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I've upgraded from 14.04 to 16.04. As per the upgrade instruction, third party sources were disabled. After the upgrade, I cannot find a possible way to enable those third party sources using command line instead of System Settings > Software & Updates > Other Software.
I need to do it on command line because changing it via Settings requires password of a Super Privileged user (idk if that's the exact term). I can do all of the
sudo su stuffs in command line with my current login, even though I've upgraded to 16.04 using
sudo do-release-upgrade (Weird right..!? idk why sysadmin set up that restriction in first place).
Also another possible solution I think of is to reinstall those packages again which will re-enable those sources I guess.