Create PostgreSQL ROLE (user) if it doesn't exist

Question

How do I write an SQL script to create a ROLE in PostgreSQL 9.1, but without raising an error if it already exists?

The current script simply has:

CR         


        

Answer 1

Bash alternative (for Bash scripting):

psql -h localhost -U postgres -tc \
"SELECT 1 FROM pg_user WHERE usename = 'my_user'" \
| grep -q 1 \
|| psql -h localhost -U postgres \
-c "CREATE ROLE my_user LOGIN PASSWORD 'my_password';"

(isn't the answer for the question! it is only for those who may be useful)