Being somewhat sick of standard Ubuntu - it’s generally pretty RAM intensive and since 18.10 gnome-shell leaks some 500 MB RAM per day, so I have to interrupt my work for a logout/login cycle way more often than I like, I’d like to give Budgie a try. “Lightweight” sounds good, very good.
With Debian and Ubuntu having excellent package managers, I’d like to extend the current installation with Budgie. Goal is to not only have Ubuntu and Ubuntu on Wayland as choice on the login screen, but also Ubuntu Budgie. Or some command to switch between these desktop environments.
Is this possible? So far I’ve found only rather old instructions: http://tipsonubuntu.com/2016/11/09/install-budgie-desktop-ubuntu-16-04-16-10/ to do this.
Some hints on how to get such a parallel installation would be great!
note - I have not tested this.
To get our packages and configuration, all you need to do is sudo apt install ubuntu-budgie-desktop. but do keep in mind that that will pull in all of our dependencies, Etc. Wondering about login manager conflicts.
In my experience, the login manager that you install via the ubuntu-budgie-desktop metapackage will be enabled automatically and replace gdm (the default Ubuntu login manager). I believe systemd only allows one service to provide the login manager.
If you want to use gdm instead after the fact (which is what you will probably want to do if you still want the lock screen to work under GNOME), just run sudo systemctl disable lightdm, then sudo systemctl enable gdm, and finally reboot for the change to take effect.
I have never done this because I don’t generally like to run two different desktop environments on my machine, but it may also be useful to set up another user to run Budgie if you want to avoid Budgie settings getting applied onto GNOME. Budgie and GNOME use the same settings backend, so themes etc. will be applied to both. Just a heads up. I have had the same theme applied to both and been quite happy with it. YMMV.
So I did a
sudo apt-get install ubuntu-budgie-desktop
During installation a menu came up, asking for a choice between lightdm and gdm3 Choice was lightdm.
Then a reboot and … nothing changed. gnome-shell still running, just gdm3 was replaced with lightdm. Choice on the login screen is still just Ubuntu and Ubuntu on Wayland.
Fiddling with systemctl doesn’t appear to work either:
$ sudo systemctl enable lightdm
Synchronizing state of lightdm.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable lightdm
The unit files have no installation config (WantedBy, RequiredBy, Also, Alias
settings in the [Install] section, and DefaultInstance for template units).
This means they are not meant to be enabled using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
.wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
D-Bus, udev, scripted systemctl call, ...).
4) In case of template units, the unit is meant to be enabled with some
instance name specified.
(same when trying to enable gdm)
Looked a bit around and found file /usr/share/ubuntu-release-upgrader/DistUpgrade.cfg. Therein:
[ubuntu-budgie-desktop]
KeyDependencies=budgie-desktop-environment, budgie-welcome, ubuntu-budgie-themes
Installed these two additional packages, still no change.
is budgie-desktop actually installed?
You’re a hero! It wasn’t. Correcting this got things working as expected.
So, to get Budgie installed in parallel to base Ubuntu, it goes like this:
sudo apt-get install ubuntu-budgie-desktop budgie-welcome ubuntu-budgie-themes budgie-desktop-environment
Then reboot and choose the environment at the login screen. Having installed both works just fine.
finish off by running budgie-welcome (menu option). It will install the snap version and promptly uninstall itself.
Should the meta package have not brought in those other packages though?
yes - that’s the very odd thing which makes my head hurt.
OK, great - I just wanted to be sure I had given the proper instructions. 
Have you some-how stopped installing recommended packages? ubuntu-budgie-desktop should have install budgie-desktop as part of the recommended packages.
Yes, of course. “Recommended” means recommended, not essential.
I’ve recorded what these commands did:
$ sudo apt-get install ubuntu-budgie-desktop
[...]
The following NEW packages will be installed:
budgie-indicator-applet budgie-lightdm-theme budgie-lightdm-theme-base
budgie-wallpapers budgie-wallpapers-cosmic gnome-themes-standard itstool
libbudgie-plugin0 libgtkd-3-0 libphobos2-ldc-shared81 libvted-3-0 lightdm
plymouth-theme-ubuntu-budgie-logo plymouth-theme-ubuntu-budgie-text
plymouth-themes python-libxml2 python3-notify2 slick-greeter
ssh-askpass-gnome tilix tilix-common ubuntu-budgie-desktop
$ sudo apt-get install budgie-welcome ubuntu-budgie-themes
The following NEW packages will be installed:
budgie-welcome ubuntu-budgie-themes
$ sudo apt-get install budgie-desktop-environment
[...]
The following NEW packages will be installed:
budgie-core budgie-desktop budgie-desktop-common budgie-desktop-environment
gir1.2-budgie-1.0 libbudgie-private0 libbudgietheme0 libraven0
qt5-style-platform-gtk2 qt5-style-plugin-blackberry
qt5-style-plugin-cleanlooks qt5-style-plugin-gtk2
qt5-style-plugin-motif qt5-style-plugin-plastique qt5-style-plugins