Introduction to Unix

UNIX Introduction

Unix or UNIX is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T Bell Labs employees including Ken Thompson, Dennis Ritchie, and Douglas McIlroy.

Today's Unix systems are split into various branches, developed over time by AT&T, several other commercial vendors, as well as several non-profit organizations, such as contributors to the GNU project.

Unix was designed to be portable, multi-tasking and multi-user in a time-sharing configuration. The Unix systems are characterized by various concepts: plain text files, command line interpreter, hierarchical file system, treating devices and certain types of inter-process communication as files, etc. In software engineering, Unix is mainly noted for its use of the C programming language and for the Unix philosophy.

The present owner of the UNIX trademark is The Open Group, while the present claimants on the rights to the UNIX source code are SCO Group and Novell (an issue that is currently being decided in court). Only systems fully compliant with and certified to the Single UNIX Specification qualify as "UNIX" (others are called "UNIX system-like" or Unix-like).

During the late 1970s and early 1980s, Unix's influence in academic circles led to massive adoption (particularly of the BSD variant, originating from the University of California, Berkeley) of Unix by commercial startups, the most notable of which is Sun Microsystems.

Sometimes, Traditional Unix may be used to describe a Unix or GNU operating system that has the characteristics of either Version 7 Unix or UNIX System V.

Beginning in the late 1980s, an open operating system standardization effort known as POSIX provided a common baseline for all operating systems; IEEE based POSIX around the structure of the Unix system. At around the same time a separate but very similar standard, the Single UNIX Specification, was also produced by the Open Group. Starting in 1998 these two standards bodies began work on merging the two standards, and the latest revisions of both are in fact identical.

In an effort towards compatibility, several Unix system vendors agreed on SVR4's ELF format as standard for binary and object code files. The common format allows substantial binary compatibility among Unix systems operating on the same CPU architecture.

The directory layout of some systems, particularly on Linux, is defined by the Filesystem Hierarchy Standard. This type of standard however is controversial among many, and even within the Linux community adoption is far from universal.

Components

The Unix system is composed of several components that are normally packaged together. By including -- in addition to the "kernel" of an operating system -- the development environment, libraries, documents, and the portable, modifiable source-code for all of these components, Unix was a self-contained software system. This was one of the key reasons it emerged into an important teaching and learning tool and had such a broad influence.

Inclusion of these components did not make the system large -- the original V7 Unix distribution, consisting of copies of all of the compiled binaries plus all of the source code and documentation occupied less than 10Mb, and arrived on a single 9-track magtape. The printed documentation was contained in two fairly thin books.

The names and filesystem locations of the Unix components has changed substantially across the history of the system. Nonetheless, the V7 implementation is considered by many to have the canonical early structure:

• Kernel -- originally found in /usr/sys, and composed of several sub-components:
  • conf -- originally found in /usr/sys/conf, and composed of configuration and machine-dependent parts, often including boot code
  • dev -- Device drivers (originally /usr/sys/dev) for control of hardware (and sometimes pseudo-hardware)
  • sys -- The "kernel" of the operating system, handling memory management, system calls, etc
  • h (or include) -- Header files, generally defining key interfaces within the system, and important system-specific invariables
• Development Environment -- Most implementations of Unix contained a development environment sufficient to recreate the system from source code. The development environment included:
  • cc -- The C language compiler
  • as -- The machine-language assembler for the machine
  • ld -- The linking loader for combining object files
  • lib -- Libraries. Originally libc, the C runtime library, was the primary library, but there have always been additional libraries for (e.g.) floating-point emulation (libm) or a database implementation. V7 Unix introduced the first consistent "Standard I/O" library stdio. Later implementations multiplied the number and type of libraries significantly.
  • include -- Header files for software development, defining standard interfaces and system invariants
  • Other (secondary) languages -- V7 Unix contained a Fortran-77 compiler, and other versions and implementations have or now contain many other language compilers and toolsets.
  • ... and a number of other tools, including an object-code archive manager (ar), symbol-table lister, compiler-development tools (e.g. yacc), make, and debugging tools.
• Commands -- Most Unix implementation make little distinction between commands (user-level programs) for system operation and maintenance (e.g. cron), commands of general utility (e.g. grep), and more general-purpose applications such as the text formatting and typesetting package. Nonetheless, some major categories are:
  • sh -- The Shell, the primary user-interface on Unix before window systems appeared, and the center of the command environment. To degrees that varied in different shell implementations, external programs (such as expr) were relied on by the shell.
  • Utilities -- the core of the Unix command set, including ls, grep, find and many others. This category could be subcategorized:
    • System utilities -- such as mkfs, fsck, and many others; and
    • User utilities -- passwd, kill, and others
  • Runoff -- Unix systems never lost their heritage as early document preparation and typesetting systems, and included many related programs such as troff, tbl, neqn, refer, plot
  • Communications -- early Unix systems contained no inter-system communication, but did include the inter-user communication programs mail and talk. V7 introduced the early inter-system communication system UUCP, and systems beginning with the BSD release included TCP/IP utilities

• Documentation -- While not strictly part of the operating system, Unix was unique in its time for including all of its documentation online in machine-readable form. The documentation included:
  • man -- Manual pages for each command, library component, system call, header file, etc
  • doc -- Longer documents detailing major subsystems, such as the C language, troff, and other systems.