# 9. The Operating System

## 9.1. File system hierarchy

### 9.1.1. 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`. [1]

   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. [2]

   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`. [3]

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) alongside this manual (or, if you have the debian-policy installed, you can try FHS (local copy)). The latest version, which may be a more recent version, may be found on FHS (upstream). 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 for more information).

### 9.1.2. 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.

### 9.1.3. 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.

### 9.1.4. `/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 Writing the scripts 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.

## 9.2. Users and groups

### 9.2.1. 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`.

### 9.2.2. 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.

65536-99999, 600100001-4294967293:

  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.

100000-600100000:

  Dynamically allocated subordinate user ids. See subuid(5). `useradd` in its default configuration (and thus `adduser`) automatically allocates a range of 65536 of these to each new non-system user created.

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.

### 9.2.3. 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.

## 9.3. Starting system services

Debian packages that provide system services should arrange for those services to be automatically started and stopped by the init system or service manager. This section describes how that is done.

### 9.3.1. Introduction

The default init system and service manager in Debian is `systemd`. Packages that wish to automatically start and stop system services must include `systemd` service units to do so, unless the service is only intended for use on systems running alternate init systems. See 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.

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.

`systemd` uses dependency and ordering information contained within the enabled unit files to decide which services to run and in which order. The `sysv-rc` runlevel system for `sysvinit` uses 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.

### 9.3.2. 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. [4] 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 Configuration 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 Configuration 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 /run and /run/lock for more information.

### 9.3.3. 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`).

#### 9.3.3.1. 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, 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.

#### 9.3.3.2. 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, 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.

### 9.3.4. Boot-time initialization

This section has been deleted.

### 9.3.5. Example

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

## 9.4. Console messages from `init.d` scripts

This section has been deleted.

## 9.5. 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 Cron job file names 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 Cron job file names. 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, 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}`.

### 9.5.1. 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.

## 9.6. 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.

## 9.7. 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.

### 9.7.1. 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. For URI schemes, the relevant MIME types are `x-scheme-handler/*` (e.g. `x-scheme-handler/https`).

### 9.7.2. Registration of media type handlers with mailcap entries

Packages that are not using desktop entries for registration should install a file in 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. [5]

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`.

### 9.7.3. Providing media types to files

The media type of a file is discovered by inspecting the file’s extension or its 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/.

## 9.8. 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.

## 9.9. 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=${BAR:-/var/lib/fubar}
    export BAR
    exec /usr/lib/foo/foo "$@"

## 9.10. 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.

## 9.11. Alternate init systems

This section has been deleted.

### 9.11.1. Event-based boot with upstart

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

[1]

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

[2]

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

[3]

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

[4]

`/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.

[5]

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.

## 9.12. Signaling that a reboot is required

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.

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.