问题
如题所示，安装时发现安装ros-melodic-desktop-full时显示一堆软件无法安装，一个一个安装显然是不现实的，找了网上方法都比较不理想，后来感觉可能会是更新源的问题。
解决方法
可以先打开source.list文件
sudo gedit /etc/apt/sources.list
如果有看到不同的两行deb同时被执行（具体可以看命令行里面的名称，一般和源的名字有关），会出现混乱。我就是这样的情况。修改如以下（我的源是ustc的）：
# deb cdrom:[Ubuntu 18.04.5 LTS _Bionic Beaver_ - Release amd64 (20200806.1)]/ bionic main restricted
# See https://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted
# deb-src https://cn.archive.ubuntu.com/ubuntu/ bionic main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted
# deb-src https://cn.archive.ubuntu.com/ubuntu/ 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 https://mirrors.ustc.edu.cn/ubuntu/ bionic universe
# deb-src https://cn.archive.ubuntu.com/ubuntu/ bionic universe
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates universe
# deb-src https://cn.archive.ubuntu.com/ubuntu/ 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 https://mirrors.ustc.edu.cn/ubuntu/ bionic multiverse
# deb-src https://cn.archive.ubuntu.com/ubuntu/ bionic multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates multiverse
# deb-src https://cn.archive.ubuntu.com/ubuntu/ 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 https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src https://cn.archive.ubuntu.com/ubuntu/ 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 https://archive.canonical.com/ubuntu bionic partner
# deb-src https://archive.canonical.com/ubuntu bionic partner
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted
# deb-src https://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://mirrors.ustc.edu.cn/ubuntu/ bionic-security universe restricted
# deb-src http://security.ubuntu.com/ubuntu bionic-security universe
网上有类似答案是改成main server后以官方源的固定格式来改source文件，但如果你的source文件原来大部分就是以某一个特定源为主，也不必要再重新改过。
之后执行
sudo apt-get update
再执行相应命令，问题解决。
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