Which user owns X display?

Question

How to determine, as root, which user owns X display :0? (on a Fedora 18 system with defaults)

Clarification: I want to know which user has original permissions to connect to :0 and grant other users permission to connect. Per current answers below, it seems that root owns the X process but the user who launched it may have permissions.

Answer 1

ps -ef|grep X will give you the info.

For example, on my computer:

user001  2721  2705  0 02:35 tty1     00:00:00 xinit /home/user001/.xinitrc -- /etc/X11/xinit/xserverrc :0 -auth /home/user001/.serverauth.2705
root      2722  2721 13 02:35 tty7     00:10:10 /usr/bin/X -nolisten tcp :0 -auth /home/user001/.serverauth.2705
user001  3475  3462  0 03:51 pts/0    00:00:00 grep X

Here, we can see that user001 launched xinit and uses the display :0. However, root is the owner of the X process.

On most installations, users don't use the xinit command (or startx which internally calls xinit). The X server is launched by root during the bootup process. In that case, root is the "owner" of the display.

Answer 2

loginctl from systemd to the rescue:

loginctl --no-legend list-sessions \
  | cut -d ' ' -f 1 \
  | while read sessionid; do \
    [ $(loginctl --property Type --value show-session $sessionid) = 'x11' ] \
      && loginctl --property Name --value show-session $sessionid; \
  done

Get the list of session IDs, see which one is of type x11 and print the user name. For Wayland the type is wayland and for MIR is mir according to the logind D-Bus API Documentation.

Interactive use:

[ciupicri@titan /]$ loginctl list-sessions 
SESSION  UID USER     SEAT  TTY
      2 5000 ciupicri seat0    
      4 6000 somebody             
      9 6000 somebody             

3 sessions listed.
[ciupicri@titan /]$ loginctl show-session 2
Id=2
User=5000
Name=ciupicri
Timestamp=Thu 2019-09-12 16:24:12 EEST
TimestampMonotonic=46881112
VTNr=1
Seat=seat0
Display=:0
Remote=no
Service=lightdm-autologin
Desktop=xfce
Scope=session-2.scope
Leader=2072
Audit=2
Type=x11
Class=user
Active=yes
State=active
IdleHint=no
IdleSinceHint=0
IdleSinceHintMonotonic=0
LockedHint=no

Answer 3

This is the exact command:

ps ho user $(pgrep X)

Answer 4

To answer your clarification: It depends.

It's possible for a user to start an X server.

However, most distros use a display manager (DM) to start the X server. This is a system service owned by root, which starts the X server as root, display a login screen, and then grant this particular user access to this display (e.g. :0) via xauth.

As to "which user can grant other users permissions to connect": Again, it depends. By default, modern distros use xauth, and restrict the X server to local user. Then, any user who is already connected can use xauth to obtain a valid MIT authorization cookie, and pass this cookie on to another user, with then in turn can connect after adding this cookie.

Unless the distro has taken additional measures to make sure only one user can connect the unix domain socket that corresponds to that display (I wouldn't be surprised if some distros do that).

Answer 5

One example that works on Ubuntu 18.04 with GDM (did not try lightdm). So for this scenario it does answer that questions. Just if someone wants to delete the answer again.... (how to find users of a display).

For GDM on Ubuntu 18.04

loginctl show-session 2 -p Display

will print Display= although being the session using the display. The solutions below worked though.

---

~$ who
user     :1           2020-03-02 07:28 (:1)
user     tty3         2020-03-03 09:39
user     pts/1        2020-03-03 10:19 (some IP)

or

~$ w
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
user     tty7     :0               15:12    1:48m  1:50   0.23s /usr/lib/gnome-
user     pts/2    SomeIP           16:58    1:39   0.04s  0.04s -bash

As you said since the Xserver is started by the DM it will run as root or similar user (as it is allowed to access HW devices of the seat). To allow another session to use an authority file is created for the User.

---

Another solution I found on Best way to identify logged-in users and their DISPLAY? (but will fail as soon as multiple users run on the same X display I think) is:

#!/bin/bash


declare -A disps usrs
usrs=()
disps=()
for i in $(users);do
    [[ $i = root ]] && continue
    usrs[$i]=1
done # unique names

#Use .*DISPLAY=(\w*:[0-9.]+).* if you want to see DISPLAY of form :0.1 
#or localhost:10.0 too
for u in "${!usrs[@]}"; do
    for i in $(sudo ps e -u "$u" | sed -rn 's/.* DISPLAY=(:[0-9]*).*/\1/p');do
        disps[$i]=$u
    done
done

for d in "${!disps[@]}";do
    echo "User: ${disps[$d]}, Display: $d"
done

It basically queries the env of all processes of a user and saves for each display which user did belong to it. (Did work for my VNC displays too).