systemd/man/systemd-machine-id-commit.service.xml

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<refentry id="systemd-machine-id-commit.service">

  <refentryinfo>
    <title>systemd-machine-id-commit.service</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Didier</firstname>
        <surname>Roche</surname>
        <email>didrocks@ubuntu.com</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-machine-id-commit.service</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-machine-id-commit.service</refname>
    <refpurpose>Commit transient machine-id to disk</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-machine-id-commit.service</filename></para>
    <para><filename>/usr/lib/systemd/systemd-machine-id-commit</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-machine-id-commit.service</filename> is a
    service responsible for committing any transient
    <filename>/etc/machine-id</filename> file to a writable file
    system. See
    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for more information about this file.</para>

    <para>This service is started shortly after
    <filename>local-fs.target</filename> if
    <filename>/etc/machine-id</filename> is an independent mount point
    (probably a tmpfs one) and /etc is writable.
    <command>systemd-machine-id-commit</command> will then write
    current machine ID to disk and unmount the transient
    <filename>/etc/machine-id</filename> file in a race-free manner to
    ensure that file is always valid for other processes.</para>

    <para>Note that the traditional way to initialize the machine ID
    in <filename>/etc/machine-id</filename> is to use
    <command>systemd-machine-id-setup</command> by system installer
    tools. You can also use
    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    to initialize the machine ID on mounted (but not booted) system
    images. The main use case for that service is
    <filename>/etc/machine-id</filename> being an empty file at boot
    and initrd chaining to systemd giving it a read only file system
    that will be turned read-write later during the boot
    process.</para>

    <para>There is no consequence if that service fails other than a
    newer machine-id will be generated during next system boot.
    </para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-machine-id-commit</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>