README for pam_xdg

PAM module that handles the XDG Base Directory Specification[1]
directories, including creation of XDG_RUNTIME_DIR and injection
of according environment variables into user sessions.

For it to work it must be included in /etc/pam.d -- to make it a
vivid part of session handling the file /etc/pam.d/common-session
seems best.  Include the following early:

   session optional pam_xdg.so [notroot] [runtime]

Use notroot argument to only handle XDG for non-root users.
Use runtime argument to only handle of XDG_RUNTIME_DIR.

  [1] https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html