I have a backup of my Ubuntu computer stored on my iMac. My hard disk died and I want get a list of the packages that was installed by just reading from the file system and without running commands like
You can do it by reading
/var/lib/dpkg/status from your backup.
In my running Ubuntu:
grep '^Package:' /var/lib/dpkg/status | cut -f2 -d' '
grep "Package" /var/lib/dpkg/status > Package.list should do the trick