Packages can declare in their control file that they have certain relationships to other packages - for example, that they may not be installed at the same time as certain other packages, and/or that they depend on the presence of others, or that they should overwrite files in certain other packages if present.
This is done using the Depends, Recommends, Suggests, Conflicts, Provides and Replaces control file fields.
These fields all have a uniform syntax. They are a list of package names separated by commas.
In Depends, Recommends, Suggests and Pre-Depends (the fields which declare dependencies of the package in which they occur on other packages) these package names may also be lists of alternative package names, separated by vertical bar symbols | (pipe symbols).
All the fields except Provides may restrict their applicability to particular versions of each named package. This is done in parentheses after each individual package name; the parentheses should contain a relation from the list below followed by a version number, in the format described in Version numbering, chapter 5.
The relations allowed are <<, <=,
=, >= and >> for
strictly earlier, earlier or equal, exactly equal, later or
equal and strictly later, respectively. The forms
< and > were used to mean
earlier/later or equal, rather than strictly earlier/later,
so they should not appear in new packages (though
dpkg
still supports them).
Whitespace may appear at any point in the version
specification, and must appear where it's necessary to
disambiguate; it is not otherwise significant. For
consistency and in case of future changes to
dpkg
it is recommended that a single space be
used after a version relationship and before a version
number; it is usual also to put a single space after each
comma, on either side of each vertical bar, and before each
open parenthesis.
For example:
Package: metamail Version: 2.7-3 Depends: libc5 (>= 5.2.18-4), mime-support, csh | tcsh
These four fields are used to declare a dependency by one package on another. They appear in the depending package's control file.
All but Pre-Depends (discussed below) take effect only when a package is to be configured. They do not prevent a package being on the system in an unconfigured state while its dependencies are unsatisfied, and it is possible to replace a package whose dependencies are satisfied and which is properly installed with a different version whose dependencies are not and cannot be satisfied; when this is done the depending package will be left unconfigured (since attempts to configure it will give errors) and will not function properly.
For this reason packages in an installation run are usually all unpacked first and all configured later; this gives later versions of packages with dependencies on later versions of other packages the opportunity to have their dependencies satisfied.
Thus Depends allows package maintainers to impose an order in which packages should be configured.
dpkg
will not configure packages whose
dependencies aren't satisfied. If it is asked to make
an installation which would cause an installed
package's dependencies to become unsatisfied it will
complain
[22], unless --auto-deconfigure is
specified, in which case those packages will be
deconfigured before the installation proceeds.
dselect
makes it hard for the user to
select packages for installation, removal or upgrade
in a way that would mean that packages'
Depends
fields would be unsatisfied. The
user can override this if they wish, for example if
they know that dselect
has an out-of-date
view of the real package relationships.
The Depends field should be used if the
depended-on package is required for the depending
package to provide a significant amount of
functionality.
dpkg
,
so that users using the command-line (who are presumed
to know what they're doing) will not be impeded.
It is treated by dselect
exactly as
Depends is; this makes it hard for the user
to select things so as to leave Recommends
fields unsatisfied, but they are able to do so by
being persistent.
The Recommends field should list packages
that would be found together with this one in all but
unusual installations.
dselect
will offer suggsted packages to
the system administrator when they select the
suggesting package, but the default is not to install
the suggested package.
dpkg
to complete installation of the
packages named before even starting the installation
of the package which declares the predependency.
dselect
checks for predependencies when
it is doing an installation run, and will attempt to
find the packages which are required to be installed
first and do so in the right order.
However, this process is slow (because it requires
repeated invocations of dpkg
) and
troublesome (because it requires guessing where to
find the appropriate files).
For these reasons, and because this field imposes
restrictions on the order in which packages may be
unpacked (which can be difficult for installations
from multipart media, for example),
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.
When the package declaring it is being configured, a
Pre-Dependency will be considered satisfied
only if the depending package has been correctly
configured, just as if an ordinary Depends
had been used.
However, when a package declaring a predependency is
being unpacked the predependency can be satisfied 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 removed 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 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 packages 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.
The dependency fields listed above are used by packages which need shared libraries to declare dependencies on the appropriate packages.
These dependencies are usually determined automatically
using dpkg-shlibdeps
and inserted in the
package control file using the control file substitution
variables mechanism; see debian/substvars
and variable substitutions, subsection 3.2.4 and
Tools for processing source packages, section 3.1.
If dpkg
would like to remove a package due to a
conflict, as described above, but this would violate a
dependency of some other package on the system,
dpkg
will usually not remove the conflicting
package and halt with an error.
However, if the --auto-deconfigure (-B)
option is used dpkg
will automatically
`deconfigure' the package with the problematic dependency,
so that the conflicting package can be removed and the
package we're trying to install can be installed. If
dpkg
is being asked to install packages (rather
than just unpacking them) it will try to reconfigure the
package when it has unpacked all its arguments, in the hope
that one of the other packages it is installing will satisfy
the problematic dependency.
dselect
supplies this argument to
dpkg
when it invokes it, so that bulk
installations proceed smoothly.
When one package declares a conflict with another
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 (Replaces - overwriting
files and replacing packages, section 8.5) 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.
dselect
makes it hard to select conflicting
packages, though the user can override this if they wish.
If they do not override it then dselect
will
select one of the packages for removal, and the user must
make sure it is the right one. In the future
dselect
will look for the presence of a
Replaces field to help decide which package should
be installed and which removed.
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 something.
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. This
aspect of installation ordering is not handled by
dselect
, so that the use Conflicts in
this way is likely to cause problems for `bulk run' upgrades
and installations.
As well as the names of actual (`concrete') packages, the package relationship fields Depends, Recommends, Suggests and Conflicts may mention 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 were the virtual package name appears.
If there are both a real and a virtual package of the same name then the dependency may be satisfied (or the conflict caused) by either the real package or any of the virtual packages which provide it. This is so that, for example, supposing we have
Package: vm Depends: emacsand someone else releases an xemacs package they can say
Package: xemacs Provides: emacsand all will work in the interim (until a purely virtual package name is decided on and the emacs and vm packages are changed to use it).
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 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 alternative before the virtual.
The Replaces control file field has two purposes, which come into play in different situations.
Virtual packages (Virtual packages - Provides, section 8.4) are not considered when looking at a Replaces field - the packages declared as being replaced must be mentioned by their real names.
Firstly, as mentioned before, it is usually an error for a package to contains files which are on the system in another package, though currently the --force-overwrite flag is enabled by default, downgrading the error to a warning,
If the overwriting package declares that it replaces the
one containing the file being overwritten then
dpkg
will proceed, and 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 conffiles details noted
in the package will be ignored, as they will have been
taken over by the replacing package(s). The package's
postrm
script will be run to allow the
package to do any final cleanup required. See Summary of ways maintainer
scripts are called, section 6.2.
In the future dpkg
will discard files which
overwrite those from another package which declares that
it replaces the one being installed (so that you can
install an older version of a package without problems).
This usage of Replaces only takes effect when both packages are at least partially on the system at once, so that it can only happen if they do not conflict or if the conflict has been overridden.
Secondly, Replaces allows dpkg
and
dselect
to resolve which package should be
removed when a conflict - see Alternative packages -
tt>Conflitt> and Replaces, section 8.3. This
usage only takes effect when the two packages do
conflict, so that the two effects do not interfere with
each other.
Ordering is significant in dependency fields.
Usually dselect will suggest to the user that they select the package with the most `fundamental' class (eg, it will prefer Base packages to Optional ones), or the one that they `most wanted' to select in some sense.
In the absence of other information dselect
will offer a default selection of the first named package in
a list of alternatives.
However, there is no way to specify the `order' of several packages which all provide the same thing, when that thing is listed as a dependency.
Therefore a dependency on a virtual package should contain a concrete package name as the first alternative, so that this is the default.
For example, consider the set of packages:
Package: glibcdoc Recommends: info-browser Package: info Provides: info-browser Package: emacs Provides: info-browser
If emacs
and info
both have the
same priority then dselect
's choice is
essentially random. Better would be
Package: glibcdoc Recommends: info | info-browserso that
dselect
defaults to selecting the
lightweight standalone info browser.