XDG RUNTIME DIR
Various daemons and applications depend on XDG_RUNTIME_DIR being set, including Wayland, PipeWire and others.
A few mechanisms exist to properly initialise this variable and its corresponding directory, as described below. Only one should be used for a user's session; configuring multiple of these mechanisms will lead to misbehaviours.
Initialising via elogind
When using elogind as a seat manager, it exports XDG_RUNTIME_DIR and other XDG environment variables automatically for each session. No further configuration is required. XDG_RUNTIME_DIR would be set as /run/user/$(id -u) by elogind.
Initialising via pam_rundir
pam_rundir is a PAM module that provides the runtime directory variable. Installing the package pam-rundir takes care of dependencies and no further configuration is required. XDG_RUNTIME_DIR would be set as /run/user/$(id -u) by pam_rundir.
Initialising via mkrundir
mkrundir is an executable that can be used to initialise the runtime directory explicitly by each user. To use mkrundir, install the package mkrundir available in testing repository. XDG_RUNTIME_DIR would be set as /run/user-$UID by mkrundir. In your shell init script (e.g.: ~/.profile include an entry as follows at the top of the file
Contents of ~/.profile
As per mkrundir website, this might have issues inside containers, due to privilege escalation.
Initialising manually in /tmp
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. Use this only on a system that's not using elogind and other solutions outlined above cannot handle this.
The XDG_RUNTIME_DIR can be initialised manually by adding below snippet to shell init scripts (e.g.: ~/.profile):
Contents of ~/.profile