# Debian packaging manual - chapter 8
Declaring relationships between packages

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.

## 8.1 Syntax of relationship 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

## 8.2 Dependencies - `Depends`, `Recommends`, `Suggests`, `Pre-Depends`

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.

`Depends`
  This declares an absolute dependency.

  `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[21], 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.

`Recommends`
  This declares a strong, but not absolute, dependency.

  `Recommends` is ignored by `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.

`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 be related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.

  `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.

`Pre-Depends`
  This field is like `Depends`, except that it also forces `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.

### 8.2.1 Dependencies on shared libraries

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.

### 8.2.2 Deconfiguration due to removal during bulk installations

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.

## 8.3 Alternative packages - `Conflicts` and `Replaces`

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.

## 8.4 Virtual packages - `Provides`

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: emacs

and someone else releases an xemacs package they can say

    Package: xemacs
    Provides: emacs

and 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.

## 8.5 `Replaces` - overwriting files and replacing packages

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.

### 8.5.1 Overwriting files in other packages

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.

### 8.5.2 Replacing whole packages, forcing their removal

Secondly, `Replaces` allows `dpkg` and `dselect` to resolve which package should be removed when a conflict - see Alternative packages - `Conflicts` 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.

## 8.6 Defaults for satisfying dependencies - ordering

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-browser

so that `dselect` defaults to selecting the lightweight standalone info browser.

Debian packaging manual - Copyright ©1996 Ian Jackson.
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