How to obtain list of installed packages

[Tom H]

On Tue, May 10, 2016 at 8:16 PM, Bret Busby <bret.busby at gmail.com> wrote:
>
> I believe that this question gets asked from time to time, but I do
> not know that a solution has been found.
>
> The question is this; does a utility exist, that can be ran, to
> produce a list of installed packages, in a way that can be used by a
> user (system administrator), to perform a system rebuild, so as to,
> after installing the system again, ensure that all of the optional
> packages that were previously installed, are again installed in the
> new system installation?
>
> I am running UbuntuMate 15.10, on a system with other Linux operating
> system versions (which are inappropriate for the hardware), that are
> not used, installed, and, with wanting to eliminate them, and,
> repartition the HDD, for a number of reasons, including occasionally
> seeing an error message that states that the partitioning is out of
> alignment, or something (I am not sure of the exact message, or, how
> to reproduce the error message), I believe that a new installation of
> the operating system, is needed, but, I want to again, have all of the
> existing installed optional packages, installed and operable.
>
> So, I figure that, if a utility exists, that can produce a list of the
> installed optional packages, that could help me to reproduce the
> system, with its current functionality, in terms of the currently
> installed, optional packages.

The best answer's the one that proposed an aptitude search because it
returned the list of packages that were installed specifically rather
than automatically.