The Commandline to call psql in solvable_files.sh seems to be quite broken.
It for example contains: -E 'UTF-8', which looks like a attempt to set the encoding, however man psql states:

       -E
       --echo-hidden
           Echo the actual queries generated by \d and other backslash commands.
           You can use this to study psql's internal operations.
           This is equivalent to setting the variable ECHO_HIDDEN to on.

Also the way the connect string is put together doesn't seem to allow connecting via socket.

I was able to get the script to work by replacing "postgresql://$db_user:$db_pwd@$db_host/$db_name" with just the name of the database and completely removing -E 'UTF-8'.