You could use a free-as-in-freedom app on your smartphone like: https://github.com/Freeyourgadget/Gadgetbridge/

GadgetBridge is the only reason I haven't canceled my Pebble Time Steel order. Hopefully I'll have time to help out with the project once my watch arrives.

Nothing for Unix systems ? I would not want to use Android for this kind of things either :/

You mean for Unix phones (a la Ubuntu Phone)? Or you want to tether it to your desktop/laptop? Either way the Gadgetbridge source should provide a good starting point if no existing software is available.

Ubuntu Phone could be OK as well, but I'd prefer to tether to my laptop for example, with command line tools if possible.