Pop!_OS comes with the GNOME desktop environment and Ubuntu before 17.10 comes with the Unity desktop environment. A desktop environment is responsible for the look and feel of the graphical desktop, and includes many of the key programs that get used every day.
Desktop environments can be installed with a single command and can be selected at the login screen (image below). All desktop environments have access to your home folder and files, so they can share data. They are basically the top, graphical layer of Pop!_OS, each with it’s own individual settings.


In Ubuntu 16.04, make sure to choose lightdm as the display manager when prompted. Choosing gdm, sddm, or another, will break the installation. Other display managers work fine in 17.04 and later. If needed, this command can be run to choose the display manager again:
sudo dpkg-reconfigure lightdm
Mate is an intuitive and attractive desktop using traditional concepts. MATE is a fork of GNOME 2.

MATE is installed with:
sudo apt install mate-desktop-environment-extras mate-dock-applet
Cinnamon is used in Linux Mint by default. Cinnamon strives to provide a traditional experience and is a fork of GNOME 3.

Cinnamon is installed with:
sudo apt install cinnamon-desktop-environment
GNOME is the default desktop environment in Pop!_OS and Ubuntu 17.10. It contains both a modern and a classic version.


In Ubuntu, GNOME is installed with:
sudo apt install ubuntu-gnome-desktop
The KDE Plasma desktop environment is a familiar working environment and looks similar to Windows’ desktop. It is highly customizable and looks clean. It is more heavy on system resources than other desktop environments, and isn’t recommended for low power usage on laptops.

KDE is installed with:
sudo apt install kubuntu-desktop
KDE may have a packaging conflict which will cause the installation to fail. If so, two packages need removed to complete the installation. After an error, please run these commands to remove the packages and finish the installation:
sudo dpkg --purge unity-scope-gdrive account-plugin-google
sudo apt install -f
Xfce embodies the traditional UNIX philosophy of modularity and re-usability. This environment provides a good bit of conformity for the programs written for it. Xfce also provides a highly customizable environment, while being leaner on resources.

Xfce can be installed with:
sudo apt install xubuntu-desktop
The Lightweight X11 Desktop Environment is a fast and energy-saving desktop environment. It looks similar to older versions of Windows. This is the leanest desktop environment and can help extend battery life in laptops.

LXDE can be installed with:
sudo apt install lubuntu-desktop
Edubuntu has been developed in collaboration with teachers and technologists in several countries and is aimed at users aged 6 to 18. It is designed for easy installation and ongoing system maintenance. It uses GNOME for it’s desktop manager.
Edubuntu can be installed with:
sudo apt install edubuntu-desktop
GNOME and KDE are heavy resource desktop environments. Installations consume more disk space than lightweight alternatives and more CPU and memory resources while in use. This is because they are full-featured; they provide the most complete and well-integrated environments.
LXDE and Xfce, on the other hand, are lightweight desktop environments. They are designed to work well on older or lower-power hardware and generally consume fewer system resources while in use. This is achieved by cutting back on extra features.
Desktop environments can interfere with each other or change universal settings. Issues with a desktop environment looking different can generally be fixed by changing the theme in the Appearance control panel.
GNOME will add it’s own login greeter, which is shown as the second image at the top of this article. To change it, run this command:
sudo gedit /etc/lightdm/lightdm.conf
And add a line at the bottom to set the preferred greeter:
Unity:
greeter-session=unity-greeter
GNOME:
greeter-session=lightdm-gtk-greeter
If you currently use automatic login to bypass the password login, a default can be set with this command:
gksudo gedit /etc/lightdm/lightdm.conf
And add a line at the bottom to set the (default):
LXDE:
user-session=Lubuntu
KDE:
user-session=kde-plasma-kf5
Unity:
user-session=ubuntu
MATE:
user-session=mate
GNOME:
user-session=gnome
-or-
user-session=gnome-classic
XFCE:
user-session=xubuntu
Edubuntu:
user-session=gnome-flashback-compiz
-or-
user-session=gnome-flashback-metacity
Many desktop environments change the startup and shutdown splash screens. To change the Plymouth splash screen, run these commands:
sudo update-alternatives --config default.plymouth
sudo update-initramfs -u
Xfce will change the notification dialog to it’s own. It can be configured with this command:
xfce4-notifyd-config
Or changed back to Unity’s with this command:
gksu gedit /usr/share/dbus-1/services/org.xfce.xfce4-notifyd.Notifications.service
And replace this line:
Exec=/usr/lib/xfce4/notifyd/xfce4-notifyd
With this one:
Exec=/usr/lib/x86_64-linux-gnu/notify-osd
If your background turns to black or white, or stops changing when you select a new background, re-enable it with this command:
gsettings set org.gnome.settings-daemon.plugins.background active true
Many desktop environments supply both a bare-bones environment, a full featured environment, or other options. Cinnamon provides both a 2d and 3d environment. Some of the extra options aren’t needed, and don’t have to be presented as options. The options available at login are in the /usr/share/xsessions folder. Simply rename or delete the files to remove them as options.
Lubuntu’s extra options:
/usr/share/xsessions/Lubuntu-Netbook.desktop
/usr/share/xsessions/LXDE.desktop
/usr/share/xsessions/openbox.desktop
Xubuntu’s extra option:
/usr/share/xsessions/xfce.desktop
Cinnamon’s extra option:
/usr/share/xsessions/cinnamon2d.desktop
Some desktop environments add addition programs to be started at boot. To change them, run the Startup Applications program, and turn off any additional startup programs that were undesirably added.
Sometimes the GNOME screensaver presents a second password unlock screen after suspending or locking the screen. It can be disabled with this command:
gsettings set org.gnome.desktop.screensaver lock-enabled false
Or, if that doesn’t stop the 2nd prompt, the screensaver can be removed with this command:
sudo apt purge gnome-screensaver
When using GNOME, the lock screens will still function normally even with the screensaver disabled.
If Cinnamon’s desktop lock screen isn’t functioning, this command will re-enable it:
gsettings set org.cinnamon.desktop.lockdown disable-lock-screen false
Ubuntu’s unity greeter signin screen will only accommodate a certain number of desktop environments. If the list is too large, then extra options can be deleted from this folder:
/usr/share/xsessions/
Another solution is to switch to the lightdm-gtk-greeter following the instructions above. This secondary greeter is installed with GNOME, or can be installed manually with this command:
sudo apt install lightdm-gtk-greeter
If you no longer want to use a desktop environment, it can be removed by using:
sudo apt purge ...
sudo apt autoremove
For example, to remove KDE:
sudo apt purge kubuntu-desktop
sudo apt autoremove
Or to remove Unity:
sudo apt purge ubuntu-desktop
sudo apt autoremove