Send A Command To psql

You can send a command to psql to be executed by using the -c flag

$ psql -c "select 'Hello, World!';"
   ?column?
---------------
 Hello, World!
(1 row)

Specify a particular database as needed

$ psql blog_prod -c 'select count(*) from posts;'
 count 
-------
     8 
(1 row)

h/t Jack Christensen