Shellscript to monitor a log file if keyword triggers then execute a command?

tail -fn0 logfile | \
while read line ; do
        echo "$line" | grep "pattern"
        if [ $? = 0 ]
        then
                ... do something ...
        fi
done

I also found that you can use awk to monitor for pattern and perform some action when pattern is found:

tail -fn0 logfile | awk '/pattern/ { print | "command" }'

This will execute command when pattern is found in the log. Command can be any unix command including shell scripts or anything else.

An even more robust approach is monit. This tool can monitor very many things, but one of them is that it will easily tail one or more logs, match against regex and then trigger a script. This is particularly useful if you have a collection of log files to watch or more than one event to trigger.