what-is-systemd.xml

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    <title>What is &systemd;?</title>
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    <para>
      &systemd; is a system and service manager for Linux operating systems. It is the default initialization system for major Linux distributions.
      &systemd; is not directly initiated by the user, but installed through the <filename>/sbin/init </filename> and started during the early boot.
      &systemd; acts as the init system that brings up and maintains user space services when run as the first process on boot (PID 1).
      PID 1 is known as <emphasis>init</emphasis> and is the first Linux user-mode process created.
      It runs until the system shutdown.
      </para>
      <para>&systemd; owns PID 1, and is started directly by the kernel. All other processes are started directly
       by &systemd; or one of its child processes. &systemd; mounts the host's file system and manages temporary files.
       It is backward compatible with the SysV init scripts. SysV is an initialization system that predates &systemd;.</para>

        <para>In &systemd;, a unit is a resource that the system knows how to operate on and manage. This is
            the primary object that the &systemd; tools use. These resources are defined with configuration files called unit files.</para>
         <para><command>systemctl</command> is the central management tool for controlling the init system. It is used to
        examine and control the state of the &systemd; system and service manager.</para>
        <para>Targets in &systemd; are groups of related units that act as synchronization points during a system boot.
          Target unit files have a <literal>.target</literal> file extension. Target units group together various &systemd;
          units through a chain of dependencies.
        </para>
        <para>For troubleshooting, you can use <command>journalctl</command>, which is
         used to query and display log messages from the &systemd; journal.</para>
         <para>For more information on &systemd;, you can refer to <link xlink:href="https://systemd.io"/> and <command>man 1 systemd</command>.</para>
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