我不是服务器专家，因此我在升级 Ubuntu 时遇到了一些问题。简单回顾一下：我的 Digital Ocean droplet 运行的是 Ubuntu 12.04 LTS，几个月前我升级到了较新的 HWE，以便继续接收 12.04 的安全更新。最近我将 Ubuntu 升级到了 14.04 LTS。一切似乎都运行正常，只是我没有收到任何有关有软件包需要更新的通知。登录我的服务器后，每日消息继续显示：
0 packages can be updated.
0 updates are security updates.
这已经是三周前的事了……？
运行sudo apt-get update没有错误，并且sudo apt-get upgrade说0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
我感觉我的系统/etc/apt/sources.list在升级到新的 HWE 内核和新的 LTS 后不知何故被破坏了。下面是当前列出的源。有人发现此列表存在任何问题吗？任何建议都将不胜感激。
$ cat /etc/apt/sources.list
#
# deb cdrom:[Ubuntu-Server 12.04.3 LTS _Precise Pangolin_ - Release i386 (20130820.2)]/ precise main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty main restricted
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty main restricted
# deb http://us.archive.ubuntu.com/ubuntu/ precise main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise main restricted
## Major bug fix updates produced after the final release of the
## distribution.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty-updates main restricted
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-updates main restricted
# deb http://us.archive.ubuntu.com/ubuntu/ precise-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise-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.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty universe
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty universe
deb http://mirrors.digitalocean.com/ubuntu/ trusty-updates universe
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-updates universe
# deb http://us.archive.ubuntu.com/ubuntu/ precise universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise universe
# deb http://us.archive.ubuntu.com/ubuntu/ precise-updates universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise-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.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty multiverse
deb http://mirrors.digitalocean.com/ubuntu/ trusty-updates multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-updates multiverse
# deb http://us.archive.ubuntu.com/ubuntu/ precise multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise multiverse
# deb http://us.archive.ubuntu.com/ubuntu/ precise-updates multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise-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.
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-backports main restricted universe multiverse
# deb http://us.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
## DigitalOcean mirrors for blazing speed!
deb http://mirrors.digitalocean.com/ubuntu/ trusty-security main restricted
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-security main restricted
deb http://mirrors.digitalocean.com/ubuntu/ trusty-security universe
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-security universe
deb http://mirrors.digitalocean.com/ubuntu/ trusty-security multiverse
deb-src http://mirrors.digitalocean.com/ubuntu/ trusty-security multiverse
# deb http://security.ubuntu.com/ubuntu precise-security main restricted
# deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
# deb http://security.ubuntu.com/ubuntu precise-security universe
# deb-src http://security.ubuntu.com/ubuntu precise-security universe
# deb http://security.ubuntu.com/ubuntu precise-security multiverse
# deb-src http://security.ubuntu.com/ubuntu precise-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 precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner
## Uncomment the following two lines to add software from Ubuntu's
## 'extras' repository.
## 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 precise main
# deb-src http://extras.ubuntu.com/ubuntu precise main
答案1
Ubuntu LTS 版本没有太多更新。事实上，你在 LTS 版本中看到的唯一更新是
- 安全更新
- 修复主要错误（文件丢失错误等）
看https://wiki.ubuntu.com/StableReleaseUpdates#When了解 LTS 版本何时进行更新。