X-Spam-Check-By: sourceware.org
Subject: Re: Piping to the 'read' command
To: cygwin AT cygwin DOT com
Date: Fri, 27 Oct 2006 01:43:08 -0600 (MDT)
CC: Jim Easton <jim AT cs DOT ualberta DOT ca>
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Message-Id: <20061027074309.0FFFE5663C@erskine.cs.ualberta.ca>
From: jim AT cs DOT ualberta DOT ca (Jim Easton)
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Hi,

Fri, 27 Oct 2006 Ian Taylor wrote:
> Can anyone explain what is happening here (using pdksh as my shell) when
> I try to set an environment variable using 'read':
...
> 
> This doesn't work:
> $ echo Test2 | read VAR2
> $ echo "VAR2 is $VAR2"
> VAR2 is
> 
> This works within the 'while' loop only:
> $ echo Test3 | while read VAR3
> > do
> > echo "VAR3 is $VAR3"
> > done
> VAR3 is Test3
> $ echo "VAR3 is $VAR3"
> VAR3 is

Hmm, I don't know but I did make a couple of observations:

This check this out:

$ echo Test2 | { read VAR2
> echo "VAR2 is $VAR2"
> }
VAR2 is Test2
$ echo second time VAR2 is $VAR2
second time VAR2 is
$ 

I also noticed in your example line 2, (the one that reads;
	$ echo "VAR2 is $VAR2")
you are being prompted with a "$" rather than a ">".  Futhermore,
VAR2 is unknown after the closing "}".

I get the same result if the commands are in a script (See below)
and on cygwin, solaris and linux.

This suggests to me that it is executing that read in a subshell that
can't pass the variable back to its parent.  This dispite the fact that
it appears to be the same process.  (see inserted echo $$)

I hope this is of some help. :-)

	Jim Easton


#!/bin/bash

echo $$
echo Test2 | { read VAR2
echo $$
echo "VAR2 is $VAR2"
}
echo second time VAR2 is $VAR2

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