UNIX was designed to allow many people to use one computer, so to protect users' files, each file has a set of attributes known as permissions that determine who can read, modify or (if the file is a program) execute the file. Normally, you should only have to modify permissions when installing a CGI script or if there's a certain area you want to keep people out of. There are three sets of permissions for each file, one set for the owner of the file, one for other users in the owner's group, and one for all other users. Each set of permissions in turn consists of three "flags" (variables that can be either on or off), and these are "r" for read, "w" for write, and "x" for execute.

If you want to see the permissions of some files and directories, just log in to your account with telnet and type the command ls -Al exactly as it appears. You will see a complete file listing with the permissions for each file on the very left and the name of each file on the very right. To change the permissions of a file or directory, you use the chmod command. For example, to set the owner permissions of an imaginary file named "example" to "rwx" so that you can read, modify and execute the file, you would type chmod o+rwx example.