Installing PostGIS after PostgreSQL Ubuntu 18.04

Question

sudo apt-get install postgresql-9.5-postgis-2.5

returns a series of errors:

The following packages have unmet dependencies:
postgresql-9.5-postgis-2.5 : 
  Depends: libgdal1i (>= 1.9.0) but it is not installable
  Depends: libjson-c2 (>= 0.11) but it is not installable
  Depends: liblwgeom-2.5-0 (>= 2.5.0~beta1) but it is not going to be installed
  Depends: libproj9 (>= 4.9.0) but it is not installable
  Depends: libsfcgal1 (>= 1.2.0) but it is not installable

E: Unable to correct problems, you have held broken packages.

I had never encountered a not installable exception to date so I ma not sure of consequences of different possible actions... Attempting lower versions leads to the same result. How can PostGIS get safely installed then?

Update

An attempt was also made, making a fresh installation of a Ubuntu 18.04 VM [following these instructions][1]

The source of the error was in adding the repository to sources.list

sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt xenial-pgdg main" >> /etc/apt/sources.list'

which should be for 18.04

sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt bionic-pgdg main" >> /etc/apt/sources.list'

[to butter things up even more, and attempt was made with bioni-pgdg main leading to its own set of errors]!

Answer 1

Unmet Dependencies

The UbuntuGIS Quickstart Guide lists a few things that need to be done before installing their packages (these packages include PostGIS).

The first is that your /etc/apt/sources.list must include the "universe" and "multiverse" repositories. Edit that file to add them. There should already be a line that lists "main". In the end, it will look like this:

deb http://archive.ubuntu.com/ubuntu bionic main  
deb http://archive.ubuntu.com/ubuntu bionic-security main  
deb http://archive.ubuntu.com/ubuntu bionic-updates main  

deb http://us.archive.ubuntu.com/ubuntu/ bionic main universe multiverse  
deb http://us.archive.ubuntu.com/ubuntu/ bionic-security main universe multiverse  
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates main universe multiverse

This is where all of your dependencies come from.

Install Repository

After that, you need to install the UbuntuGIS repository

sudo add-apt-repository ppa:ubuntugis/ppa

Once this is done, you should be able to install postgis in the typical way:

sudo apt install postgis

Note: there will be a lot of items to install.

Answer 2

Ensure that the proper repository is added to the sources.list

sudo lsb_release -a 

ensure the codename is the same in the add repository command:

sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt **Codename**-pgdg main" >> /etc/apt/sources.list'