What is the difference between apt-cache pkgnames and dpkg --list?

Question

After reading this question, I'm still confused about two other commands, namely apt-cache pkgnames and dpkg --list. apt-cache pkgnames doesn't return the same list as apt list, a command mentioned in the other question. Could you tell me what the difference between the three is?

Answer 1

apt-cache pkgnames is designed for use in tab-completion: it quickly generates a list of all the package names apt knows about for the system’s main architecture (whether installed, available remotely, or simply mentioned in some other package’s relationships). It’s not intended for direct human consumption, and doesn’t spend much time filtering, so it lists packages other commands won’t necessarily show (such as virtual packages).

dpkg --list lists all the packages dpkg knows about, along with their status, description, and version; dpkg doesn’t know about remote packages, so this mostly lists packages which are installed or removed (but not purged).

apt list lists all the packages apt knows about, along with the repositories they’re available in, whether they’re installed, the available versions, and whether they’re upgradable. It shares the same basic list of packages as apt-cache pkgnames, but it includes any configured architecture; for example on typical multiarch amd64/i386 setups, apt-cache pkgnames won’t list i386-only packages, but apt list will.

dpkg --list and apt list support various options and filters to limit what they display.