Programs All programs are files. To run a program you just have to type its name. The shell will find the program and run it for you. UNIX tells the difference between programs and other files by means of a number called the file's mode . Every file has several pieces of status information associated with it, of which the mode is one.
Every file also has an owner and a group .
The -lg in the command is a set of switches to the ls command. In this case, the arguments are expected to be the names of files, but this is not always so. Each is owned by joe and each has a group of users . Well, suppose that joe wanted only a certain set of people to be able to see these files. For example, if he worked for the Advancement Office, he might want only people in the Advancement Office to see certain files.