You can process (build) a single DITA topic by using its name in place of a ditamap's name in any of the Ant drivers.
Note: If you want to try processing a single file, you can modify one of the Ant scripts in the groceryshopping/ant_scripts directory to build one of the topics you created in this chapter. If you don't feel confident doing that yet, work through the maps chapter (following this one) first, where you'll learn more about processing DITA files with Ant.