• Write small programs to do one thing, and do it well.
• Write programs that work together.
• Read input and write output as human-readable text, because that is the universal interface.

The Unix philosophy originated with Ken Thompson as a set of approaches to software development based on the experiences that he had in leading the developers of the Unix operating system. The philosophy emphasizes building short, simple, clear, modular, and extendable code that can be easily maintained and re-purposed by others.

“This is the Unix philosophy: Write programs that do one thing and do it well. Write programs that work together. Write programs that handle text streams, because that is a universal interface.”
–Doug McIlroy

• 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 code that is complex, long, and unreadable.

• Rule of Clarity: Developers should write programs as if the most important communication is to the developer, including him- or herself, who 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.

• Rule of Least Surprise: Developers should design programs that build on top of the potential 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.

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.

• http://en.wikipedia.org/wiki/Unix_philosophy