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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