Howto control log output format

I'm currently getting what appears to be json output on a single line in my logstash log file (-l §LS_LOG_FILE), how can I control format for this output so it might be more readable or at least line separated?

Logstash's log format currently isn't configurable (and it's not JSON), but there's been some recent work in the context of the issue below so perhaps it'll be addressed in the next release.

Thanks for the Info, Magnus!