Remove ubuntu-gnome-desktop?
The bad thing about installing other Desktop environments is they install a lot of extra packages and it is sometimes hard to find and remove all of them.
Start by running this command:
sudo apt-get purge abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview alacarte argyll cups-pk-helper epiphany-browser epiphany-browser-data evolution evolution-common evolution-plugins fonts-cantarell fonts-lyx gdm gir1.2-accountsservice-1.0 gir1.2-caribou-1.0 gir1.2-clutter-gst-2.0 gir1.2-evince-3.0 gir1.2-gck-1 gir1.2-gconf-2.0 gir1.2-gcr-3 gir1.2-gdesktopenums-3.0 gir1.2-gkbd-3.0 gir1.2-gnomedesktop-3.0 gir1.2-gst-plugins-base-1.0 gir1.2-gstreamer-1.0 gir1.2-ibus-1.0 gir1.2-mutter-3.0 gir1.2-networkmanager-1.0 gir1.2-panelapplet-4.0 gir1.2-polkit-1.0 gir1.2-telepathyglib-0.12 gir1.2-telepathylogger-0.2 gir1.2-upowerglib-1.0 gir1.2-xkl-1.0 gjs glchess glines gnect gnibbles gnobots2 gnome-applets gnome-applets-data gnome-color-manager gnome-desktop-data gnome-dictionary gnome-games gnome-games-extra-data gnome-icon-theme-extras gnome-mahjongg gnome-packagekit gnome-packagekit-data gnome-panel gnome-panel-data gnome-search-tool gnome-session-fallback gnome-shell gnome-shell-common gnome-software-manager gnome-sushi gnome-tweak-tool gnome-update-viewer gnotravex gnotski gnumeric gnumeric-common gnumeric-doc gstreamer1.0-alsa gstreamer1.0-plugins-base-apps gstreamer1.0-pulseaudio gstreamer1.0-tools gtali iagno itstool libabiword-2.9 libaudit0 libcaribou-common libcaribou0 libcolamd2.7.1 libcolord-gtk1 libedataserverui-3.0-4 libevolution libgdict-1.0-6 libgdict-common libgdome2-0 libgdome2-cpp-smart0c2a libgjs0c libgoffice-0.8-8 libgoffice-0.8-8-common libgtkmathview0c2a libicc2 libimdi0 libiptcdata0 liblink-grammar4 libloudmouth1-0 libmail-spf-perl libmozjs185-1.0 libmutter0 libnetaddr-ip-perl libots0 libpst4 libtidy-0.99-0 libtracker-extract-0.14-0 libtracker-miner-0.14-0 libtracker-sparql-0.14-0 libwv-1.2-4 libytnef0 lightsoff link-grammar-dictionaries-en mutter mutter-common packagekit packagekit-backend-aptcc packagekit-tools plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text python-cloudfiles python-packagekit quadrapassel re2c simple-scan spamassassin spamc swell-foop tracker tracker-extract tracker-gui tracker-miner-fs tracker-utils ubuntu-gnome-default-settings ubuntu-gnome-desktop xsltproc yelp-tools
It will purge most of the extra packages you installed.
Then run sudo apt-get autoremove to remove extra packages and dependencies no longer needed on your system.
You might (or might not, depending on if gdm was removed already) need to use these commands to change your login screen back to Unity's default (which is lightDM) run
sudo dpkg-reconfigure gdm
Hit enter at the prompt and then select lightdm from the options.
After that you can purge gdm with
sudo apt-get purge gdm
Finally run:
sudo apt-get install --reinstall ubuntu-desktop
In case the removal of gnome grabbed any bits of the main desktop.
This answer is based on Seth's answer, and is intended to solve a small problem that I was facing on my machine (Ubuntu 14.04) when I tried to execute his first command:
sudo apt-get purge abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview alacarte argyll cups-pk-helper epiphany-browser epiphany-browser-data evolution evolution-common evolution-plugins fonts-cantarell fonts-lyx gdm gir1.2-accountsservice-1.0 gir1.2-caribou-1.0 gir1.2-clutter-gst-2.0 gir1.2-evince-3.0 gir1.2-gck-1 gir1.2-gconf-2.0 gir1.2-gcr-3 gir1.2-gdesktopenums-3.0 gir1.2-gkbd-3.0 gir1.2-gnomedesktop-3.0 gir1.2-gst-plugins-base-1.0 gir1.2-gstreamer-1.0 gir1.2-ibus-1.0 gir1.2-mutter-3.0 gir1.2-networkmanager-1.0 gir1.2-panelapplet-4.0 gir1.2-polkit-1.0 gir1.2-telepathyglib-0.12 gir1.2-telepathylogger-0.2 gir1.2-upowerglib-1.0 gir1.2-xkl-1.0 gjs glchess glines gnect gnibbles gnobots2 gnome-applets gnome-applets-data gnome-color-manager gnome-desktop-data gnome-dictionary gnome-games gnome-games-extra-data gnome-icon-theme-extras gnome-mahjongg gnome-packagekit gnome-packagekit-data gnome-panel gnome-panel-data gnome-search-tool gnome-session-fallback gnome-shell gnome-shell-common gnome-software-manager gnome-sushi gnome-tweak-tool gnome-update-viewer gnotravex gnotski gnumeric gnumeric-common gnumeric-doc gstreamer1.0-alsa gstreamer1.0-plugins-base-apps gstreamer1.0-pulseaudio gstreamer1.0-tools gtali iagno itstool libabiword-2.9 libaudit0 libcaribou-common libcaribou0 libcolamd2.7.1 libcolord-gtk1 libedataserverui-3.0-4 libevolution libgdict-1.0-6 libgdict-common libgdome2-0 libgdome2-cpp-smart0c2a libgjs0c libgoffice-0.8-8 libgoffice-0.8-8-common libgtkmathview0c2a libicc2 libimdi0 libiptcdata0 liblink-grammar4 libloudmouth1-0 libmail-spf-perl libmozjs185-1.0 libmutter0 libnetaddr-ip-perl libots0 libpst4 libtidy-0.99-0 libtracker-extract-0.14-0 libtracker-miner-0.14-0 libtracker-sparql-0.14-0 libwv-1.2-4 libytnef0 lightsoff link-grammar-dictionaries-en mutter mutter-common packagekit packagekit-backend-aptcc packagekit-tools plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text python-cloudfiles python-packagekit quadrapassel re2c simple-scan spamassassin spamc swell-foop tracker tracker-extract tracker-gui tracker-miner-fs tracker-utils ubuntu-gnome-default-settings ubuntu-gnome-desktop xsltproc yelp-tools
For me, due to some reason, apt-get sees that a few packages don't exist and just quits. So I purged all the packages one by one instead, by doing:
for i in abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview alacarte argyll cups-pk-helper epiphany-browser epiphany-browser-data evolution evolution-common evolution-plugins fonts-cantarell fonts-lyx gdm gir1.2-accountsservice-1.0 gir1.2-caribou-1.0 gir1.2-clutter-gst-2.0 gir1.2-evince-3.0 gir1.2-gck-1 gir1.2-gconf-2.0 gir1.2-gcr-3 gir1.2-gdesktopenums-3.0 gir1.2-gkbd-3.0 gir1.2-gnomedesktop-3.0 gir1.2-gst-plugins-base-1.0 gir1.2-gstreamer-1.0 gir1.2-ibus-1.0 gir1.2-mutter-3.0 gir1.2-networkmanager-1.0 gir1.2-panelapplet-4.0 gir1.2-polkit-1.0 gir1.2-telepathyglib-0.12 gir1.2-telepathylogger-0.2 gir1.2-upowerglib-1.0 gir1.2-xkl-1.0 gjs glchess glines gnect gnibbles gnobots2 gnome-applets gnome-applets-data gnome-color-manager gnome-desktop-data gnome-dictionary gnome-games gnome-games-extra-data gnome-icon-theme-extras gnome-mahjongg gnome-packagekit gnome-packagekit-data gnome-panel gnome-panel-data gnome-search-tool gnome-session-fallback gnome-shell gnome-shell-common gnome-software-manager gnome-sushi gnome-tweak-tool gnome-update-viewer gnotravex gnotski gnumeric gnumeric-common gnumeric-doc gstreamer1.0-alsa gstreamer1.0-plugins-base-apps gstreamer1.0-pulseaudio gstreamer1.0-tools gtali iagno itstool libabiword-2.9 libaudit0 libcaribou-common libcaribou0 libcolamd2.7.1 libcolord-gtk1 libedataserverui-3.0-4 libevolution libgdict-1.0-6 libgdict-common libgdome2-0 libgdome2-cpp-smart0c2a libgjs0c libgoffice-0.8-8 libgoffice-0.8-8-common libgtkmathview0c2a libicc2 libimdi0 libiptcdata0 liblink-grammar4 libloudmouth1-0 libmail-spf-perl libmozjs185-1.0 libmutter0 libnetaddr-ip-perl libots0 libpst4 libtidy-0.99-0 libtracker-extract-0.14-0 libtracker-miner-0.14-0 libtracker-sparql-0.14-0 libwv-1.2-4 libytnef0 lightsoff link-grammar-dictionaries-en mutter mutter-common packagekit packagekit-backend-aptcc packagekit-tools plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text python-cloudfiles python-packagekit quadrapassel re2c simple-scan spamassassin spamc swell-foop tracker tracker-extract tracker-gui tracker-miner-fs tracker-utils ubuntu-gnome-default-settings ubuntu-gnome-desktop xsltproc yelp-tools; do sudo apt-get -y purge $i; done
After doing the above, follow the rest of Seth's instructions.
Hope you get a gnome-shell-free desktop when all is done!
Update: I can confirm that Seth's procedure (mostly) worked on my Ubuntu 14.04. There are still minor remnants of Gnome behavior, but they are tolerable.
Addon to Seth's answer:
apt-get autoremove
does not remove configuration files. If you want them to be removed too use
apt-get autoremove --purge
Reference: https://corlewsolutions.com/articles/article-13-removing-packages-with-apt-get-on-ubuntu