Ubuntu Desktop comes with GNOME as the default desktop environment along with all the necessary GNOME applications and tools. You can install KDE applications using apt, but it might not integrate or look well in GNOME.
A complete KDE suite just like Kubuntu can be installed on Ubuntu. You can choose whether to use GNOME or KDE's Plasma desktop every time you log in to the system. You can also set KDE as your default environment.
$ sudo apt update [sudo] password for user: Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB] Hit:2 http://jp.archive.ubuntu.com/ubuntu focal InRelease Get:3 http://jp.archive.ubuntu.com/ubuntu focal-updates InRelease [111 kB] Get:4 http://jp.archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB] Get:5 http://security.ubuntu.com/ubuntu focal-security/main amd64 DEP-11 Metadata [21.2 kB] Get:6 http://security.ubuntu.com/ubuntu focal-security/universe amd64 DEP-11 Metadata [35.8 kB] Fetched 374 kB in 2s (205 kB/s) Reading package lists... Done Building dependency tree Reading state information... Done All packages are up to date.
$ sudo apt dist-upgrade --assume-yes Reading package lists... Done Building dependency tree Reading state information... Done Calculating upgrade... Done 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
$ sudo apt install --assume-yes kubuntu-desktop Reading package lists... Done Building dependency tree Reading state information... Done The following additional packages will be installed: appmenu-gtk-module-common appmenu-gtk3-module apport-kde apt-config-icons-large apt-config-icons-large-hidpi apt-xapian-index ark baloo-kf5 bluedevil breeze breeze-cursor-theme breeze-gtk-theme breeze-icon-theme catdoc cdrdao cryfs cryptsetup cryptsetup-bin ##### snipped 0 upgraded, 671 newly installed, 0 to remove and 0 not upgraded. Need to get 402 MB of archives. After this operation, 1,490 MB of additional disk space will be used. ##### snipped
This will install a complete KDE Plasma/Kubuntu packages and experience on your Ubuntu system.
SDDM is the default display manager for Kubuntu/KDE.
$ sudo reboot
Select Ubuntu or Ubuntu on Wayland (Wayland) if you want to log in to GNOME desktop environment.
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