\set var val
\echo :var
... \gset

-- only one line should be resulted
select now() as curr_time \gset

https://psql-tips.org/psql_tips_all.html#tip037

$ cat query.sql 
select * from :tablename;
$ psql -v tablename=test -f query.sql 
 id | value 
----+-------
  1 | test
(1 row)

$ cat query2.sql 
select * from test where value = :'var';
$ psql --set var=test -f query2.sql 
 id | value 
----+-------
  1 | test
(1 row)


select oid as tsoid from pg_tablespaces where spcname = 'ts' \gset
select datname from pg_database where oid in (select pg_tablespace_databases(:tsoid));