2002-04-18 at 01:43, Dan Nelson wrote:
> > > In the last episode (Apr 18), Jim C. said:
> > > > On Wed, 2002-04-17 at 23:34, Dan Nelson wrote:
> > > > > In the last episode (Apr 17), Michael E Mercer said:
> > > > > > another way to do it is this with the same outcome:
> > > > > > 
> > > > > > while read address ; do
> > > > > >   somecommand $address
> > > > > > done < file.txt
> > > > > 
> > > > > But that puts the input file at the end of the command, which
> > > > > makes it harder to see where the input is coming from,
> > > > > especially if you have a lot of stuff in your loop.
> > > > 
> > > > But its still more resource intensive.  If you pipe a huge file
> > > > into while the machine has to store that file into memory before
> > > > processing it whereas with the redirection it simply gets
> > > > filtered straight into the while loop line by line.
> > > 
> > > No it doesn't; 
> > 
> > It does. (not significantly but it does)
> > 
> > > the 'read' command will pull one line at a time out of the pipe.
> >
> > Correct.  However, the pipe "fills up" before read starts to take lines
> > out of it.  This is where the resources get taken.
> 
> Pipes aren't infinite buffers.  The pipe fills up at 16k, and simply
> blocks cat until the script empties it a bit.  The resources are taken
> by the extra syscalls and the context switches between the shell and
> cat.
>  
> > Based on these tests I guarantee you the pipe is taking more system
> > resources (albeit not much) than the redirect is.
> 
> No argument there.  But I'd have switched to C long before trying to
> process a 15MB file line-by-line in a scripting language :)
> 
hahaha...yes.  I suppose so that would have been the best route to go in
such a situation.

> I think we've strayed into bikeshed territory anyhow.

indeed.  =P

- Jim

> 
> -- 
> 	Dan Nelson
> 	dnelson@allantgroup.com
> 



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