Chapter 6. Basics of the Debian package management system

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  Package A replaces Package B when files installed by B are removed and (in some cases)

over written by files in A.

  Package A provides Package B when all of the files and functionality of B are incorporated

into A. This mechanism provides a way for users with constrained disk space to get only

that part of package A which they really need.

More detailed information on the use of each these terms can be found in the Policy manual.

6.10 What is meant by Pre Depends?

 Pre Depends  is a special dependency. In the case of most packages,

dpkg

will unpack its

archive file (i.e., its

.deb

file) independently of whether or not the files on which it depends

exist on the system. Simplistically, unpacking means that

dpkg

will extract the files from the

archive file that were meant to be installed on your file system, and put them in place. If those

packages depend on the existence of some other packages on your system,

dpkg

will refuse

to complete the installation (by executing its  configure  action) until the other packages are

installed.

However, for some packages,

dpkg

will refuse even to unpack them until certain dependencies

are resolved. Such packages are said to  Pre depend  on the presence of some other packages.

The Debian project provided this mechanism to support the safe upgrading of systems from

a.out

format to

ELF

format, where the order in which packages were unpacked was critical.

There are other large upgrade situations where this method is useful, e.g. the packages with

the required priority and their LibC dependency.

As before, more detailed information about this can be found in the Policy manual.

6.11 What is meant by unknown, install, remove purge and hold in

the package status?

These  want  flags tell what the user wanted to do with a package (as indicated either by the

user's actions in the  Select  section of

dselect

, or by the user's direct invocations of

dpkg

).

Their meanings are:

  unknown   the user has never indicated whether he wants the package

  install   the user wants the package installed or upgraded

  remove   the user wants the package removed, but does not want to remove any existing

configuration files.

  purge   the user wants the package to be removed completely, including its configuration

files.