在Ubuntu系统下，使用sudo apt-get update有时会出现“W: 无法下载 http://cn.archive.ubuntu.com/ubuntu/.... 校验和不符”，
xj@xjVir:~/test$ sudo apt-get update 获取:1 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB] 获取:2 http://cn.archive.ubuntu.com/ubuntu bionic InRelease [242 kB] 获取:3 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages [573 kB] 获取:4 http://security.ubuntu.com/ubuntu bionic-security/main i386 Packages [404 kB] 错误:2 http://cn.archive.ubuntu.com/ubuntu bionic InRelease 连接超时 [IP: 91.189.91.24 80] 获取:5 http://cn.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB] 忽略:4 http://security.ubuntu.com/ubuntu bionic-security/main i386 Packages 获取:6 http://security.ubuntu.com/ubuntu bionic-security/main Translation-en [190 kB] 获取:7 http://security.ubuntu.com/ubuntu bionic-security/main amd64 DEP-11 Metadata [38.5 kB] 获取:8 http://security.ubuntu.com/ubuntu bionic-security/main DEP-11 48x48 Icons [17.6 kB] 获取:9 http://security.ubuntu.com/ubuntu bionic-security/main DEP-11 64x64 Icons [41.5 kB] 错误:5 http://cn.archive.ubuntu.com/ubuntu bionic-updates InRelease 连接超时 [IP: 91.189.91.24 80] 获取:10 http://cn.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB] 获取:11 http://security.ubuntu.com/ubuntu bionic-security/restricted amd64 Packages [13.2 kB] 获取:12 http://security.ubuntu.com/ubuntu bionic-security/restricted i386 Packages [4,280 B] 获取:13 http://security.ubuntu.com/ubuntu bionic-security/restricted Translation-en [4,292 B] 获取:14 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [621 kB] 忽略:14 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages 获取:15 http://security.ubuntu.com/ubuntu bionic-security/universe i386 Packages [597 kB] 错误:10 http://cn.archive.ubuntu.com/ubuntu bionic-backports InRelease 连接超时 [IP: 91.189.91.26 80] 获取:16 http://security.ubuntu.com/ubuntu bionic-security/universe Translation-en [207 kB] 忽略:16 http://security.ubuntu.com/ubuntu bionic-security/universe Translation-en 获取:17 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 DEP-11 Metadata [42.1 kB] 获取:18 http://security.ubuntu.com/ubuntu bionic-security/universe DEP-11 48x48 Icons [16.4 kB] 获取:19 http://security.ubuntu.com/ubuntu bionic-security/universe DEP-11 64x64 Icons [111 kB] 获取:20 http://security.ubuntu.com/ubuntu bionic-security/multiverse i386 Packages [4,292 B] 获取:21 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 Packages [5,904 B] 获取:22 http://security.ubuntu.com/ubuntu bionic-security/multiverse Translation-en [2,568 B] 获取:23 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 DEP-11 Metadata [2,464 B] 获取:24 http://security.ubuntu.com/ubuntu bionic-security/multiverse DEP-11 48x48 Icons [29 B] 获取:25 http://security.ubuntu.com/ubuntu bionic-security/multiverse DEP-11 64x64 Icons [2,638 B] 获取:4 http://security.ubuntu.com/ubuntu bionic-security/main i386 Packages [404 kB] 获取:14 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [621 kB] 获取:16 http://security.ubuntu.com/ubuntu bionic-security/universe Translation-en [207 kB] 已下载 2,987 kB，耗时 5分 3秒 (9,868 B/s) 正在读取软件包列表... 完成 W: 无法下载 http://cn.archive.ubuntu.com/ubuntu/dists/bionic/InRelease 连接超时 [IP: 91.189.91.24 80] W: 无法下载 http://cn.archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease 连接超时 [IP: 91.189.91.24 80] W: 无法下载 http://cn.archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease 连接超时 [IP: 91.189.91.26 80] W: 部分索引文件下载失败。如果忽略它们，那将转而使用旧的索引文件。
解决方法如下：sudo gedit /etc/apt/sources.list
然后将 文件中http://cn.archive.ubuntu.com/ubuntu/
替换为http://mirrors.163.com/ubuntu或http://mirrors.sohu.com/ubuntu，
再次sudo apt-get update。
如若还出现类似的问题，建议多尝试更换，部分用163，部分用sohu。