Chapter 7. Declaring relationships between packages

49

or any other concrete package which provides the virtual package with the name in question.

This is so that, for example, supposing we have

Package: foo

Depends: bar

and someone else releases an enhanced version of the

bar

package (for example, a non US

variant), they can say:

Package: bar plus

Provides: bar

and the

bar plus

package will now also satisfy the dependency for the

foo

package.

If a dependency or a conflict has a version number attached then only real packages will be

considered to see whether the relationship is satisfied (or the prohibition violated, for a conflict)

  it is assumed that a real package which provides the virtual package is not of the  right 

version. So, a

Provides

field may not contain version numbers, and the version number of

the concrete package which provides a particular virtual package will not be looked at when

considering a dependency on or conflict with the virtual package name.

It is likely that the ability will be added in a future release of

dpkg

to specify a version number

for each virtual package it provides. This feature is not yet present, however, and is expected

to be used only infrequently.

If you want to specify which of a set of real packages should be the default to satisfy a particular

dependency on a virtual package, you should list the real package as an alternative before the

virtual one.

7.5 Overwriting files and replacing packages  

Replaces

Packages can declare in their control file that they should overwrite files in certain other pack 

ages, or completely replace other packages. The

Replaces

control file field has these two

distinct purposes.

7.5.1 Overwriting files in other packages

Firstly, as mentioned before, it is usually an error for a package to contain files which are on

the system in another package.

However, if the overwriting package declares that it

Replaces

the one containing the file

being overwritten, then

dpkg

will replace the file from the old package with that from the new.

The file will no longer be listed as  owned  by the old package.

If a package is completely replaced in this way, so that

dpkg

does not know of any files it

still contains, it is considered to have  disappeared . It will be marked as not wanted on the

system (selected for removal) and not installed. Any

conffile

s details noted for the package

will be ignored, as they will have been taken over by the overwriting package. The package's