Ceph » RADOS

as for the mysterious newline in front of the json output, here's the root cause from src/ceph.in:1275

            # hack: old code printed status line before many json outputs
            # (osd dump, etc.) that consumers know to ignore.  Add blank line
            # to satisfy consumers that skip the first line, but not annoy
            # consumers that don't.
            if parsed_args.output_format and \
               parsed_args.output_format.startswith('json'):
                print()

this newline is present in all cases when --format=json* is given. We will need a separate ticket if we want to get rid of this.