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Re: Possible bug with grep/sed/tail?

From: Paolo Bonzini
Subject: Re: Possible bug with grep/sed/tail?
Date: Thu, 20 Nov 2008 17:01:00 +0100
User-agent: Thunderbird (Macintosh/20081105)

> A) # tail trace.txt | grep "com"
> - WORKS: produces output
> B) # tail trace.txt | grep "com" | cat
> - WORKS: produces output
> C) # tail -f trace.txt | grep "com"
> - WORKS: produces output, then waits and reports new lines
> D) # tail -f trace.txt | grep "com" | cat
> - FAILS: no output from existing lines, never gets new data
> To me, it seems completely counterintuitive that A, B, and C would
> work, but D does not.  Each line of input read by tail should be
> passed to STDOUT, which is then read as STDIN by grep/sed, then passed
> to STDOUT and read by cat.  It should not matter if tail is "done"
> reading the output or not, as clearly that works fine in case C.

Buffering occurs line by line in cases A and C, in bigger blocks in
cases B and D.  So the data is stuck in grep (or sed's) buffers until
enough of it is produced.  If it is never produced, it is stuck unless
sed/grep see an end-of-file condition on stdin -- which they do with
tail, but not with tail -f.


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