The instructions on the QGIS download site are not as intuitive as I'd like, but they do work when you know the command syntax: First, what version of Linux (Ubuntu / Mint) are you running?

The codename of your APT repository will depend on the version of the OS and which version of QGIS you can run. QGIS 3.x will run on Ubuntu 18.04 (bionic) and Linux Mint 19 (bionic) and above.

QGIS 2.18.x will run on Ubuntu 16.04 (xenial) and Linux Mint 18.x (xenial) and below.

Add links to QGIS repositories

sudo sh -c 'echo "deb http://qgis.org/ubuntu bionic main" >> /etc/apt/sources.list'

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

wget -O - http://qgis.org/downloads/qgis-2017.gpg.key | gpg --import

gpg --fingerprint CAEB3DC3BDF7FB45

gpg --export --armor CAEB3DC3BDF7FB45 | sudo apt-key add -

Install QGIS sudo apt-get update

sudo apt-get install qgis python-qgis