nixpkgs/nixos/doc/manual/administration/user-sessions.xml

<chapter xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-user-sessions">
 <title>User Sessions</title>
 <para>
  Systemd keeps track of all users who are logged into the system (e.g. on a
  virtual console or remotely via SSH). The command <command>loginctl</command>
  allows querying and manipulating user sessions. For instance, to list all
  user sessions:
<screen>
<prompt>$ </prompt>loginctl
   SESSION        UID USER             SEAT
        c1        500 eelco            seat0
        c3          0 root             seat0
        c4        500 alice
</screen>
  This shows that two users are logged in locally, while another is logged in
  remotely. (“Seats” are essentially the combinations of displays and input
  devices attached to the system; usually, there is only one seat.) To get
  information about a session:
<screen>
<prompt>$ </prompt>loginctl session-status c3
c3 - root (0)
           Since: Tue, 2013-01-08 01:17:56 CET; 4min 42s ago
          Leader: 2536 (login)
            Seat: seat0; vc3
             TTY: /dev/tty3
         Service: login; type tty; class user
           State: online
          CGroup: name=systemd:/user/root/c3
                  ├─ 2536 /nix/store/10mn4xip9n7y9bxqwnsx7xwx2v2g34xn-shadow-4.1.5.1/bin/login --
                  ├─10339 -bash
                  └─10355 w3m nixos.org
</screen>
  This shows that the user is logged in on virtual console 3. It also lists the
  processes belonging to this session. Since systemd keeps track of this, you
  can terminate a session in a way that ensures that all the session’s
  processes are gone:
<screen>
# loginctl terminate-session c3
</screen>
 </para>
</chapter>