I am running Ubuntu 12.10 and, if possible, I would like to get the list of configuration files available for a given package that is already installed.
I would prefer to do this using dpkg or apt if possible, but no problem if other tools such as aptitude or dselect are needed/recommended.
There's no need to use anything other than cat, if I understood your intention correctly:
cat /var/lib/dpkg/info/<package>.conffiles
should give you what you're after. For instance for package zsh:
% cat /var/lib/dpkg/info/zsh.conffiles
/etc/zsh/zlogin
/etc/zsh/zlogout
/etc/zsh/zprofile
/etc/zsh/zshenv
/etc/zsh/zshrc
/etc/zsh/newuser.zshrc.recommended
Regarding a case where there's no such file for a given package - it's up to the package maintainer to designate certain files as configuration. If this hasn't been done properly, you should file a bug where appropriate.
In such cases you have a couple of options.
List files belonging to the package that are in /etc/:
dpkg -L package | grep '/etc'
Fetch and inspect the source package to find out how it was compiled (which should also show you where it expects its configuration files to be located).
apt-get source package
less package-x.y.z/debian/rules
Look up the upstream project page to find documentation.
Let's for example test the package apt to get the config file(s).
It can be tricky to understand what .conffiles you should check to see the info so I suggest using grep to find the clue.
locate *.conffiles | grep apt
/var/lib/dpkg/info/apt-config-icons.conffiles
/var/lib/dpkg/info/apt.conffiles
/var/lib/dpkg/info/aptdaemon.conffiles
/var/lib/dpkg/info/apturl-common.conffiles
/var/lib/dpkg/info/libatk-adaptor:amd64.conffiles
And to cat any of these in particular if you are interested according to Marcin's Kaminski answer.
Another trick is to read the manual, for instance man apt will lead you to SEE ALSO section from where you can call man apt.conf where you will see the location of the config file for apt in this case: /etc/apt/apt.conf.
However, config file /etc/apt/apt.conf may not even exist. Be aware of that when searching for the config files.
In Linux config files should be inside the /etc/ directory. You may use dpkg-query -L your_package | grep etc to list all package files and directories inside the /etc/ directory.
How about dpkg -S [package-name]? Cant see it being mentioned here.
root@homehub:/# dpkg -S nginx
nginx-common: /usr/share/nginx/html
nginx-full: /usr/share/man/man8/nginx.8.gz
libnginx-mod-http-xslt-filter: /usr/share/doc/libnginx-mod-http-xslt-filter/copyright
nginx-common: /etc/logrotate.d/nginx
libnginx-mod-mail: /usr/share/doc/libnginx-mod-mail/changelog.gz
libnginx-mod-http-upstream-fair: /usr/share/doc/libnginx-mod-http-upstream-fair
libnginx-mod-http-auth-pam: /usr/share/doc/libnginx-mod-http-auth-pam/changelog.Debian.gz
libnginx-mod-http-image-filter: /usr/lib/nginx/modules/ngx_http_image_filter_module.so
nginx-common: /etc/nginx/snippets
libnginx-mod-mail: /usr/share/doc/libnginx-mod-mail/copyright
nginx-common: /etc/nginx/snippets/fastcgi-php.conf
libnginx-mod-http-xslt-filter: /usr/share/doc/libnginx-mod-http-xslt-filter/changelog.Debian.gz
nginx-full: /usr/share/doc/nginx-full/copyright
nginx-common: /usr/share/doc/nginx-common
nginx-common: /etc/ufw/applications.d/nginx
libnginx-mod-http-upstream-fair: /usr/share/doc/libnginx-mod-http-upstream-fair/changelog.gz
libnginx-mod-stream: /usr/share/nginx/modules-available/mod-stream.conf
nginx-common: /etc/nginx/sites-enabled
libnginx-mod-http-upstream-fair: /usr/share/doc/libnginx-mod-http-upstream-fair/copyright
libnginx-mod-http-subs-filter: /usr/share/doc/libnginx-mod-http-subs-filter/changelog.gz
libnginx-mod-http-dav-ext: /usr/share/nginx/modules-available/mod-http-dav-ext.conf
nginx-common: /etc/nginx/koi-utf
nginx-common: /etc/nginx/mime.types
nginx-common: /usr/share/vim/addons/syntax/nginx.vim
nginx-common: /usr/share/doc/nginx-common/NEWS.Debian.gz
nginx-common: /usr/share/doc/nginx-common/README.Debian
nginx-full: /usr/share/doc/nginx-full
libnginx-mod-mail: /usr/share/nginx/modules-available/mod-mail.conf
libnginx-mod-http-auth-pam: /usr/share/doc/libnginx-mod-http-auth-pam/changelog.gz
nginx: /usr/share/doc/nginx
nginx-common: /etc/nginx/win-utf
libnginx-mod-http-dav-ext: /usr/share/doc/libnginx-mod-http-dav-ext
....
And sometimes depending on filename structure it might be good if piped out to grep names containing ".conf":
root@homehub:/# dpkg -S nginx |grep ".conf"
nginx-common: /etc/nginx/snippets/fastcgi-php.conf
libnginx-mod-stream: /usr/share/nginx/modules-available/mod-stream.conf
libnginx-mod-http-dav-ext: /usr/share/nginx/modules-available/mod-http-dav-ext.conf
libnginx-mod-mail: /usr/share/nginx/modules-available/mod-mail.conf
libnginx-mod-http-auth-pam: /usr/share/nginx/modules-available/mod-http-auth-pam.conf
nginx-common: /etc/nginx/fastcgi.conf
nginx-common: /etc/init/nginx.conf
nginx-common: /etc/nginx/conf.d
libnginx-mod-http-subs-filter: /usr/share/nginx/modules-available/mod-http-subs-filter.conf
libnginx-mod-http-geoip: /usr/share/nginx/modules-available/mod-http-geoip.conf
libnginx-mod-http-echo: /usr/share/nginx/modules-available/mod-http-echo.conf
libnginx-mod-http-upstream-fair: /usr/share/nginx/modules-available/mod-http-upstream-fair.conf
nginx-common: /etc/init/nginx.conf
libnginx-mod-http-xslt-filter: /usr/share/nginx/modules-available/mod-http-xslt-filter.conf
nginx-common: /etc/nginx/snippets/snakeoil.conf
nginx-common: /etc/nginx/nginx.conf
libnginx-mod-http-image-filter: /usr/share/nginx/modules-available/mod-http-image-filter.conf
