Run a startup script with lightdm

I would like to add that display-setup-script=/path/to/some/script goes into the [Seat:*] ([SeatDefaults] in older versions) section of /etc/lightdm/lightdm.conf.

I use the script to setup the correct resolution and screen orientation for the greeter. This looks like:

xrandr --output DVI-0  --mode 1920x1200 --rotate left --primary
xrandr --output HDMI-0 --mode 1920x1080

You might want to try adding pre-start or post-start scripts to

/etc/init/lightdm.conf

(see http://upstart.ubuntu.com/getting-started.html for an introduction to upstart that parses this file)

Also /etc/lightdm.conf allows to specify scripts to be loaded. You have a detailed description of all (many) the options that can be used in the configuration file lightdm.conf at /usr/share/doc/lightdm/lightdm.conf.

Try insert your script in ~/.xprofile file, like below

#! /bin/sh
# ~/.xprofile: execute commands at the beginning of the X user
#              session - before the window manager is started.
#
# The xprofile files are natively sourced by the following
# display managers:
#
#     GDM     - /etc/gdm/Xsession
#     LightDM - /etc/lightdm/Xsession
#     LXDM    - /etc/lxdm/Xsession
#     SDDM    - /usr/share/sddm/scripts/Xsession
#
# More in https://wiki.archlinux.org/index.php/Autostarting

/usr/bin/nm-applet &