Chapter 7. Declaring relationships between packages

47

For this reason packages in an installation run are usually all unpacked first and all config 

ured later; this gives later versions of packages with dependencies on later versions of other

packages the opportunity to have their dependencies satisfied.

The

Depends

field thus allows package maintainers to impose an order in which packages

should be configured.

The meaning of the five dependency fields is as follows:

Depends

This declares an absolute dependency. A package will not be configured unless all

of the packages listed in its

Depends

field have been correctly configured.

The

Depends

field should be used if the depended on package is required for the de 

pending package to provide a significant amount of functionality.

The

Depends

field should also be used if the

postinst

,

prerm

or

postrm

scripts re 

quire the package to be present in order to run. Note, however, that the

postrm

cannot

rely on any non essential packages to be present during the

purge

phase.

Recommends

This declares a strong, but not absolute, dependency.

The

Recommends

field should list packages that would be found together with this one

in all but unusual installations.

Suggests

This is used to declare that one package may be more useful with one or more

others. Using this field tells the packaging system and the user that the listed packages

are related to this one and can perhaps enhance its usefulness, but that installing this one

without them is perfectly reasonable.

Enhances

This field is similar to Suggests but works in the opposite direction. It is used to

declare that a package can enhance the functionality of another package.

Pre Depends

This field is like

Depends

, except that it also forces

dpkg

to complete instal 

lation of the packages named before even starting the installation of the package which

declares the pre dependency, as follows:

When a package declaring a pre dependency is about to be unpacked the pre dependency

can be satisfied if the depended on package is either fully configured, or even if the

depended on package(s) are only unpacked or half configured, provided that they have

been configured correctly at some point in the past (and not removed or partially re 

moved since). In this case, both the previously configured and currently unpacked or

half configured versions must satisfy any version clause in the

Pre Depends

field.

When the package declaring a pre dependency is about to be configured, the pre 

dependency will be treated as a normal

Depends

, that is, it will be considered satisfied

only if the depended on package has been correctly configured.

Pre Depends

should be used sparingly, preferably only by packages whose premature

upgrade or installation would hamper the ability of the system to continue with any

upgrade that might be in progress.

Pre Depends

are also required if the

preinst

script depends on the named package.

It is best to avoid this situation if possible.