File Systems and Management: File Access Rights

File Access Rights

After defining a fairly wide range of possible operations on files we shall now look at the file system which supports all these services on behalf of the OS. In the preamble of this chapter we defined a file system as that software which allows users and applications to organize and manage their files. The organization of information may involve access, updates, and movement of information between devices. Our first major concern is access.

Access permissions: Typically a file may be accessed to read or write or execute.

The usage is determined usually by the context in which the file is created. For instance, a city bus timetable file is created by a transport authority for the benefit of its customers. So this file may be accessed by all members of public. While they can access it for a read operation, they cannot write into it. An associated file may be available to the supervisor who assigns duties to drivers. He can, not only read but also write in to the files that assign drivers to bus routes. The management at the transport authority can read, write and even execute some files that generate the bus schedules. In other words, a file system must manage access by checking the access rights of users. In general, access is managed by keeping access rights information for each file in a file system.

Who can access files?: Unix recognizes three categories of users of files, e.g. user (usually the user who created it and owns it), the group, and others. The owner may be a person or a program (usually an application or a system-based utility). The notion of “group” comes from software engineering and denotes a team effort. The basic concept is that users in a group may share files for a common project. Group members often need to share files to support each other's activity. Others has the connotation of public usage as in the example above. Unix organizes access as a three bit information for each i.e. owner, group, and others. So the access rights are defined by 9 bits as rwx rwx rwx respectively for owner, group and others. The rwx can be defined as an octal number too. If all bits are set then we have a pattern 111 111 111 (or 777 in octal) which means the owner has read, write, and execute rights, and the group to which he belongs has also read, write and execute rights, and others have read, write and execute rights as well. A pattern of 111 110 100 (or 764 octal, also denoted as rwx rw- r--) means the owner has read, write, and execute permissions; the group has read and write permissions but no execute permission and others have only the read permission. Note that Unix group permissions are for all or none. Windows 2000 and NTFS permit a greater degree of refinement on a group of users. Linux allows individual users to make up groups.