Installing QGIS on Ubuntu xenial - QGIS nightly repo?

Question

I'm having major problems installing QGIS3 on my Ubuntu 16.04 system...

I followed a recently posted answer. Even after adding the deb qgis.org xxx line to /etc/apt/sources.list, the only thing it installs is a really old one, QGIS 2.8.

I also tried to edit the sources file to only contain https://qgis.org/debian-nightly/. I tried the following sequence of commands on a) a fresh (was acutally a fresh install) and b) a fully commented /etc/apt/sources.list file:

sudo sh -c 'echo "deb http://qgis.org/debian-nightly xenial main " >> /etc/apt/sources.list'
sudo sh -c 'echo "deb-src http://qgis.org/debian-nightly xenial main " >> /etc/apt/sources.list'

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys CAEB3DC3BDF7FB45
sudo apt-get update
sudo apt-get install qgis

For a): QGIS 2.8 was installed

For b): Package 'qgis' has no installation candidate

I just switched to Linux from Win, so I might not know all the tricks.. I also saw this post on how to specify the repository when you're installing smth. I didn't really understand how to make this work with the QGIS nightly repo: sudo apt-get install -t xenial qgis?! I don't think so..

Is anyone patient enough to quickly outline the steps for me?

Answer 1

On my machine, I have the following in my /etc/apt/sources.list.d/additional-repositories.list:

# deb http://qgis.org/debian xenial main
deb http://apt.postgresql.org/pub/repos/apt/ xenial-pgdg main 
#deb http://qgis.org/debian-nightly/ xenial main
#deb-src http://qgis.org/debian-nightly/ xenial main
deb http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu xenial main
deb http://qgis.org/ubuntugis-nightly xenial main

This leads to being offered a new nightly QGIS build of 2.99 every morning (or so) in the update manager.