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When using a systemd user instance (systemd --user), every user-created daemons are owned by this instance. For example:

  1. Let's create a daemon: nohup sleep 100 0<&- &>/dev/null &. It has PID 12549. Kill the parent terminal process (or directly the shell) so that it becomes a daemon
  2. The new parent of the process is the systemd user instance:

    UID        PID  PPID  C STIME TTY          TIME CMD
    phylliade   12549 20280  0 17:21 ?        00:00:00 sleep 100
    phylliade   20280     1  0 09:23 ?        00:00:00 /usr/lib/systemd/systemd --user
    

So, how is this happening? When a process parent dies, the kernel normally assigns init (here the main systemd) as its new parent; Is the system systemd then assigning the daemon to the user systemd?

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Kill the parent […] so that it becomes a daemon

No it doesn't. A dæmon is not associated with a controlling terminal or a login session. A process running via nohup most definitely still is. After all, if it were not, there would be no point to the gynmastics of ignoring controlling terminal hangup.

When a process parent dies, the kernel normally assigns init […] as its new parent

No it doesn't. It assigns the nearest ancestor process that was marked as a child process reaper, falling back to process #1 if there is no such ancestor process. The world has changed.

Upstart, the nosh toolset's service-manager, and systemd all make use of this. Per-user service management in all three involves local reapers.

Further reading

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  • I meant kill the terminal emulator process (which also kills the shell), editing
    – Phylliade
    Apr 22, 2018 at 19:10

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