x-window-system.md

# 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

### Linux Example

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 in ssh client:

```console
 local $ ssh -X username@cluster-name.it4i.cz
```

### PuTTY 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.

![](../../../img/cygwinX11forwarding.png)

Then log in as usual.

### WSL (Bash on Windows)

```console
 local $ export DISPLAY=localhost:0.0
 local $ ssh -X username@cluster-name.it4i.cz
```

## Verify the Forwarding

To verify the forwarding, type

```console
$ echo $DISPLAY
```

if you receive something like

```console
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
    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.

```console
$ ml intel (idb and gvim not installed yet)
$ gvim &
```

```console
$ 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

```console
$ 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.

```console
$ ssh -X r24u35n680
```

In this example, we log in on the r24u35n680 compute node, with the X11 forwarding enabled.

## 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:

```console
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

```console
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:

```console
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](#putty-on-windows) or [Bash on Windows](#wsl-bash-on-windows). On the cluster, run the gnome-session command.

```console
$ 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