But TMTOWTDI (SQL standard compliant):

select schema_name
from information_schema.schemata
where schema_name <> 'information_schema' -- exclude 'system' schemata
  and schema_name !~ E'^pg_'              -- exclude more 'system' (pg
+-specific)
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In psql there is \dn to retrieve schema names. If PGPASSFILE is set (and possibly more PG* environment variables like PGDATABASE), you can just talk to the database from the commandline:

echo '\dn' | psql
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