As an example, because Larry created the files in his home directory, Larry owns those files, and has access to them. UNIX also allows files to be shared between users and groups of users. If Larry so desired, he could cut off access to his files, such that no other user could access them. As explained above, every file is owned by a particular user. However, files are also owned by a particular group , which is a system-defined group of users.
Every user is placed into at least one group when that user is created.
Groups are usually defined by the type of users which access the machine. For example, on a university UNIX system, users may be placed into the groups student , staff , faculty or guest . Write permission allows the user to write to and modify the file. For directories, write permission allows the user to create new files or delete files within that directory. For directories, having execute permission allows the user to cd into the directory in question.