# X Window System
The X Window system is a principal way to get GUI access to the clusters. The **X Window System** (commonly known as **X11**, based on its current major version being 11, or shortened to simply **X**, and sometimes informally **X-Windows**) is a computer software system and network [protocol](http://en.wikipedia.org/wiki/Protocol_%28computing%29 "Protocol (computing)") that provides a basis for [graphical user interfaces](http://en.wikipedia.org/wiki/Graphical_user_interface "Graphical user interface") (GUIs) and rich input device capability for [networked computers](http://en.wikipedia.org/wiki/Computer_network "Computer network").
!!! tip
The X display forwarding must be activated and the X server running on client side
## X display
In order to display graphical user interface GUI of various software tools, you need to enable the X display forwarding. On Linux and Mac, log in using the -X option tho ssh client:
```bash
local $ ssh -X username@cluster-name.it4i.cz
```
## X Display Forwarding on Windows
On Windows use the PuTTY client to enable X11 forwarding. In PuTTY menu, go to Connection-SSH-X11, mark the Enable X11 forwarding checkbox before logging in. Then log in as usual.
To verify the forwarding, type
```bash
$ echo $DISPLAY
```
if you receive something like
```bash
localhost:10.0
```
then the X11 forwarding is enabled.
## X Server
In order to display graphical user interface GUI of various software tools, you need running X server on your desktop computer. For Linux users, no action is required as the X server is the default GUI environment on most Linux distributions. Mac and Windows users need to install and run the X server on their workstations.
## X Server on OS X
Mac OS users need to install [XQuartz server](https://www.xquartz.org).
## X Server on Windows
There are variety of X servers available for Windows environment. The commercial Xwin32 is very stable and rich featured. The Cygwin environment provides fully featured open-source XWin X server. For simplicity, we recommend open-source X server by the [Xming project](http://sourceforge.net/projects/xming/). For stability and full features we recommend the
[XWin](http://x.cygwin.com/) X server by Cygwin
| How to use Xwin | How to use Xming |
| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------- |
| [Install Cygwin](http://x.cygwin.com/) Find and execute XWin.exe to start the X server on Windows desktop computer.[If no able to forward X11 using PuTTY to CygwinX](#if-no-able-to-forward-x11-using-putty-to-cygwinx) | Use Xlaunch to configure the Xming. Run Xming to start the X server on Windows desktop computer. |
Read more on [http://www.math.umn.edu/systems_guide/putty_xwin32.html](http://www.math.umn.edu/systems_guide/putty_xwin32.shtml)
## Running GUI Enabled Applications
!!! Note "Note"
Make sure that X forwarding is activated and the X server is running.
Then launch the application as usual. Use the & to run the application in background.
```bash
$ module load intel (idb and gvim not installed yet)
$ gvim &
```
```bash
$ xterm
```
In this example, we activate the intel programing environment tools, then start the graphical gvim editor.
## GUI Applications on Compute Nodes
Allocate the compute nodes using -X option on the qsub command
```bash
$ qsub -q qexp -l select=2:ncpus=24 -X -I
```
In this example, we allocate 2 nodes via qexp queue, interactively. We request X11 forwarding with the -X option. It will be possible to run the GUI enabled applications directly on the first compute node.
**Better performance** is obtained by logging on the allocated compute node via ssh, using the -X option.
```bash
$ ssh -X r24u35n680
```
In this example, we log in on the r24u35n680 compute node, with the X11 forwarding enabled.
## The Gnome GUI Environment
The Gnome 2.28 GUI environment is available on the clusters. We recommend to use separate X server window for displaying the Gnome environment.
## Gnome on Linux and OS X
To run the remote Gnome session in a window on Linux/OS X computer, you need to install Xephyr. Ubuntu package is
xserver-xephyr, on OS X it is part of [XQuartz](http://xquartz.macosforge.org/landing/). First, launch Xephyr on local machine:
```bash
local $ Xephyr -ac -screen 1024x768 -br -reset -terminate :1 &
```
This will open a new X window with size 1024 x 768 at DISPLAY :1. Next, ssh to the cluster with DISPLAY environment variable set and launch gnome-session
```bash
local $ DISPLAY=:1.0 ssh -XC yourname@cluster-name.it4i.cz -i ~/.ssh/path_to_your_key
... cluster-name MOTD...
yourname@login1.cluster-namen.it4i.cz $ gnome-session &
```
On older systems where Xephyr is not available, you may also try Xnest instead of Xephyr. Another option is to launch a new X server in a separate console, via:
```bash
xinit /usr/bin/ssh -XT -i .ssh/path_to_your_key yourname@cluster-namen.it4i.cz gnome-session -- :1 vt12
```
However this method does not seem to work with recent Linux distributions and you will need to manually source
/etc/profile to properly set environment variables for PBS.
## Gnome on Windows
Use Xlaunch to start the Xming server or run the XWin.exe. Select the "One window" mode.
Log in to the cluster, using PuTTY. On the cluster, run the gnome-session command.
```bash
$ gnome-session &
```
In this way, we run remote gnome session on the cluster, displaying it in the local X server
Use System-Log Out to close the gnome-session
### If no able to forward X11 using PuTTY to CygwinX
```bash
[usename@login1.anselm ~]$ gnome-session &
[1] 23691
[usename@login1.anselm ~]$ PuTTY X11 proxy: unable to connect to forwarded X server: Network error: Connection refused
PuTTY X11 proxy: unable to connect to forwarded X server: Network error: Connection refused
(gnome-session:23691): WARNING **: Cannot open display:**
```
1. Locate and modify Cygwin shortcut that uses [startxwin](http://x.cygwin.com/docs/man1/startxwin.1.html)
locate
C:cygwin64binXWin.exe
change it
to
C:_cygwin64binXWin.exe -listen tcp_
1. Check Putty settings:
Enable X11 forwarding
