How to conditionally stop a psql script (based on a variable value)?

There is an option in psql which stops executing commands on error, this is ON_ERROR_STOP. If we could raise an error somehow, this would do what we want.

The problem is that we have to test the variable and produce an error somehow. Since one can't use control structures in psql (because there are none)*, my only idea was to use SQL for testing. Well, producing an error conditionally is something which pl/pgsql is quite good at, so I wrote a function which would generate an error. I can now call this function from a simple CASE structure. A simple example:

-- let's assume for clarity that there is no function with this name in the database
CREATE OR REPLACE FUNCTION error_generator()
RETURNS boolean AS
$body$
BEGIN
    RAISE 'Meaningful error message here';
    RETURN FALSE; -- just for aesthetical purposes
END;
$body$
LANGUAGE plpgsql;

\set ON_ERROR_STOP on

BEGIN;

-- test for the variable value
-- notice that if :var is not set, it fails as well (with a syntax error)
SELECT CASE WHEN 1 = :var THEN error_generator() ELSE TRUE END;

INSERT INTO test_table (integer_value, text_value)
VALUES (:var, 'something');

COMMIT;

*: You can use any shell commands after \! and conditionals of the shell, but since \! opens a new shell, executing anything there does not have any effect for the current psql script.

PostgreSQL 10

PostgreSQL 10 brings conditionals to psql. This is no longer an issue.

\if :db_to_run_on = 'dev_database'
  TRUNCATE the_most_important_table;
\endif

I guess you could also use DO..

\if :db_to_run_on != 'dev_database'
do $$
  BEGIN
    RAISE 'Meaningful error message here';
  END;
$$ LANGUAGE plpgsql;
\endif

A more concise version of dezso's answer:

CREATE OR REPLACE FUNCTION pg_temp.err(msg varchar) RETURNS boolean     
AS $$ BEGIN RAISE '%',msg; END; $$ LANGUAGE plpgsql;

You can then call this like:

\set ON_ERROR_STOP on

SELECT CASE WHEN (
  SELECT COUNT(*) FROM mytable
) > 0 THEN pg_temp.err('Already loaded') END;