Setup PostgreSQL/PostGIS on Ubuntu

First, install PostgreSQL and Postgis

$ sudo apt-get update
$ sudo apt-get install postgresql-10 postgresql-10-postgis-2.4 postgis
$ sudo service postgresql restart

If the package postgresql-10 couldn't be located, try:

$ sudo apt-get install wget ca-certificates
$ wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
$ sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -cs`-pgdg main" >> /etc/apt/sources.list.d/pgdg.list'
$ sudo apt-get update
$ sudo apt-get install postgresql-10 postgresql-10-postgis-2.4
$ sudo service postgresql restart

Setup a password for postgres user

$ sudo su - postgres
$ psql
postgres=# \password postgres
Enter new password: <new-password>
postgres=# \q

Locate and edit 'pg_hba.conf'

$ sudo vim /etc/postgresql/10/main/pg_hba.conf

The line:

local   all             postgres                                peer

Should be:

local   all             postgres                                md5

Restart PostgreSQL service

$ sudo service postgresql restart

You can now use the psql -U postgres <...> commands.