Rats in the wild eat a wide variety of food sources, and they are often called “true omnivores” because of this ability to eat almost anything in order to survive.
But in general, rats prefer to eat the same types of foods that humans do, and will go for these types of foods first if they are available.
Various cereals and grains that grow in the wild provide the bulk of the rats diet, and that is supplemented by a wide variety of fruits and vegetables that they may obtain.
Rats will also eat various insects such as grasshoppers in order to supplement their diet.
But if food sources are short, the rat can eat almost any organic substance in order to sustain itself, and therefore can eat cardboard boxes, etc. if need be.