Manual page and help for the zerofree linux command. The zerofree command finds non-zero unused blocks in an ext2, ext3, or ext4 filesystem (such as / dev / hda1) and populates them with zeros (or another value of your choice). Filling unused areas with zeros is useful if the file system device is a disk image. In this case, depending on the type of disk image, a secondary utility may be able to reduce the size of the disk image after running the zerofree program.