OSX with HFS Plus defragments files automatically when you open them and doesn't really fragment them to begin with. Linux does the same. Even if you did manage to fragment something in those it would resolve itself if you just let the OS to its own devices. The OS knows what it wants the file system to look like and even if you're going to defrag the drive it's best to let the OS do it.
It's really only a problem on Windows these days if you're really fucking with the HDD. For instance I was continuously writing 64GB files to my HDD the other day for several hours and even that didn't manage to fragment the drive. Although, if I had managed to fragment the drive Windows automatically checks and defragments the drives when the computer is idle