SDDM

SDDM, the Simple Desktop Display Manager, is a display manager for Linux and other Unix-like systems. It provides a graphical login screen, session selection, and startup of the user’s desktop environment. It supports both X11 and Wayland sessions and is used across distributions, particularly those that ship KDE software. SDDM aims to be lightweight, fast, and highly customizable through themes.

Implementation and design: SDDM is written in C++ and uses the Qt toolkit for its user interface and theming. Its architecture separates the greeter (login screen) from the session launcher, allowing themes and authentication backends to be modified independently. It can integrate with PAM for authentication and with systemd-logind to manage user sessions.

Configuration and theming: SDDM includes a default set of themes and supports additional themes installed from packages or manually added to the themes directory. The appearance and behavior can be configured in sddm.conf (typically /etc) or via drop-in fragments. The Theme option selects the active theme, and other options control autologin, fonts, and locale.

Usage and deployment: On boot, SDDM presents the login screen. After a login, it starts the selected desktop environment and returns to the login screen when the user logs out. It is commonly used by KDE-oriented ecosystems and is also employed by various non-KDE distributions.

Development and licensing: SDDM is an open-source project maintained by the KDE community and released under the GNU General Public License. It competes with display managers such as GDM and LightDM and continues to receive updates for X11 and Wayland support.