As requested here is a summary of openSUSE's usage of scripting facilities offered by gdm and xdm.


Available scripting facilities in xdm and gdm
=============================================

xdm         gdm
--------------------------------------------------------------------------------
Xsetup      Init            runs after starting X server, before the greeter
            PostLogin       runs after authentication, before session setup
                            (including pam_open_session)
Xstartup    PreSession      runs after session has been initialized
Xsession    Xsession        wraps the session, runs as the user logging in
Xreset      PostSession     runs after session and X server have been terminated


xdm scripting use in openSUSE
=============================

Xsetup
------
* expected environment:
    DISPLAY        the associated display name
    PATH           the value of DisplayManager.DISPLAY.systemPath
    SHELL          the value of DisplayManager.DISPLAY.systemShell
    XAUTHORITY     may be set to an authority file
* configures external display in case of laptops with XRANDR 1.2 available
* starts xvkbd in case of tablet pcs
* sets root window background
* sets numlock status with numlockx
* reads in Xresources
* enables screensaver
* sets keymap using XKB or Xmodmap

Xstartup
--------
* expected environment:
    DISPLAY        the associated display name
    HOME           the initial working directory of the user
    LOGNAME        the user name
    USER           the user name
    PATH           the value of DisplayManager.DISPLAY.systemPath
    SHELL          the value of DisplayManager.DISPLAY.systemShell
    XAUTHORITY     may be set to an authority file
    WINDOWPATH     may be set to the "window path" leading to the X server
* kill xvkbd in case of tablet
* check for /etc/nologin, show error and exit if it exists
* enforces /etc/sysconfig/displaymanager $DISPLAYMANAGER_ROOT_LOGIN_REMOTE
* if DISPLAY is local execute /etc/X11/xdm/GiveDevices
* kill preload
* registers session in utmp by exec sessreg

Xsession
--------
* arguments: session name, language
* expected environment:
    DISPLAY        the associated display name
    HOME           the initial working directory of the user
    LOGNAME        the user name
    USER           the user name
    PATH           the value of DisplayManager.DISPLAY.userPath
    SHELL          the user's default shell (from getpwnam)
    XAUTHORITY     may be set to a non-standard authority file
    KRB5CCNAME     may be set to a Kerberos credentials cache name
    WINDOWPATH     may be set to the "window path" leading to the X server
* if $SHELL not in /etc/shells disallow login
* redirect errors to user logfile
* handle login manager arguments (session, language)
    * session
        * if default/custom, do nothing
        * if failsafe, execute xterm
        * else set WINDOWMANAGER
    * language
        * if given, set LANG, otherwise fall back to /etc/profile.d/lang.sh
* if WINDOWMANAGER not set above, get system default from
  /etc/profile.d/profile.sh
* source /etc/X11/xinit/xinitrc.common
    * choose WINDOWMANAGER again if not specified yet
    * start xim server if installed
    * apply numlock settings
    * load nvidia settings
    * load xbkmap or xmodmmap
    * merge Xresources
* if WINDOWMANAGER was passed through xdm and /etc/X11/xdm/sys.xsession exists,
  execute it with $SHELL
* otherwise check for ~/.session, ~/.xinitrc, /etc/X11/xdm/sys.xsession,
  /etc/X11/xinit/xinitrc, and execute it with $SHELL
    * /etc/X11/xdm/sys.xsession
        * starts gnome-keyring if exists
        * starts ssh-agent if exists
        * starts seahorse-agent if exists and WINDOWMANAGER is gnome, else
          start gpg-agent if exists
        * checks for ~/.xinitrc, /etc/X11/xinit/xinitrc and executes with bash
          (see below), otherwise source /etc/X11/xinit/xinitrc.common
            * see above
        * execute WINDOWMANAGER
    * /etc/X11/xinit/xinitrc
        * executes scripts in /etc/X11/xinit/xinitrc.d/
        * sources /etc/X11/xinit/xinitrc.common
        * if WINDOWMANAGER is twm set root window background
        * executes WINDOWMANAGER
* othwise fall back to failsafe session

Xreset
------
* if DISPLAY is local execute /etc/X11/xdm/TakeDevices
* kill Xconsole
* execute sessreg in order to remove utmp entry


gdm scripting use in openSUSE
=============================

Init
----
runs Xsetup

PostLogin
---------
empty

PreSession
----------
runs Xstartup

Xsession
--------
runs Xsession $1 $GDM_LANG

PostSession
-----------
runs Xreset