Unix philosophy

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The Unix philosophy, originated by Ken Thompson, is a set of cultural norms and philosophical approaches to developing small yet capable software based on the experience of leading developers of the Unix operating system. The Unix philosophy emphasizes building short, simple, clear, modular, and extendable code that can be easily maintained and repurposed by developers other than its creators.

McIlroy: A Quarter Century of Unix

Doug McIlroy, then head of the Bell Labs CSRC and contributor to Unix pipes,[1] summarised Unix philosophy as follows:[2]

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.

Eric Raymond’s 17 Unix Rules

In his book The Art of Unix Programming that was first published in 2003,[3] Eric S. Raymond, an American programmer and open source advocate, summarizes the Unix philosophy as KISS Principle of "Keep it Simple, Stupid."[4] He provides a series of design rules:[5]

  • Rule of Modularity: Developers should build a program out of simple parts connected by well defined interfaces, so problems are local, and parts of the program can be replaced in future versions to support new features. This rule aims to save time on debugging complex code that is complex, long, and unreadable.
  • Rule of Clarity: Developers should write programs as if the most important communication is to the developers, including him- or herself, whom will read and maintain the program rather than the computer. This rule aims to make code readable and comprehensible for whomever works on the code in future.
  • Rule of Composition: Developers should write programs that can communicate easily with other programs. This rule aims to allow developers to break down projects into small, simple programs rather than overly complex monolithic programs.
  • Rule of Separation: Developers should separate the mechanisms of the programs from the policies of the programs; one method is to divide a program into a front-end interface and back-end engine that interface communicates with. This rule aims to let policies be changed without destabilizing mechanisms and consequently reducing the number of bugs.
  • Rule of Simplicity: Developers should design for simplicity by looking for ways to break up program systems into small, straightforward cooperating pieces. This rule aims to discourage developers’ affection for writing “intricate and beautiful complexities” that are in reality bug prone programs.
  • Rule of Parsimony: Developers should avoid writing big programs. This rule aims to prevent overinvestment of development time in failed or suboptimal approaches caused by the owners of the program’s reluctance to throw away visibly large pieces of work. Smaller programs are not only easier to optimize and maintain; they are easier to delete when deprecated.
  • Rule of Transparency: Developers should design for visibility and discoverability by writing in a way that their thought process can lucidly be seen by future developers working on the project and using input and output formats that make it easy to identify valid input and correct output. This rule aims to reduce debugging time and extend the lifespan of programs.
  • Rule of Robustness: Developers should design robust programs by designing for transparency and discoverability, because code that is easy to understand is easier to stress test for unexpected conditions that may not be foreseeable in complex programs. This rule aims to help developers build robust, reliable products.
  • Rule of Representation: Developers should choose to make data more complicated rather than the procedural logic of the program when faced with the choice, because it is easier for humans to understand complex data compared with complex logic. This rule aims to make programs more readable for any developer working on the project, which allows the program to be maintained.[6]
  • Rule of Least Surprise: Developers should design programs that build on top of the potentials users' expected knowledge; for example, ‘+’ should always mean addition in a calculator program. This rule aims to encourage developers to build intuitive products that are easy to use.
  • Rule of Silence: Developers should design programs so that they do not print unnecessary output. This rule aims to allows other programs and developers to pick out the information they need from a program's output without having to parse verbosity.
  • Rule of Repair: Developers should design programs that fail in a manner that is easy to localize and diagnose or in other words “fail noisily”. This rule aims to prevent incorrect output from a program from becoming an input and corrupting the output of other code undetected.
  • Rule of Economy: Developers should value developer time over machine time, because machine cycles as of the year 2013 are relatively inexpensive compared to prices in the 1970s. This rule aims to reduce development costs of projects.
  • Rule of Generation: Developers should avoid writing code by hand and instead write abstract high-level programs that generate code. This rule aims to reduce humans errors and save time.
  • Rule of Optimization: Developers should prototype software before polishing it. This rule aims to prevent developers from spending too much time for marginal gains.
  • Rule of Diversity: Developers should design their programs to be flexible and open. This rule aims to make programs flexible, allowing them to be used in other ways than their developers intended.
  • Rule of Extensibility: Developers should design for the future by making their protocols extensible, allowing for easy plugins without modification to the program's architecture by other developers, noting the version of the program, and more. This rule aims to extend the lifespan and enhance the utility of the code the developer writes.

Mike Gancarz: The UNIX Philosophy

In 1994 Mike Gancarz (a member of the team that designed the X Window System), 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:

  1. Small is beautiful.
  2. Make each program do one thing well.
  3. Build a prototype as soon as possible.
  4. Choose portability over efficiency.
  5. Store data in flat text files.
  6. Use software leverage to your advantage.
  7. Use shell scripts to increase leverage and portability.
  8. Avoid captive user interfaces.
  9. Make every program a filter.

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 (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 (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 such as read(), write(), open(), and select(). 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.


  • "Unix is simple. It just takes a genius to understand its simplicity." – Dennis Ritchie
  • "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 Spencer

See also


  1. ^ http://cm.bell-labs.com/cm/cs/who/dmr/mdmpipe.html
  2. ^ Basics of the Unix Philosophy
  3. ^ Raymond, Eric (2003-09-19). The Art of Unix Programming. Addison-Wesley. ISBN 0-13-142901-9. Retrieved 2009-02-09.
  4. ^ Raymond, Eric (2003-09-19). "The Unix Philosophy in One Lesson". The Art of Unix Programming. Addison-Wesley. ISBN 0-13-142901-9. Retrieved 2009-02-09.
  5. ^ Raymond, Eric. "The Art of Unix Programming". Thyrus Enterprises. Retrieved 13 February 2013.
  6. ^ Raymond, Eric (19 September 2003). "Basics of the Unix Philosophy". The Art of Unix Programming. Addison-Wesley. ISBN 0-13-142901-9. Retrieved 2009-02-09.


External links