我想 ：
sudo apt-get update
并得到：
ravit@ubuntu:/$ sudo apt-get update
Ign http://extras.ubuntu.com trusty InRelease
Ign http://ppa.launchpad.net trusty InRelease
Get:1 http://extras.ubuntu.com trusty Release.gpg [72 B]
Get:2 http://extras.ubuntu.com trusty Release [11.9 kB]
Ign http://ppa.launchpad.net trusty InRelease
Get:3 http://extras.ubuntu.com trusty/main Sources [14 B]
Ign http://us.archive.ubuntu.com trusty InRelease
Ign http://security.ubuntu.com trusty-security InRelease
Ign http://ppa.launchpad.net trusty Release.gpg
Get:4 http://security.ubuntu.com trusty-security Release.gpg [933 B]
Get:5 http://extras.ubuntu.com trusty/main i386 Packages [14 B]
Get:6 http://security.ubuntu.com trusty-security Release [63.5 kB]
Ign http://ppa.launchpad.net trusty Release.gpg
Get:7 http://security.ubuntu.com trusty-security/main Sources [87.4 kB]
Ign http://us.archive.ubuntu.com trusty-updates InRelease
Ign http://ppa.launchpad.net trusty Release
Get:8 http://security.ubuntu.com trusty-security/restricted Sources [2,061 B]
Get:9 http://security.ubuntu.com trusty-security/universe Sources [28.1 kB]
Ign http://ppa.launchpad.net trusty Release
Get:10 http://security.ubuntu.com trusty-security/multiverse Sources [2,337 B]
Ign http://us.archive.ubuntu.com trusty-backports InRelease
Get:11 http://us.archive.ubuntu.com trusty Release.gpg [72 B]
Get:12 http://us.archive.ubuntu.com trusty-updates Release.gpg [933 B]
Get:13 http://security.ubuntu.com trusty-security/main i386 Packages [290 kB]
Get:14 http://us.archive.ubuntu.com trusty-backports Release.gpg [933 B]
Get:15 http://us.archive.ubuntu.com trusty Release [11.9 kB]
Get:16 http://us.archive.ubuntu.com trusty-updates Release [63.5 kB]
Get:17 http://us.archive.ubuntu.com trusty-backports Release [63.5 kB]
Ign http://extras.ubuntu.com trusty/main Translation-en_US
Ign http://extras.ubuntu.com trusty/main Translation-en
Get:18 http://us.archive.ubuntu.com trusty/main Sources [14 B]
Get:19 http://us.archive.ubuntu.com trusty-updates/main Sources [212 kB]
Get:20 http://security.ubuntu.com trusty-security/restricted i386 Packages [8,846 B]
Get:21 http://security.ubuntu.com trusty-security/universe i386 Packages [111 kB]
Get:22 http://security.ubuntu.com trusty-security/multiverse i386 Packages [3,854 B]
Get:23 http://us.archive.ubuntu.com trusty-updates/restricted Sources [3,433 B]
Get:24 http://security.ubuntu.com trusty-security/main Translation-en [159 kB]
Get:25 http://us.archive.ubuntu.com trusty-updates/universe Sources [123 kB]
Err http://ppa.launchpad.net trusty/main i386 Packages
404 Not Found
Ign http://ppa.launchpad.net trusty/main Translation-en_US
Ign http://ppa.launchpad.net trusty/main Translation-en
Err http://ppa.launchpad.net trusty/main i386 Packages
404 Not Found
Get:26 http://us.archive.ubuntu.com trusty-updates/multiverse Sources [5,151 B]
Get:27 http://security.ubuntu.com trusty-security/multiverse Translation-en [1,679 B]
Get:28 http://security.ubuntu.com trusty-security/restricted Translation-en [2,266 B]
Get:29 http://security.ubuntu.com trusty-security/universe Translation-en [64.0 kB]
Ign http://ppa.launchpad.net trusty/main Translation-en_US
Get:30 http://us.archive.ubuntu.com trusty-updates/main i386 Packages [548 kB]
Ign http://ppa.launchpad.net trusty/main Translation-en
Get:31 http://us.archive.ubuntu.com trusty-updates/restricted i386 Packages [11.8 kB]
Get:32 http://us.archive.ubuntu.com trusty-updates/universe i386 Packages [292 kB]
Get:33 http://us.archive.ubuntu.com trusty-updates/multiverse i386 Packages [12.1 kB]
Get:34 http://us.archive.ubuntu.com trusty-updates/main Translation-en [271 kB]
Get:35 http://us.archive.ubuntu.com trusty-updates/multiverse Translation-en [6,148 B]
Get:36 http://us.archive.ubuntu.com trusty-updates/restricted Translation-en [2,774 B]
Get:37 http://us.archive.ubuntu.com trusty-updates/universe Translation-en [154 kB]
Get:38 http://us.archive.ubuntu.com trusty-backports/main Sources [5,851 B]
Get:39 http://us.archive.ubuntu.com trusty-backports/restricted Sources [28 B]
Get:40 http://us.archive.ubuntu.com trusty-backports/universe Sources [28.4 kB]
Get:41 http://us.archive.ubuntu.com trusty-backports/multiverse Sources [1,898 B]
Get:42 http://us.archive.ubuntu.com trusty-backports/main i386 Packages [6,285 B]
Get:43 http://us.archive.ubuntu.com trusty-backports/restricted i386 Packages [28 B]
Get:44 http://us.archive.ubuntu.com trusty-backports/universe i386 Packages [32.6 kB]
Get:45 http://us.archive.ubuntu.com trusty-backports/multiverse i386 Packages [1,552 B]
Get:46 http://us.archive.ubuntu.com trusty-backports/main Translation-en [3,645 B]
Get:47 http://us.archive.ubuntu.com trusty-backports/multiverse Translation-en [1,215 B]
Get:48 http://us.archive.ubuntu.com trusty-backports/restricted Translation-en [14 B]
Get:49 http://us.archive.ubuntu.com trusty-backports/universe Translation-en [29.6 kB]
Fetched 2,731 kB in 11s (242 kB/s)
W: Failed to fetch http://ppa.launchpad.net/octave/stable/ubuntu/dists/trusty/main/binary-i386/Packages 404 Not Found
W: Failed to fetch http://ppa.launchpad.net/picaso/octave/ubuntu/dists/trusty/main/binary-i386/Packages 404 Not Found
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/Release Unable to find expected entry 'restricted/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Some index files failed to download. They have been ignored, or old ones used instead.
有什么问题？（我正在使用 Ubuntu DISTRIB_RELEASE=14.04）
答案1
一个简单的、经常重复出现的问题在错误的地方被问到，无法提供详细的答案，但有一个简单的解决方案，在这种情况下，因为您收到了两种不同类型的常见包管理错误消息，所以有两个解决方案。
从软件源中删除不再存在的 PPA。从 Dash 中选择软件与更新， 然后其他软件tab -> 取消勾选，删除所有不再存在的 PPA 软件源。您需要删除的软件源是导致您
404 Not Found在运行时收到错误消息的软件源sudo apt-get update。您的软件源中还有一行格式不正确。请
/etc/apt/sources.list按照如何从我的 sources.list 中删除格式错误的行？。
答案2
尝试这个答案：
显然 apt 无法处理太多软件源，因此我可以通过取消选择我需要访问的源以外的所有源来解决此问题。如果经常这样做，确实很麻烦，但总比什么都不做要好。摘自网站：
在“其他软件”选项卡中，禁用所有 PPA 存储库，只保留官方 Ubuntu 存储库。启用太多 PPA 存储库（非官方）可能会导致包管理器变得不稳定。