Now it's time to talk about security. Many PC users have little experience with file security because Windows does not have any. It doesn't need security because it is a single-user operating system. Linux is a multi-user operating system, so it has security to prevent people from accessing each other's confidential files. Unfortunately, the security system in Linux is not very flexible, but we'll talk about that later. In our zoo, we don't want anyone to modify the primate files except for those workers who take care of the primates. It will take quite a bit of explanation before we can show you how to arrange this sort of security. When you execute an "ls" command, you are not given any information about the security of the files, because by default "ls" only lists the names of files. You can get more information by using an "option" with the "ls" command. All options start with a '-'. For example, to execute "ls" with the "long listing" option, you would type ls -l When you do so, each file will be listed on a separate line in long format. There is an example in the window on the right. There are lots of other options you can use with the ls command, but we won't need them to accomplish our goals at the zoo. Click the right arrow to continue.