ubuntu20.04 LTS系统默认源sources.list文件的修改

sudo gedit /etc/apt/sources.list

#deb cdrom:[Ubuntu 20.04 LTS _Focal Fossa_ - Release amd64 (20200423)]/ focal main restricted# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to# newer versions of the distribution.deb http://cn.archive.ubuntu.com/ubuntu/ focal main restricted# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal main restricted## Major bug fix updates produced after the final release of the## distribution.deb http://cn.archive.ubuntu.com/ubuntu/ focal-updates main restricted# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-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://cn.archive.ubuntu.com/ubuntu/ focal universe# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal universedeb http://cn.archive.ubuntu.com/ubuntu/ focal-updates universe# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-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://cn.archive.ubuntu.com/ubuntu/ focal multiverse# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal multiversedeb http://cn.archive.ubuntu.com/ubuntu/ focal-updates multiverse# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-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://cn.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-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 focal partner# deb-src http://archive.canonical.com/ubuntu focal partnerdeb http://security.ubuntu.com/ubuntu focal-security main restricted# deb-src http://security.ubuntu.com/ubuntu focal-security main restricteddeb http://security.ubuntu.com/ubuntu focal-security universe# deb-src http://security.ubuntu.com/ubuntu focal-security universedeb http://security.ubuntu.com/ubuntu focal-security multiverse# deb-src http://security.ubuntu.com/ubuntu focal-security multiverse# This system was installed using small removable media# (e.g. netinst, live or single CD). The matching "deb cdrom"# entries were disabled at the end of the installation process.# For information about how to configure apt package sources,# see the sources.list(5) manual.

sudo apt update