Ubuntu18.04 默认源 恢复默认源 恢复备份源

方法1：进入/etc/apt文件夹，如果有备份过，直接将备份文件sources.list.bak文件的后缀名.bak删去，使用普通的文本编辑器或者vim打开，就是原来的默认源。

方法2：直接复制下面的文本：

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://ports.ubuntu.com/ubuntu-ports/ bionic main restricted
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-updates main restricted
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-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://ports.ubuntu.com/ubuntu-ports/ bionic universe
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic universe
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-updates universe
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-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://ports.ubuntu.com/ubuntu-ports/ bionic multiverse
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic multiverse
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-updates multiverse
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-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://ports.ubuntu.com/ubuntu-ports/ bionic-backports main restricted universe multiverse
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-backports main restricted universe 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 bionic partner
# deb-src http://archive.canonical.com/ubuntu bionic partner

deb http://ports.ubuntu.com/ubuntu-ports/ bionic-security main restricted
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-security main restricted
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-security universe
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-security universe
deb http://ports.ubuntu.com/ubuntu-ports/ bionic-security multiverse
# deb-src http://ports.ubuntu.com/ubuntu-ports/ bionic-security multiverse

保存后别忘记更新配置：

sudo apt-get install update
sudo apt-get install upgrade