How to restart X Window Server from command line?

NOTE: This will forcefully quit all graphical programs, you'll lose any unsaved work, and you'll be logged out. Non-graphical programs will not be affected.

TL;DR: on systems with systemd (Ubuntu 15.04 and newer)

sudo systemctl restart display-manager

This will restart the appropriate display manager service (lightdm till 17.04, gdm3 after, sddm in Kubuntu, etc.). You can replace display-manager with lightdm, gdm3, sddm, etc. if needed, but this should be enough.

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For other Ubuntu versions, first find which display manager your ubuntu is having with following command:

cat /etc/X11/default-display-manager

Than depending on what display manager, you can use one of the following commands:

• Default Ubuntu (with LightDM)

  sudo systemctl restart lightdm  
  

• Gnome (with GDM)

  sudo systemctl restart gdm
  

• KDE (with KDM)

  sudo systemctl restart kdm
  

  Note: From 12.10 to 15.04, Kubuntu also uses LightDM.

• For MDM (e.g. for Mint Cinnamon)

  sudo systemctl restart mdm
  

For 11.04 and earlier:

sudo service gdm restart

For 11.10 and later:

sudo service lightdm restart

Found out that you can do sudo pkill X

and it seems to work for me!