Answer by Thomas for Reconfigure the display-manager non-interactively

After reading up the .post*, '.pre*' and .config scripts of gdm3 and lightdm I have found a way to run dpkg-reconfigure interactively to change between the display managers and keep the magic of the mentioned pre, post and config scripts.

All you have to do is to update the /etc/X11/default-display-manager file with the display manager binary you want to change to. Then run dpkg-reconfigure non-interactively and you are done. Well, gdm3 does not update the debconf database, whereas lightdm does, but that is OK to do that manually.

Here we go...

Change from lightdm to gdm3.

$ echo "/usr/sbin/gdm3"> /etc/X11/default-display-manager$ DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true dpkg-reconfigure gdm3$ echo set shared/default-x-display-manager gdm3 | debconf-communicate

Change from gdm3 to lightdm.

$ echo "/usr/sbin/lightdm"> /etc/X11/default-display-manager$ DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true dpkg-reconfigure lightdm$ echo set shared/default-x-display-manager lightdm | debconf-communicate

As already mentioned the last step echo set shared/default-x-display-manager lightdm | debconf-communicate is not really necessary here as the scripts of lightdm will take care of it. But for the sake of a simplified script it does not hurt to set it.

Putting things together into a shell script could be as follows. One could add more control if needed.

$ cat set_dm.sh#!/bin/bashset_dm() {    DISPLAY_MANAGER="gdm3"    DISPLAY_MANAGER_SERVICE="/etc/systemd/system/display-manager.service"    DEFAULT_DISPLAY_MANAGER_FILE="/etc/X11/default-display-manager"    if [ -n "${1}" ]    then        DISPLAY_MANAGER="$1"    fi    DISPLAY_MANAGER_BIN="/usr/sbin/${DISPLAY_MANAGER}"    if [ ! -e "${DISPLAY_MANAGER_BIN}" ]    then        echo "${DISPLAY_MANAGER} seems not to be a valid display manager or is not installed."    exit 1    fi    echo "${DISPLAY_MANAGER_BIN}"> "${DEFAULT_DISPLAY_MANAGER_FILE}"    DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true dpkg-reconfigure "${DISPLAY_MANAGER}"    echo set shared/default-x-display-manager "${DISPLAY_MANAGER}" | debconf-communicate &> /dev/null     echo -n "systemd service is set to: "    readlink "${DISPLAY_MANAGER_SERVICE}"    echo -n "${DEFAULT_DISPLAY_MANAGER_FILE} is set to: "    cat "${DEFAULT_DISPLAY_MANAGER_FILE}"    echo -n "debconf is set to: "    echo get shared/default-x-display-manager | debconf-communicate }set_dm $1

The two variables DEBIAN_FRONTEND=noninteractive and DEBCONF_NONINTERACTIVE_SEEN=true are needed to really run interactively.

What also helped was the environment variable DEBCONF_DEBUG to see what debconf is doing. It can be set to

• user
• developer
• db
• .* which is all of the above