Monitor the rate of log file writes matching a pattern - linux

Question

I have a timestamped log file that I am watching. Every so often the log writes a message, "OK"
How could I capture the number of "OK"s written per second? E.g. output like
10.5 OK/sec
9 OK/sec
20 OK/sec
The log is pretty big, its not practical to do a grep on the entire file. I started with
tail -f my.log | grep OK
which is helpful but doesn't quite get me the rate.
Thanks!
First solution below is great (and other is good too), I altered it a bit and this is what I came up with
tail -f my.log | perl -lne 'if(/(\d+:\d+:\d+).*OK/) { print $1; }'| uniq -c

Answer 1

When your log is in form like:
Wed Jul 10 22:44:00 CEST 2013 OK
Wed Jul 10 22:44:00 CEST 2013 OK
Wed Jul 10 22:44:00 CEST 2013 OK
Wed Jul 10 22:44:00 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
Wed Jul 10 22:44:01 CEST 2013 OK
so, in form timestamp OK, you can try the simple:
tail -f logfile | uniq -c
and you will get the next result:
4 Wed Jul 10 22:44:00 CEST 2013 OK
8 Wed Jul 10 22:44:01 CEST 2013 OK
^
+---- number of OK in the same timestamp

Answer 2

Here's a quick script to do it:
#! /bin/bash
log="/var/log/messages"
seconds="1"
filter="OK"
while true; do
echo "$(timeout $seconds tail -n0 -f $log|grep $filter|wc -l) OK/sec"
done
There's a bit of a race condition where it might lose one or two lines during the loop, but it is unlikely. Also, not every Linux distro has timeout installed by default, so you might want to check.