Edit: Thanks for the response. This looks like a do-able fix to the problem. But the question remains: Why? Apparently, I'm not the only one struck by this error. How/Why does it happen?
On my Bionic Beaver (x86-64) system, I noticed today that I could not install anything (with "apt install"). Further, when I do "apt-get update", it returns immediately. Upon investigation, I found that my /etc/apt/source.list file is entirely commented out (so there is nothing to get stuff from). The contents of the file is listed below. How do I get back a default (normal) sources.list file? (No, I don't have any backups of the file). Note that there is also a source.list.save file in the same directory (/etc/apt), but it is identical to sources.list. I have no idea how this happened - I didn't do anything - it happened on October 27 (of last year).
(Edit to add: Note that every non-empty line below has a pound sign (#) at the beginning. This seems not to show up when I view the post)
# deb cdrom:[Ubuntu 18.04 LTS _Bionic Beaver_ - Release amd64 (20180426)]/ bionic main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
## Major bug fix updates produced after the final release of the
## distribution.
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu bionic partner
# deb-src http://archive.canonical.com/ubuntu bionic partner
# deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted
# deb-src http://security.ubuntu.com/ubuntu bionic-security universe
# deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse
