That's a great and practical question! The answer really depends on the specific city and even the neighborhood you're in. In most major cities, public trash cans maintained by the municipal sanitation department do come with liners. These heavy-duty plastic bags are installed and replaced by city workers during their regular collection rounds. The goal is to keep the bins cleaner, make emptying faster, and prevent waste from sticking to the bottom.
However, it's not a universal rule. Some smaller towns or parks might use unlined cans to reduce plastic bag costs. You might also occasionally find a public bin where the liner is torn or missing, especially right after it's been emptied.
So, what should you do? As a general rule, you don't need to supply your own liner for public bins. Just deposit your trash inside. If the bin is overflowing or has no liner, it's best to find another one or hold onto your waste temporarily. Trying to add your own flimsy bag might actually create more mess for the workers.
For your own home or business curb can, that's a different story—you are almost always responsible for supplying and using your own liner. When in doubt, check your local city government's sanitation website for their specific guidelines.