How can I extract certain lines from a logfile (txt)

You can have a process that runs after the workspace from your log is finished, that reads this log as textfile and use a tester to look for lines with the Errors and Warns (Using "Contains") that you want to filter then save it.

Or a LogMessageStreamer in your workspace, then select the TAG that you want and save it. You can be more specific using a tester using "Contains" in the log message after the TAG selection.

FME has the option to filter log messages.

Have a look at the options menu in FME Desktop:

Using the python shutdown script is the way I go about parsing the log files.

Thanks for the quick reply everybody!

@rodrigo_ferrao I think I will go with your first option. I was trying with the stringsearcher before but I kept getting stuck building the regular exporession.

@erik_jan Thanks for the answer. But the process is not runing in FME (server). It's a totally different process and by scanning that log we hope to get a better view on things when it goes wrong. (kind of a black box right now)

@warrengis thanks for the answer: I'm almost there using the tester transformer but this info can get in handy another time!