It's better for users if programs don't do "significant" things too easily, and should be especially conservative when called without any arguments whatsoever. So far systemd would would try to launch itself as a user manager and fail on some cgroup permission stuff. systemd --user is run execlusively from user@.service and there we call it with --user. Calls to the binary without any arguments as non-pid1 are almost always a mistake. https://github.com/systemd/systemd/issues/18419#issuecomment-779422571
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