Systemd target units with same names but different contents

Question

I am using Linux Mint 20.3 Cinnamon

$ systemctl --user get-default
default.target
$ ls -al /lib/systemd/system/default.target
lrwxrwxrwx 1 root 16 Jan 10 05:56 /lib/systemd/system/default.target -> graphical.target

$ \cat /lib/systemd/system/default.target
#  SPDX-License-Identifier: LGPL-2.1+
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.
[Unit]
Description=Graphical Interface
Documentation=man:systemd.special(7)
Requires=multi-user.target
Wants=display-manager.service
Conflicts=rescue.service rescue.target
After=multi-user.target rescue.service rescue.target display-manager.service
AllowIsolate=yes

• I have default.target located in 2 different locations with different contents as follows:

$ \cat ~/.config/systemd/user/default.target
#  SPDX-License-Identifier: LGPL-2.1+
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.
[Unit]
Description=Main User Target
Documentation=man:systemd.special(7)
Requires=basic.target
After=basic.target
AllowIsolate=yes

Question

1. How to specify /lib/systemd/system/default.target inside a service unit file, for a user unit?
2. How to know which default target is used by a particular service unit file?
3. systemctl --user get-default refers to which of these default.target?

Answer 1

For knowing the current locations of a target, just query the status, e.g.:

systemctl --user status default.target
● default.target - Main User Target
     Loaded: loaded (/usr/lib/systemd/user/default.target; static)
[...]
systemctl status default.target
● graphical.target - Graphical Interface
     Loaded: loaded (/lib/systemd/system/graphical.target; indirect; vendor preset: enabled)
[...]

The order of where units are searched is given in the manuals (as system/user unit search path)

That means if I just create a dummy target in my local .config-dir and start it, it gets loaded first, as its position is higher in the search path:

$cat ~/.config/systemd/user/default.target
[Unit]
Description=just exists
$systemctl --user daemon-reload
$systemctl --user start default.target
$systemctl --user status default.target
● default.target - just exists
     Loaded: loaded (/home/felixjn/.config/systemd/user/default.target; static)

I.e. the unit serach path is just like the $PATH variable from shells: the first for a unit match will be taken.

The important part is to know, that user and system paths differ and do not overlap!