How to obtain list of installed packages

[Bret Busby]

Hello.

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.

Thank you in anticipation.

-- 

Bret Busby
Armadale
West Australia

..............

"So once you do know what the question actually is,
 you'll know what the answer means."
- Deep Thought,
 Chapter 28 of Book 1 of
 "The Hitchhiker's Guide to the Galaxy:
 A Trilogy In Four Parts",
 written by Douglas Adams,
 published by Pan Books, 1992

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