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From: root@progress.COM (Root of all Evil)
Newsgroups: comp.unix.shell
Subject: cat, pipes, and filters
Summary: redirecting file to itself
Keywords: cat pipe filter
Message-ID: <1991May31.165446.1530@progress.com>
Date: 31 May 91 16:54:46 GMT
Sender: usenet@progress.com (Mr. Usenet)
Organization: Progress Software Corp.
Lines: 36
Nntp-Posting-Host: ericf


Hi,

   I've got a question regarding the way cat behaves in a pipeline.
(I know, his fur gets all oily %+})  Can I cat the contents of a
file | pipe the output to a filter (such as sed) | then cat the
filtered output back to the original file?  I've tried this with the
following commands:

   cat $FILE | sed s/"$ENTRY"/"$NEWENTRY"/ > $FILE

   cat $FILE | sed s/"$ENTRY"/"$NEWENTRY"/ | cat > $FILE

Both command produce identical results: $FILE is truncated to 0-length.

   However, the following command gives me the result I want:

   cat $FILE | sed s/"$ENTRY"/"$NEWENTRY"/ | tee $FILE 1>/dev/null

So now my script works but I don't really understand why.  I've tested
this on SunOs 4.1 and Interactive SysV 2.2, no difference.  Is there
something simple about pipes or I/O redirection that I'm not grasping?
Or is this a feature of 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
me know.

                              Curious Rich

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Rich Lenihan                 UUCP: mit-eddie!progress!rich
Progress Software Corp.      Internet: rich@progress.com
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