How to Install postgresql-client-11 on Ubuntu 18.04?

Import the repository key from https://www.postgresql.org/media/keys/ACCC4CF8.asc:

sudo apt install curl ca-certificates gnupg
curl https://www.postgresql.org/media/keys/ACCC4CF8.asc | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/apt.postgresql.org.gpg >/dev/null

Create /etc/apt/sources.list.d/pgdg.list. The distributions are called codename-pgdg. In the example, replace buster with the actual distribution you are using. File contents:

deb http://apt.postgresql.org/pub/repos/apt buster-pgdg main

(You may determine the codename of your distribution by running lsb_release -c). For a script version of the above file creation, presuming you are using a supported release:

sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'

Finally, update the package lists, and start installing packages:

sudo apt update

sudo apt install postgresql-client-11