export XDG_RUNTIME_DIR='$XDG_RUNTIME_DIR'
@domi I'm surprised that relative paths are allowed there (i.e. not ignored).
@robryk it's very much application dependent, I think
XDG base dirs spec says they should be ignored (" All paths set in these environment variables must be absolute. If an implementation encounters a relative path in any of these variables it should consider the path invalid and ignore it."), but obviously not everyone follows it.
