ubuntu

# Type the following to get the value of the DISPLAY variable, we will need in in a couple of steps.
echo $DISPLAY;
# It will print out :0 or :1 or similar
# Give temporary access to user gdm to access control list and to applications that need a monitor:
sudo xhost +SI:localuser:gdm;
# Output will be something like the following:
# localuser:gdm being added to access control list
# Switch to the user (su) gdm using bash shell
su gdm -l -s /bin/bash;
# Set the DISPLAY variable to the value you got before (could be :0 or :1 or similar):
export DISPLAY=:0;
# Disable the user list by setting the disable-user-list flag to true:
gsettings set org.gnome.login-screen disable-user-list true;

# Type the following to get the value of the DISPLAY variable, we will need in in a couple of steps.
echo $DISPLAY;
# It will print out :0 or :1 or similar
# Give temporary access to user gdm to access control list and to applications that need a monitor:
sudo xhost +SI:localuser:gdm;
# Output will be something like the following:
# localuser:gdm being added to access control list
# Switch to the user (su) gdm using bash shell
su gdm -l -s /bin/bash;
# Set the DISPLAY variable to the value you got before (could be :0 or :1 or similar):
export DISPLAY=:0;
# Enable to the user list by resetting it to its default value:
gsettings reset org.gnome.login-screen disable-user-list;