For this to work, you need to have the sudo package installed and configured. On the other hand, you can just start aptitude as a normal user and it will ask for the root user's password when it's time to do some system-wide changes. This is pretty cool.
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Install reccommended pakages automatically
Heh, that's the easy way.
If you start aptitude as root, the changes will be written to /root/.aptitude/config but if you start it as a normal user, then the changes will go to ~/.aptitude/config . Person'lly, I like to keep all APT related configurations in just one place: /etc/apt/apt.conf .
Aptitude has an extensive documentation in /usr/share/aptitude/README .