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One approach to establish a parameter within a plpgsql function for utilization in a bash script is to use a combination of a plpgsql function with an OUT parameter and a programmatic call to the function from the bash script.

Here's an example:

First, create a plpgsql function that takes a parameter and returns its value through an OUT parameter:

CREATE OR REPLACE FUNCTION get_param(p_name text, OUT p_value text)
AS $$
BEGIN
  SELECT value INTO p_value FROM parameters WHERE name = p_name;
END;
$$
LANGUAGE plpgsql;

In this example, the function get_param takes a parameter p_name and returns its value through the OUT parameter p_value. The SELECT statement retrieves the value from a parameters table that presumably stores various parameters.

Next, in your bash script, define a variable to receive the value returned by the plpgsql function using psql to execute the function programmatically:

#!/bin/bash

PARAM_NAME="some_param"
PARAM_VALUE=$(psql -d <database_name> -U <database_user> -t -A -c "SELECT get_param('$PARAM_NAME')")
echo "Parameter $PARAM_NAME = $PARAM_VALUE"

In this example, we define a bash variable PARAM_NAME which corresponds to the p_name parameter in the plpgsql function. We execute the plpgsql function using psql, passing in the PARAM_NAME variable, and capture the result into PARAM_VALUE. Finally, we print out the parameter name and value.

Note that you will need to substitute <database_name> and <database_user> with your actual database name and user, and ensure that the appropriate PostgreSQL client tools (e.g. psql) are installed on your system.