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Some packages, have a Provides: header. For example, the package postfix on Debian Wheezy has Provides: mail-transport-agent.

The package mail-transport-agent which doesn't exist physically is considered as a "virtual" package.

How can I know, on a Debian based system, if a given virtual package is "installed" (or "provided")? Can I list every "provided" virtual package?

Hint: to list every existing virtual package, installed or not, do: aptitude search "~v"

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I don't have a debian at the moment, but wouldn't a dry-run install do the job? – jthill Nov 23 '13 at 0:45
up vote 10 down vote accepted

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.

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Accepting this answer because is uses standard aptitude. I like the explaination of @umläute though. – Totor Dec 31 '13 at 14:41
On Debian Jessie, I just tried aptitude search '~Pmysql-server ~i' which gives mariadb-server-10.0 and mariadb-server-core-10.0, but when I do aptitude search '~Rprovides:~i ~v', I do not see mysql-server in the list... However, I do see virtual-mysql-client, virtual-mysql-client-core, virtual-mysql-server and virtual-mysql-client-core in the list. – Totor Jun 9 at 15:14

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"
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Note: the commands grep-available and grep-status are available once you installed the dctrl-tools package. – Totor Oct 17 '13 at 14:17

Ok, virtual packages are alias name for a real package, or may be can say is a pointer to a package: mail-transport-agent : MTA , You can install postfix, sendmail , exim4 or every MTA for it.just one, They have conflict.

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This doesn't answer the question: OP is looking for a way to list all "provided" packages. Please use comments to clarify your understanding of what the questioner is seeking before answering. – jasonwryan Oct 17 '13 at 0:18

You can get all of the virtual packages from the AUTHORITATIVE LIST OF VIRTUAL PACKAGE NAMES. As for determining if a virtual package is installed I use dpkg -l and the fact that it has an exit code of 0 if the package is installed and an exit code of 1 if it is not installed.

dpkg -l mail-transport-agent &> /dev/null; echo $?
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No, this doesn't work. dpkg -l mail-transport-agent can return 0 even if there is no package providing it that is currently installed. I think it's based on presence in /var/dpkg/lib/available but I'm not sure. I did check that dpkg -l foo returns 1 for a package that I've never installed but 0 for a package that I just purged. – Gilles Oct 17 '13 at 10:17

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