problem searching packages

[Bernard <bdebreil@teaser.fr>]

Hi there !

Newcomer on Debian (former user of RedHat 7.2), I find the Debian packages
kind of hard to manage. With RedHat "rpm", I could find very easily
whether a package was installed or not, what was the version number etc..
With Debian, I have not yet found the way to go ; I suppose that I should
read a greater number of pages, but there is so much doc that it is not
easy to find the right one right away.

#apt-cache search package_name

gives me some information on said package, but not the version number. In
fact, I suspect that this command does not read in my local base, but in
the distant servers, so that I don't know if a given package is being
installed on my system or not.

For instance :

#apt-cache search hplip

lists a number of related packages that, I think, are supposed de operate
together... but this does not tell me whether hplip is installed or not in
my system. I think it is not installed, since

#man hplip

tells me that there is no manual entry for hplip.

In fact, hplip has been in turn installed, removed, re-installed and
re-removed on my system, so it should not be there at this time, except if
I got mixed up somewhere. I can't remember what hplip version I had, and I
don't plan to reinstall blindly as I had done before.

Now, if I try

#dpkg -I hplip

I get : dpkg-deb: unable to read the archive "hplip", no file or directory
of this type.

But, if I try the same command with "cups" or "gimp" as arguments, I get
the same kind of answers, despites that cups and gimp are properly
installed. I imagine that I should write, as an argument, not a guessed
package name such as 'gimp', but a real package name with a version number
and .deb in the end... but where am I to find such information on debian
packages ?

My first question will be :

How to check whether hplip (or any other package) is installed on my
system or not ?

The second question : how to investigate what packages are installed in my
system, with all informations (package name, version etc...)

Thanks in advance for any useful input