psql also allows you to set local variables and interpolate them into SQL. Values can be assigned from query results or with shell style back ticks! Powerful and frightening!

From the manual:

  => SELECT 'hello' AS var1, 10 AS var2
  -> \gset
  => \echo :var1 :var2
  hello 10

  testdb=> \set content `cat my_file.txt`
  testdb=> INSERT INTO my_table VALUES (:'content');