How can I specify a display?

When I run some programs over SSH, such as firefox &, I get an error

Error: no display specified 

I would like to open many displays, still showing the stdout of each program.

Initial Question: How can I specify the display to get a many-displayed program?

Pablo Santa Cruz gives me the following code as a solution. I do not understand it.

$ export

$ firefox &

What are yourmachine and in the command?


The way that X works is the same as the way any network program works. You have a server of some description (in this case, the X display server) which runs on a specific machine, and you have X clients (like firefox) that try to connect to that server to get their information displayed.

Often (on "home" machines), the client and server run on the same box and there's only one server, but X is powerful enough that this doesn't need to happen. It was built with the server/client separation built in from the start.

This allows you to do such wondrous things such as log on to your box (in text mode) halfway around the planet, tell it that the display server is the box you're currently on and, voila, the windows suddenly start appearing locally.

In order for a client to interact with a user, it needs to know how to find the server. There are a number of ways to do this. Many clients allow the -display or --displayoption to specify it:

xeyes -display

Many will use the DISPLAY environment variable if a display isn't specifically given. You can set this variable like any other:; export DISPLAY # in .profile
export # in your shell firefox & # for that command (shell permitting)

The first part of the DISPLAY variable is just the address of the display server machine. It follows the same rule as any other IP address; it can be a resolvable DNS name (including localhost) or a specific IP address (such as

The second part is X-specific. It gives the X "display" (X server) number and screen number to use. The first (display number) generally refers to a group of devices containing one or more screens but with a single keyboard and mouse (i.e., one input stream). The screen number generally gives the specific screen within that group.

An example would be:

||                      |
+-----------------+                      |
|                                        |
|  +----------+----+  +----------+----+  |
|  |Display :0|    |  |Display :1|    |  |
|  +----------+    |  +----------+    |  |
|  |               |  |               |  |
|  | +-----------+ |  |               |  |
|  | |Screen :0.0| |  |               |  |
|  | +-----------+ |  |               |  |
|  | +-----------+ |  |               |  |
|  | |Screen :0.1| |  |               |  |
|  | +-----------+ |  |               |  |
|  | +-----------+ |  | +-----------+ |  |
|  | |Screen :0.2| |  | |Screen :1.0| |  |
|  | +-----------+ |  | +-----------+ |  |
|  | +-----------+ |  | +-----------+ |  |
|  | |Screen :0.3| |  | |Screen :1.1| |  |
|  | +-----------+ |  | +-----------+ |  |
|  | +-----------+ |  | +-----------+ |  |
|  | | Keyboard  | |  | |  Keyboard | |  |
|  | +-----------+ |  | +-----------+ |  |
|  | +-----------+ |  | +-----------+ |  |
|  | |   Mouse   | |  | |   Mouse   | |  |
|  | +-----------+ |  | +-----------+ |  |
|  +---------------+  +---------------+  |
|                                        |

Here you have a single machine ( with two display servers. The first has four screens and the second has two. The possibilities are then:

depending on where you want your actual windows to appear and which input devices you want to use.

$ export
$ firefox &

When you are connecting to another machine over SSH, you can enable X-Forwarding in SSH, so that X windows are forwarded encrypted through the SSH tunnel back to your machine. You can enable X forwarding by appending -X to the ssh command line or setting ForwardX11 yes in your SSH config file.

To check if the X-Forwarding was set up successfully (the server might not allow it), just try if echo $DISPLAY outputs something like localhost:10.0.


export DISPLAY=localhost:0.0

Try installing the xorg-x11-xauth package.

I ran into a similar issue, so maybe this answer will help someone.

The reason for the Error: no display specified error is that Firefox is being launched, but there is no X server (GUI) running on the remote host. You can use X11 forwarding to run Firefox on the remote host, but display it on your local host. On Mac OS X, you will need to download XQuartz in order to use X11 forwarding. Without it, you won't have a $DISPLAY variable set, so if you try and echo $DISPLAY, it will be blank.

Please do NOT try to set $DISPLAY manually when connecting over SSH. If you connect via SSH -X and $DISPLAY stays empty, this usually means that no encrypted channel could be established.

Most likely you are missing the package xauth or xorg-x11-xauth. Try to install it on the remote machine using:

sudo apt-get install xauth


sudo apt-get install xorg-x11-xauth

After that end and restart your SSH connection. Don't forget to use SSH -X so that X Window output is forwarded to your local machine.

Now try echo $DISPLAYagain to see if $DISPLAY has been set automatically by the SSH demon. It should show you a line with an IP address and a port.

I faced similar problem today. So, here's a simple solution: While doing SSH to the machine, just add Ctrl - Y.

ssh user@ip_address -Y

After login, type firefox &. And you are good to go.

login to your server via

ssh -X root@yourIP

edit /etc/ssh/sshd_config file, and add this line to it.

X11UseLocalhost no

Restart sshd. for CentOS (check your distribution)

/sbin/service sshd restart

check your DISPLAY


you should see this




for more info

Even i faced the same in CentOS 6.8.

yum reinstall xorg*

End your current session and open another session in tool like mobiXterm. Make sure session has X11 forwarding enabled in the tool.

I through vnc to understand the X11 more. To specify the display to get a many-displayed program, export DISPLAY=IP:DisplayNum.ScreenNum

For example,

vncserver :2
vncserver -list
export DISPLAY=:2  # export DISPLAY=IP:DisplayNum or export DISPLAY=:DisplayNum for localhost; So that can vnc connect and see the vnc desktop :2 if $DISPLAY is not :2.

Need Your Help

return statement in for loops


I have been working on this assignment for school and I just can't figure out what why I cant get this program to work properly. I am trying to get the program to allow the user to enter three anim...

Load files in xcode unit tests

xcode unit-testing nsbundle

I have an Xcode 5 unit test project and some test xml files related to it. I've tried a bunch of approaches but I can't seem to load the xml files.