https://en.wikipedia.org/wiki/X_display_manager_(program_type)
Concept well defined in terms of an X11 API it seems, may not apply to other windowing systems.
Shows the initial login screen where you can:
- enter your username and password
- shutdown the computer
- select which desktop environment to run (GNOME, Unity, KDE Plasma, etc.)
Some implementations:
- LightDM (lightweight), Ubuntu
- KDM, KDE
- GDM (gnome) fedora
Find your default X display manager:
cat /etc/X11/default-display-manager
Log out from the graphical TTY and go back to the display manger login screen http://askubuntu.com/questions/180628/how-can-i-logout-from-the-gui-using-cli:
kill -9 -1
man kill tells us that here:
-9is for signal number 9,SIGKILL-1is a special number, which means all PIDs except for init and the kill process itself.
For X11: https://www.gulshansingh.com/posts/how-to-write-a-display-manager/