\set somevar1 '\'2018-01-01\''
\set somevar2 '\'2017-01-01\''
\echo Starting
\pset fieldsep ','
\pset format unaligned
\pset null '(null)'
\o 'your-output.csv'
-- here your query
select
  field1, field2
from a_table;
\o
\echo finished