Definition of a Filesystem

Probably the simplest definition of a filesystem is a data structure or a collection of files. In Unix, filesystem can refer to two very distinct things; the directory tree or the arrangement of files on disk partitions. The latter can be thought of as the physical filesystem as it has a tangible physical location.

The physical filesystem is divided first by disk partitions. Partition size determines the number of blocks that the filesystem uses. Each filesystem has a superblock, inodes and data blocks. The superblock holds the control information for the system. Inodes contain similar information for individual files. The data blocks hold data, the information in the files.

The logical filesystem refers to a hierarchy of connected directories made of all the files (or disk partitions) that are accessible to the user. The Unix filesystem is arranged in a tree or inverted pyramid, where all files are logically contained in the root file.

There are slight differences in the arrangement of directories between types of Unix. However, the overall structure is basically the same. Please note that although only one branch of the tree is shown for each level, each of the directories potentially contains subdirectories.

The Unix Filesystem

BSD directory structure

This image was taken from the O'Reilly & Associates Inc. book, Essential System Administration, written by Æleen Frisch, ISBN 0-937175-80-3. It has been used by permission of the publisher.

For information on the contents of each directory, click on that directory.

SVR4 directory structure

For information on the contents of each directory, click on that directory.

Terms used: partition, superblock, inode, block, pseudo terminal, cron.