Is this info part of APT cache or does it create a separate file to compare with new cache? Where is it stored then?
If I'm not mistaken the files you are looking for are:
This is a local package database for binary and source packages respectively. It is updated every time you run
To extract the information from the apt cache, you use
will list the complete package information.
Synaptic is essentially a GUI wrapper around apt, so its apt that is keeping track of the repository and package information and Synaptic is querying it.
apt-cache wrapped in bash scripts is probably the easiest way to do something similar on your own. Synaptic uses libapt-pkg* via C++, but there are also libraries for Perl and Python.
Finding new packages in aptitude
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