Chapter 6. Basics of the Debian package management system

30

Additionally, some packages are marked as Essential since they are absolutely necessary for

the proper functioning of the system. The package management tools will refuse to remove

these.

6.8 What is a Virtual Package?

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 should therefore satisfy any dependency of a program that required a news

reader on a system, in order to work or to be useful. They are therefore both said to provide

the  virtual package  called

news reader

.

Similarly,

smail

and

sendmail

both provide the functionality of a mail transport agent. They

are therefore said to provide the virtual package,  mail transport agent . If either one is in 

stalled, then any program depending on the installation of a

mail transport agent

will

be satisfied by the existence of this virtual package.

Debian provides a mechanism so that, if more than one package which provide the same

virtual package is installed on a system, then system administrators can set one as the pre 

ferred package. The relevant command is

update alternatives

, and is described further

in `Some users like mawk, others like gawk; some like vim, others like elvis; some like trn,

others like tin; how does Debian support diversity?' on page

57

.

6.9 What is meant by saying that a package Depends, Recommends,

Suggests, Conflicts, Replaces or Provides another package?

The Debian package system has a range of package  dependencies  which are designed to in 

dicate (in a single flag) the level at which Program A can operate independently of the existence

of Program B on a given system:

  Package A depends on Package B if B absolutely must be installed in order to run A. In

some cases, A depends not only on B, but on a version of B. In this case, the version

dependency is usually a lower limit, in the sense that A depends on any version of B

more recent than some specified version.

  Package A recommends Package B, if the package maintainer judges that most users would

not want A without also having the functionality provided by B.

  Package A suggests Package B if B contains files that are related to (and usually enhance)

the functionality of A.

  Package A conflicts with Package B when A will not operate if B is installed on the system.

Most often, conflicts are cases where A contains files which are an improvement over

those in B.  Conflicts  are often combined with  replaces .