Dynamic PL/pgSQL

I plan to run an SQL script - using JDBC - to initialize a database.
Right now I have the need to execute serveral statements only under
certain conditions.

for example:
IF NOT EXISTS (
SELECT 1 FROM pg_catalog.pg_user WHERE ussername = 'Jon' ) THEN
CREATE USER 'Jon';
END IF;

Within a PL/pgSQL function this would be easy, but I need to store the
complete initialization script in a text file and execute it as a whole.
Is there any way to solve this? Any suggestions are welcome
TIA
Rüdiger

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