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"
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"
You can detect the presence of a given virtual package by using apt-cache showpkg <virtual> to display a list of candidate packages, and then dpkg -l <canddate> <candidate> ... to display the installation status of the candidates.
For example:
$ apt-cache showpkg awk
$ dpkg -l original-awk mawk gawk
Here is a full printout:
$ apt-cache showpkg awk
Package: awk
Versions:
Reverse Depends:
base-files,awk
base-files,awk
Dependencies:
Provides:
Reverse Provides:
mawk:i386 1.3.3-17ubuntu2
gawk:i386 1:4.0.1+dfsg-2.1ubuntu2
original-awk 2012-12-20-1
mawk 1.3.3-17ubuntu2
gawk 1:4.0.1+dfsg-2.1ubuntu2
$ dpkg -l original-awk mawk gawk
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-==============-============-============-=================================
ii gawk 1:4.0.1+dfsg amd64 GNU awk, a pattern scanning and p
ii mawk 1.3.3-17ubun amd64 a pattern scanning and text proce
dpkg-query: no packages found matching original-awk
I believe dpkg and apt-cache are lower level tools than aptitude, grep-available and grep-status. Using lower level tools may or may not be a benefit, depending upon your particular requirements.
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 $?
