Linux Mint: QGIS won't install

Got this problem a long time ago on Ubuntu.

Now I'm using Mint and I've installed QGIS without any problems with:

# add-apt-repository ppa:ubuntugis/ubuntugis-unstable

# nano /etc/apt/sources.list

Edit sources list and add two lines:

deb http://qgis.org/ubuntugis xenial main
deb-src http://qgis.org/ubuntugis xenial main

Update and install QGIS:

# apt-get update

# apt-get install qgis

Before you try, please purge your current QGIS installation.

According to https://launchpad.net/~ubuntugis/+archive/ubuntu/ubuntugis-unstable/+index?field.series_filter=trusty, the QGIS package you have loaded is from the ubuntugis unstable ppa. This version is slightly outdated by now.

If you use the qgis.org/ubuntugis repo, you can have the latest QGIS 2.18.4 by adding to your repo list:

deb http://qgis.org/ubuntugis trusty main
deb-src http://qgis.org/ubuntugis trusty main

If you want to stick to the LTR version, qgis.org/ubuntugis-ltr offers now QGIS 2.14.12 by adding

deb http://qgis.org/ubuntugis-ltr trusty main
deb-src http://qgis.org/ubuntugis-ltr trusty main

In any case, stick to the Ubuntu dist version you are running (i.e. trusty), and update and upgrade all other packages before updating QGIS with

sudo apt-get update
sudo apt-get upgrade

You might as well have to upgrade the keys for QGIS and ubuntugis.

If you decide to switch from ubuntugis to non-ubuntugis repos, make sure to clean and purge anything related to QGIS.