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From: wallace@ynotme.enet.dec.com (Ray Wallace)
Newsgroups: comp.unix.shell
Subject: Re: cat, pipes, and filters
Message-ID: <1991May31.204314.27493@engage.pko.dec.com>
Date: 31 May 91 20:32:07 GMT
Sender: newsdaemon@engage.pko.dec.com (USENET News Daemon)
Organization: Digital Equipment Corporation
Lines: 22


In article <1991May31.165446.1530@progress.com>, root@progress.COM (Root of all Evil) writes...
>   I've got a question regarding the way cat behaves in a pipeline.
> 
>   cat $FILE | sed s/"$ENTRY"/"$NEWENTRY"/ > $FILE
>Both command produce identical results: $FILE is truncated to 0-length.
One of the first things the shell does when parsing the line, is to handle I/O
redirection. So before any of the commands are executed the ">$FILE" part of
the line causes the shell to create an empty file which just happens to be
the file that you are trying to read (cat).

>   Any enlightenment would be appreciated.  Also, if you can think of
>a better way to do the same thing (short of using perl), please let
  cat $FILE | sed s/"$ENTRY"/"$NEWENTRY"/ > ${FILE}.new ; mv ${FILE}.new $FILE
is a different way, not neccessarily a better way.

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