Lightdm will run your scripts, but there are some complicated prerequisites ...

[lutusp]

Using lightdm 1.26.0 on Debian 11 and xfce4 4.16 environment. After several days of struggle I managed to get Lightdm to change monitor resolutions in advance of logging on. Some points:

• If an external shell script is used, it must be:

  • Located under /usr/share,
  • Owned by lightdm, in group lightdm,
  • It cannot print or log to any destination not accessible to lightdm.
  • There is no flexibility about this, and no warnings/errors are provided.

• To test a configuration:

  • Install xserver-xephyr:
    # apt install xserver-xephyr
  • Run the test as user lightdm:
    # sudo -u lightdm lightdm --test-mode --debug

• Put the shell script reference in /etc/lightdm/lightdm.conf:

        [Seat:*]
        display-setup-script=/usr/share/your_script_name.sh 

• Again, the script must be owned by lightdm and be located under /usr/share.

I hope this helps.

[o-alquimista]

Hey,

I was able to get it working with a script in /usr/local/bin instead of /usr/share. The only thing I did was to set its ownership to the lightdm user and group, as suggested; and making it executable.

$ ls -l /usr/local/bin/dpms-enable 
-rwxr-xr-x 1 lightdm lightdm 41 fev  6 00:21 /usr/local/bin/dpms-enable

Contents of /usr/local/bin/dpms-enable

#!/bin/sh

(sleep 5; xset dpms 0 0 30) &

Contents of /etc/lightdm/lightdm.conf.d/99-dpms.conf:

[Seat:*]
display-setup-script=/usr/local/bin/dpms-enable

I'm using this script to force the display off after 30 seconds at the greeter, to work around a bug in elementary OS 8. LightDM is version 1.30.0.