Add xephyr page.

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+---
+tags:
+    - linux
+    - unix
+    - utils
+    - xephyr
+---
+
+import Tabs from '@theme/Tabs';
+import TabItem from '@theme/TabItem';
+
+# Xephyr
+
+Xephyr is a nested X server that runs as an X application.
+
+## Instalation
+
+<Tabs>
+  <TabItem value="arch" label="archlinux" default>
+        ```bash
+        $ sudo pacman -S xorg-server-xephyr
+        ```
+  </TabItem>
+</Tabs>
+
+## Usage
+
+To run a nested X window, you will need to specify a new display:
+```bash
+$ Xephyr -br -ac -noreset -screen 800x600 :1
+```
+This will launch a new Xephyr window with a DISPLAY of ":1". In order to launch an application in that window, you would need to specify that display:
+```bash
+$ DISPLAY=:1 xterm
+```
+
+### Launching window managers
+
+If you want to launch a specific WM, [awesomewm](https://awesomewm.org) for example, you would type:
+```bash
+$ DISPLAY=:1 awesome
+```
+
+### Grabbing and un-grabbing user input
+
+Pressing `Ctrl+Shift` should lock/unlock your mouse pointer and your keystrokes inside Xephyr window exclusively if possible.
+
+## Other examples for situations where Xephyr can be useful
+
+- A testing environment for an X application, or feature, in which the tester would like to keep working in their usual X environment,
+  yet defending the other applications from failures of the application under test. Example script for running tested app(for examples
+  window manager) inside Xephyr:
+  ```bash title="xephyr.sh"
+  #!/bin/bash
+
+  Xephyr -br -ac -reset -screen 1920x1080 :5 &
+  sleep 1s
+  export DISPLAY=:5
+  $1 &
+  ```
+  To run [awesomewm](https://awesomewm.org) inside Xephyr you can run:
+  ```bash
+  $ ./xephyr.sh awesome
+  ```
+