How to use classic Gnome shell instead of Unity desktop

There are cases where I prefer to install and use Gnome fallback mode (Gnome Flashback in recent system versions) instead of full-blown Unity desktop. I will give you simple instructions on how to change the default session manager.

Ubuntu 14.04 Trusty Tahr

Enable universe repository component.

$ sudo add-apt-repository universe

Update the package list.

$ sudo apt-get update

Install gnome-session-fallback package, which provides the required functionality.

$ sudo apt-get install gnome-session-fallback

Set gnome-fallback as the default session manager.

$ sudo sed -i '/\[SeatDefaults\]/a user-session=gnome-flashback-metacity' \
              /etc/lightdm/lightdm.conf

Ubuntu 15.04 Vivid Vervet and 15.10 Wily Werewolf

Enable universe repository component.

$ sudo add-apt-repository universe

Update the package list.

$ sudo apt-get update

Install gnome-session-flashback package, which provides the required functionality.

$ sudo apt-get install gnome-session-flashback

Set gnome-flashback-metacity as the default session manager.

$ sudo sed -i '/[SeatDefaults]/a user-session=gnome-flashback-metacity'
              /etc/lightdm/lightdm.conf

Additional notes

Use the following command to create /etc/lightdm/lightdm.conf configuration file if it is missing.

$ cat << EOF | sudo tee /etc/lightdm/lightdm.conf
[SeatDefaults]
user-session=gnome-flashback-metacity
EOF