Why not have the grepper listen on the log file directly? You'll need to have a timestamp in the log and know where the log is, but those can be known at the time of writing the service script.
There's no such thing as "the log file". There's the log backendS, which can be one or more automatically rotated log files in a directory, status files, network backends, etc. The only way to access the log stream in a reliable and generic way is to read it on the way to the backends. You have no control over what happens later on. -- Laurent
