Re: bsdgrep does not work with tail -f | grep combination

On Tue, 03 Aug 2010 20:21:56 +0200 Gabor Kovesdan <gabor_at_FreeBSD.org>  wrote:
> Em 2010.08.03. 19:25, poyopoyo_at_puripuri.plala.or.jp escreveu:
> > Hi,
> >
> > It seems bsdgrep does not work when piped from tail -f.
> > I'm running r210728.
> >
> > term0$ jot 10>  /tmp/1
> > term0$ tail -f /tmp/1 | grep 0
> > [no output]
> >
> > otherterm$ jot 10>>  /tmp/1
> > [no output to term0]
> >
> > =====
> >
> > with GNU grep:
> >
> > term0$ tail -f /tmp/1 | gnugrep 0
> > 10
> > otherterm$ jot 10>>  /tmp/1
> > [on term0]
> > 10
> > 10
> >    
> I've checked on 8.0 and GNU grep doesn't output anything either for me. 
> If you use tail -f, you will enter more lines and end it with EOF, won't 
> you? And then BSD grep will process the input and print out matches. I 
> don't think it's bad behaviour in itself but if you can explain why you 
> think it's bad I'm willing to change it.

This is more fundamental, not just limited to grep.  tail -f
never closes its stdout channel so the next process in the
pipeline will never seen an EOF on its stdin and must
continue processing its input. Try this:

    rm -f /tmp/1; touch /tmp/1
    tail -f /tmp/1 | cat &
    while sleep 1; do date >> /tmp/1; done

Notice how cat doesn't quit. In fact

    tail -f /tmp/1 | bsdgrep ''

must behave exactly the same as 

    tail -f /tmp/1 | cat

and so must this:

    tail -f /tmp/1 | cat | bsdgrep ''

bsdgrep when used this way doesn't quit but doesn't do
anything either (including printing what tail -f spits out
from existing file data). This is just a bug.