The command-line client is typically invoked by just running
I would suggest you look at the option reference to get a list of options as the first thing you do. It's easy:
This will always write out a new file. I strongly suggest using
Now nothing will be written to stdout (thus myfile_pretty.css will have zero bytes) when there are errors or warnings.
There are a lot of warnings in the tool. I welcome additions, in case you want to help other web developers from making the same mistakes. Also, some things can get automatically corrected and perhaps you'll just accept the changes and not worry about seeing the warnings.
This is very similar to the command that my work is currently using when pretty printing their CSS files. Their target is IE8 (thus we don't care about IE7), let the pretty printer autocorrect things, ignore the W3C recommendation about relative units and ignore the unofficial things that browsers understand but are not in the CSS spec.
This example shows you how to ignore various codes, and some of those codes have associated values. For instance, you can use
A list of warning and error codes is available at Error/Warning Codes.