Comment # 4 on bug 1205109 from Zim Tsui

(In reply to Dr. Werner Fink from comment #3)
> just try 
> 
>    sudo /usr/bin/emacs-nox
> 
> and then try with installed emacs-x11
> 
>    sudo /usr/bin/emacs-gtk
> 
> and see what happens ... I see e.g.

If I run `sudo emacs-nox` directly, no errors occurred.

Only if I run `sudo emacs`, it prints errors.

This is because the script `/usr/bin/emacs` set the variable `XDG_RUNTIME_DIR`
to `/run/user/0` when I run `sudo emacs`, while the directory `/run/user/0`
doesn't exist.

The reason why `/run/user/0` doesn't exist is that `sudo` doesn't create a new
real session to login as root(UID:0), it merely SIMULATES a login shell. The
systemd is not aware of someone loging in as root, and will not create the
directory `/run/user/0`.

> The aim is to replace XDG_RUNTIME_DIR to the correct one

The variable `XDG_RUNTIME_DIR` should be managed only by systemd rather than by
user scripts, because only systemd knows what to do else with the variable,
such as creating the corresponding directories. It's dangerous to set the XDG_
variables manually.

The simplist solution to this issue is not to set the XDG variables in
`/usr/bin/emacs`, as if the variables are not set when `emacs-nox` is run
directly.