Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!amdahl!pacbell!sactoh0!tree!stever
From: stever@tree.UUCP (Steve Rudek)
Newsgroups: comp.unix.questions
Subject: Re: Help with strings in Bourne shell
Summary: Questions on this file descriptor "slight of hand"
Message-ID: <275@tree.UUCP>
Date: 27 Apr 89 20:20:53 GMT
References: <10166@orstcs.CS.ORST.EDU> <1493@vsedev.VSE.COM> <870@marvin.Solbourne.COM>
Distribution: usa
Organization: TREE BBS (916)-349-0385 Sacramento, Ca
Lines: 36

In article <870@marvin.Solbourne.COM>, dce@Solbourne.COM (David Elliott) writes:
> 	exec 3<&0 0<"$INPUTFILE"
> 	while read DEFINITION
> 	do
> 		echo "$DEFINITION"
> 	done
> 	exec 0<&3
> 
> The first exec makes fd 3 a duplicate of fd 0 (stdin), and
> redirects stdin.  The second exec changes fd 0 back to what
> it was.
I've never seen anything quite like this.  It is beautiful, exciting and
rather bizarre; shell at its best! :-) I can guess that you're creating
a file descriptor 3 *strictly* to preserve the (address??) of stdin.  But the
0<"$INPUTFILE" bit boggles my mind.  How is this different from just <$INPUTFILE
I guess the leading "exec" is important in understanding this?  Could you
explain?

By the way, how would:
  while read DEFINITION <$INPUTFILE 
  do
  done

differ from: 
  while read DEFINITION
  do
  done <$INPUTFILE

Or wouldn't it?


If not too much trouble, I'd appreciate a *copy* of any responses via email
since I really have a considerable interest in the answer and fear I might
otherwise miss a newsgroup response.
----------
Steve Rudek  {ucbvax!ucdavis!csusac OR ames!pacbell!sactoh0} !tree!stever