How do you switch from Wayland back to Xorg in Ubuntu 17.10?

When you boot your system and get to the GDM login screen you should find a cogwheel (⚙️) next to the sign in button. If you click on the cogwheel you should find an Ubuntu on Xorg option which will start an Xorg session instead of a Wayland session.

If you wish to do it permanently, edit /etc/gdm3/custom.conf and uncomment the line:

#WaylandEnable=false

by removing the # in front of it.

Save the file and then on reboot you will never see the cog asking for which session to use.

EDIT: Apparently @doug beat me to this answer. I didn't see it earlier - It was in a comment that was hidden initially.

You may want to remove wayland session to prevent accidental logins.

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Your package maintainers will be proud of you if you do it as follows:

sudo mkdir /usr/share/wayland-sessions/hidden
sudo dpkg-divert --rename \
      --divert /usr/share/wayland-sessions/hidden/ubuntu.desktop \
      --add /usr/share/wayland-sessions/ubuntu.desktop

What this does is to instruct the package manager to remember a new location for the file. This has several advantages over the other answers:

• It guarantees a future package install/upgrade won't revert your change
• It works with other display managers (lxdm for example lists .backup entries)

• You can revert it easily if you change your mind with:

  sudo dpkg-divert --rename --remove /usr/share/wayland-sessions/ubuntu.desktop

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Source: https://askubuntu.com/a/500813/602695