Bug 79252

* Overview

In my machine `systemctl --user` works perfectly, while `systemd-run --user` fails.

* Steps to Reproduce & Actual Results

$ systemd-run --user /usr/bin/ls
Failed to create bus connection: No such file or directory

* Expected Results

I would expect it to create a transient unit to run the program. I can manually create an unit in ~/.config/systemd/user and start it with `systemctl --user` so why not with `systemd-run --user`?

* Platform
 - ArchLinux x86_64
 - systemd 212

* Additional Information

I've found that it is because `systemd-run --user` tries to connect to the systemd daemon using the session dbus-daemon, while `systemctl --user` connects to the systemd daemon directly, using the private socket.

If I configure the session dbus-daemon then it will work fine, but since the `systemctl --user` works without it, I don't see why it should be required by `systemd-run --user`.

It can be proved that the session dbus-daemon is not a requirement by running the following command:

$ gdbus call -a unix:path=/run/user/$(id -u)/systemd/private \
  -o /org/freedesktop/systemd1 \
  -m org.freedesktop.systemd1.Manager.StartTransientUnit \
  run-$RANDOM.service fail \
  "[('ExecStart', <[('/usr/bin/ls', ['/usr/bin/ls'], false)]>)]" \
  "[]"

It does create the transient unit, equivalent to `systemd-run --user /usr/bin/ls`, but using the private socket instead of the session dbus-daemon.

Now that we are into it, why is there a command `systemd-run` instead of `systemctl run`? Too different syntax?

TIA