How to check whether hplip (or any other package) is installed on my system or not ?
dpkg -l | grep packagename
The second question : how to investigate what packages are installed in my system, with all informations (package name, version etc...)
dpkg -l :) There are graphical frontends, if you prefer those. Look for synaptic. Regards. -- http://arhuaco.org http://emQbit.com