As soon as I run this function I lose both logs in that file and on Stdout ...

I think these are actually 3 separate problems.

• The first is that os.Create() truncates the file. You can use os.OpenFile() to open for writing without truncate. To resolve this on the sawmill side, I could add a helper function to the handler/writer package. Such as writer.NewFileAppender(path string, mode int). Which would open the file for appending, creating it only if it doesn't exist.
• Then, the bufio.NewWriter() is creating an async write buffer, that doesn't get flushed automatically. So unless an explicit call to logfileWriter.Flush() is made, log entries may be missing. For this, I might be open to adding a Stop() method to the handler interface, this could be called when a handler is removed from the logger (logger is shut down). The handler/writer package would then make sure Close() is called on the underlying writer which would flush the buffer.
• In regards to losing STDOUT, sm.NewLogger() creates a completely empty logger. It doesn't output to STDOUT or anything. If you call InitStdStreams(), that will set up STDOUT/STDERR on the logger.

Lastly, I can add some more examples which show use cases such as creating a custom logger which writes to multiple destinations.

I'll take a look at these things this weekend (maybe sooner if I have some time).

And thank you for the feedback :-)

Just an FYI, #58 was merged which adds an append helper.

I have an idea how I'm going to do the close/flush thing, but I want to give it some more thought as it might be a significant design change.