How can I switch user in a shell and use the existing gnome display session?

Question

If I switch user in a terminal.

su bob

I can't open gedit because bob doesn't own the display.

If I execute

xhost +

before switching to bob I can open the display for some applications but not all. I get the following output when trying to execute gedit:

(crashreporter:4415): GnomeUI-WARNING *: While connecting to session manager: None of the authentication protocols specified are supported.
* GLib-GIO:ERROR:/build/buildd/glib2.0-2.28.6/./gio/gdbusconnection.c:2279:initable_init: assertion failed: (connection->initialization_error == NULL)

Solution

If you want to allow user bob to access your existing X session from the command line, you can enter these commands at the terminal prompt:

xauth nextract - $DISPLAY | sudo -u bob xauth nmerge -

Thius uses the xauth command to extract the authentication information for the current user (first part of the command, before the |) and then merge it into user bob's $HOME/.Xauthority file. Note that it requires /etc/sudoers to be configured so that you are allowed to run commands as user bob (this is the default if you're the admin user on a Ubuntu box).

You can then switch to user bob with su -l.

Alternatively, you can issue these separate commands:

xauth nextract - $DISPLAY > /tmp/xauth.temporary.file
su -l bob -c 'xauth nmerge - < /tmp/xauth.temporary.file'
rm /tmp/xauth.temporary.file

Note that:

1) the temporary file must have permissions such that user bob can read it (which often means that any user on the system can read it);

2) there is a time window during which any user that can read the temporary file can get access to your X display.

Therefore, it is better to use the sudo approach as it does not have these downsides.

Comments

[Oxwivi]

Try command line editors like nano.

[jefe goode]

@Oxwivi Sure, and I do use nano a lot but in this case gedit is just one of the GUI applications I wasn't able to open.

[Oxwivi]

The only program you can't open as another user? You should include it in the question.

[Riccardo Murri]

Try to avoid xhost + if you're on a networked computer, as that disables X security and gives anyone on the network permission to connect to your X display and do things like reading your keyboard from remote...

[jefe goode]

@Oxwivi It could also be firefox, that doesn't work either, it's just in general, I want to be able to do this. gedit was a poor example, you are right about that.

[jefe goode]

I got this to work, but: su: must be run from a terminal

[Riccardo Murri]

@z7sg: Yes, of course it must be run in a terminal. Isn't that what your question is about? How to switch to another user in a terminal and still be able to launch X applications from that same terminal?

[jefe goode]

@Ricardo Yes, but su doesn't like being run from a pipe. It's a security issue.

[Riccardo Murri]

@z7sg Oh, I see. You're right, but you can use sudo instead of su and not run into this problem. I'll edit my answer with details.

[djb]

From my desktop running xubuntu, I run ssh -X otherhost. From that host, DISPLAY is "localhost:10.0". When I try xauth extract - $DISPLAY I get an error, No matches found, authority file "-" not written

[Riccardo Murri]

@djb You should not need to do any xauth magic: ssh already handles the details for you.

[djb]

Thanks, @RiccardoMurri - I left out something. From my desktop, I ssh -X somehost to login as myself and X works. From there, I want to sudo and have that command use my desktop display. But after doing ssh -X, the remote shell has DISPLAY set to a local proxy (localhost:10.0 for example) and this can't be passed to sudu (or other su userid commands). The pipeline above fails because localhost is not known to xauth. But this works: [[ xauth nextract - ${DISPLAY/localhost/$(hostname)/unix} | sudo xauth nmerge - ]] followed by sudo xcommand . I must change localhost to $(hostname)/unix for xauth