That's a great and practical question! It really depends on the specific city and even the particular location within it. In most major cities, public trash cans maintained by the municipal sanitation department do come with liners. These are heavy-duty plastic bags installed and replaced by city workers during their regular collection and cleaning rounds. The liners are crucial for hygiene, preventing liquids and food waste from soaking into the bin itself, which makes cleaning faster and reduces odors and pests.
However, you might occasionally find an empty can, usually because a liner has torn or the bin was just emptied and hasn't been re-lined yet. In parks or at public events, sometimes volunteers or different agencies manage the bins, and practices can vary. As a general rule, you are not expected to supply your own liner. Your responsibility is simply to place your trash inside the bin. If a can is overflowing or has no liner, it's best to find another bin or hold onto your trash temporarily rather than placing it loosely in or around the container. For the most accurate information, checking your own city's sanitation department website will give you the definitive local policy.