You still need to run it via index.php and provide route that should be executed (notice the space after .php):
Code:
php -f /opt/bitnami/apache2/htdocs/index.php /cron_job
All the CodeIgniter related code is included in index.php file, like setting up folders and loading core functionality.
I'm also 99% certain you can skip -f flag, I'm pretty sure I only have to do php index.php, but it could be different for different PHP versions.
If that method does not work (some servers have different settings), you can provide directory/controller/method with d= c= and m= attributes:
Code:
php -f /opt/bitnami/apache2/htdocs/index.php c=cron_job
