How to check if a postgres user exists?

SELECT 1 FROM pg_roles WHERE rolname="USR_NAME"

And in terms of command line (thanks to Erwin):

psql postgres -tXAc "SELECT 1 FROM pg_roles WHERE rolname="USR_NAME""

Yields 1 if found and nothing else.

That is:

psql postgres -tXAc "SELECT 1 FROM pg_roles WHERE rolname="USR_NAME"" | grep -q 1 || createuser ...