The Operating System
====================

.. _s9.1:

File system hierarchy
---------------------

.. _s-fhs:

File System Structure
~~~~~~~~~~~~~~~~~~~~~

The location of all files and directories must comply with the
Filesystem Hierarchy Standard (FHS), version 3.0, with the exceptions
noted below, and except where doing so would violate other terms of
Debian Policy. The following exceptions to the FHS apply:

1.  The FHS requirement that architecture-independent
    application-specific static files be located in ``/usr/share`` is
    relaxed to a suggestion. In particular, a subdirectory of
    ``/usr/lib`` may be used by a package (or a collection of packages)
    to hold a mixture of architecture-independent and
    architecture-dependent files. However, when a directory is entirely
    composed of architecture-independent files, it should be located in
    ``/usr/share``.

2.  The optional rules related to user specific configuration files for
    applications are stored in the user's home directory are relaxed. It
    is recommended that such files start with the '``.``' character (a
    "dot file"), and if an application needs to create more than one dot
    file then the preferred placement is in a subdirectory with a name
    starting with a '.' character, (a "dot directory"). In this case it
    is recommended the configuration files not start with the '.'
    character.

3.  Only the dynamic linker and libc are allowed to install files
    in ``/lib64``.

4.  The requirement for object files, internal binaries, and libraries,
    including ``libc.so.*``, to be located directly under ``/lib{,32}``
    and ``/usr/lib{,32}`` is amended, permitting files to instead be
    installed to ``/lib/triplet`` and ``/usr/lib/triplet``, where
    ``triplet`` is the value returned by ``dpkg-architecture -qDEB_HOST_MULTIARCH`` for the architecture of the
    package. Packages must not install files to any triplet path other
    than the one matching the architecture of that package; for
    instance, an ``Architecture:  amd64`` package containing 32-bit x86
    libraries must not install these libraries to
    ``/usr/lib/i386-linux-gnu``.  [#]_

    Packages for 64-bit architectures must not install files in ``/usr/lib64``
    or in a subdirectory of it.

    The requirement for C and C++ headers files to be accessible through
    the search path ``/usr/include/`` is amended, permitting files to be
    accessible through the search path ``/usr/include/triplet`` where
    ``triplet`` is as above.  [#]_

    Applications may also use a single subdirectory under
    ``/usr/lib/triplet``.

    The execution time linker/loader, ld\*, must still be made available
    in the existing location under /lib or /lib64 since this is part of
    the ELF ABI for the architecture.

5.  The requirement that ``/usr/local/share/man`` be "synonymous" with
    ``/usr/local/man`` is relaxed to a recommendation

6.  The requirement that window managers with a single configuration file
    call it ``system.*wmrc`` is removed, as is the restriction that the
    window manager subdirectory be named identically to the window
    manager name itself.

7.  The requirement that boot manager configuration files live in
    ``/etc``, or at least are symlinked there, is relaxed to a
    recommendation.

8.  ``/var/run`` is required to be a symbolic link to ``/run``,
    and ``/var/lock`` is required to be a symbolic link to ``/run/lock``.

9.  The ``/var/www`` directory is additionally allowed.

10. The requirement for ``/usr/local/share/color`` to exist if
    ``/usr/share/color`` exists is relaxed to a recommendation.

11. The requirement for ``/usr/local/libqual`` to exist if ``/libqual``
    or ``/usr/libqual`` exists (where ``libqual`` is a variant of
    ``lib`` such as ``lib32`` or ``lib64``) is removed.

12. On GNU/Hurd systems, the following additional directories are
    allowed in the root filesystem: ``/hurd`` and ``/servers``.  [#]_

13. As an exception to the requirement for there to be no subdirectories
    in ``/usr/bin``, the ``mh`` mail-handling suite may create
    ``/usr/bin/mh/``, as was allowed in FHS version 2.3. Other
    subdirectories are not allowed.

The version of this document referred here can be found in the
``debian-policy`` package or on `FHS (Debian
copy) <https://www.debian.org/doc/packaging-manuals/fhs/>`_ alongside
this manual (or, if you have the debian-policy installed, you can try
`FHS (local copy) <file:///usr/share/doc/debian-policy/fhs/>`_). The
latest version, which may be a more recent version, may be found on `FHS
(upstream) <http://refspecs.linuxfoundation.org/fhs.shtml>`_. Specific
questions about following the standard may be asked on the
``debian-devel`` mailing list, or referred to the FHS mailing list
(see the `FHS web site <http://refspecs.linuxfoundation.org/fhs.shtml>`_
for more information).

.. _s9.1.2:

Site-specific programs
~~~~~~~~~~~~~~~~~~~~~~

As mandated by the FHS, packages must not place any files in
``/usr/local``, either by putting them in the file system archive to be
unpacked by ``dpkg`` or by manipulating them in their maintainer
scripts.

However, the package may create empty directories below ``/usr/local``
so that the system administrator knows where to place site-specific
files. These are not directories *in* ``/usr/local``, but are children
of directories in ``/usr/local``. These directories
(``/usr/local/*/dir/``) should be removed on package removal if they are
empty.

Note that this applies only to directories *below* ``/usr/local``, not
*in* ``/usr/local``. Packages must not create sub-directories in the
directory ``/usr/local`` itself, except those listed in FHS, section
4.9. However, you may create directories below them as you wish. You
must not remove any of the directories listed in 4.9, even if you
created them.

If ``/etc/staff-group-for-usr-local`` does not exist, ``/usr/local``
and all subdirectories created by packages should have permissions
0755 and be owned by ``root:root``.  If
``/etc/staff-group-for-usr-local`` exists, ``/usr/local`` and
subdirectories should have permissions 2775 (group-writable and
set-group-id) and be owned by ``root:staff``.

Since ``/usr/local`` can be mounted read-only from a remote server,
``/usr/local/*/dir/`` directories must be created and removed by the
``postinst`` and ``prerm`` maintainer scripts and not be included in
the ``.deb`` archive. These scripts must not fail if either of these
operations fail.

For example, the ``emacsen-common`` package could contain something like

::

    if [ ! -e /usr/local/share/emacs ]; then
        if mkdir /usr/local/share/emacs 2>/dev/null; then
            if test -e /etc/staff-group-for-usr-local ; then
                if chown root:staff /usr/local/share/emacs; then
                    chmod 2775 /usr/local/share/emacs || true
                fi
            elif chown root:root /usr/local/share/emacs; then
                chmod 0755 /usr/local/share/emacs || true
            fi
        fi
    fi

in its ``postinst`` script, and

::

    rmdir /usr/local/share/emacs/site-lisp 2>/dev/null || true
    rmdir /usr/local/share/emacs 2>/dev/null || true

in the ``prerm`` script. (Note that this form is used to ensure that if
the script is interrupted, the directory ``/usr/local/share/emacs`` will
still be removed.)

If you do create a directory in ``/usr/local`` for local additions to a
package, you should ensure that settings in ``/usr/local`` take
precedence over the equivalents in ``/usr``.

However, because ``/usr/local`` and its contents are for exclusive use
of the local administrator, a package must not rely on the presence or
absence of files or directories in ``/usr/local`` for normal operation.

.. _s9.1.3:

The system-wide mail directory
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The system-wide mail directory is ``/var/mail``. This directory is part
of the base system and should not be owned by any particular mail
agents. The use of the old location ``/var/spool/mail`` is deprecated,
even though the spool may still be physically located there.

.. _s-fhs-run:

``/run`` and ``/run/lock``
~~~~~~~~~~~~~~~~~~~~~~~~~~

The directory ``/run`` is cleared at boot, normally by being a mount
point for a temporary file system. Packages therefore must not assume
that any files or directories under ``/run`` other than ``/run/lock``
exist unless the package has arranged to create those files or
directories since the last reboot. Normally, this is done by the package
via an init script. See :ref:`s-writing-init` for more
information.

Packages must not include files or directories under ``/run``, or under
the older ``/var/run`` and ``/var/lock`` paths. The latter paths will
normally be symlinks or other redirections to ``/run`` for backwards
compatibility.

.. _s9.2:

Users and groups
----------------

.. _s9.2.1:

Introduction
~~~~~~~~~~~~

The Debian system can be configured to use either plain or shadow
passwords.

Some user ids (UIDs) and group ids (GIDs) are reserved globally for use
by certain packages. Because some packages need to include files which
are owned by these users or groups, or need the ids compiled into
binaries, these ids must be used on any Debian system only for the
purpose for which they are allocated. This is a serious restriction, and
we should avoid getting in the way of local administration policies. In
particular, many sites allocate users and/or local system groups
starting at 100.

Apart from this we should have dynamically allocated ids, which should
by default be arranged in some sensible order, but the behavior should
be configurable. When maintainers choose a new hardcoded or dynamically
generated username for packages to use, they should start this username
with an underscore. This minimizes collisions with locally created user
accounts.

Packages other than ``base-passwd`` must not modify ``/etc/passwd``,
``/etc/shadow``, ``/etc/group`` or ``/etc/gshadow``.

.. _s9.2.2:

UID and GID classes
~~~~~~~~~~~~~~~~~~~

The UID and GID numbers are divided into classes as follows:

0-99:
    Globally allocated by the Debian project, the same on every Debian
    system. These ids will appear in the ``passwd`` and ``group`` files
    of all Debian systems, new ids in this range being added
    automatically as the ``base-passwd`` package is updated.

    Packages which need a single statically allocated uid or gid should
    use one of these; their maintainers should ask the ``base-passwd``
    maintainer for ids.

100-999:
    Dynamically allocated system users and groups. Packages which need a
    user or group, but can have this user or group allocated dynamically
    and differently on each system, should use ``adduser --system`` to
    create the group and/or user. ``adduser`` will check for the
    existence of the user or group, and if necessary choose an unused id
    based on the ranges specified in ``adduser.conf``.

1000-59999:
    Dynamically allocated user accounts. By default ``adduser`` will
    choose UIDs and GIDs for user accounts in this range, though
    ``adduser.conf`` may be used to modify this behavior.

60000-64999:
    Globally allocated by the Debian project, but only created on
    demand. The ids are allocated centrally and statically, but the
    actual accounts are only created on users' systems on demand.

    These ids are for packages which are obscure or which require many
    statically-allocated ids. These packages should check for and create
    the accounts in ``/etc/passwd`` or ``/etc/group`` (using ``adduser``
    if it has this facility) if necessary. Packages which are likely to
    require further allocations should have a "hole" left after them in
    the allocation, to give them room to grow.

65000-65533:
    Reserved.

65534:
    User ``nobody``. The corresponding gid refers to the group
    ``nogroup``.

65535:
    This value *must not* be used, because it was the error return
    sentinel value when ``uid_t`` was 16 bits.

65536-4294967293:
    Dynamically allocated user accounts. By default ``adduser`` will not
    allocate UIDs and GIDs in this range, to ease compatibility with
    legacy systems where ``uid_t`` is still 16 bits.

4294967294:
    ``(uid_t)(-2) == (gid_t)(-2)`` *must not* be used, because it is
    used as the anonymous, unauthenticated user by some NFS
    implementations.

4294967295:
    ``(uid_t)(-1) == (gid_t)(-1)`` *must not* be used, because it is the
    error return sentinel value.

.. _s-nonexistent:

Non-existent home directories
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The canonical non-existent home directory is ``/nonexistent``.  Users
who should not have a home directory should have their home directory
set to this value.

The Debian autobuilders set HOME to ``/nonexistent`` so that packages
which try to write to a home directory will fail to build.

.. _s-services:

Starting system services
------------------------

.. _s-services-intro:

Introduction
~~~~~~~~~~~~

Packages that include system services should include ``systemd`` service
units to start or stop those services.  See :manpage:`systemd.service(5)`
for details on the syntax of a service unit file.  In the common case that
a package includes a single system service, the service unit should have
the same name as the package plus the ``.service`` extension.

If the package does not include a service unit (if, for example, no one
has yet written one), including an init script, as described below, to
start the service is encouraged.

Packages including a service unit may optionally include an init script to
support other init systems.  In this case, the init script should have the
same name as the ``systemd`` service unit so that ``systemd`` will ignore
it and use the service unit instead.  Packages may also support other init
systems by including configuration in the native format of those init
systems.

If a service unit is not present, ``systemd`` uses dependency information
contained within the init scripts and symlinks in ``/etc/rcn.d`` to decide
which scripts to run and in which order.  The ``sysv-rc`` runlevel system
for ``sysvinit`` uses the same symlinks in ``/etc/rcn.d`` to decide which
scripts to run and in which order at boot time and when the init state (or
"runlevel") is changed.  See the ``README.runlevels`` file shipped with
``sysv-rc`` for implementation details.  Other alternatives might exist.

The sections below describe how to write those scripts and configure those
symlinks.

.. _s-writing-init:

Writing the scripts
~~~~~~~~~~~~~~~~~~~

Init scripts are placed in ``/etc/init.d``.  In the common case that a
package starts a single service, they should be named
``/etc/init.d/package``.  They should accept one argument, saying what to
do:

``start``
    start the service,

``stop``
    stop the service,

``restart``
    stop and restart the service if it's already running, otherwise
    start the service

``try-restart``
    restart the service if it's already running, otherwise just report
    success.

``reload``
    cause the configuration of the service to be reloaded without
    actually stopping and restarting the service,

``force-reload``
    cause the configuration to be reloaded if the service supports this,
    otherwise restart the service.

``status``
    report the current status of the service

The ``start``, ``stop``, ``restart``, and ``force-reload`` options should
be supported by all init scripts. Supporting ``status`` is encouraged.
The ``reload`` and ``try-restart`` options are optional.

The ``init.d`` scripts must ensure that they will behave sensibly (i.e.,
returning success and not starting multiple copies of a service) if
invoked with ``start`` when the service is already running, or with
``stop`` when it isn't, and that they don't kill unfortunately-named
user processes. The best way to achieve this is usually to use
``start-stop-daemon`` with the ``--oknodo`` option.

Be careful of using ``set -e`` in ``init.d`` scripts. Writing correct
``init.d`` scripts requires accepting various error exit statuses when
daemons are already running or already stopped without aborting the
``init.d`` script, and common ``init.d`` function libraries are not safe
to call with ``set -e`` in effect.  [#]_ For ``init.d`` scripts, it's
often easier to not use ``set -e`` and instead check the result of each
command separately.

If a service reloads its configuration automatically (as in the case of
``cron``, for example), the ``reload`` option of the ``init.d`` script
should behave as if the configuration has been reloaded successfully.

The ``/etc/init.d`` scripts must be treated as configuration files,
either (if they are present in the package, that is, in the .deb file)
by marking them as ``conffile``\ s, or, (if they do not exist in the
.deb) by managing them correctly in the maintainer scripts (see
:ref:`s-config-files`). This is important since we want
to give the local system administrator the chance to adapt the scripts
to the local system, e.g., to disable a service without de-installing
the package, or to specify some special command line options when
starting a service, while making sure their changes aren't lost during
the next package upgrade.

These scripts should not fail obscurely when the configuration files
remain but the package has been removed, as configuration files remain
on the system after the package has been removed. Only when ``dpkg`` is
executed with the ``--purge`` option will configuration files be
removed. In particular, as the ``/etc/init.d/package`` script itself is
usually a ``conffile``, it will remain on the system if the package is
removed but not purged. Therefore, you should include a ``test``
statement at the top of the script, like this:

::

    test -f program-executed-later-in-script || exit 0

Often there are some variables in the ``init.d`` scripts whose values
control the behavior of the scripts, and which a system administrator is
likely to want to change. As the scripts themselves are frequently
``conffile``\ s, modifying them requires that the administrator merge in
their changes each time the package is upgraded and the ``conffile``
changes. To ease the burden on the system administrator, such
configurable values should not be placed directly in the script.
Instead, they should be placed in a file in ``/etc/default``, which
typically will have the same base name as the ``init.d`` script. This
extra file should be sourced by the script when the script runs. It must
contain only variable settings and comments in POSIX.1-2017 ``sh`` format. It
must either be a ``conffile`` or a configuration file maintained by the
package maintainer scripts. See :ref:`s-config-files` for
more details.

To ensure that vital configurable values are always available, the
``init.d`` script should set default values for each of the shell
variables it uses, either before sourcing the ``/etc/default/`` file or
afterwards using something like the ``: ${VAR:=default}`` syntax. Also,
the ``init.d`` script must behave sensibly and not fail if the
``/etc/default`` file is deleted.

Files and directories under ``/run``, including ones referred to via the
compatibility paths ``/var/run`` and ``/var/lock``, are normally stored
on a temporary filesystem and are normally not persistent across a
reboot. The ``init.d`` scripts must handle this correctly. This will
typically mean creating any required subdirectories dynamically when the
``init.d`` script is run. See :ref:`s-fhs-run` for more
information.

.. _s9.3.3:

Interfacing with init systems
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Maintainer scripts for packages including init scripts must use
``update-rc.d`` as described below to interact with the service manager
for requests such as enabling or disabling services.  They should use
``invoke-rc.d`` as described below to invoke init scripts for requests
such as starting and stopping service.

Directly managing the ``/etc/rc?.d`` links and directly invoking the
``/etc/init.d/`` init scripts should be done only by packages providing
the init script subsystem (such as ``init-system-helpers``).

.. _s9.3.3.1:

Managing the links
^^^^^^^^^^^^^^^^^^

The program ``update-rc.d`` is provided for package maintainers to
arrange for the proper creation and removal of ``/etc/rcn.d`` symbolic
links, or their functional equivalent if another method is being used.
It is intended for use in package maintainer scripts.

You must not include any ``/etc/rcn.d`` symbolic links in the actual
archive or manually create or remove the symbolic links in maintainer
scripts; you must use the ``update-rc.d`` program instead. (The former
will fail if an alternative method of maintaining runlevel information
is being used.) You must not include the ``/etc/rcn.d`` directories
themselves in the archive either. (Only the ``init-system-helpers``
package is permitted to do so.)

To get the default behavior for your package, put in your ``postinst``
script::

    update-rc.d package defaults

and in your ``postrm``::

    if [ "$1" = purge ]; then
        update-rc.d package remove
    fi

The default behaviour is to enable autostarting your package's daemon.
The local administrator can override this using the command
``update-rc.d package disable``.  If, however, the daemon should not
be autostarted unless the local administrator has explicitly requested
this, instead add to your ``postinst`` script::

    update-rc.d package defaults-disabled

and add a dependency on ``init-system-helpers (>= 1.50)``, which
introduced the ``defaults-disabled`` option.  Then the local
administrator can enable autostarting the daemon using the command
``update-rc.d package enable``.

An older practice, which should not be used, was to include a line
like ``DISABLED=yes`` in the package's ``/etc/default`` file.  The
package's init script would not start the service until the local
system administrator changed this to ``DISABLED=no``, or similar.  The
problem with this approach was that it hides from the init system
whether or not the daemon should actually be started, which leads to
inconsistent and confusing behavior: ``service <package> start`` could
return success but not start the service; services with a dependency
on this service will be started even though the service isn't running;
and init system status commands could incorrectly claim that the service
was started.

Note that if your package changes runlevels or priority, you may have to
remove and recreate the links, since otherwise the old links may
persist. Refer to the documentation of ``update-rc.d``.

For more information about using ``update-rc.d``, please consult its man
page, :manpage:`update-rc.d(8)`.

It is easiest for packages not to call ``update-rc.d`` directly, but
instead use debhelper programs that add the required ``update-rc.d``
calls automatically. See ``dh_installinit``, ``dh_installsystemd``, etc.

.. _s9.3.3.2:

Running init scripts
^^^^^^^^^^^^^^^^^^^^

The program ``invoke-rc.d`` is provided to make it easier for package
maintainers to properly invoke an init script, obeying runlevel and other
locally-defined constraints that might limit a package's right to start,
stop and otherwise manage services. This program may be used by
maintainers in their packages' scripts.

The package maintainer scripts must use ``invoke-rc.d`` to invoke the
``/etc/init.d/*`` init scripts or equivalent instead of calling them
directly.

By default, ``invoke-rc.d`` will pass any action requests (start, stop,
reload, restart...) to the ``/etc/init.d`` script, filtering out
requests to start or restart a service out of its intended runlevels.

Most packages will simply use:

::

    invoke-rc.d package action

in their ``postinst`` and ``prerm`` scripts.

A package should register its init script services using ``update-rc.d``
before it tries to invoke them using ``invoke-rc.d``. Invocation of
unregistered services may fail.

For more information about using ``invoke-rc.d``, please consult its man
page, :manpage:`invoke-rc.d(8)`.

It is easiest for packages not to call ``invoke-rc.d`` directly, but
instead use debhelper programs that add the required ``invoke-rc.d``
calls automatically. See ``dh_installinit``, ``dh_installsystemd``, etc.

.. _s9.3.4:

Boot-time initialization
~~~~~~~~~~~~~~~~~~~~~~~~

This section has been deleted.

.. _s9.3.5:

Example
~~~~~~~

Examples on which you can base your ``systemd`` service units are
available in the man page :manpage:`systemd.unit(5)`. An example on which
you can base your init scripts is available in the man page
:manpage:`init-d-script(5)`.

.. _s9.4:

Console messages from ``init.d`` scripts
----------------------------------------

This section has been deleted.

.. _s-cron-jobs:

Cron jobs
---------

Packages must not modify the configuration file ``/etc/crontab``, and
they must not modify the files in ``/var/spool/cron/crontabs``.

If a package wants to install a job that has to be executed via cron, it
should place a file named as specified in
:ref:`s-cron-files` into one or more of the following
directories:

-  ``/etc/cron.hourly``

-  ``/etc/cron.daily``

-  ``/etc/cron.weekly``

-  ``/etc/cron.monthly``

As these directory names imply, the files within them are executed on an
hourly, daily, weekly, or monthly basis, respectively. The exact times
are listed in ``/etc/crontab``.

All files installed in any of these directories must be scripts (e.g.,
shell scripts or Perl scripts) so that they can easily be modified by
the local system administrator. In addition, they must be treated as
configuration files.

If a certain job has to be executed at some other frequency or at a
specific time, the package should install a file in ``/etc/cron.d`` with
a name as specified in :ref:`s-cron-files`. This file
uses the same syntax as ``/etc/crontab`` and is processed by ``cron``
automatically. The file must also be treated as a configuration file.
(Note that entries in the ``/etc/cron.d`` directory are not handled by
``anacron``. Thus, you should only use this directory for jobs which may
be skipped if the system is not running.)

Unlike ``crontab`` files described in the IEEE Std 1003.1-2008 (POSIX.1)
available from `The Open
Group <https://www.opengroup.org/onlinepubs/9699919799/>`_, the files
in ``/etc/cron.d`` and the file ``/etc/crontab`` have seven fields;
namely:

1. Minute [0,59]

2. Hour [0,23]

3. Day of the month [1,31]

4. Month of the year [1,12]

5. Day of the week ([0,6] with 0=Sunday)

6. Username

7. Command to be run

Ranges of numbers are allowed. Ranges are two numbers separated with a
hyphen. The specified range is inclusive. Lists are allowed. A list is a
set of numbers (or ranges) separated by commas. Step values can be used
in conjunction with ranges.

The scripts or ``crontab`` entries in these directories should check if
all necessary programs are installed before they try to execute them.
Otherwise, problems will arise when a package was removed but not purged
since configuration files are kept on the system in this situation.

Any ``cron`` daemon must provide ``/usr/bin/crontab`` and support normal
``crontab`` entries as specified in POSIX. The daemon must also support
names for days and months, ranges, and step values. It has to support
``/etc/crontab``, and correctly execute the scripts in ``/etc/cron.d``.
The daemon must also correctly execute scripts in
``/etc/cron.{hourly,daily,weekly,monthly}``.

.. _s-cron-files:

Cron job file names
~~~~~~~~~~~~~~~~~~~

The file name of a cron job file should normally match the name of the
package from which it comes.

If a package supplies multiple cron job files files in the same
directory, the file names should all start with the name of the package
(possibly modified as described below) followed by a hyphen (``-``) and
a suitable suffix.

A cron job file name must not include any period or plus characters
(``.`` or ``+``) characters as this will cause cron to ignore the file.
Underscores (``_``) should be used instead of ``.`` and ``+``
characters.

.. _s-menus:

Menus
-----

Packages shipping applications that comply with minimal requirements
described below for integration with desktop environments should
register these applications in the desktop menu, following the
*FreeDesktop* standard, using text files called *desktop entries*. Their
format is described in the *Desktop Entry Specification* at
https://standards.freedesktop.org/desktop-entry-spec/latest/ and
complementary information can be found in the *Desktop Menu
Specification* at https://standards.freedesktop.org/menu-spec/latest/.

The desktop entry files are installed by the packages in the directory
``/usr/share/applications`` and the FreeDesktop menus are refreshed
using *dpkg triggers*. It is therefore not necessary to depend on
packages providing FreeDesktop menu systems.

Entries displayed in the FreeDesktop menu should conform to the
following minima for relevance and visual integration.

-  Unless hidden by default, the desktop entry must point to a PNG or
   SVG icon with a transparent background, providing at least the 22×22
   size, and preferably up to 64×64. The icon should be neutral enough
   to integrate well with the default icon themes. It is encouraged to
   ship the icon in the default *hicolor* icon theme directories, or to
   use an existing icon from the *hicolor* theme.

-  If the menu entry is not useful in the general case as a standalone
   application, the desktop entry should set the ``NoDisplay`` key to
   true, so that it can be configured to be displayed only by those who
   need it.

-  In doubt, the package maintainer should coordinate with the
   maintainers of menu implementations through the *debian-desktop*
   mailing list in order to avoid problems with categories or bad
   interactions with other icons. Especially for packages which are part
   of installation tasks, the contents of the
   ``NotShowIn``/``OnlyShowIn`` keys should be validated by the
   maintainers of the relevant environments.

Since the FreeDesktop menu is a cross-distribution standard, the desktop
entries written for Debian should be forwarded upstream, where they will
benefit to other users and are more likely to receive extra
contributions such as translations.

If a package installs a FreeDesktop desktop entry, it must not also
install a Debian menu entry.

.. _s-mime:

Multimedia handlers
-------------------

Media types (formerly known as MIME types, Multipurpose Internet Mail
Extensions, RFCs 2045-2049) is a mechanism for encoding files and data
streams and providing meta-information about them, in particular their
type and format (e.g. ``image/png``, ``text/html``, ``audio/ogg``).

Registration of media type handlers allows programs like mail user
agents and web browsers to invoke these handlers to view, edit or
display media types they don't support directly.

There are two overlapping systems to associate media types to programs
which can handle them. The *mailcap* system is found on a large number
of Unix systems. The *FreeDesktop* system is aimed at Desktop
environments. In Debian, FreeDesktop entries are automatically
translated in mailcap entries, therefore packages already using desktop
entries should not use the mailcap system directly.

.. _s-media-types-freedesktop:

Registration of media type handlers with desktop entries
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Packages shipping an application able to view, edit or point to files of
a given media type, or open links with a given URI scheme, should list
it in the ``MimeType`` key of the application's `desktop
entry <#s-menus>`_. For URI schemes, the relevant MIME types are
``x-scheme-handler/*`` (e.g. ``x-scheme-handler/https``).

.. _s-mailcap:

Registration of media type handlers with mailcap entries
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Packages that are not using desktop entries for registration should
install a file in :manpage:`mailcap(5)` format (RFC 1524) in the directory
``/usr/lib/mime/packages/``. The file name should be the binary package's
name.

The mailcap package provides the ``update-mime`` program, which
integrates these registrations in the ``/etc/mailcap`` file, using dpkg
triggers.  [#]_

Packages installing desktop entries should not install mailcap entries
for the same program, because the mailcap package already reads
desktop entries.

Packages using these facilities *should not* depend on, recommend, or
suggest ``mailcap``.

.. _s-file-media-type:

Providing media types to files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The media type of a file is discovered by inspecting the file's extension
or its :manpage:`magic(5)` pattern, and interrogating a database
associating them with media types.

To support new associations between media types and files, their
characteristic file extensions and magic patterns should be registered
to the IANA (Internet Assigned Numbers Authority). See
https://www.iana.org/assignments/media-types and RFC 6838 for details.
This information will then propagate to the systems discovering file
media types in Debian, provided by the shared-mime-info, media-types
and file packages. If registration and propagation can not be waited
for, support can be asked to the maintainers of the packages mentioned
above.

For files that are produced and read by a single application, it is also
possible to declare this association to the *Shared MIME Info* system by
installing in the directory ``/usr/share/mime/packages`` a file in the
XML format specified at
https://standards.freedesktop.org/shared-mime-info-spec/latest/.

.. _s9.8:

Keyboard configuration
----------------------

To achieve a consistent keyboard configuration so that all applications
interpret a keyboard event the same way, all programs in the Debian
distribution must be configured to comply with the following guidelines.

The following keys must have the specified interpretations:

``<--``
    delete the character to the left of the cursor

``Delete``
    delete the character to the right of the cursor

``Control+H``
    emacs: the help prefix

The interpretation of any keyboard events should be independent of the
terminal that is used, be it a virtual console, an X terminal emulator,
an rlogin/telnet session, etc.

The following list explains how the different programs should be set up
to achieve this:

-  ``<--`` generates ``KB_BackSpace`` in X.

-  ``Delete`` generates ``KB_Delete`` in X.

-  X translations are set up to make ``KB_Backspace`` generate ASCII
   DEL, and to make ``KB_Delete`` generate ``ESC [ 3 ~`` (this is the vt220 escape code for the "delete
   character" key). This must be done by loading the X resources using
   ``xrdb`` on all local X displays, not using the application defaults,
   so that the translation resources used correspond to the ``xmodmap``
   settings.

-  The Linux console is configured to make ``<--`` generate DEL, and
   ``Delete`` generate ``ESC [ 3 ~``.

-  X applications are configured so that ``<`` deletes left, and
   ``Delete`` deletes right. Motif applications already work like this.

-  Terminals should have ``stty erase ^?`` .

-  The ``xterm`` terminfo entry should have ``ESC [ 3 ~`` for ``kdch1``,
   just as for ``TERM=linux`` and ``TERM=vt220``.

-  Emacs is programmed to map ``KB_Backspace`` or the ``stty erase``
   character to ``delete-backward-char``, and ``KB_Delete`` or ``kdch1``
   to ``delete-forward-char``, and ``^H`` to ``help`` as always.

-  Other applications use the ``stty erase`` character and ``kdch1`` for
   the two delete keys, with ASCII DEL being "delete previous character"
   and ``kdch1`` being "delete character under cursor".

This will solve the problem except for the following cases:

-  Some terminals have a ``<--`` key that cannot be made to produce
   anything except ``^H``. On these terminals Emacs help will be
   unavailable on ``^H`` (assuming that the ``stty erase`` character
   takes precedence in Emacs, and has been set correctly). ``M-x help`` or ``F1`` (if available) can be used instead.

-  Some operating systems use ``^H`` for ``stty erase``. However, modern
   telnet versions and all rlogin versions propagate ``stty`` settings,
   and almost all UNIX versions honour ``stty erase``. Where the
   ``stty`` settings are not propagated correctly, things can be made to
   work by using ``stty`` manually.

-  Some systems (including previous Debian versions) use ``xmodmap`` to
   arrange for both ``<--`` and ``Delete`` to generate ``KB_Delete``. We
   can change the behavior of their X clients using the same X resources
   that we use to do it for our own clients, or configure our clients
   using their resources when things are the other way around. On
   displays configured like this ``Delete`` will not work, but ``<--``
   will.

-  Some operating systems have different ``kdch1`` settings in their
   ``terminfo`` database for ``xterm`` and others. On these systems the
   ``Delete`` key will not work correctly when you log in from a system
   conforming to our policy, but ``<--`` will.

.. _s9.9:

Environment variables
---------------------

Programs installed on the system PATH (``/bin``, ``/usr/bin``,
``/sbin``, ``/usr/sbin``, or similar directories) must not depend on
custom environment variable settings to get reasonable defaults. This is
because such environment variables would have to be set in a system-wide
configuration file such as a file in ``/etc/profile.d``, which is not
supported by all shells.

If a program usually depends on environment variables for its
configuration, the program should be changed to fall back to a
reasonable default configuration if these environment variables are not
present. If this cannot be done easily (e.g., if the source code of a
non-free program is not available), the program must be replaced by a
small "wrapper" shell script that sets the environment variables if they
are not already defined, and calls the original program.

Here is an example of a wrapper script for this purpose:

::

    #!/bin/sh
    BAR=${BAR:-/var/lib/fubar}
    export BAR
    exec /usr/lib/foo/foo "$@"

.. _s-doc-base:

Registering Documents using doc-base
------------------------------------

The doc-base package implements a mechanism for handling and
presenting documentation. Debian packages that provides online
documentation (other than just manual pages) may register these
documents with doc-base by installing a doc-base control file in
``/usr/share/doc-base/``.

Please refer to the documentation that comes with the doc-base package
for information and details.

.. _s-alternateinit:

Alternate init systems
----------------------

This section has been deleted.

.. _s-upstart:

Event-based boot with upstart
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``upstart`` event-based boot system is no longer maintained in
Debian, so this section has been removed.

.. [#]
   This is necessary in order to reserve the directories for use in
   cross-installation of library packages from other architectures, as
   part of ``multiarch``.

.. [#]
   This is necessary for architecture-dependent headers file to coexist
   in a ``multiarch`` setup.

.. [#]
   These directories are used to store translators and as a set of
   standard names for mount points, respectively.

.. [#]
   ``/lib/lsb/init-functions``, which assists in writing LSB-compliant
   init scripts, may fail if ``set -e`` is in effect and echoing status messages to the
   console fails, for example.

.. [#]
   Creating, modifying or removing a file in
   ``/usr/lib/mime/packages/`` using maintainer scripts will not
   activate the trigger. In that case, it can be done by calling
   ``dpkg-trigger --no-await /usr/lib/mime/packages`` from the
   maintainer script after creating, modifying, or removing the file.

.. index::
   pair: signaling; reboot

.. _s-signalingreboot:

Signaling that a reboot is required
-----------------------------------

.. index::
   single: reboot-required
   single: reboot-required.pkgs

Programs can signal that a reboot is required by ``touch``\ing
``/run/reboot-required``.  It is conventional to add the name of the
package(s) requiring the reboot to
``/run/reboot-required.pkgs``. Programs should not add a package name
to ``/run/reboot-required.pkgs`` if it is already present there.

.. index:
   single: postinst

The ``/run/reboot-required`` mechanism is used when a reboot is
needed to fully apply the changes introduced by package
installation or upgrade.  Typically it is the ``postinst``
maintainer script that touches ``/run/reboot-required``, at the end
of a successful configuration of the package.

There are no guarantees provided by the ``/run/reboot-required``
convention as to when or whether the requested reboot will occur.
