bsdgrep does not work with tail -f | grep combination

jhell jhell at
Wed Aug 4 14:26:36 UTC 2010

On 08/03/2010 14:21, Gabor Kovesdan wrote:
> Em 2010.08.03. 19:25, poyopoyo at 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 a common functionality gnu-grep. tail -f never exits and grep 
keeps grepping until it gets a EOF which is never hit unless you ^C.

A good example for such a use is monitoring a all.log log while looking 
for non-exact situations. something like

% tail -f all.log |egrep -v "(sendmail|sm-mta|cron)"

which would remove all lines that contain sendmail sm-mta & cron from 
the output and continue to read output from tail -f until it is ^C.

You can turn on your all.log through /etc/syslog.conf after creating the 
mode 600 file under /var/log for toying with.

There is quite a few other cases but I don't think I need to mention 
them. I rely on this for continuous firewall log trolling.

No offense but If the functionality exists in gnu-grep then the same 
functionality needs to exist in bsd-grep, ``period''. At least for the 




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