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Debian GNU/Linux Reference Guide
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2.2.7 Virtual packages

A virtual package is a generic name that applies to any one of a group of packages, all of which provide similar basic functionality. For example, both the tin and trn programs are news readers, and either one should therefore satisfy the need of a program that requires a news reader on the system in order to be useful. They are therefore both said to Provide the "virtual package" called news-reader.

Similarly, many packages such as exim, exim4, sendmail, and postfix, provide the functionality of a mail transport agent. They are therefore said to Provide the virtual package mail-transport-agent. If either one is installed, then any program that Depends on the installation of a mail transport agent will be satisfied by the existence of this virtual package.

Debian has a mechanism such that, if more than one package which Provides the same virtual package is installed on a system, the system administrator can set one as the preferred package. The relevant command is update-alternatives, and is described further in Alternative commands, Section 6.5.3.

Debian GNU/Linux Reference Guide
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  Published under the terms of the GNU General Public License Design by Interspire