Linux GUI

Major Concepts

1. Display Server

• What it is: The low-level system that handles communication between your hardware (GPU, keyboard, mouse) and graphical apps.
• Examples:
  • X11 (Xorg)—legacy but still common.
  • Wayland—modern, secure, faster.

2. Display Manager (DM)

• What it is: The graphical login screen that lets you log into your user account and choose a desktop environment.
• Examples:
  • GDM (GNOME Display Manager)
  • LightDM
  • SDDM (used by KDE)
  • LXDM

3. Desktop Environment (DE)

• What it is: The full graphical interface: panel, file manager, window manager, settings, etc.
• Examples:
  • GNOME (Ubuntu default)
  • KDE Plasma
  • XFCE
  • LXQt
  • Cinnamon, MATE

4. Window Manager (WM)

• What it is: Controls how windows behave—how they’re drawn, moved, minimized, tiled, etc.
• • Types:
    • Stacking WMs: GNOME Shell, Xfwm
    • Tiling WMs: i3, bspwm, awesome

Login Flow (Graphical Sessions)

🔄 High-Level Flow:

System Boot → systemd → DM → Login → Session (.desktop) → DE + WM on top of Display Server

🧩 Step-by-Step Breakdown:

1. System Boot

• Bootloader (GRUB) loads the kernel.
• The Linux kernel initializes the system.
• systemd (or another init system) takes over.

2. Display Manager (DM) Starts

• systemd starts display-manager.service
• It:
  • Launches the display server (like Xorg or Wayland)
  • Presents the graphical login screen
  • Lets you choose the session (e.g., GNOME, XFCE, custom WM)
  • Authenticates the user

GDM Configuration

This file /etc/gdm3/custom.conf is the GDM (GNOME Display Manager) configuration file. It defines how GDM behaves during system startup and login. Most of the times, we disable Wayland:

WaylandEnable=false

It is useful for compatibility with remote desktop (like xrdp), NVIDIA drivers, or legacy apps. To apply changes (like WaylandEnable=false), you usually need to reboot or restart GDM:

sudo systemctl restart gdm3

3. User Login → Session Launch

• After successful login, the display manager:
  • Reads the selected .desktop session file
    • Found in /usr/share/xsessions/ or /usr/share/wayland-sessions/
• This file defines the session’s startup command:

Exec=env GNOME_SHELL_SESSION_MODE=ubuntu /usr/bin/gnome-session --session=ubuntu

Ubuntu Session

/usr/share/xsessions/ubuntu.desktop — available system-wide to all users.

When you log in graphically and select the "Ubuntu" session, the display manager reads this file and runs:

env GNOME_SHELL_SESSION_MODE=ubuntu /usr/bin/gnome-session --session=ubuntu

This starts the Ubuntu-flavored GNOME desktop environment.

4. Session Starts DE or WM

• The command in the .desktop file launches either:
  • A Desktop Environment (DE): full suite (panel, file manager, settings, etc.)
    • e.g., GNOME, KDE, XFCE
  • Or just a Window Manager (WM): lightweight control over windows only
    • e.g., i3, bspwm