Re: "Tailing" grep (Was: cat and grep without destroying file)

On Sun, 2007-09-30 at 19:37 -0500, Brian Fahrlander wrote:
> Joel Goguen wrote:
> > On Sun, 2007-09-30 at 17:04 -0500, Brian Fahrlander wrote:
> >> While we've got the attention of the most-able on this subject, I'd
> >> like to reopen a discussion about a very similar tool: a 'grep' that
> >> happens on tailing a file. There have been dozens of times that I'm
> >> looking for a message number in /var/log/mail.log and would love to see
> >> only the lines mentioning that message.
> >>
> >> A couple of years ago I asked and got a "maybe you could write
> >> something", but I actually never could. It's not as easy as it looks.
> >>
> >> Any chance you guys have seen this done?
> >>
> > I have a great deal of success with 'tail -f /var/log/mail.log | grep
> > XX' where XX is whatever you're looking for. I actually use that exact
> > set of commands on a daily basis when I'm at work.
>
> Wow; that's strange- am I dreaming, or is that a different
> behaviour than before?
>

I don't know if it's different from any previous version, but it makes perfect sense to me that it would work this way. 'tail' sends its output to the standard output stream, which I'm piping through to 'grep'. If I don't give a file to 'grep' (which would be the second parameter that isn't there in this example) it listens on its standard input stream for input instead of reading a file, and here the standard input stream is being redirected to be the output from from 'tail'. 'grep' will listen until the standard input stream closes, which it doesn't until it encounters a 'Ctrl+D' character, 'tail' exits, or you press Ctrl+C to stop execution.

Of course, this halts execution of a script until 'grep' exits, so it's not useful if you want to run it in a script. You could background it I suppose...

-- 
Joel Goguen
http://jgoguen.net/
The human mind treats a new idea the way the body treats a strange
protein -- it rejects it.  -- P. Medawar

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