How to obtain list of installed packages

[Ralf Mardorf]

Hi Tom,

On Wed, 11 May 2016 12:06:29 +0200, Tom H wrote:
>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.

this isn't a good advice.

A pitfall could be that some packages are installed with the
recommended packages, but others were not installed with recommended
packages. This is important even when installing the same Ubuntu
release and fails when following your advice Tom.
Solvable by getting a list of all packages, including the automatically
installed packages and then installing this list with the
--no-install-recommends option. Changed hard dependencies anyway will
get resolved. Unlikely hat not needed, not automatically installed
packages still exist for another release, so your advice would install
more unneeded packages, than my advice. OTOH the problem with my advice
is

[root at moonstudio ~]# apt-get install --no-install-recommends pluma hdgsfsg
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package hdgsfsg
[root at moonstudio ~]# apt-get install --no-install-recommends --ignore-missing pluma hdgsfsg
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package hdgsfsg

a package that is missing for a new release, but this isn't an issue
when reinstalling the same release and even is an issue if official
Ubuntu repositories and /or third party repos dropped a packages that
were explicitly installed, which often happens.

I suspect this gets solved by running "dselect-upgrade".

Another pitfall could be, that by another Ubuntu release a package could
be split into two packages, that not necessarily are a recommended or
even suggested dependency. There's no solution to automatically solve
this issue. However, this rarely happens. I only noticed this once,
within a development release.

Regards,
Ralf