The at.allow and at.deny files

The at and batch commands use the files /usr/lib/cron/at.allow and /usr/lib/cron/at.deny to restrict usage. If the at.allow file exists, only users listed in the file are allowed to use at and batch. If the file does not exist, at.deny is checked to see if the user is explicitly denied permission. If neither of the files exists, only the root user is permitted to use the commands.