Classic Shell Scripting

Appendix B. Files and Filesystems

Effective use of computers requires an understanding of files and filesystems. This appendix presents an overview of the important features of Unix filesystems: what a file is, how files are named and what they contain, how they are grouped into a filesystem hierarchy, and what properties they have.

What Is a File?

Simply put, a file is a collection of data that resides in a computer system, and that can be referenced as a single entity from a computer program. Files provide a mechanism for data storage that survives process execution, and generally, restarts of the computer.^[1]

In the early days of computers, files were external to the computer system: they usually resided on magnetic tape, paper tape, or punched cards. Their management was left up to their owner, who was expected to try very hard not to drop a stack of punched cards on the floor!

Later, magnetic disks became common, and their physical size decreased sharply, from as large as the span of your arms, to some as small as the width of your thumb, while their capacity increased by several orders of magnitude, from about 5MB in the mid-1950s to about 400,000MB in 2004. Costs and access times have dropped by at least three orders of magnitude. Today, there are about as many magnetic disks in existence as there are humans.

Optical storage devices, such as CD-ROMs and DVDs, are inexpensive and capacious: in the 1990s, CD-ROMs largely replaced removable flexible magnetic disks (floppies) and ...