LPIC-1 106.1 — Install and Configure X11

Exam weight: 2 — LPIC-1 v5, Exam 102

What You Need to Know

From the official LPIC-1 objectives:

• Understanding of the X11 architecture.
• Basic understanding and knowledge of the X Window configuration file.
• Overwrite specific aspects of Xorg configuration, such as keyboard layout.
• Understand the components of desktop environments, such as display managers and window managers.
• Manage access to the X server and display applications on remote X servers.
• Awareness of Wayland.

Key files, terms, and utilities: /etc/X11/xorg.conf, /etc/X11/xorg.conf.d/, ~/.xsession-errors, xhost, xauth, DISPLAY, X.

X Window System Architecture

The X Window System (also known as X11 or just X) is a software stack used to display text and graphics on a screen. It is divided into a client and a server:

• The X server controls the physical display and handles input devices (mouse, keyboard, trackpad). It draws what clients request.
• An X client is any graphical application — a terminal emulator, a browser, a game. Each client informs the server about its window location and content.

The X protocol (X11) is the communication mechanism between client and server. In modern Linux distributions, the primary implementation is Xorg version 11.

X is network-capable: multiple X clients from different computers on a network can make drawing requests to a single remote X server. This allows running a graphical application on a remote system and displaying it locally.

The X Window System is modular — newer features are added as extensions to the X server without changing the core X11 protocol. Examples of Xorg extension libraries: libXrandr, libXcursor, libX11, libxkbfile.

A display manager provides a graphical login screen. It launches after boot, starts an X server session for the authenticated user, and keeps the X server running. Common display managers: GDM, SDDM, LightDM.

The DISPLAY Variable

Each running X server instance has a display name to identify it:

hostname:displaynumber.screennumber

• hostname — the system that will display the application; omitted for localhost.
• displaynumber — identifies the collection of screens in use; starts at 0.
• screennumber — identifies an individual screen within the display; default is 0 and can be omitted.

The display name of a running X session is stored in the DISPLAY environment variable:

$ echo $DISPLAY
:0

This output means:

1. X server is on the local system (nothing to the left of the colon).
2. First X server session (display 0).
3. Single logical screen (no screen number shown).

Multi-monitor examples:

• (A) Single monitor, single display :0 — one screen.
• (B) Two physical monitors configured as one logical screen :0 — windows move freely between monitors.
• (C) Two monitors as independent screens :0.0 and :0.1 — windows cannot be moved between them.

To start an application on a specific screen:

$ DISPLAY=:0.1 firefox &

X Server Configuration

The primary configuration file is /etc/X11/xorg.conf. On modern distributions, X auto-configures at runtime, so this file may not exist. The file is divided into sections, each starting with Section "name" and ending with EndSection.

InputDevice

Configures a specific keyboard or mouse model.

InputClass

Configures a class of devices (e.g. all keyboards). Typically placed in /etc/X11/xorg.conf.d/. Example /etc/X11/xorg.conf.d/00-keyboard.conf:

Section "InputClass"
        Identifier "system-keyboard"
        MatchIsKeyboard "on"
        Option "XkbLayout" "us"
        Option "XkbModel" "pc105"
EndSection

XkbLayout sets the key layout (language, Dvorak, etc.). XkbModel defines the keyboard type. Layout files are in /usr/share/X11/xkb.

Monitor

Describes the physical monitor and its connection port:

Section "Monitor"
        Identifier  "DP2"
        Option      "Primary" "true"
EndSection

Device

Describes the video card, its kernel driver, and PCI location:

Section "Device"
        Identifier  "Device0"
        Driver      "i915"
        BusID       "PCI:0:2:0"
EndSection

Screen

Ties a Monitor and Device together:

Section "Screen"
        Identifier "Screen0"
        Device     "Device0"
        Monitor    "DP2"
EndSection

ServerLayout

Groups all sections (screens, input devices) into one X Window System interface:

Section "ServerLayout"
    Identifier   "Layout-1"
    Screen       "Screen0" 0 0
    InputDevice  "mouse1" "CorePointer"
    InputDevice  "system-keyboard" "CoreKeyboard"
EndSection

Config file locations and priority:

• /etc/X11/xorg.conf.d/ — user-specified drop-in files (parsed first)
• /usr/share/X11/xorg.conf.d/ — distribution-provided configs
• /etc/X11/xorg.conf — main config (parsed after .conf.d/)

Keyboard Layout

Session-only — change layout for the current X session:

$ setxkbmap -model chromebook -layout "gr(polytonic)"

setxkbmap uses the X Keyboard Extension (XKB). The change lasts until the session ends.

Persistent — write to /etc/X11/xorg.conf.d/00-keyboard.conf directly, or use localectl (systemd) which creates that file automatically:

$ localectl --no-convert set-x11-keymap "gr(polytonic)" chromebook

--no-convert prevents localectl from also modifying the console keymap.

xdpyinfo

xdpyinfo displays information about a running X server instance: display name, version, vendor, extensions, screen dimensions, and more.

$ xdpyinfo
name of display:    :0
version number:    11.0
vendor string:     The X.Org Foundation
...
number of extensions:   25
    BIG-REQUESTS
    Composite
    DAMAGE
    DRI3
    GLX
    RANDR
    RENDER
    SECURITY
    SHAPE
    XKEYBOARD
    ...
default screen number:   0
number of screens:   1

screen #0:
  dimensions:    3840x1080 pixels (1016x286 millimeters)
  resolution:    96x96 dots per inch
  depths (7):    24, 1, 4, 8, 15, 16, 32
  ...

Creating a Basic Xorg Configuration File

Even though X auto-configures on modern systems, you can generate a permanent /etc/X11/xorg.conf:

$ sudo Xorg -configure

If an X session is already running, specify a different display:

$ sudo Xorg :1 -configure

This creates xorg.conf.new in the current directory, derived from detected hardware. Move it to its final location:

$ sudo mv xorg.conf.new /etc/X11/xorg.conf

On some distributions, X is a symbolic link to Xorg and can be used instead.

Man pages for further reference: xorg.conf(5), Xserver(1), X(1), Xorg(1).

Wayland

Wayland is the newer display protocol designed to replace X. Many modern distributions use it as the default display server. It is lighter on resources and has a smaller footprint.

Key differences from X:

• No separate server — there is no X server instance between the client and the kernel.
• A client window works with its own rendering code (GTK+ 3, Qt 5, etc.) and sends rendering requests to the Linux kernel via the Wayland protocol.
• The Wayland compositor handles device input, window management, and composition (combines rendered elements into visual output).

Modern toolkits (GTK+ 3, Qt 5) support both X and Wayland. Applications still targeting X can run under XWayland — a separate X server instance that runs inside a Wayland client.

The Wayland equivalent of DISPLAY is WAYLAND_DISPLAY:

$ echo $WAYLAND_DISPLAY
wayland-0

This variable is not set on systems running plain X.

Quick Reference

# Check current display
echo $DISPLAY          # :0  (local, first session, single screen)
                       # lab01:0.2  (host lab01, display 0, third independent screen)
echo $WAYLAND_DISPLAY  # wayland-0  (only set on Wayland systems)

# Run app on specific screen
DISPLAY=:0.1 firefox &

# Show X server info (extensions, screen size, version)
xdpyinfo

# Generate xorg.conf from detected hardware
sudo Xorg -configure                      # creates xorg.conf.new in CWD
sudo Xorg :1 -configure                   # use if X session is already running
sudo mv xorg.conf.new /etc/X11/xorg.conf

# Keyboard layout — current session only (uses XKB extension)
setxkbmap -model pc105 -layout us
setxkbmap -model chromebook -layout "gr(polytonic)"

# Keyboard layout — persistent (creates /etc/X11/xorg.conf.d/00-keyboard.conf)
localectl --no-convert set-x11-keymap us pc105
localectl --no-convert set-x11-keymap "gr(polytonic)" chromebook
# --no-convert: do not also change the console keymap

# Config file locations and parse order
# 1st: /etc/X11/xorg.conf.d/      (user drop-in files)
# 2nd: /usr/share/X11/xorg.conf.d/ (distro files)
# 3rd: /etc/X11/xorg.conf          (main config)

# xorg.conf section structure
Section "InputDevice"    # specific keyboard or mouse model
Section "InputClass"     # class of devices (e.g. all keyboards)
Section "Monitor"        # physical monitor and port
Section "Device"         # video card, driver, BusID
Section "Screen"         # ties Monitor + Device together
Section "ServerLayout"   # ties all sections into one interface

# Related man pages
man xorg.conf    # xorg.conf(5)
man Xserver      # Xserver(1)
man Xorg         # Xorg(1)

Exam Questions

1. What is the format of the DISPLAY variable? → hostname:displaynumber.screennumber
2. What does :0 mean in DISPLAY? → Local X server, first display session, single logical screen.
3. What does lab01:0.2 mean? → System lab01, display 0, third independent screen (index 2).
4. How to run an application on the second independent screen? → DISPLAY=:0.1 app &
5. What command shows X server info including extensions and screen dimensions? → xdpyinfo
6. Which xorg.conf section configures a class of input devices (e.g. all keyboards)? → InputClass
7. Which section configures a specific keyboard or mouse model? → InputDevice
8. Which section ties Monitor and Device together? → Screen
9. Which section groups all sections into one interface? → ServerLayout
10. Where should user drop-in Xorg config files be placed? → /etc/X11/xorg.conf.d/
11. Which config location is parsed first — xorg.conf or xorg.conf.d/? → /etc/X11/xorg.conf.d/ is parsed first.
12. How to change keyboard layout for the current X session only? → setxkbmap -layout us
13. How to persistently set the X11 keyboard layout? → localectl --no-convert set-x11-keymap
14. What does --no-convert do in localectl? → Prevents changing the console keymap at the same time.
15. What command generates a new xorg.conf from detected hardware? → sudo Xorg -configure
16. What file does Xorg -configure create, and where? → xorg.conf.new in the current working directory.
17. What component is responsible for keeping the X server running after login? → The display manager (GDM, SDDM, LightDM).
18. What is XWayland? → A separate X server that runs inside a Wayland client, allowing X-only apps to work under Wayland.
19. What environment variable identifies the display on a Wayland system? → WAYLAND_DISPLAY
20. What Xorg libraries extend X server functionality? → libXrandr, libXcursor, libX11, libxkbfile.

Exercises

Exercise 1 — Identifying Xorg Extensions

What command would you use to determine what Xorg extensions are available on a system?

$ xdpyinfo

Exercise 2 — New Mouse Configuration

You have just received a brand new 10-button mouse that requires extra configuration. Without modifying the rest of the X server configuration, what directory would you use to create a new configuration file for this mouse, and what specific configuration section would be used?

Exercise 3 — What Keeps X Running?

What component of a Linux installation is responsible for keeping an X server running?

Exercise 4 — Generating xorg.conf

What command line switch is used with the X command (or Xorg) to create a new xorg.conf configuration file?

$ sudo Xorg -configure

Exercise 5 — Reading the DISPLAY Variable

What would the contents of the DISPLAY environment variable be on a system named lab01 using a single display configuration, where the terminal is on the third independent screen?

lab01:0.2

Exercise 6 — Persistent Keyboard Configuration

What command can be used to create a keyboard configuration file for use by the X Window System that persists after reboot?

$ localectl --no-convert set-x11-keymap us pc105

LPIC-1 Study Notes | Topic 106: User Interfaces and Desktops