Wayland

Wayland is a new display protocol that aims to replace X11.

Multiple compositor implementations exist, including Sway, Mutter (GNOME's compositor) and Kwin (KDE's compositor). A more exhaustive list is available in the ArchWiki.

XDG_RUNTIME_DIR

As per the protocol spec, Wayland compositors require the XDG_RUNTIME_DIR variable to be set. There are a few ways to configure this variable:

A login manager such as elogind can configure this and other XDG environment variables automatically.
pam-rundir can enable this for logins. To use this PAM is required.
Setting it up manually

Configuring XDG_RUNTIME_DIR manually

Generally, care should be taken when configuring the XDG_* variables manually as this configuration may have errors or conflict with other utilities that do this automatically.

On a system that's not using elogind nor any pam module that handles this, it's often necessary to set up XDG_RUNTIME_DIR manually. This can be done by adding a snippet like this one to shell init scripts (e.g.: ~/.profile):

Contents of ~/.profile

if test -z "${XDG_RUNTIME_DIR}"; then
  export XDG_RUNTIME_DIR=/tmp/$(id -u)-runtime-dir
  if ! test -d "${XDG_RUNTIME_DIR}"; then
    mkdir "${XDG_RUNTIME_DIR}"
    chmod 0700 "${XDG_RUNTIME_DIR}"
  fi
fi

XDG_RUNTIME_DIR

Configuring XDG_RUNTIME_DIR manually

See also