今天我遇到了一个奇怪的问题，我发现“软件和更新”的“其他软件”选项卡中 Canonical 合作伙伴和独立存储库不见了。我还注意到“main”、“universe”、“multiverse”和“restricted”以及“trusy-security”、“trusty-updates”、“trusy-proposed”和“trusty-backports”都被禁用了。
问题是：我如何添加规范和独立的存储库以便重新启用它们？ 这可能是什么原因造成的？
先感谢您。
答案1
打开终端（Ctrl+Alt+T）然后运行：
sudo gedit /etc/apt/sources.list
将所有文本替换为以下内容：
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ trusty main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ trusty universe
# deb-src http://archive.ubuntu.com/ubuntu/ trusty universe
deb http://archive.ubuntu.com/ubuntu/ trusty-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu/ trusty multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ trusty multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu trusty-security main restricted
# deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
# deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
# deb-src http://security.ubuntu.com/ubuntu trusty-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu trusty partner
# deb-src http://archive.canonical.com/ubuntu trusty partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu trusty main
# deb-src http://extras.ubuntu.com/ubuntu trusty main
保存，关闭 gedit，然后运行：
sudo apt-get update