Better way to process boolean query result in shell-like situations?

Hi,

I regularly run into the problem that I want to query a
PostgreSQL database in a script/program and depending on a
boolean result do one thing or the other.  A typical example
would be a Puppet Exec that creates a user only if it does
not exist yet.

But unfortunately psql always returns with the exit code 0
if the query was run without errors.  In a shell script I
can use a query that returns an empty string for failure and
something else for success and then test that à la:

| if [ -n "$(psql -Atc "[…]") ]; then echo Success.; fi

but for example in Puppet this requires putting around
'/bin/bash -c "[…]"' with yet another level of quoting.

The best idea I had so far was to cause a runtime error
(here with the logic reversed: If the user exists, psql re-
turns failure, otherwise success):

| [tim@passepartout ~]$ psql -c "SELECT usename::INT FROM pg_user WHERE usename = 'tim';"; echo $?
| FEHLER:  ungültige Eingabesyntax für ganze Zahl: »tim«
| 1
| [tim@passepartout ~]$ psql -c "SELECT usename::INT FROM pg_user WHERE usename = 'does-not-exist';"; echo $?
|  usename
| ---------
| (0 rows)

| 0
| [tim@passepartout ~]$

But this (in theory) could fail if usename could be con-
verted to a number, and for example 'a'::INT will fail al-
ways.

Are there better ways?  The environment I am most interested
in is 9.3 on Ubuntu Trusty.

Tim

P. S.: I /can/ write providers or inline templates for Pup-
       pet in Ruby to deal with these questions; but here I
       am only looking for a solution that is more "univer-
       sal" and relies solely on psql or another utility
       that is already installed.