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.