journal: Serialize __MONOTONIC_TIMESTAMP metadata field as well

[thirdparty/systemd.git] / man / os-release.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="os-release" xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>os-release</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>os-release</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>os-release</refname>
    <refname>initrd-release</refname>
    <refname>extension-release</refname>
    <refpurpose>Operating system identification</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><simplelist>
      <member><filename>/etc/os-release</filename></member>
      <member><filename>/usr/lib/os-release</filename></member>
      <member><filename>/etc/initrd-release</filename></member>
      <member><filename>/usr/lib/extension-release.d/extension-release.<replaceable>IMAGE</replaceable></filename></member>
    </simplelist></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>The <filename>/etc/os-release</filename> and
    <filename>/usr/lib/os-release</filename> files contain operating
    system identification data.</para>

    <para>The format of <filename>os-release</filename> is a newline-separated list of
    environment-like shell-compatible variable assignments. It is possible to source the configuration from
    Bourne shell scripts, however, beyond mere variable assignments, no shell features are supported (this
    means variable expansion is explicitly not supported), allowing applications to read the file without
    implementing a shell compatible execution engine. Variable assignment values must be enclosed in double
    or single quotes if they include spaces, semicolons or other special characters outside of A–Z, a–z,
    0–9. (Assignments that do not include these special characters may be enclosed in quotes too, but this is
    optional.) Shell special characters ("$", quotes, backslash, backtick) must be escaped with backslashes,
    following shell style. All strings should be in UTF-8 encoding, and non-printable characters should not
    be used. Concatenation of multiple individually quoted strings is not supported. Lines beginning with "#"
    are treated as comments. Blank lines are permitted and ignored.</para>

    <para>The file <filename>/etc/os-release</filename> takes
    precedence over <filename>/usr/lib/os-release</filename>.
    Applications should check for the former, and exclusively use its
    data if it exists, and only fall back to
    <filename>/usr/lib/os-release</filename> if it is missing.
    Applications should not read data from both files at the same
    time. <filename>/usr/lib/os-release</filename> is the recommended
    place to store OS release information as part of vendor trees.
    <filename>/etc/os-release</filename> should be a relative symlink
    to <filename>/usr/lib/os-release</filename>, to provide
    compatibility with applications only looking at
    <filename>/etc/</filename>. A relative symlink instead of an
    absolute symlink is necessary to avoid breaking the link in a
    chroot or initrd environment.</para>

    <para><filename>os-release</filename> contains data that is
    defined by the operating system vendor and should generally not be
    changed by the administrator.</para>

    <para>As this file only encodes names and identifiers it should
    not be localized.</para>

    <para>The <filename>/etc/os-release</filename> and
    <filename>/usr/lib/os-release</filename> files might be symlinks
    to other files, but it is important that the file is available
    from earliest boot on, and hence must be located on the root file
    system.</para>

    <para><filename>os-release</filename> must not contain repeating keys. Nevertheless, readers should pick
    the entries later in the file in case of repeats, similarly to how a shell sourcing the file would. A
    reader may warn about repeating entries.</para>

    <para>For a longer rationale for <filename>os-release</filename>
    please refer to the <ulink
    url="https://0pointer.de/blog/projects/os-release">Announcement of <filename>/etc/os-release</filename></ulink>.</para>

    <refsect2>
      <title><filename>/etc/initrd-release</filename></title>

      <para>In the <ulink
      url="https://docs.kernel.org/admin-guide/initrd.html">initrd</ulink>,
      <filename>/etc/initrd-release</filename> plays the same role as <filename>os-release</filename> in the
      main system. Additionally, the presence of that file means that the system is in the initrd phase.
      <filename>/etc/os-release</filename> should be symlinked to <filename>/etc/initrd-release</filename>
      (or vice versa), so programs that only look for <filename>/etc/os-release</filename> (as described
      above) work correctly.</para>

      <para>The rest of this document that talks about <filename>os-release</filename> should be understood
      to apply to <filename>initrd-release</filename> too.</para>
    </refsect2>

    <refsect2>
      <title><filename>/usr/lib/extension-release.d/extension-release.<replaceable>IMAGE</replaceable></filename></title>

      <para><filename>/usr/lib/extension-release.d/extension-release.<replaceable>IMAGE</replaceable></filename>
      plays the same role for extension images as <filename>os-release</filename> for the main system, and
      follows the syntax and rules as described in the <ulink
      url="https://systemd.io/PORTABLE_SERVICES">Portable Services</ulink> page. The purpose of this
      file is to identify the extension and to allow the operating system to verify that the extension image
      matches the base OS. This is typically implemented by checking that the <varname>ID=</varname> options
      match, and either <varname>SYSEXT_LEVEL=</varname> exists and matches too, or if it is not present,
      <varname>VERSION_ID=</varname> exists and matches. This ensures ABI/API compatibility between the
      layers and prevents merging of an incompatible image in an overlay.</para>

      <para>In order to identify the extension image itself, the same fields defined below can be added to the
      <filename>extension-release</filename> file with a <varname>SYSEXT_</varname> prefix (to disambiguate
      from fields used to match on the base image). E.g.: <varname>SYSEXT_ID=myext</varname>,
      <varname>SYSEXT_VERSION_ID=1.2.3</varname>.</para>

      <para>In the <filename>extension-release.<replaceable>IMAGE</replaceable></filename> filename, the
      <replaceable>IMAGE</replaceable> part must exactly match the file name of the containing image with the
      suffix removed. In case it is not possible to guarantee that an image file name is stable and doesn't
      change between the build and the deployment phases, it is possible to relax this check: if exactly one
      file whose name matches <literal><filename>extension-release.*</filename></literal> is present in this
      directory, and the file is tagged with a <varname>user.extension-release.strict</varname>
      <citerefentry project='man-pages'><refentrytitle>xattr</refentrytitle><manvolnum>7</manvolnum></citerefentry> set to the
      string <literal>0</literal>, it will be used instead.</para>

      <para>The rest of this document that talks about <filename>os-release</filename> should be understood
      to apply to <filename>extension-release</filename> too.</para>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <para>The following OS identifications parameters may be set using
    <filename>os-release</filename>:</para>

    <refsect2>
      <title>General information identifying the operating system</title>

      <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, a default of <literal>NAME=Linux</literal> may
          be used.</para>

          <para>Examples: <literal>NAME=Fedora</literal>, <literal>NAME="Debian GNU/Linux"</literal>.
          </para></listitem>
        </varlistentry>

        <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. Note that even though this string may not include
          characters that require shell quoting, quoting may nevertheless be used.</para>

          <para>Examples: <literal>ID=fedora</literal>, <literal>ID=debian</literal>.</para></listitem>
        </varlistentry>

        <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.</para>

          <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>

        <varlistentry>
          <term><varname>PRETTY_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, a default of <literal>PRETTY_NAME="Linux"</literal> may be used</para>

          <para>Example: <literal>PRETTY_NAME="Fedora 17 (Beefy Miracle)"</literal>.</para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>CPE_NAME=</varname></term>

          <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>

          <para>Example: <literal>CPE_NAME="cpe:/o:fedoraproject:fedora:17"</literal></para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>VARIANT=</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>

          <para>Examples: <literal>VARIANT="Server Edition"</literal>, <literal>VARIANT="Smart Refrigerator
          Edition"</literal>.</para>

          <para>Note: this field is for display purposes only. The <varname>VARIANT_ID</varname> field should
          be used for making programmatic decisions.</para>

          <xi:include href="version-info.xml" xpointer="v220"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>VARIANT_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>

          <para>Examples: <literal>VARIANT_ID=server</literal>, <literal>VARIANT_ID=embedded</literal>.
          </para>

          <xi:include href="version-info.xml" xpointer="v220"/></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2>
      <title>Information about the version of the operating system</title>

      <variablelist class='environment-variables'>
        <varlistentry>
          <term><varname>VERSION=</varname></term>

          <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>

          <para>Examples: <literal>VERSION=17</literal>, <literal>VERSION="17 (Beefy Miracle)"</literal>.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>VERSION_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 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>

          <para>Examples: <literal>VERSION_ID=17</literal>, <literal>VERSION_ID=11.04</literal>.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>VERSION_CODENAME=</varname></term>

          <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>Examples: <literal>VERSION_CODENAME=buster</literal>,
          <literal>VERSION_CODENAME=xenial</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v231"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>BUILD_ID=</varname></term>

          <listitem><para>A string uniquely identifying the system image originally used as the installation
          base. In most cases, <varname>VERSION_ID</varname> or
          <varname>IMAGE_ID</varname>+<varname>IMAGE_VERSION</varname> are updated when the entire system
          image is replaced during an update. <varname>BUILD_ID</varname> may be used in distributions where
          the original installation image version is important: <varname>VERSION_ID</varname> would change
          during incremental system updates, but <varname>BUILD_ID</varname> would not. This field is
          optional.</para>

          <para>Examples: <literal>BUILD_ID="2013-03-20.3"</literal>, <literal>BUILD_ID=201303203</literal>.
          </para>

          <xi:include href="version-info.xml" xpointer="v200"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>IMAGE_ID=</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>

          <para>Examples: <literal>IMAGE_ID=vendorx-cashier-system</literal>,
          <literal>IMAGE_ID=netbook-image</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v249"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>IMAGE_VERSION=</varname></term>

          <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>Examples: <literal>IMAGE_VERSION=33</literal>, <literal>IMAGE_VERSION=47.1rc1</literal>.
          </para>

          <xi:include href="version-info.xml" xpointer="v249"/></listitem>
        </varlistentry>
      </variablelist>

      <para>To summarize: if the image updates are built and shipped as comprehensive units,
      <varname>IMAGE_ID</varname>+<varname>IMAGE_VERSION</varname> is the best fit. Otherwise, if updates
      eventually completely replace previously installed contents, as in a typical binary distribution,
      <varname>VERSION_ID</varname> should be used to identify major releases of the operating system.
      <varname>BUILD_ID</varname> may be used instead or in addition to <varname>VERSION_ID</varname> when
      the original system image version is important.</para>
    </refsect2>

    <refsect2>
      <title>Presentation information and links</title>

      <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>SUPPORT_END=</varname></term>

          <listitem><para>The date at which support for this version of the OS ends. (What exactly "lack of
          support" means varies between vendors, but generally users should assume that updates, including
          security fixes, will not be provided.) The value is a date in the ISO 8601 format
          <literal>YYYY-MM-DD</literal>, and specifies the first day on which support <emphasis>is
          not</emphasis> provided.</para>

          <para>For example, <literal>SUPPORT_END=2001-01-01</literal> means that the system was supported
          until the end of the last day of the previous millennium.</para>

          <xi:include href="version-info.xml" xpointer="v252"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>LOGO=</varname></term>

          <listitem><para>A string, specifying the name of an icon as defined by <ulink
          url="https://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>

          <xi:include href="version-info.xml" xpointer="v240"/></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>

        <varlistentry>
          <term><varname>VENDOR_NAME=</varname></term>

          <listitem><para>The name of the OS vendor. This is the name of the organization or company which
          produces the OS. This field is optional.</para>

          <para>This name is intended to be exposed in "About this system" UIs or software update UIs when
          needed to distinguish the OS vendor from the OS itself. It is intended to be human readable.</para>

          <para>Examples: <literal>VENDOR_NAME="Fedora Project"</literal> for Fedora Linux,
          <literal>VENDOR_NAME="Canonical"</literal> for Ubuntu.</para>

          <xi:include href="version-info.xml" xpointer="v254"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>VENDOR_URL=</varname></term>

          <listitem><para>The homepage of the OS vendor. This field is optional. The
          <varname>VENDOR_NAME=</varname> field should be set if this one is, although clients must be
          robust against either field not being set.</para>

          <para>The value 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. Only one URL shall be listed in the
          setting.</para>

          <para>Examples: <literal>VENDOR_URL="https://fedoraproject.org/"</literal>,
          <literal>VENDOR_URL="https://canonical.com/"</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v254"/></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>

          <xi:include href="version-info.xml" xpointer="v248"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>ARCHITECTURE=</varname></term>
          <listitem><para>A string that specifies which CPU architecture the userspace binaries require.
          The architecture identifiers are the same as for <varname>ConditionArchitecture=</varname>
          described in <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
          The field is optional and should only be used when just single architecture is supported.
          It may provide redundant information when used in a GPT partition with a GUID type that already
          encodes the architecture. If this is not the case, the architecture should be specified in
          e.g., an extension image, to prevent an incompatible host from loading it.
          </para>

          <xi:include href="version-info.xml" xpointer="v252"/></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 <filename>/usr/lib/extension-release.d/extension-release.<replaceable>IMAGE</replaceable></filename>,
          <ulink url="https://docs.kernel.org/admin-guide/initrd.html">initrd</ulink> and
          <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>

          <xi:include href="version-info.xml" xpointer="v248"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>CONFEXT_LEVEL=</varname></term>

          <listitem><para>Semantically the same as <varname>SYSEXT_LEVEL=</varname> but for confext images.
          See <filename>/etc/extension-release.d/extension-release.<replaceable>IMAGE</replaceable></filename>
          for more information.</para>

          <para>Examples: <literal>CONFEXT_LEVEL=2</literal>, <literal>CONFEXT_LEVEL=15.14</literal>.
          </para>

          <xi:include href="version-info.xml" xpointer="v254"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>SYSEXT_SCOPE=</varname></term>
          <listitem><para>Takes a space-separated list of one or more of the strings
          <literal>system</literal>, <literal>initrd</literal> and <literal>portable</literal>. This field is
          only supported in <filename>extension-release.d/</filename> files and indicates what environments
          the system extension is applicable to: i.e. to regular systems, to initrds, or to portable service
          images. If unspecified, <literal>SYSEXT_SCOPE=system portable</literal> is implied, i.e. any system
          extension without this field is applicable to regular systems and to portable service environments,
          but not to initrd environments.</para>

          <xi:include href="version-info.xml" xpointer="v250"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>CONFEXT_SCOPE=</varname></term>

          <listitem><para>Semantically the same as <varname>SYSEXT_SCOPE=</varname> but for confext images.</para>

          <xi:include href="version-info.xml" xpointer="v254"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>PORTABLE_PREFIXES=</varname></term>
          <listitem><para>Takes a space-separated list of one or more valid prefix match strings for the
          <ulink url="https://systemd.io/PORTABLE_SERVICES">Portable Services</ulink> logic.
          This field serves two purposes: it is informational, identifying portable service images as such
          (and thus allowing them to be distinguished from other OS images, such as bootable system images).
          It is also used when a portable service image is attached: the specified or implied portable
          service prefix is checked against the list specified here, to enforce restrictions how images may
          be attached to a system.</para>

          <xi:include href="version-info.xml" xpointer="v250"/></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2>
      <title>Notes</title>

      <para>If you are using this file 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>
    <title>Examples</title>

    <example>
      <title><filename>os-release</filename> file for Fedora Workstation</title>

      <programlisting>NAME=Fedora
VERSION="32 (Workstation Edition)"
ID=fedora
VERSION_ID=32
PRETTY_NAME="Fedora 32 (Workstation Edition)"
ANSI_COLOR="0;38;2;60;110;180"
LOGO=fedora-logo-icon
CPE_NAME="cpe:/o:fedoraproject:fedora:32"
HOME_URL="https://fedoraproject.org/"
DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/f32/system-administrators-guide/"
SUPPORT_URL="https://fedoraproject.org/wiki/Communicating_and_getting_help"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=32
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=32
PRIVACY_POLICY_URL="https://fedoraproject.org/wiki/Legal:PrivacyPolicy"
VARIANT="Workstation Edition"
VARIANT_ID=workstation</programlisting>
    </example>

    <example>
      <title><filename>extension-release</filename> file for an extension for Fedora Workstation 32</title>

      <programlisting>ID=fedora
VERSION_ID=32</programlisting>
    </example>

    <example>
      <title>Reading <filename>os-release</filename> in
      <citerefentry project='man-pages'><refentrytitle>sh</refentrytitle><manvolnum>1</manvolnum></citerefentry></title>

      <programlisting><xi:include href="check-os-release.sh" parse="text" /></programlisting>
    </example>

    <example>
      <title>Reading <filename>os-release</filename> in
      <citerefentry project='die-net'><refentrytitle>python</refentrytitle><manvolnum>1</manvolnum></citerefentry> (versions &gt;= 3.10)</title>

      <programlisting><xi:include href="check-os-release-simple.py" parse="text" /></programlisting>

      <para>See docs for <ulink url="https://docs.python.org/3/library/platform.html#platform.freedesktop_os_release">
      <function>platform.freedesktop_os_release</function></ulink> for more details.
      </para>
    </example>

    <example>
      <title>Reading <filename>os-release</filename> in
      <citerefentry project='die-net'><refentrytitle>python</refentrytitle><manvolnum>1</manvolnum></citerefentry> (any version)</title>

      <programlisting><xi:include href="check-os-release.py" parse="text" /></programlisting>

      <para>Note that the above version that uses the built-in implementation is preferred
      in most cases, and the open-coded version here is provided for reference.</para>
    </example>

  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para><simplelist type="inline">
        <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry project='die-net'><refentrytitle>lsb_release</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      </simplelist></para>
  </refsect1>

</refentry>