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UNIX File System

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

The UNIX file system is a hierarchical, tree-structured namespace that is designed to help users organize and access files. The namespace consists of directories that hold files. UNIX file systems consist of the following:

  • Disk-based file systems store files on magnetic media, CD-ROMs, and other media. Sun Microsystems Solaris uses three formats, including ufs (UNIX file system), which is based on the BSD file allocation table file system; hsfs (High Sierra and ISO 9660 file system), a CD-ROM file system; and pcfs (PC file system), which provides access to data on DOS-formatted disks.

  • Network file systems provide network-wide file sharing and file access. NFS (Network File System) and rfs (remote file sharing) file systems are commonly used.

  • Pseudo file systems are virtual file systems that exist in temporary memory and provide a place to store files or directory entries for fast access.

File systems can be mounted and unmounted at any time. A mounted file system is available for use. If it is a network file system, it is available for remote users on the network to access.

As mentioned, the UNIX file system is hierarchical in structure and starts with a root directory (called /) from which all other directories branch. The root directory and subdirectories hold other files and other subdirectories. UNIX includes some unique subdirectories where system files are stored, including /bin, /etc, and /dev. A typical UNIX directory structure is pictured in the following illustration.

Illustration 13 (see book, page 1289)

UNIX has three access file permissions-read, write, and execute-and these are issued to three categories of users: owner, group, and others. Each of these is described here:

  • read (r)    Allows the user to read a file or list the contents of a directory

  • write (w)    Allows the user to write to (change) a file or do a number of things in a directory, including create, rename, and delete a file

  • execute (x)    Allows the user to run a program or search a directory

The three categories of users are described here:

  • owner (u)    The owner of a file

  • group (g)    A group of users to which the owner of a file belongs

  • others (o)    All other users

Information on viewing and setting permissions is available at the UNIX file permissions page listed on the related entries page.

Commands in the UNIX environment such as ls (list), cat (catalog), and the FTP (File Transfer Protocol) utility display file permissions in a format similar to the following:

-rw-r--r-- 1 Tom Research 1009 Nov 11  1996 stars.gif
drwxrwxr-x 2 DC Research 512 Sep 16 02:03 stats

The first column of the file listing displays the permissions, and the remaining columns define the following in order: number of links, owner, group, file size, date, and filename. The permissions are listed as shown here:

Illustration 14 (see book, page 1290)

The UNIX system also defines special types of files that represent physical devices such as printers, terminals, and tape drives. Because devices are defined in this way, it is possible to direct the output of some command to the devices as if you were storing information in a file. If the device is a printer, it will print the output. Special device files are stored in the /dev directory. UNIX pipes are temporary files that store information that is being directed from one place to another.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.