apt - How to list all installed packages

I'd like to output a list of all installed packages into a text file so that I can review it and bulk-install on another system. How would I do this?

The apt tool on Ubuntu 14.04 and above makes this very easy.

apt list --installed

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To get a list of packages installed locally do this in your terminal:

dpkg --get-selections | grep -v deinstall

^{(The -v tag "inverts" grep to return non-matching lines)}

To get a list of a specific package installed:

dpkg --get-selections | grep postgres

To save that list to a text file called packages on your desktop do this in your terminal:

dpkg --get-selections | grep -v deinstall > ~/Desktop/packages

Alternatively, simply use

dpkg -l

(you don't need to run any of these commands as the superuser, so no sudo or any other variants necessary here)