2.2.7.
Virtual packages
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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.
