Get:1 http://mirrors.sohu.com/ubuntu xenial InRelease [8 B] Err:1 http://mirrors.sohu.com/ubuntu xenial InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) Get:2 http://mirrors.sohu.com/ubuntu xenial-updates InRelease [8 B] Err:2 http://mirrors.sohu.com/ubuntu xenial-updates InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) Get:3 http://mirrors.sohu.com/ubuntu xenial-backports InRelease [8 B] Err:3 http://mirrors.sohu.com/ubuntu xenial-backports InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) Get:4 http://mirrors.sohu.com/ubuntu xenial-security InRelease [8 B] Err:4 http://mirrors.sohu.com/ubuntu xenial-security InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) Fetched 32 B in 0s (156 B/s) Reading package lists... Done E: Failed to fetch http://mirrors.sohu.com/ubuntu/dists/xenial/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) E: Failed to fetch http://mirrors.sohu.com/ubuntu/dists/xenial-updates/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) E: Failed to fetch http://mirrors.sohu.com/ubuntu/dists/xenial-backports/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) E: Failed to fetch http://mirrors.sohu.com/ubuntu/dists/xenial-security/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) E: Some index files failed to download. They have been ignored, or old ones used instead.
解决方法如下：
1. 打开系统设置，双击软件与更新；
2. 修改Download from为Main Server ,然后点击关闭，弹出密码输入框，输入密码后确定弹出提示框，选择reload按钮。
3. 返回命令窗口，执行命令：sudo apt-get update或者sudo apt-get install ****;
执行成功。