deb http://mirrors4.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main restricted universe multiverse
deb http://mirrors4.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse
deb http://mirrors4.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse
deb http://mirrors4.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse