How to access postgres
sudo -u <username> psql

Disconnect from postgres (psql)
\q

List Connection Information (psql)
\conninfo

Create a new User/Role
sudo -u postgres createuser --interactive

List all databases (psql)
\l

List all users in postgres (psql)
\du

Create a Database (psql)
create database <database name>

Steps to add a new user (psql)

sudo -u postgres createuser --interactive
sudo -u postgres createdb <username>
sudo adduser <username>
Set user's password 
ALTER USER <username> PASSWORD 'newPassword';
OR
\password <username>

ACCESS PROVILEGES
GRANT CONNECT ON DATABASE my_db TO my_user;
GRANT USAGE ON SCHEMA public TO my_user;
GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA public TO my_user;
GRANT ALL PRIVILEGES ON ALL SEQUENCES IN SCHEMA public TO my_user;

Get the version of Postgresql (psql)

SELECT version();

Get the size of a single database (psql)

SELECT pg_database_size(current_database());

Get the size of all databases (psql)

SELECT sum(pg_database_size(datname)) from pg_database;

Get the uptime of the server (psql)

SELECT date_trunc('second', current_timestamp - pg_postmaster_start_time()) as uptime;

Get total size of a database

SELECT pg_database_size(current_database());

Get total size of all Databases

SELECT sum(pg_database_size(datname)) from pg_database;

Restart Postgres server
sudo systemctl restart postgresql