UNIX Files

JMU

UNIX Files
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Before Getting Started

A Common Usage:
- A file is a sequence of bytes that persists beyond the life of a program
The UNIX/Linux/Posix Usage:
- A file is a sequence of bytes

Before Getting Started (cont.)

Theoretical Implications of the UNIX Usage:
- A file is an abstraction
Practical Implications of the UNIX Usage:
- Files are used to model every I/O device (e.g., disks, keyboards, displays, networks)
- Files provide a uniform view of varied I/O devices

File Types

Regular Files:
- Contain arbitrary data (Note: To the kernel, there is no distinction between text files and binary files, all files contain bytes.)
Directories:
- Contain a collection of (hard) links, each of which maps a filename to a file
Sockets:
- Used to communicate across a network
Pipes:
- A (unidirectional) byte stream with limited capacity

File Types (cont.)

Symbolic (Soft) Links:
- A file whose data is the name of another file (which may or may not exist)
Character Devices:
- Devices that handle data on a character-by-character basis (e.g., terminals and keyboards)
Block Devices:
- Devices that handle data a block at a time (e.g., disks)

Text Files and Binary Files

Applications:
- Often distinguish between text files that contain only ASCII or Unicode characters and binary files
- Text files are often line-oriented (i.e., contain line-feed characters [UNIX], carriage returns, or both)
The Kernel:
- Does not distinguish between text and binary files

Organization of Files

The Hierarchy:
- Anchored at a directory named / (called the root directory)
Some Important Directories:
- /dev contains devices
- /usr/include contains header files
Some Important Directory Entries:
- . a link to the directory itself
- .. a link to the parent directory