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ERROR: cannot download default sources list from:
raw.githubusercontent.com/ros/rosdist…
Website may be down.
解决办法：
pip install rosdepc
sudo rosdepc init
rosdep update
错误:30 mirrors.tuna.tsinghua.edu.cn/ros/ubuntu bionic Release.gpg
下列签名无效： EXPKEYSIG F42ED6FBAB17C654 Open Robotics info@osrfoundation.org
解决办法： 更新密钥：
sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
E: 无法定位软件包
解决方法： 更新软件源 修改/etc/apt/sources.list
sudo gedit /etc/apt/sources.list
适用于ubuntu16.04 使用aliyun，将sources.list内容修改为下面内容
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
适用于ubuntu18.04 使用aliyun，将sources.list内容修改为下面内容
deb http://mirrors.aliyun.com/ubuntu bionic main multiverse restricted universe
deb http://mirrors.aliyun.com/ubuntu bionic-updates main multiverse restricted universe
deb http://mirrors.aliyun.com/ubuntu bionic-security main multiverse restricted universe
deb http://mirrors.aliyun.com/ubuntu bionic-proposed main multiverse restricted universe
deb http://mirrors.aliyun.com/ubuntu bionic-backports main multiverse restricted universe
适用于ubuntu20.04 使用tsinghua，将sources.list内容修改为下面内容
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main universe restricted multiverse
最后更新软件包
sudo apt update
or
sudo apt-get update
