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The Art of Unix Programming
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Unix Programming - Basics of the Unix Philosophy

The ‘Unix philosophy’ originated with Ken Thompson's early meditations on how to design a small but capable operating system with a clean service interface. It grew as the Unix culture learned things about how to get maximum leverage out of Thompson's design. It absorbed lessons from many sources along the way.

The Unix philosophy is not a formal design method. It wasn't handed down from the high fastnesses of theoretical computer science as a way to produce theoretically perfect software. Nor is it that perennial executive's mirage, some way to magically extract innovative but reliable software on too short a deadline from unmotivated, badly managed, and underpaid programmers.

The Unix philosophy (like successful folk traditions in other engineering disciplines) is bottom-up, not top-down. It is pragmatic and grounded in experience. It is not to be found in official methods and standards, but rather in the implicit half-reflexive knowledge, the expertise that the Unix culture transmits. It encourages a sense of proportion and skepticism — and shows both by having a sense of (often subversive) humor.

Doug McIlroy, the inventor of Unix pipes and one of the founders of the Unix tradition, had this to say at the time [McIlroy78]:

(i) Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new features.

(ii) 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.

(iii) 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.

(iv) 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.

He later summarized it this way (quoted in A Quarter Century of Unix [Salus]):

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.

Rob Pike, who became one of the great masters of C, offers a slightly different angle in Notes on C Programming [Pike]:

Ken Thompson, the man who designed and implemented the first Unix, reinforced Pike's rule 4 with a gnomic maxim worthy of a Zen patriarch:

When in doubt, use brute force.

More of the Unix philosophy was implied not by what these elders said but by what they did and the example Unix itself set. Looking at the whole, we can abstract the following ideas:

  1. Rule of Modularity: Write simple parts connected by clean interfaces.

  2. Rule of Clarity: Clarity is better than cleverness.

  3. Rule of Composition: Design programs to be connected to other programs.

  4. Rule of Separation: Separate policy from mechanism; separate interfaces from engines.

  5. Rule of Simplicity: Design for simplicity; add complexity only where you must.

  6. Rule of Parsimony: Write a big program only when it is clear by demonstration that nothing else will do.

  7. Rule of Transparency: Design for visibility to make inspection and debuggingeasier.

  8. Rule of Robustness: Robustness is the child of transparency and simplicity.

  9. Rule of Representation: Fold knowledge into data so program logic can be stupid and robust.

  10. Rule of Least Surprise: In interface design, always do the least surprising thing.

  11. Rule of Silence: When a program has nothing surprising to say, it should say nothing.

  12. Rule of Repair: When you must fail, fail noisily and as soon as possible.

  13. Rule of Economy: Programmer time is expensive; conserve it in preference to machine time.

  14. Rule of Generation: Avoid hand-hacking; write programs to write programs when you can.

  15. Rule of Optimization: Prototype before polishing. Get it working before you optimize it.

  16. Rule of Diversity: Distrust all claims for “one true way”.

  17. Rule of Extensibility: Design for the future, because it will be here sooner than you think.

If you're new to Unix, these principles are worth some meditation. Software-engineering texts recommend most of them; but most other operating systems lack the right tools and traditions to turn them into practice, so most programmers can't apply them with any consistency. They come to accept blunt tools, bad designs, overwork, and bloated code as normal — and then wonder what Unix fans are so annoyed about.

As Brian Kernighan once observed, “Controlling complexity is the essence of computer programming” [Kernighan-Plauger]. Debugging dominates development time, and getting a working system out the door is usually less a result of brilliant design than it is of managing not to trip over your own feet too many times.

Assemblers, compilers, flowcharting, procedural programming, structured programming, “artificial intelligence”, fourth-generation languages, object orientation, and software-development methodologies without number have been touted and sold as a cure for this problem. All have failed as cures, if only because they ‘succeeded’ by escalating the normal level of program complexity to the point where (once again) human brains could barely cope. As Fred Brooks famously observed [Brooks], there is no silver bullet.

The only way to write complex software that won't fall on its face is to hold its global complexity down — to build it out of simple parts connected by well-defined interfaces, so that most problems are local and you can have some hope of upgrading a part without breaking the whole.


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The Art of Unix Programming
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