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

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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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 a transient machine ID to disk</refpurpose>
  </refnamediv>

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

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-machine-id-commit.service</filename> is an
    early boot service responsible for committing transient
    <filename>/etc/machine-id</filename> files to a writable disk file
    system. See
    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for more information about machine IDs.</para>

    <para>This service is started after
    <filename>local-fs.target</filename> in case
    <filename>/etc/machine-id</filename> is a mount point of its own
    (usually from a memory file system such as
    <literal>tmpfs</literal>) and /etc is writable. The service will
    invoke <command>systemd-machine-id-setup --commit</command>, which
    writes the current transient machine ID to disk and unmount the
    <filename>/etc/machine-id</filename> file in a race-free manner to
    ensure that file is always valid and accessible for other
    processes. See
    <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    for details.</para>

    <para>The main use case of this service are systems where
    <filename>/etc/machine-id</filename> is read-only and initially
    not initialized. In this case, the system manager will generate a
    transient machine ID file on a memory file system, and mount it
    over <filename>/etc/machine-id</filename>, during the early boot
    phase. This service is then invoked in a later boot phase, as soon
    as <filename>/etc</filename> has been remounted writable and the
    ID may thus be committed to disk to make it permanent.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</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>