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Ubuntu Policy Manual
Chapter 3 - Binary packages


The Ubuntu distribution is based on the Debian package management system, called dpkg. Thus, all packages in the Ubuntu distribution must be provided in the .deb file format.


3.1 The package name

Every package must have a name that's unique within the Ubuntu archive.

The package name is included in the control field Package, the format of which is described in Package, Section 5.6.7. The package name is also included as a part of the file name of the .deb file.


3.2 The version of a package

Every package has a version number recorded in its Version control file field, described in Version, Section 5.6.12.

The package management system imposes an ordering on version numbers, so that it can tell whether packages are being up- or downgraded and so that package system front end applications can tell whether a package it finds available is newer than the one installed on the system. The version number format has the most significant parts (as far as comparison is concerned) at the beginning.

If an upstream package has problematic version numbers they should be converted to a sane form for use in the Version field.


3.2.1 Version numbers based on dates

In general, Ubuntu packages should use the same version numbers as the upstream sources.

However, in some cases where the upstream version number is based on a date (e.g., a development "snapshot" release) the package management system cannot handle these version numbers without epochs. For example, dpkg will consider "96May01" to be greater than "96Dec24".

To prevent having to use epochs for every new upstream version, the date based portion of the version number should be changed to the following format in such cases: "19960501", "19961224". It is up to the maintainer whether they want to bother the upstream maintainer to change the version numbers upstream, too.

Note that other version formats based on dates which are parsed correctly by the package management system should not be changed.

Native Debian or Ubuntu packages (i.e., packages which have been written especially for Debian or Ubuntu) whose version numbers include dates should always use the "YYYYMMDD" format.


3.3 The maintainer of a package

Every package must have a Debian maintainer (the maintainer may be one person or a group of people reachable from a common email address, such as a mailing list). The maintainer is responsible for ensuring that the package is placed in the appropriate distributions.

The maintainer must be specified in the Maintainer control field with their correct name and a working email address. If one person maintains several packages, they should try to avoid having different forms of their name and email address in the Maintainer fields of those packages.

The format of the Maintainer control field is described in Maintainer, Section 5.6.2.

If the maintainer of a package quits from the Debian project, "Debian QA Group" packages@qa.debian.org takes over the maintainer-ship of the package until someone else volunteers for that task. These packages are called orphaned packages.[8]

Ubuntu: Packages that are modified in Ubuntu should have an Ubuntu-specific Maintainer field.[9] All Ubuntu binary packages, and Ubuntu source packages that are modified relative to Debian (that is, its version number contains the string "ubuntu"), should have their Maintainer field adjusted as follows:

If the Maintainer field is modified, then the old value must be saved in a field named XSBC-Original-Maintainer. Because it is mandated and very common, it is not necessary or appropriate to document this change in debian/changelog, unless it is the only change involved in the upload.


3.4 The description of a package

Every Ubuntu package must have an extended description stored in the appropriate field of the control record. The technical information about the format of the Description field is in Description, Section 5.6.13.

The description should describe the package (the program) to a user (system administrator) who has never met it before so that they have enough information to decide whether they want to install it. This description should not just be copied verbatim from the program's documentation.

Put important information first, both in the synopsis and extended description. Sometimes only the first part of the synopsis or of the description will be displayed. You can assume that there will usually be a way to see the whole extended description.

The description should also give information about the significant dependencies and conflicts between this package and others, so that the user knows why these dependencies and conflicts have been declared.

Instructions for configuring or using the package should not be included (that is what installation scripts, manual pages, info files, etc., are for). Copyright statements and other administrivia should not be included either (that is what the copyright file is for).


3.4.1 The single line synopsis

The single line synopsis should be kept brief - certainly under 80 characters.

Do not include the package name in the synopsis line. The display software knows how to display this already, and you do not need to state it. Remember that in many situations the user may only see the synopsis line - make it as informative as you can.


3.4.2 The extended description

Do not try to continue the single line synopsis into the extended description. This will not work correctly when the full description is displayed, and makes no sense where only the summary (the single line synopsis) is available.

The extended description should describe what the package does and how it relates to the rest of the system (in terms of, for example, which subsystem it is which part of).

The description field needs to make sense to anyone, even people who have no idea about any of the things the package deals with.[10]


3.5 Dependencies

Every package must specify the dependency information about other packages that are required for the first to work correctly.

For example, a dependency entry must be provided for any shared libraries required by a dynamically-linked executable binary in a package.

Packages are not required to declare any dependencies they have on other packages which are marked Essential (see below), and should not do so unless they depend on a particular version of that package.[11]

Sometimes, a package requires another package to be installed and configured before it can be installed. In this case, you must specify a Pre-Depends entry for the package.

You should not specify a Pre-Depends entry for a package before this has been discussed on the ubuntu-devel mailing list and a consensus about doing that has been reached.

The format of the package interrelationship control fields is described in Declaring relationships between packages, Chapter 7.


3.6 Virtual packages

Sometimes, there are several packages which offer more-or-less the same functionality. In this case, it's useful to define a virtual package whose name describes that common functionality. (The virtual packages only exist logically, not physically; that's why they are called virtual.) The packages with this particular function will then provide the virtual package. Thus, any other package requiring that function can simply depend on the virtual package without having to specify all possible packages individually.

All packages should use virtual package names where appropriate, and arrange to create new ones if necessary. They should not use virtual package names (except privately, amongst a cooperating group of packages) unless they have been agreed upon and appear in the list of virtual package names. (See also Virtual packages - Provides, Section 7.5)

The latest version of the authoritative list of virtual package names can be found in the debian-policy package. It is also available from the Debian web mirrors at /doc/packaging-manuals/virtual-package-names-list.txt.

The procedure for updating the list is described in the preface to the list.


3.7 Base system

The base system is a minimum subset of the Ubuntu system that is installed before everything else on a new system. Only very few packages are allowed to form part of the base system, in order to keep the required disk usage very small.

The base system consists of all those packages with priority required or important. Many of them will be tagged essential (see below).


3.8 Essential packages

Essential is defined as the minimal set of functionality that must be available and usable on the system at all times, even when packages are in an unconfigured (but unpacked) state. Packages are tagged essential for a system using the Essential control file field. The format of the Essential control field is described in Essential, Section 5.6.9.

Since these packages cannot be easily removed (one has to specify an extra force option to dpkg to do so), this flag must not be used unless absolutely necessary. A shared library package must not be tagged essential; dependencies will prevent its premature removal, and we need to be able to remove it when it has been superseded.

Since dpkg will not prevent upgrading of other packages while an essential package is in an unconfigured state, all essential packages must supply all of their core functionality even when unconfigured. If the package cannot satisfy this requirement it must not be tagged as essential, and any packages depending on this package must instead have explicit dependency fields as appropriate.

Maintainers should take great care in adding any programs, interfaces, or functionality to essential packages. Packages may assume that functionality provided by essential packages is always available without declaring explicit dependencies, which means that removing functionality from the Essential set is very difficult and is almost never done. Any capability added to an essential package therefore creates an obligation to support that capability as part of the Essential set in perpetuity.

You must not tag any packages essential before this has been discussed on the ubuntu-devel mailing list and a consensus about doing that has been reached.


3.9 Maintainer Scripts

The package installation scripts should avoid producing output which is unnecessary for the user to see and should rely on dpkg to stave off boredom on the part of a user installing many packages. This means, amongst other things, using the --quiet option on install-info.

Errors which occur during the execution of an installation script must be checked and the installation must not continue after an error.

Note that in general Scripts, Section 10.4 applies to package maintainer scripts, too.

You should not use dpkg-divert on a file belonging to another package without consulting the maintainer of that package first.

All packages which supply an instance of a common command name (or, in general, filename) should generally use update-alternatives, so that they may be installed together. If update-alternatives is not used, then each package must use Conflicts to ensure that other packages are de-installed. (In this case, it may be appropriate to specify a conflict against earlier versions of something that previously did not use update-alternatives; this is an exception to the usual rule that versioned conflicts should be avoided.)


3.9.1 Prompting in maintainer scripts

Package maintainer scripts may prompt the user if necessary. Prompting must be done by communicating through a program, such as debconf, which conforms to the Debian Configuration Management Specification, version 2 or higher.

Packages which are essential, or which are dependencies of essential packages, may fall back on another prompting method if no such interface is available when they are executed.

The Debian Configuration Management Specification is included in the debconf_specification files in the debian-policy package. It is also available from the Debian web mirrors at /doc/packaging-manuals/debconf_specification.html.

Packages which use the Debian Configuration Management Specification may contain an additional config script and a templates file in their control archive[12]. The config script might be run before the preinst script, and before the package is unpacked or any of its dependencies or pre-dependencies are satisfied. Therefore it must work using only the tools present in essential packages.[13]

Packages which use the Debian Configuration Management Specification must allow for translation of their user-visible messages by using a gettext-based system such as the one provided by the po-debconf package.

Packages should try to minimize the amount of prompting they need to do, and they should ensure that the user will only ever be asked each question once. This means that packages should try to use appropriate shared configuration files (such as /etc/papersize and /etc/news/server), and shared debconf variables rather than each prompting for their own list of required pieces of information.

It also means that an upgrade should not ask the same questions again, unless the user has used dpkg --purge to remove the package's configuration. The answers to configuration questions should be stored in an appropriate place in /etc so that the user can modify them, and how this has been done should be documented.

If a package has a vitally important piece of information to pass to the user (such as "don't run me as I am, you must edit the following configuration files first or you risk your system emitting badly-formatted messages"), it should display this in the config or postinst script and prompt the user to hit return to acknowledge the message. Copyright messages do not count as vitally important (they belong in /usr/share/doc/package/copyright); neither do instructions on how to use a program (these should be in on-line documentation, where all the users can see them).

Any necessary prompting should almost always be confined to the config or postinst script. If it is done in the postinst, it should be protected with a conditional so that unnecessary prompting doesn't happen if a package's installation fails and the postinst is called with abort-upgrade, abort-remove or abort-deconfigure.


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Ubuntu Policy Manual

version 3.8.2.0ubuntu1, 2009-06-19

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