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man: reorder fields in os-release

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Let's order the fields from the most general to least: os name, os variant, os
version, machine-parseable version details, metadata, special settings. I added
section headers to roughly group the settings. The division is not strict,
because for example CPE_NAME also includes the version, and PRETTY_NAME may
too, but it still makes it easier to find the right name.

Also split out Examples to separate paragraphs:
almost all descriptions had "Example:" at the end, where multiple
examples were listed. Splitting this out to separate paragraphs
makes the whole thing much easier to read.

Add missing markup and punctuation while at it.

About
- If not set, defaults to <literal>NAME=Linux</literal>.
+ If not set, a default of <literal>NAME=Linux</literal> may be used.
and similar changes: in many circumstances, if this is not set, no value should
be used. The fallback mostly make sense when we need to present something to the
user. So let's reword this to not imply that the default is necessary.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2021-05-20 14:57:06 +02:00
parent 354dadb30f
commit 00e3abe024
1 changed files with 254 additions and 266 deletions
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520
man/os-release.xml
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@ -87,315 +87,303 @@
<para>The following OS identifications parameters may be set using
<filename>os-release</filename>:</para>
<variablelist class='environment-variables'>
<refsect2>
<title>General information identifying the operating system</title>
<varlistentry>
<term><varname>NAME=</varname></term>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>NAME=</varname></term>
<listitem><para>A string identifying the operating system,
without a version component, and suitable for presentation to
the user. If not set, defaults to
<literal>NAME=Linux</literal>. Example:
<literal>NAME=Fedora</literal> or <literal>NAME="Debian
GNU/Linux"</literal>.</para></listitem>
</varlistentry>
<listitem><para>A string identifying the operating system, without a version component, and
suitable for presentation to the user. If not set, a default of <literal>NAME=Linux</literal> may
be used.</para>
<varlistentry>
<term><varname>VERSION=</varname></term>
<para>Examples: <literal>NAME=Fedora</literal>, <literal>NAME="Debian GNU/Linux"</literal>.
</para></listitem>
</varlistentry>
<listitem><para>A string identifying the operating system
version, excluding any OS name information, possibly including
a release code name, and suitable for presentation to the
user. This field is optional. Example:
<literal>VERSION=17</literal> or <literal>VERSION="17 (Beefy
Miracle)"</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ID=</varname></term>
<varlistentry>
<term><varname>ID=</varname></term>
<listitem><para>A lower-case string (no spaces or other characters outside of 0–9, a–z, ".", "_"
and "-") identifying the operating system, excluding any version information and suitable for
processing by scripts or usage in generated filenames. If not set, a default of
<literal>ID=linux</literal> may be used.</para>
<listitem><para>A lower-case string (no spaces or other
characters outside of 0–9, a–z, ".", "_" and "-") identifying
the operating system, excluding any version information and
suitable for processing by scripts or usage in generated
filenames. If not set, defaults to
<literal>ID=linux</literal>. Example:
<literal>ID=fedora</literal> or
<literal>ID=debian</literal>.</para></listitem>
</varlistentry>
<para>Examples: <literal>ID=fedora</literal>, <literal>ID=debian</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ID_LIKE=</varname></term>
<varlistentry>
<term><varname>ID_LIKE=</varname></term>
<listitem><para>A space-separated list of operating system
identifiers in the same syntax as the <varname>ID=</varname>
setting. It should list identifiers of operating systems that
are closely related to the local operating system in regards
to packaging and programming interfaces, for example listing
one or more OS identifiers the local OS is a derivative from.
An OS should generally only list other OS identifiers it
itself is a derivative of, and not any OSes that are derived
from it, though symmetric relationships are possible. Build
scripts and similar should check this variable if they need to
identify the local operating system and the value of
<varname>ID=</varname> is not recognized. Operating systems
should be listed in order of how closely the local operating
system relates to the listed ones, starting with the closest.
This field is optional. Example: for an operating system with
<literal>ID=centos</literal>, an assignment of
<literal>ID_LIKE="rhel fedora"</literal> would be appropriate.
For an operating system with <literal>ID=ubuntu</literal>, an
assignment of <literal>ID_LIKE=debian</literal> is
appropriate.</para></listitem>
</varlistentry>
<listitem><para>A space-separated list of operating system identifiers in the same syntax as the
<varname>ID=</varname> setting. It should list identifiers of operating systems that are closely
related to the local operating system in regards to packaging and programming interfaces, for
example listing one or more OS identifiers the local OS is a derivative from. An OS should
generally only list other OS identifiers it itself is a derivative of, and not any OSes that are
derived from it, though symmetric relationships are possible. Build scripts and similar should
check this variable if they need to identify the local operating system and the value of
<varname>ID=</varname> is not recognized. Operating systems should be listed in order of how
closely the local operating system relates to the listed ones, starting with the closest. This
field is optional.</para>
<varlistentry>
<term><varname>VERSION_CODENAME=</varname></term>
<para>Examples: for an operating system with <literal>ID=centos</literal>, an assignment of
<literal>ID_LIKE="rhel fedora"</literal> would be appropriate. For an operating system with
<literal>ID=ubuntu</literal>, an assignment of <literal>ID_LIKE=debian</literal> is appropriate.
</para></listitem>
</varlistentry>
<listitem><para>
A lower-case string (no spaces or other characters outside of
0–9, a–z, ".", "_" and "-") identifying the operating system
release code name, excluding any OS name information or
release version, and suitable for processing by scripts or
usage in generated filenames. This field is optional and may
not be implemented on all systems.
Examples:
<literal>VERSION_CODENAME=buster</literal>,
<literal>VERSION_CODENAME=xenial</literal>
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>PRETTY_NAME=</varname></term>
<varlistentry>
<term><varname>VERSION_ID=</varname></term>
<listitem><para>A pretty operating system name in a format suitable for presentation to the
user. May or may not contain a release code name or OS version of some kind, as suitable. If not
set, a default of <literal>PRETTY_NAME="Linux"</literal> may be used</para>
<listitem><para>A lower-case string (mostly numeric, no spaces
or other characters outside of 0–9, a–z, ".", "_" and "-")
identifying the operating system version, excluding any OS
name information or release code name, and suitable for
processing by scripts or usage in generated filenames. This
field is optional. Example: <literal>VERSION_ID=17</literal>
or <literal>VERSION_ID=11.04</literal>.</para></listitem>
</varlistentry>
<para>Example: <literal>PRETTY_NAME="Fedora 17 (Beefy Miracle)"</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>PRETTY_NAME=</varname></term>
<varlistentry>
<term><varname>CPE_NAME=</varname></term>
<listitem><para>A pretty operating system name in a format
suitable for presentation to the user. May or may not contain
a release code name or OS version of some kind, as suitable.
If not set, defaults to
<literal>PRETTY_NAME="Linux"</literal>. Example:
<literal>PRETTY_NAME="Fedora 17 (Beefy
Miracle)"</literal>.</para></listitem>
</varlistentry>
<listitem><para>A CPE name for the operating system, in URI binding syntax, following the <ulink
url="http://scap.nist.gov/specifications/cpe/">Common Platform Enumeration Specification</ulink> as
proposed by the NIST. This field is optional.</para>
<varlistentry>
<term><varname>ANSI_COLOR=</varname></term>
<para>Example: <literal>CPE_NAME="cpe:/o:fedoraproject:fedora:17"</literal></para></listitem>
</varlistentry>
<listitem><para>A suggested presentation color when showing the OS name on the console. This should
be specified as string suitable for inclusion in the ESC [ m ANSI/ECMA-48 escape code for setting
graphical rendition. This field is optional. Example: <literal>ANSI_COLOR="0;31"</literal> for red,
<literal>ANSI_COLOR="1;34"</literal> for light blue, or
<literal>ANSI_COLOR="0;38;2;60;110;180"</literal> for Fedora blue.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>VARIANT=</varname></term>
<varlistentry>
<term><varname>CPE_NAME=</varname></term>
<listitem><para>A string identifying a specific variant or edition of the operating system suitable
for presentation to the user. This field may be used to inform the user that the configuration of
this system is subject to a specific divergent set of rules or default configuration settings. This
field is optional and may not be implemented on all systems.</para>
<listitem><para>A CPE name for the operating system, in URI
binding syntax, following the
<ulink url="http://scap.nist.gov/specifications/cpe/">Common
Platform Enumeration Specification</ulink> as proposed by the
NIST. This field is optional. Example:
<literal>CPE_NAME="cpe:/o:fedoraproject:fedora:17"</literal>
</para></listitem>
</varlistentry>
<para>Examples: <literal>VARIANT="Server Edition"</literal>, <literal>VARIANT="Smart Refrigerator
Edition"</literal>.</para>
<varlistentry>
<term><varname>HOME_URL=</varname></term>
<term><varname>DOCUMENTATION_URL=</varname></term>
<term><varname>SUPPORT_URL=</varname></term>
<term><varname>BUG_REPORT_URL=</varname></term>
<term><varname>PRIVACY_POLICY_URL=</varname></term>
<para>Note: this field is for display purposes only. The <varname>VARIANT_ID</varname> field should
be used for making programmatic decisions.</para></listitem>
</varlistentry>
<listitem><para>Links to resources on the Internet related to
the operating system.
<varname>HOME_URL=</varname> should refer to the homepage of
the operating system, or alternatively some homepage of the
specific version of the operating system.
<varname>DOCUMENTATION_URL=</varname> should refer to the main
documentation page for this operating system.
<varname>SUPPORT_URL=</varname> should refer to the main
support page for the operating system, if there is any. This
is primarily intended for operating systems which vendors
provide support for. <varname>BUG_REPORT_URL=</varname> should
refer to the main bug reporting page for the operating system,
if there is any. This is primarily intended for operating
systems that rely on community QA.
<varname>PRIVACY_POLICY_URL=</varname> should refer to the
main privacy policy page for the operating system, if there is
any. These settings are optional, and providing only some of
these settings is common. These URLs are intended to be
exposed in "About this system" UIs behind links with captions
such as "About this Operating System", "Obtain Support",
"Report a Bug", or "Privacy Policy". The values should be in
<ulink url="https://tools.ietf.org/html/rfc3986">RFC3986
format</ulink>, and should be <literal>http:</literal> or
<literal>https:</literal> URLs, and possibly
<literal>mailto:</literal> or <literal>tel:</literal>. Only
one URL shall be listed in each setting. If multiple resources
need to be referenced, it is recommended to provide an online
landing page linking all available resources. Examples:
<literal>HOME_URL="https://fedoraproject.org/"</literal> and
<literal>BUG_REPORT_URL="https://bugzilla.redhat.com/"</literal></para></listitem>
</varlistentry>
<varlistentry>
<term><varname>VARIANT_ID=</varname></term>
<varlistentry>
<term><varname>BUILD_ID=</varname></term>
<listitem><para>A lower-case string (no spaces or other characters outside of 0–9, a–z, ".", "_" and
"-"), identifying a specific variant or edition of the operating system. This may be interpreted by
other packages in order to determine a divergent default configuration. This field is optional and
may not be implemented on all systems.</para>
<listitem><para>A string uniquely identifying the system image
used as the origin for a distribution (it is not updated with
system updates). The field can be identical between different
VERSION_IDs as BUILD_ID is an only a unique identifier to a
specific version. Distributions that release each update as a
new version would only need to use VERSION_ID as each build is
already distinct based on the VERSION_ID. This field is
optional. Example: <literal>BUILD_ID="2013-03-20.3"</literal>
or <literal>BUILD_ID=201303203</literal>.
<para>Examples: <literal>VARIANT_ID=server</literal>, <literal>VARIANT_ID=embedded</literal>.
</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
</para></listitem>
</varlistentry>
<refsect2>
<title>Information about the version of the operating system</title>
<varlistentry>
<term><varname>VARIANT=</varname></term>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>VERSION=</varname></term>
<listitem><para>
A string identifying a specific variant or edition of the
operating system suitable for presentation to the user. This
field may be used to inform the user that the configuration of
this system is subject to a specific divergent set of rules or
default configuration settings. This field is optional and may
not be implemented on all systems.
Examples:
<literal>VARIANT="Server Edition"</literal>,
<literal>VARIANT="Smart Refrigerator Edition"</literal>
Note: this field is for display purposes only. The
<varname>VARIANT_ID</varname> field should be used for making
programmatic decisions.
</para></listitem>
</varlistentry>
<listitem><para>A string identifying the operating system version, excluding any OS name
information, possibly including a release code name, and suitable for presentation to the
user. This field is optional.</para>
<varlistentry>
<term><varname>VARIANT_ID=</varname></term>
<para>Examples: <literal>VERSION=17</literal>, <literal>VERSION="17 (Beefy Miracle)"</literal>.
</para></listitem>
</varlistentry>
<listitem><para>
A lower-case string (no spaces or other characters outside of
0–9, a–z, ".", "_" and "-"), identifying a specific variant or
edition of the operating system. This may be interpreted by
other packages in order to determine a divergent default
configuration. This field is optional and may not be
implemented on all systems.
Examples:
<literal>VARIANT_ID=server</literal>,
<literal>VARIANT_ID=embedded</literal>
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>VERSION_ID=</varname></term>
<varlistentry>
<term><varname>LOGO=</varname></term>
<listitem><para>A lower-case string (mostly numeric, no spaces or other characters outside of 0–9,
a–z, ".", "_" and "-") identifying the operating system version, excluding any OS name information
or release code name, and suitable for processing by scripts or usage in generated filenames. This
field is optional.</para>
<listitem><para>
A string, specifying the name of an icon as defined by <ulink
url="http://standards.freedesktop.org/icon-theme-spec/latest">
freedesktop.org Icon Theme Specification</ulink>. This can be
used by graphical applications to display an operating
system's or distributor's logo. This field is optional and
may not necessarily be implemented on all systems.
Examples:
<literal>LOGO=fedora-logo</literal>,
<literal>LOGO=distributor-logo-opensuse</literal>
</para></listitem>
</varlistentry>
<para>Examples: <literal>VERSION_ID=17</literal>, <literal>VERSION_ID=11.04</literal>.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DEFAULT_HOSTNAME=</varname></term>
<varlistentry>
<term><varname>VERSION_CODENAME=</varname></term>
<listitem><para>A string specifying the hostname if
<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry> is not
present and no other configuration source specifies the hostname. Must be either a single DNS label
(a string composed of 7-bit ASCII lower-case characters and no spaces or dots, limited to the format
allowed for DNS domain name labels), or a sequence of such labels separated by single dots that forms
a valid DNS FQDN. The hostname must be at most 64 characters, which is a Linux limitation (DNS allows
longer names).</para>
<listitem><para>A lower-case string (no spaces or other characters outside of 0–9, a–z, ".", "_"
and "-") identifying the operating system release code name, excluding any OS name information or
release version, and suitable for processing by scripts or usage in generated filenames. This field
is optional and may not be implemented on all systems.</para>
<para>See
<citerefentry><refentrytitle>org.freedesktop.hostname1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for a description of how
<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
determines the fallback hostname.</para></listitem>
</varlistentry>
<para>Examples: <literal>VERSION_CODENAME=buster</literal>,
<literal>VERSION_CODENAME=xenial</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>SYSEXT_LEVEL=</varname></term>
<varlistentry>
<term><varname>BUILD_ID=</varname></term>
<listitem><para>A lower-case string (mostly numeric, no spaces or other characters outside of 0–9,
a–z, ".", "_" and "-") identifying the operating system extensions support level, to indicate which
extension images are supported (See:
<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>).
Example: <literal>SYSEXT_LEVEL=2</literal> or
<literal>SYSEXT_LEVEL=15.14</literal>.</para></listitem>
</varlistentry>
<listitem><para>A string uniquely identifying the system image used as the origin for a distribution
(it is not updated with system updates). The field can be identical between different
<varname>VERSION_ID</varname>s as <varname>BUILD_ID</varname> is an only a unique identifier to a
specific version. Distributions that release each update as a new version would only need to use
<varname>VERSION_ID</varname> as each build is already distinct based on the
<varname>VERSION_ID</varname>. This field is optional.</para>
<varlistentry>
<term><varname>IMAGE_ID=</varname></term>
<para>Examples: <literal>BUILD_ID="2013-03-20.3"</literal>, <literal>BUILD_ID=201303203</literal>.
</para></listitem>
</varlistentry>
<listitem><para> A lower-case string (no spaces or other characters outside of 0–9, a–z, ".", "_" and
"-"), identifying a specific image of the operating system. This is supposed to be used for
environments where OS images are prepared, built, shipped and updated as comprehensive, consistent OS
images. This field is optional and may not be implemented on all systems, in particularly not on those
that are not managed via images but put together and updated from individual packages and on the
local system. Examples: <literal>IMAGE_ID=vendorx-cashier-system</literal>,
<literal>IMAGE_ID=netbook-image</literal> </para></listitem>
</varlistentry>
<varlistentry>
<term><varname>IMAGE_ID=</varname></term>
<varlistentry>
<term><varname>IMAGE_VERSION=</varname></term>
<listitem><para> A lower-case string (no spaces or other characters outside of 0–9, a–z, ".", "_"
and "-"), identifying a specific image of the operating system. This is supposed to be used for
environments where OS images are prepared, built, shipped and updated as comprehensive, consistent
OS images. This field is optional and may not be implemented on all systems, in particularly not on
those that are not managed via images but put together and updated from individual packages and on
the local system.</para>
<listitem><para>A lower-case string (mostly numeric, no spaces or other characters outside of 0–9,
a–z, ".", "_" and "-") identifying the OS image version. This is supposed to be used together with
<varname>IMAGE_ID</varname> described above, to discern different versions of the same
image. Examples: <literal>IMAGE_VERSION=33</literal>,
<literal>IMAGE_VERSION=47.1rc1</literal> </para></listitem>
</varlistentry>
<para>Examples: <literal>IMAGE_ID=vendorx-cashier-system</literal>,
<literal>IMAGE_ID=netbook-image</literal>.</para></listitem>
</varlistentry>
</variablelist>
<varlistentry>
<term><varname>IMAGE_VERSION=</varname></term>
<para>If you are reading this file from C code or a shell script
to determine the OS or a specific version of it, use the
<varname>ID</varname> and <varname>VERSION_ID</varname> fields,
possibly with <varname>ID_LIKE</varname> as fallback for
<varname>ID</varname>. When looking for an OS identification
string for presentation to the user use the
<varname>PRETTY_NAME</varname> field.</para>
<listitem><para>A lower-case string (mostly numeric, no spaces or other characters outside of 0–9,
a–z, ".", "_" and "-") identifying the OS image version. This is supposed to be used together with
<varname>IMAGE_ID</varname> described above, to discern different versions of the same image.
</para>
<para>Note that operating system vendors may choose not to provide
version information, for example to accommodate for rolling
releases. In this case, <varname>VERSION</varname> and
<varname>VERSION_ID</varname> may be unset. Applications should
not rely on these fields to be set.</para>
<para>Examples: <literal>IMAGE_VERSION=33</literal>, <literal>IMAGE_VERSION=47.1rc1</literal>.
</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
<para>Operating system vendors may extend the file
format and introduce new fields. It is highly
recommended to prefix new fields with an OS specific
name in order to avoid name clashes. Applications
reading this file must ignore unknown fields. Example:
<literal>DEBIAN_BTS="debbugs://bugs.debian.org/"</literal></para>
<refsect2>
<title>Presentation information and links</title>
<para>Container and sandbox runtime managers may make the host's
identification data available to applications by providing the host's
<filename>/etc/os-release</filename> (if available, otherwise
<filename>/usr/lib/os-release</filename> as a fallback) as
<filename>/run/host/os-release</filename>.</para>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>HOME_URL=</varname></term>
<term><varname>DOCUMENTATION_URL=</varname></term>
<term><varname>SUPPORT_URL=</varname></term>
<term><varname>BUG_REPORT_URL=</varname></term>
<term><varname>PRIVACY_POLICY_URL=</varname></term>
<listitem><para>Links to resources on the Internet related to the operating system.
<varname>HOME_URL=</varname> should refer to the homepage of the operating system, or alternatively
some homepage of the specific version of the operating system.
<varname>DOCUMENTATION_URL=</varname> should refer to the main documentation page for this
operating system. <varname>SUPPORT_URL=</varname> should refer to the main support page for the
operating system, if there is any. This is primarily intended for operating systems which vendors
provide support for. <varname>BUG_REPORT_URL=</varname> should refer to the main bug reporting page
for the operating system, if there is any. This is primarily intended for operating systems that
rely on community QA. <varname>PRIVACY_POLICY_URL=</varname> should refer to the main privacy
policy page for the operating system, if there is any. These settings are optional, and providing
only some of these settings is common. These URLs are intended to be exposed in "About this system"
UIs behind links with captions such as "About this Operating System", "Obtain Support", "Report a
Bug", or "Privacy Policy". The values should be in <ulink
url="https://tools.ietf.org/html/rfc3986">RFC3986 format</ulink>, and should be
<literal>http:</literal> or <literal>https:</literal> URLs, and possibly <literal>mailto:</literal>
or <literal>tel:</literal>. Only one URL shall be listed in each setting. If multiple resources
need to be referenced, it is recommended to provide an online landing page linking all available
resources.</para>
<para>Examples: <literal>HOME_URL="https://fedoraproject.org/"</literal>,
<literal>BUG_REPORT_URL="https://bugzilla.redhat.com/"</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LOGO=</varname></term>
<listitem><para>A string, specifying the name of an icon as defined by <ulink
url="http://standards.freedesktop.org/icon-theme-spec/latest">freedesktop.org Icon Theme
Specification</ulink>. This can be used by graphical applications to display an operating system's
or distributor's logo. This field is optional and may not necessarily be implemented on all
systems.</para>
<para>Examples: <literal>LOGO=fedora-logo</literal>, <literal>LOGO=distributor-logo-opensuse</literal>
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ANSI_COLOR=</varname></term>
<listitem><para>A suggested presentation color when showing the OS name on the console. This should
be specified as string suitable for inclusion in the ESC [ m ANSI/ECMA-48 escape code for setting
graphical rendition. This field is optional.</para>
<para>Examples: <literal>ANSI_COLOR="0;31"</literal> for red, <literal>ANSI_COLOR="1;34"</literal>
for light blue, or <literal>ANSI_COLOR="0;38;2;60;110;180"</literal> for Fedora blue.
</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
<refsect2>
<title>Distribution-level defaults and metadata</title>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>DEFAULT_HOSTNAME=</varname></term>
<listitem><para>A string specifying the hostname if
<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry> is not
present and no other configuration source specifies the hostname. Must be either a single DNS label
(a string composed of 7-bit ASCII lower-case characters and no spaces or dots, limited to the
format allowed for DNS domain name labels), or a sequence of such labels separated by single dots
that forms a valid DNS FQDN. The hostname must be at most 64 characters, which is a Linux
limitation (DNS allows longer names).</para>
<para>See <citerefentry><refentrytitle>org.freedesktop.hostname1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for a description of how
<citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
determines the fallback hostname.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>SYSEXT_LEVEL=</varname></term>
<listitem><para>A lower-case string (mostly numeric, no spaces or other characters outside of 0–9,
a–z, ".", "_" and "-") identifying the operating system extensions support level, to indicate which
extension images are supported. See:
<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>)
for more information.</para>
<para>Examples: <literal>SYSEXT_LEVEL=2</literal>, <literal>SYSEXT_LEVEL=15.14</literal>.
</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
<refsect2>
<title>Notes</title>
<para>If you are reading this file from C code or a shell script to determine the OS or a specific
version of it, use the <varname>ID</varname> and <varname>VERSION_ID</varname> fields, possibly with
<varname>ID_LIKE</varname> as fallback for <varname>ID</varname>. When looking for an OS identification
string for presentation to the user use the <varname>PRETTY_NAME</varname> field.</para>
<para>Note that operating system vendors may choose not to provide version information, for example to
accommodate for rolling releases. In this case, <varname>VERSION</varname> and
<varname>VERSION_ID</varname> may be unset. Applications should not rely on these fields to be
set.</para>
<para>Operating system vendors may extend the file format and introduce new fields. It is highly
recommended to prefix new fields with an OS specific name in order to avoid name clashes. Applications
reading this file must ignore unknown fields.</para>
<para>Example: <literal>DEBIAN_BTS="debbugs://bugs.debian.org/"</literal>.</para>
<para>Container and sandbox runtime managers may make the host's identification data available to
applications by providing the host's <filename>/etc/os-release</filename> (if available, otherwise
<filename>/usr/lib/os-release</filename> as a fallback) as
<filename>/run/host/os-release</filename>.</para>
</refsect2>
</refsect1>
<refsect1>