User interfaces

Teletype

The teletype, short tty, interface originates in the time of terminals. The tty is a text only interface and is usually implemented directly in the Kernel. Special functionality like colors are handled using escape sequences. Small executables like agetty from util-linux or getty from FreeBSD are used to open the tty line. On modern computers terminals are emulated, the tty experiences keyboard and monitor as a terminal.

X

X is a secification how gui clients talk to a server compositing multiple windows onto a desktop. The current version is X11. The server implementation used virtually everywhere is X.Org. It is a display server, thus doing the rendering part. There are many window managers to be used on to of X.Org, that mainly change the appearance and layout of the desktop, e.g. i3.

Notable points:

• Most programs support X.
• X does not properly support multiple monitors with different refresh rates.
• X does not properly support color management, like HDR and 10bit color depth.
• X is mostly considered deprecated.

Wayland

Wayland is another specification how gui clients talk to the server compositing them. Wayland does not split the display server and window manager. A Wayland server doing both of these is called a compositor. An example of a Wayland compositor is Hyprland.

Notable points:

• Many programs support Wayland.
• XWayland as an X emulator inside a Wayland compositor works very well for programs not supporting Wayland.
• Wayland is not network transparent
• Wayland does properly support multiple monitors with differen refresh rates.
• Wayland does properly support color management, like HDR and 10bit color depth.
• Most Wayland compositors depend on libinput which depends upon libudev. They thus technically depend on systemd, but udev and libudev are currently still independently usable through projects like systemd-utils and libudev-zero.
• Wayland compositors usually need a seat management daemon, mainly to mediate access to the GPU. Current options are seatd and elogind on non-systemd systems. Systemd has its own seat management daemon: logind.

Terminal emulator

Terminal emulators work exactly as the ttys described above, with the same escape sequences. They are, however, not implemented in Kernel, but as clients to X or Wayland. They are the current standard for displaying text based interfaces. Terminal emulators mostly invoke the executable specified for the current user at /etc/passwd. This is usually a shell like GNU Bash, that then interprets the user input.

Secure shell

SSH is a means to create a secure connection. Usually, upon connection and authentification, the executable specified at /etc/passwd for the authenticated user is ran. Thus, the SSH client is by default executed in a terminal emulator, with a shell listening and reacting on the server side. SSH can also be used to forward X windows. Examples of SSH implementations containing both a client and a server program are OpenSSH and Dropbear.

Arcan

Arcan is both a protocol and its implementation. Arcan is based on having a display server running Appls. Appls being, games and even the window manager itself is an Appl.

Notable points:

• Almost no programs support Arcan.
• Arcan has Wayland emulation and X emulation.
• Arcan is network transparent.
• Arcan does properly support multiple monitors with differen refresh rates.
• Arcan does properly support color management, like HDR and 10bit color depth.
• Arcan provides a way to create text based interfaces without terminal emulations. See here and here.