這是一個非常受歡迎的問題，但請耐心等待，因為我已經嘗試了你們所建議的一切。這就是我得到的：
sudo apt-get install ubuntu-desktop
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help resolve the situation:
The following packages have unmet dependencies:
ubuntu-desktop : Depends: ubuntu-session but it is not going to be installed
Depends: unity-control-center but it is not going to be installed
Depends: unity-settings-daemon but it is not going to be installed
Recommends: xul-ext-webaccounts but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
所以我嘗試安裝unity-control-center
Depends: unity-settings-daemon but it is not going to be installed
所以我嘗試安裝unity-settings-daemon
Depends: gnome-settings-daemon-schemas (< 3.10) but 3.12.2-0ubuntu1~trusty2 is to be installed
根本問題似乎是gnome-settings-daemon-schemas需要 3.10 或更低版本，但由於某種原因無法獲得它。
所以您可能會想嘗試以下操作：
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo apt-get clean
sudo apt-get autoclean
sudo dpkg --configure -a
sudo -f
我已登入 gnome，當我嘗試啟動lightgdm系統時，系統在有關印表機假脫機的操作後立即掛起。
我目前有 gdm、kdm 和 lightgdm。
我希望我的電腦恢復為 gnome 和 ubuntu unity。有什麼想法嗎？ （重新安裝是不可能的）
答案1
Unity是基於gnome的。許多 unity 套件（和ubuntu-desktop）套件依賴 gnome 套件。
所以這裡的問題似乎是增加了 Gnome 3.12 ppa。
gnome-settings-daemon-schemas
是 Gnome 的一部分。您嘗試安裝的軟體包 ( ubuntu-desktop) 需要 gnome 提供的版本 3.10，但它無法取得版本 3.10，因為 ppa 告訴它從 3.12 儲存庫取得它。
如果你想恢復 Unity，你需要安裝以前的版本：
sudo apt-get install gnome-settings-daemon-schemas=3.8.6.1-0ubuntu11.2
但這會因為依賴關係而刪除一些 - 所以 gnome 3.12 將破壞...
The following packages will be REMOVED
gdm gnome-control-center gnome-session gnome-settings-daemon gnome-shell
The following packages will be DOWNGRADED:
gnome-settings-daemon-schemas