psql error could not connect to server no such file or directory ubuntu code example

Example 1: psql: error: could not connect to server: No such file or directory

sudo service postgresql start

#bpn

Example 2: psql: error: could not connect to server: could not connect to server: no such file or directory

psql: could not connect to server: No such file or directory

Is the server running locally and accepting connections on 
Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?