I recently messed up with my Ubuntu 12.04 /etc/apt/sources.list file. In order to restore it you need to run software-properties-gtk This will cause a window to appear, I have chosen only Canonical-supported free and open-source software(main), you can choose other options you want. This adds the following line to my blank/previously in-existent /etc/apt/sources.list deb http://archive.ubuntu.com/ubuntu precise main Now, run apt-get update and other apt-get commands as usual.
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