How to create a json file from postgres table with parameters as a filename

You could likely accomplish what you're looking to do in a couple of different ways, though both would require you to GRANT a combination of pg_write_server_files and/or pg_execute_server_program to the executing role/user.

Method 1: Using Dynamic SQL (with pg_write_server_files)

Here, you'd extrapolate upon your initial plan of using the COPY command, changing

COPY v_originaltext to '/var/lib/postgresql/sftp/smrptesting1.json';

to instead DECLARE a SQL string variable (v_sql) within the function block that you'd then EXECUTE, i.e. something like

CREATE OR REPLACE FUNCTION json_putter(_parameters TEXT)
RETURNS VOID
LANGUAGE plpgsql
AS $function$
DECLARE
    v_sql TEXT DEFAULT NULL;
    v_originaltext JSONB;
    v_filename TEXT;
BEGIN
    -- < ... code to populate v_originaltext here ... >
    v_filename := '/var/lib/postgresql/sftp/' || _parameters || '.json';
    v_sql := 'COPY $1 TO $2';
    
    EXECUTE v_sql USING v_originaltext, v_filename;

-- or, alternatively, with literal single quotes:

    v_sql := 'COPY (SELECT v_originaltext) TO ' || CHR(39) || v_filename || CHR(39);
    EXECUTE v_sql;
END
$function$

which will build up the COPY statement as if you were manually running from the client.

Method 2: Using server-side psql command (with pg_execute_server_program)

You can output the json directly from the psql command, and redirect the stdout to a file descriptor that you generate from the parameters.

To implement this, you'd probably want to create two atomic functions for simplicity's sake:

• One that acts as a wrapper to generate the value of v_originaltext [e.g. fx_originaltext(_parameters => text)]
• One that will run a generic command via psql using the COPY ... FROM PROGRAM utility and consumes the stdout with a temp table or similar:

--

CREATE OR REPLACE FUNCTION psql_runner(_function TEXT, _parameters TEXT)

RETURNS VOID
LANGUAGE plpgsql
AS $function$
DECLARE
    v_filename TEXT;
BEGIN
    v_filename := '/var/lib/postgresql/sftp/' || _parameters || '.json';
    CREATE TEMP TABLE _out ( stdout text );
    COPY _out FROM PROGRAM ('psql -tq -c "SELECT ' || _function || '(_parameters :=' || quote_literal(_parameters) || ') " > ' || v_filename);
END
$function$

Notably, the second method requires a larger scope of privileges, and depending on which version of postgres you're using, may need to execute the COPY as dynamic SQL as in the first method.