Chapter 7. Declaring relationships between packages

48

When selecting which level of dependency to use you should consider how important the

depended on package is to the functionality of the one declaring the dependency. Some pack 

ages are composed of components of varying degrees of importance. Such a package should

list using

Depends

the package(s) which are required by the more important components. The

other components' requirements may be mentioned as Suggestions or Recommendations, as

appropriate to the components' relative importance.

7.3 Conflicting binary packages  

Conflicts

When one binary package declares a conflict with another using a

Conflicts

field,

dpkg

will

refuse to allow them to be installed on the system at the same time.

If one package is to be installed, the other must be removed first   if the package being installed

is marked as replacing (see `Overwriting files and replacing packages  

Replaces

' on the

facing page) the one on the system, or the one on the system is marked as deselected, or both

packages are marked

Essential

, then

dpkg

will automatically remove the package which

is causing the conflict, otherwise it will halt the installation of the new package with an error.

This mechanism is specifically designed to produce an error when the installed package is

Essential

, but the new package is not.

A package will not cause a conflict merely because its configuration files are still installed; it

must be at least half installed.

A special exception is made for packages which declare a conflict with their own package

name, or with a virtual package which they provide (see below): this does not prevent their

installation, and allows a package to conflict with others providing a replacement for it. You

use this feature when you want the package in question to be the only package providing some

feature.

A

Conflicts

entry should almost never have an  earlier than  version clause. This would

prevent

dpkg

from upgrading or installing the package which declared such a conflict until

the upgrade or removal of the conflicted with package had been completed.

7.4 Virtual packages  

Provides

As well as the names of actual ( concrete ) packages, the package relationship fields

Depends

,

Recommends

,

Suggests

,

Enhances

,

Pre Depends

,

Conflicts

,

Build Depends

,

Build Depends Indep

,

Build Conflicts

and

Build Conflicts Indep

may men 

tion  virtual packages .

A virtual package is one which appears in the

Provides

control file field of another package.

The effect is as if the package(s) which provide a particular virtual package name had been

listed by name everywhere the virtual package name appears. (See also `Virtual packages' on

page

14

)

If there are both concrete and virtual packages of the same name, then the dependency may

be satisfied (or the conflict caused) by either the concrete package with the name in question