possible false positive in package_check

[keegoid]

Premise: It's better to check if a package doesn't exist, because a false positive here does no harm.

commit comment

use apt-cache policy instead of dpkg -s to check if pkg is NOT installed
sometimes dpkg -s will find Version information for packages that have
been removed, resulting in a false positive

reasoning

1. If we check that a package exists and get a false positive, that package will never get installed.
2. If we check that a package doesn't exist and get a false positive, apt-get will attempt to install it and notice that it is already installed.

example

remove virtualbox-5.0 and check if it still exists

sudo apt-get remove virtualbox-5.0
dpkg -s virtualbox-5.0 2>&1 | grep 'Version:'

This results in a false positive due to 'Version:' being found in several places.

Version: 5.0.14-105127~Ubuntu~trusty
Config-Version: 5.0.14-105127~Ubuntu~trusty
Python-Version: 2.7

check with apt-cache policy instead

apt-cache policy virtualbox-5.0

The result is not a false positive.

virtualbox-5.0:
  Installed: (none)
  Candidate: 5.0.14-105127~Ubuntu~trusty

[keegoid]

Perhaps a combination of the two will work better.

if [ "$(dpkg -s ${1} 2>&1 | grep 'Version:')" ]; then
   [ -n "$(apt-cache policy ${1} | grep 'Installed: (none)')" ] && return 0 || return 1
else
   return 0
fi

[jeremyfelt]

Thanks for the excellent pull request, @keegoid. This makes sense.