$ apt apt 1.6.12 (amd64) Usage: apt [options] command apt is a commandline package manager and provides commands for searching and managing as well as querying information about packages. It provides the same functionality as the specialized APT tools, like apt-get and apt-cache, but enables options more suitable for interactive use by default. Most used commands: list - list packages based on package names search - search in package descriptions show - show package details install - install packages remove - remove packages autoremove - Remove automatically all unused packages update - update list of available packages upgrade - upgrade the system by installing/upgrading packages full-upgrade - upgrade the system by removing/installing/upgrading packages edit-sources - edit the source information file See apt(8) for more information about the available commands. Configuration options and syntax is detailed in apt.conf(5). Information about how to configure sources can be found in sources.list(5). Package and version choices can be expressed via apt_preferences(5). Security details are available in apt-secure(8). This APT has Super Cow Powers.
How to list all installed packages
The apt tool on Ubuntu 14.04 and above makes this very easy.
apt list --installed