[ubuntu] How to add software sources with console

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http://ubuntuforums.org – Hi. I recently installed Ubuntu Server 11.10. But I cannot install any software using the "apt-get" command. I think that's because the software sources for my country (Bangladesh) don't really work. In Ubuntu 11.10 I managed to change the sources from the software center. I use Ubuntu main server as the source and it works. But how can I change the source in Ubuntu Server edition with only the command line? I am fairly new to the Linux world so please give me a step by step instruction :D Thanks. (Hardware)