The official QGIS installation documentation is a little hard to follow, because it doesn't give a detailed example.
@zorkerz provided an answer to my installation question for Ubuntu 15.04. I modified these instructions to 18.04. Is my interpretation correct?
To install the newest version of QGIS, add the QGIS repositories to the /etc/apt/sources.list file:
sudo nano /etc/apt/sources.list
Arrow the cursor to the bottom and insert these lines:
deb http://qgis.org/ubuntu bionic main
deb-src http://qgis.org/ubuntu bionic main
After those commands, I followed the rest of the instructions on the official QGIS installation page starting with the following.
"After that type the commands below to install QGIS:"
sudo apt update
sudo apt install qgis qgis-plugin-grass
While completing the step to verify the fingerprint, the following part of the output seemed off:
gpg: key 51F523511C7028C3: 1 signature not checked due to a missing key gpg: key 51F523511C7028C3: "QGIS Archive Automatic Signing Key (2019) " not changed gpg: Total number processed: 1 gpg: unchanged: 1
When adding the key, the following part of the output was concerning. Is this a problem on my end or for QGIS to resolve?
gpg: key 51F523511C7028C3: 1 signature not checked due to a missing key