Problem:

How to know if a app is installed in my computer, using Ubuntu.

Solution:

To solve it, you should get installed app list from both package managers: snap and apt.

Step by step:

  • Using snap: 
    snap list
  • Using apt: 
    apt list --installed
  • For older versions of Ubuntu 14.04: 
    dpkg --get-selections | grep -v deinstall

    And list packages for a specific package; this example is for the postgres package:

    dpkg --get-selections | grep postgres

Source:

https://manpages.ubuntu.com/manpages/noble/man8/apt.8.html
https://askubuntu.com/questions/17823/how-to-list-all-installed-packages