package management - /etc/apt/sources.list Permissions? - Ask Ubuntu

this question has answer here:

• how restore sources.list file? [duplicate] 4 answers
• how restore default repositories? 3 answers
• cannot find softwares installed via synaptic manager after accidentally deleting sources.list 1 answer
• incorrect filename extension 4 answers

i have started using terminal, , have been having lots of problems installing programs ubuntu software center (minecraft, tor, tails installer, etc.)

when run sudo apt-get update

n: ignoring file '50unattended-upgrades.ucf-old' in directory '/etc/apt/apt.conf.d/' has invalid filename extension

i system error message when boot in relation deb-src http://deb.torproject.org/torproject.org xenial main

when try open /etc/apt/sources.list various commands seems it's not there, unless open sudo gedit /etc/apt/sources.list, blank document

so guess have accidentally removed or something...

what do ?

just edit in response duplicate flags

the invalid file name sorted (it needed removing). problem adding lines sources list. tried making new file 1 of flagged duplicate threads moved file , made new 1 gedit. seemed restore defaults , came 2x canonical brothers lines. , when try add line nothing happens.

-just found out when use sudo su can add lines sources list

sudo su -c "echo 'deb http://www.duinsoft.nl/pkg debs all' >> /etc/apt/sources.list 

have tried changing repository 'main server'? whenever change , close, prompt reload. should restore defaults.

if not work, here copy of mine:

deb http://archive.ubuntu.com/ubuntu xenial main restricted deb-src http://archive.ubuntu.com/ubuntu xenial restricted universe main multiverse deb http://archive.ubuntu.com/ubuntu xenial-updates main restricted deb-src http://archive.ubuntu.com/ubuntu xenial-updates restricted universe main multiverse deb http://archive.ubuntu.com/ubuntu xenial universe deb http://archive.ubuntu.com/ubuntu xenial-updates universe deb http://archive.ubuntu.com/ubuntu xenial multiverse deb http://archive.ubuntu.com/ubuntu xenial-updates multiverse deb http://archive.ubuntu.com/ubuntu xenial-security main restricted deb-src http://archive.ubuntu.com/ubuntu xenial-security restricted universe main multiverse deb http://archive.ubuntu.com/ubuntu xenial-security universe deb http://archive.ubuntu.com/ubuntu xenial-security multiverse deb http://archive.canonical.com/ubuntu xenial partner deb-src http://archive.canonical.com/ubuntu xenial partner