How can I know if a virtual package is "installed" on a Debian system?
To list packages providing mail-transport-agent:
$ aptitude search '~Pmail-transport-agent'
p citadel-mta - complete and feature-rich groupware server
p courier-mta - Courier mail server - ESMTP daemon
p dma - lightweight mail transport agent
p esmtp-run - user configurable relay-only MTA - the reg
p exim4-daemon-heavy - Exim MTA (v4) daemon with extended feature
p exim4-daemon-light - lightweight Exim MTA (v4) daemon
p masqmail - mail transport agent for intermittently co
p msmtp-mta - light SMTP client with support for server
p nullmailer - simple relay-only mail transport agent
i postfix - High-performance mail transport agent
p sendmail-bin - powerful, efficient, and scalable Mail Tra
p ssmtp - extremely simple MTA to get mail off the s
p xmail - advanced, fast and reliable ESMTP/POP3 mai
Make that aptitude search '~Pmail-transport-agent ~i' to only list installed packages (if any).
To list all virtual packages provided by currently installed packages:
aptitude search '~Rprovides:~i ~v'
See the aptitude manual for an explanation of the search patterns.
the problem here is that there is a subtle difference between virtual packages and packages provided by other packages.
the difference is, that a package may provide a real package as well, not only a virtual package.
anyhow, the following will search for all packages that provide a package and will print both the package name and the packages it provides:
grep-available -sPackage -sProvides -FProvides -e '^.'
to find whether any package is installed on your system that provides a given one (e.g. mail-transport-agent), use
grep-status -sPackage -sProvides -FProvides "mail-transport-agent"