Unix philosophy
Encyclopedia
The Unix philosophy is a set of cultural norms and philosophical approaches to developing software
based on the experience of leading developers of the Unix
operating system
.
, the inventor of Unix pipes
and one of the founders of the Unix tradition, summarized the philosophy as follows:
This is usually abridged to "Write programs that do one thing and do it well."
, in his book The Art of Unix Programming
, summarizes the Unix philosophy as the widely-used KISS Principle
of "Keep it Simple, Stupid." He also provides a series of design rules:
), drew on his own experience with Unix, as well as discussions with fellow programmers and people in other fields who depended on Unix, to produce The UNIX Philosophy which sums it up in 9 paramount precepts:
For example, in the early days Unix was a monolithic kernel
(which means that user processes carried out kernel system calls all on the user stack). If a signal was delivered to a process while it was blocked on a long-term I/O
in the kernel, then what should be done? Should the signal be delayed, possibly for a long time (maybe indefinitely) while the I/O completed? The signal handler could not be executed when the process was in kernel mode, with sensitive kernel data on the stack. Should the kernel back-out the system call, and store it, for replay and restart later, assuming that the signal handler completes successfully?
In these cases Ken Thompson and Dennis Ritchie
favored simplicity over perfection. The Unix system would occasionally return early from a system call with an error stating that it had done nothing—the "Interrupted System Call", or an error number 4 (
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....
based on the experience of leading developers of the Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
.
McIlroy: A Quarter Century of Unix
Doug McIlroyDouglas McIlroy
Malcolm Douglas McIlroy is a mathematician, engineer, and programmer. As of 2007 he is an Adjunct Professor of Computer Science at Dartmouth College. Dr...
, the inventor of Unix pipes
Pipeline (Unix)
In Unix-like computer operating systems , a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input to the next one. Each connection is implemented by an anonymous pipe...
and one of the founders of the Unix tradition, summarized the philosophy as follows:
This is usually abridged to "Write programs that do one thing and do it well."
Eric Raymond
Eric S. RaymondEric S. Raymond
Eric Steven Raymond , often referred to as ESR, is an American computer programmer, author and open source software advocate. After the 1997 publication of The Cathedral and the Bazaar, Raymond was for a number of years frequently quoted as an unofficial spokesman for the open source movement...
, in his book The Art of Unix Programming
The Art of Unix Programming
The Art of Unix Programming by Eric S. Raymond is a book about the history and culture of Unix programming from its earliest days in 1969 to 2003 when it was published, covering both genetic derivations such as BSD and conceptual ones such as Linux....
, summarizes the Unix philosophy as the widely-used KISS Principle
KISS principle
KISS is an acronym for the design principle Keep it simple, Stupid!. Other variations include "keep it simple and stupid", "keep it short and simple", "keep it simple sir", "keep it simple or be stupid" or "keep it simple and straightforward"...
of "Keep it Simple, Stupid." He also provides a series of design rules:
- Rule of ModularityModularity (programming)Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
: Write simple parts connected by clean interfaces. - Rule of Clarity: Clarity is better than cleverness.
- Rule of Composition: Design programs to be connected to other programs.
- Rule of Separation: Separate policy from mechanism; separate interfaces from engines.
- Rule of Simplicity: Design for simplicity; add complexity only where you must.
- Rule of Parsimony: Write a big program only when it is clear by demonstration that nothing else will do.
- Rule of Transparency: Design for visibility to make inspection and debugging easier.
- Rule of Robustness: Robustness is the child of transparency and simplicity.
- Rule of Representation: Fold knowledge into data so program logic can be stupid and robust.
- Rule of Least Surprise: In interface design, always do the least surprising thing.
- Rule of Silence: When a program has nothing surprising to say, it should say nothing.
- Rule of Repair: When you must fail, fail noisily and as soon as possible.
- Rule of Economy: Programmer time is expensive; conserve it in preference to machine time.
- Rule of Generation: Avoid hand-hacking; write programs to write programs when you can.
- Rule of OptimizationOptimization (computer science)In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
: PrototypePrototypeA prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
before polishing. Get it working before you optimize it. - Rule of Diversity: Distrust all claims for "one true way".
- Rule of Extensibility: Design for the future, because it will be here sooner than you think.
Mike Gancarz: The UNIX Philosophy
In 1994 Mike Gancarz (a member of the team that designed the X Window SystemX Window System
The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...
), drew on his own experience with Unix, as well as discussions with fellow programmers and people in other fields who depended on Unix, to produce The UNIX Philosophy which sums it up in 9 paramount precepts:
- Small is beautiful.
- Make each program do one thing well.
- Build a prototype as soon as possible.
- Choose portability over efficiency.
- Store data in flat text fileText fileA text file is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists within a computer file system...
s. - Use software leverage to your advantage.
- Use shell scriptShell scriptA shell script is a script written for the shell, or command line interpreter, of an operating system. It is often considered a simple domain-specific programming language...
s to increase leverage and portability. - Avoid captive user interfaces.
- Make every program a filterFilter (Unix)In Unix and Unix-like operating systems, a filter is a program that gets most of its data from its standard input and writes its main results to its standard output . Unix filters are often used as elements of pipelines...
.
Worse is better
Richard P. Gabriel suggests that a key advantage of Unix was that it embodied a design philosophy he termed "worse is better", in which simplicity of both the interface and the implementation are more important than any other attributes of the system—including correctness, consistency, and completeness. Gabriel argues that this design style has key evolutionary advantages, though he questions the quality of some results.For example, in the early days Unix was a monolithic kernel
Monolithic kernel
A monolithic kernel is an operating system architecture where the entire operating system is working in the kernel space and alone as supervisor mode...
(which means that user processes carried out kernel system calls all on the user stack). If a signal was delivered to a process while it was blocked on a long-term I/O
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
in the kernel, then what should be done? Should the signal be delayed, possibly for a long time (maybe indefinitely) while the I/O completed? The signal handler could not be executed when the process was in kernel mode, with sensitive kernel data on the stack. Should the kernel back-out the system call, and store it, for replay and restart later, assuming that the signal handler completes successfully?
In these cases Ken Thompson and Dennis Ritchie
Dennis Ritchie
Dennis MacAlistair Ritchie , was an American computer scientist who "helped shape the digital era." He created the C programming language and, with long-time colleague Ken Thompson, the UNIX operating system...
favored simplicity over perfection. The Unix system would occasionally return early from a system call with an error stating that it had done nothing—the "Interrupted System Call", or an error number 4 (
EINTR) in today's systems. Of course the call had been aborted in order to call the signal handler. This could only happen for a handful of long-running system calls, i.e. read, write, open, select, etc. On the plus side, this made the I/O system many times simpler to design and understand. The vast majority of user programs were never affected because they didn't handle or experience signals other than SIGINT or Control-C and would die right away if one was raised. For the few other programs—things like shells or text editors that respond to job control key presses—small wrappers could be added to system calls so as to retry the call right away if this EINTR error was raised. Thus, the problem was solved in a simple manner.Quotes
- "Unix is simple. It just takes a genius to understand its simplicity." – Dennis RitchieDennis RitchieDennis MacAlistair Ritchie , was an American computer scientist who "helped shape the digital era." He created the C programming language and, with long-time colleague Ken Thompson, the UNIX operating system...
- "UNIX was not designed to stop its users from doing stupid things, as that would also stop them from doing clever things." – Doug Gwyn
- "Unix never says 'please." – Rob Pike
- "Unix is user-friendly. It just isn't promiscuous about which users it's friendly with." – Steven King
- "Those who don't understand UNIX are condemned to reinvent it, poorly." – Henry SpencerHenry SpencerHenry Spencer is a Canadian computer programmer and space enthusiast. He wrote "regex", a widely-used software library for regular expressions, and co-wrote C News, a Usenet server program. He also authored The Ten Commandments for C Programmers. He is coauthor, with David Lawrence, of the book...
See also
- Unix architectureUnix architectureA Unix architecture is a computer operating system system architecture that embodies the Unix philosophy. It may adhere to standards such as the Single UNIX Specification or similar POSIX IEEE standard...
- Plan 9 from Bell LabsPlan 9 from Bell LabsPlan 9 from Bell Labs is a distributed operating system. It was developed primarily for research purposes as the successor to Unix by the Computing Sciences Research Center at Bell Labs between the mid-1980s and 2002...
- Pipes and filters
- The UNIX-HATERS HandbookThe UNIX-HATERS HandbookThe UNIX-HATERS Handbook is a semi-humorous edited compilation of messages to the UNIX-HATERS mailing list. The book was edited by Simson Garfinkel, Daniel Weise and Steven Strassmann and published in 1994....
- Software engineeringSoftware engineeringSoftware Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
- Hacker (programmer subculture)
- List of software development philosophies
External links
- The Unix Philosophy: A Brief Introduction - by The Linux Information Project (LINFO)
- Joel on Software - Biculturalism
- The truth about Unix: "The user interface is horrid" (1981)
- The Unix Philosophy
- Why the Unix Philosophy still matters