Two things in one: First of all, when using logging, use it to log events, not for user interfaces; second, log events in a machine readable way, not necessarily an human readable format.
For a long time, I used logs to show to the user what was happening. To me, it
was logical to use something where I could mark errors as errors, general
information as information and, if the user requested more information, print
more information on what was going on. So I just added logging, defined normal
info, errors as
errors, information that may help me find
debug and use only the logging system for all output of the
But that's not what logs are targeted for -- and now I'm having to rethink most of the stuff I already wrote.
Use the standard output to inform the user what's going on, in a human readable format; use the standard error output to inform the user when things go wrong; but use the logs to capture something that you'll have to process later, so you should probably use a format that it is easier to parse, even if it is not so friendly.
As an example, let's say you're connecting to a server. You could use the standard output to say "Connecting to server", to give the user a feedback about what's going on; at the same time, you could log "CONNECTION [SERVER]", with the IP/Name of the server you're connecting. Surely, the "CONNECTION" word is not that friendly to the user, but if you had to parse the line, it would be really easy, wouldn't it?
Another example: If your application is adding a record to the database, there is nothing wrong logging "ADDING_RECORD: field=value; field=value; field=value" 'cause, in case something goes wrong while saving the record, you could have the values to try to figure out why it failed -- surely, logging why it failed also helps, but you know what I mean. This is an example of something that makes complete sense in logs, but not in user interfaces.