ubuntu更换桌面环境（unity-->gnome）

ubuntu 12.04 中安装gnome桌面的命令为：
sudo apt-get install gnome-session-fallback

也可以用：sudo apt-get install gnome-panel

注：

GNOME  Flashback is a session for Gnome 3 which was initially called "Fallback Mode", and shipped as a stand-alone session in Debian and Ubuntu. It provides a similar user experience to the Gnome 2.x series sessions. As Fallback mode, it was used in situations where the GNOME Shell could not launch due to a computer not meeting its higher hardware demands such ascompositing and further desktop effects, although it could also be toggled to be activated by the user.

Fallback mode was removed in GNOME 3.8 and replaced with a suite of officially supported GNOME extensions named 'GNOME Classic'.

gnome-shell   vs    gnome-panel

gnome-panel:

GNOME Panel is a highly configurable launcher and taskbar for GNOME. It formed a core part of the GNOME desktop in versions 1.x and 2.x. It has been replaced in GNOME 3.x by default with GNOME Shell, which only works with the Mutter window manager.

GNOME Panel served as Fallback Mode when Mutter could not be executed until GNOME 3.8, where it was replaced with a suite of officially supported GNOME extensions named 'GNOME Classic'.

gnome-shell:

GNOME Shell is the graphical user interface of the GNOME desktop environment starting with version 3. it is intended to replace GNOME Panel and other software components from GNOME 2 to offer a user experience that breaks from the previous model of desktop metaphor, used in earlier versions of GNOME.

在删除unity桌面之前，把ubuntu默认的登录界面改为gnome，设置登录界面为 gnome classic命令如下：

sudo /usr/lib/lightdm/lightdm-set-defaults -s gnome-classic

设置登录界面为gnome3命令如下：

sudo /usr/lib/lightdm/lightdm-set-defaults -s gnome-shell

卸载unity:
sudo apt-get -y –auto-remove purge unity
sudo apt-get -y –auto-remove purge unity-common
sudo apt-get -y –auto-remove purge unity-lens*
sudo apt-get -y –auto-remove purge unity-services
sudo apt-get -y –auto-remove purge unity-asset-pool