The Unix philosophy, originated by Ken Thompson, is a set of cultural norms and philosophical approaches to minimalist, modular software development. It is based on the experience of leading developers of the Unix operating system. Early Unix developers were important in bringing the concepts of modularity and reusability into software engineering practice, spawning a "software tools" movement. Over time, the leading developers of Unix (and programs that ran on it) established a set of cultural norms for developing software; these norms became as important and influential as the technology of Unix itself, and have been termed the "Unix philosophy." The Unix philosophy emphasizes building simple, compact, clear, modular, and extensible code that can be easily maintained and repurposed by developers other than its creators. The Unix philosophy favors composability as opposed to monolithic design.
Origin In their Unix paper of 1974, Ritchie and Thompson quote the following design considerations:
Make it easy to write, test, and run programs. Interactive use instead of batch processing. Economy and elegance of design due to size constraints ("salvation through suffering"). Self-supporting system: all Unix software is maintained under Unix. In 1978, Doug McIlroy documented a set of principles encapsulating the "characteristic style" that had emerged among Unix system users and developers:
Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features". Expect the output of every program to become the input to another, as yet unknown, program. Don't clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don't insist on interactive input. Design and build software, even operating systems, to be tried early, ideally within weeks. Don't hesitate to throw away the clumsy parts and rebuild them. Use tools in preference to unskilled help to lighten a programming task, even if you have to detour to build the tools and expect to throw some of them out after you've finished using them. Later, in 1994, it was summarized with the explicit name "The Unix philosophy" by Peter H. Salus, who credited it to McIlroy:
Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.
Parts
The UNIX Programming Environment In their preface to the 1984 book, The UNIX Programming Environment, Brian Kernighan and Rob Pike, both from Bell Labs, give a brief description of the Unix design and the Unix philosophy:
Even though the UNIX system introduces a number of innovative programs and techniques, no single program or idea makes it work well. Instead, what makes it effective is the approach to programming, a philosophy of using the computer. Although that philosophy can't be written down in a single sentence, at its heart is the idea that the power of a system comes more from the relationships among programs than from the programs themselves. Many UNIX programs do quite trivial things in isolation, but, combined with other programs, become general and useful tools. The authors further write that their goal for this book is "to communicate the UNIX programming philosophy."
Program Design in the UNIX Environment
In October 1984, Brian Kernighan and Rob Pike published a paper called Program Design in the UNIX Environment. In this paper, they criticize the accretion of program options and features found in some newer Unix systems such as 4.2BSD and System V, and explain the Unix philosophy of software tools, each performing one general function:
Much of the power of the UNIX operating system comes from a style of program design that makes programs easy to use and, more important, easy to combine with other programs. This style has been called the use of software tools, and depends more on how the programs fit into the programming environment and how they can be used with other programs than on how they are designed internally. [...] This style was based on the use of tools: using programs separately or in combination to get a job done, rather than doing it by hand, by monolithic self-sufficient subsystems, or by special-purpose, one-time programs. The authors contrast Unix tools such as cat with larger program suites used by other systems.
The design of cat is typical of most UNIX programs: it implements one simple but general function that can be used in many different applications (including many not envisioned by the original author). Other commands are used for other functions. For example, there are separate commands for file system tasks like renaming files, deleting them, or telling how big they are. Other systems instead lump these into a single "file system" command with an internal structure and command language of its own. (The PIP file copy program found on operating systems like CP/M or RSX-11 is an example.) That approach is not necessarily worse or better, but it is certainly against the UNIX philosophy.
Doug McIlroy on Unix programming
McIlroy, then head of the Bell Labs Computing Sciences Research Center, and inventor of the Unix pipe, summarized the Unix philosophy as follows:
This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface. Beyond these statements, he has also emphasized simplicity and minimalism in Unix programming:
The notion of "intricate and beautiful complexities" is almost an oxymoron. Unix programmers vie with each other for "simple and beautiful" honors — a point that's implicit in these rules, but is well worth making overt. Conversely, McIlroy has criticized modern Linux as having software bloat, remarking that, "adoring admirers have fed Linux goodies to a disheartening state of obesity." He contrasts this with the earlier approach taken at Bell Labs when developing and revising Research Unix:
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