PostgreSQL 9.5: Describe function in UNIX terminal

If you start psql with key -E (psql -E) and run your \df+ functionName, you will see, that it takes definition from pg_catalog.pg_proc, so you can just query it like select prosrc from pg_proc where proname = 'functionName';'

Same for \sf functionName - it's is a wrap up for pg_catalog.pg_get_functiondef.

Lastly if you do it your way, with \df+, just run \x before it and Source code will look much better.

Anyway in ALL those case indentation is saved:

b=# create function p() returns text
b-# as
b-# $$
b$# begin
b$#   --tab
b$#   --two spaces
b$#    --three spaces
b$#   return 't';
b$# end;
b$# $$ language plpgsql
b-# ;
CREATE FUNCTION
b=# \x
Expanded display is on.
b=# \df+ p
List of functions
-[ RECORD 1 ]-------+------------------
Schema              | public
Name                | p
Result data type    | text
Argument data types |
Type                | normal
Security            | invoker
Volatility          | volatile
Owner               | postgres
Language            | plpgsql
Source code         |                  +
                    | begin            +
                    |   --tab          +
                    |   --two spaces   +
                    |    --three spaces+
                    |   return 't';    +
                    | end;             +
                    |
Description         |

b=# \sf p
CREATE OR REPLACE FUNCTION public.p()
 RETURNS text
 LANGUAGE plpgsql
AS $function$
begin
  --tab
  --two spaces
   --three spaces
  return 't';
end;
$function$