docs/unix-phil
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| author | meillo@marmaro.de |
|---|---|
| date | Wed, 24 Feb 2010 10:42:02 +0100 |
| parents | f89ca78f289e |
| children | e2240a387a84 |
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22 .\"----------------------------------------
23 .TL
24 Why the Unix Philosophy still matters
25 .AU
26 markus schnalke <meillo@marmaro.de>
27 .AB
28 .ti \n(.iu
29 This paper discusses the importance of the Unix Philosophy in software design.
30 Today, few software designers are aware of these concepts,
31 and thus most modern software is limited and does not make use of software leverage.
32 Knowing and following the tenets of the Unix Philosophy makes software more valuable.
33 .AE
35 .\".if t .2C
37 .FS
38 .ps -1
39 This paper was prepared for the seminar ``Software Analysis'' at University Ulm.
40 Mentor was professor Schweiggert. 2010-02-05
41 .br
42 You may get this document from my website
43 .CW \s-1http://marmaro.de/docs
44 .FE
46 .NH 1
47 Introduction
48 .LP
49 Building a software is a process from an idea of the purpose of the software
50 to its release.
51 No matter \fIhow\fP the process is run, two things are common:
52 the initial idea and the release.
53 The process in between can be of any shape.
54 The the maintenance work after the release is ignored for the moment.
55 .PP
56 The process of building splits mainly in two parts:
57 the planning of what and how to build, and implementing the plan by writing code.
58 This paper focuses on the planning part \(en the designing of the software.
59 .PP
60 Software design is the plan of how the internals and externals of the software should look like,
61 based on the requirements.
62 This paper discusses the recommendations of the Unix Philosophy about software design.
63 .PP
64 The here discussed ideas can get applied by any development process.
65 The Unix Philosophy does recommend how the software development process should look like,
66 but this shall not be of matter here.
67 Similar, the question of how to write the code is out of focus.
68 .PP
69 The name ``Unix Philosophy'' was already mentioned several times, but it was not explained yet.
70 The Unix Philosophy is the essence of how the Unix operating system and its toolchest was designed.
71 It is no limited set of rules, but what people see to be common to typical Unix software.
72 Several people stated their view on the Unix Philosophy.
73 Best known are:
74 .IP \(bu
75 Doug McIlroy's summary: ``Write programs that do one thing and do it well.''
76 .[
77 %A M. D. McIlroy
78 %A E. N. Pinson
79 %A B. A. Taque
80 %T UNIX Time-Sharing System Forward
81 %J The Bell System Technical Journal
82 %D 1978
83 %V 57
84 %N 6
85 %P 1902
86 .]
87 .IP \(bu
88 Mike Gancarz' book ``The UNIX Philosophy''.
89 .[
90 %A Mike Gancarz
91 %T The UNIX Philosophy
92 %D 1995
93 %I Digital Press
94 .]
95 .IP \(bu
96 Eric S. Raymond's book ``The Art of UNIX Programming''.
97 .[
98 %A Eric S. Raymond
99 %T The Art of UNIX Programming
100 %D 2003
101 %I Addison-Wesley
102 %O .CW \s-1http://www.faqs.org/docs/artu/
103 .]
104 .LP
105 These different views on the Unix Philosophy have much in common.
106 Especially, the main concepts are similar for all of them.
107 But there are also points on which they differ.
108 This only underlines what the Unix Philosophy is:
109 A retrospective view on the main concepts of Unix software;
110 especially those that were successful and unique to Unix.
111 .\" really?
112 .PP
113 Before we will have a look at concrete concepts,
114 we discuss why software design is important
115 and what problems bad design introduces.
118 .NH 1
119 Importance of software design in general
120 .LP
121 Why should we design software at all?
122 It is general knowledge, that even a bad plan is better than no plan.
123 Ignoring software design is programming without a plan.
124 This will lead pretty sure to horrible results.
125 .PP
126 The design of a software is its internal and external shape.
127 The design talked about here has nothing to do with visual appearance.
128 If we see a program as a car, then its color is of no matter.
129 Its design would be the car's size, its shape, the number and position of doors,
130 the ratio of passenger and cargo transport, and so forth.
131 .PP
132 A software's design is about quality properties.
133 Each of the cars may be able to drive from A to B,
134 but it depends on its properties whether it is a good car for passenger transport or not.
135 It also depends on its properties if it is a good choice for a rough mountain area.
136 .PP
137 Requirements to a software are twofold: functional and non-functional.
138 Functional requirements are easier to define and to verify.
139 They are directly the software's functions.
140 Functional requirements are the reason why software gets written.
141 Someone has a problem and needs a tool to solve it.
142 Being able to solve the problem is the main functional requirement.
143 It is the driving force behind all programming effort.
144 .PP
145 On the other hand, there are also non-functional requirements.
146 They are called \fIquality\fP requirements, too.
147 The quality of a software is about properties that are not directly related to
148 the software's basic functions.
149 Quality aspects are about the properties that are overlooked at first sight.
150 .PP
151 Quality is of few matter when the software gets initially built,
152 but it will be of matter in usage and maintenance of the software.
153 A short-sighted might see in developing a software mainly building something up.
154 Reality shows, that building the software the first time is only a small amount
155 of the overall work.
156 Bug fixing, extending, rebuilding of parts \(en short: maintenance work \(en
157 does soon take over the major part of the time spent on a software.
158 Not to forget the usage of the software.
159 These processes are highly influenced by the software's quality.
160 Thus, quality should never be neglected.
161 The problem is that you hardly ``stumble over'' bad quality during the first build,
162 but this is the time when you should care about good quality most.
163 .PP
164 Software design is not about the basic function of a software;
165 this requirement will get satisfied anyway, as it is the main driving force behind the development.
166 Software design is about quality aspects of the software.
167 Good design will lead to good quality, bad design to bad quality.
168 The primary functions of the software will be affected modestly by bad quality,
169 but good quality can provide a lot of additional gain from the software,
170 even at places where one never expected it.
171 .PP
172 The ISO/IEC 9126-1 standard, part 1,
173 .[
174 %I International Organization for Standardization
175 %T ISO Standard 9126: Software Engineering \(en Product Quality, part 1
176 %C Geneve
177 %D 2001
178 .]
179 defines the quality model as consisting out of:
180 .IP \(bu
181 .I Functionality
182 (suitability, accuracy, inter\%operability, security)
183 .IP \(bu
184 .I Reliability
185 (maturity, fault tolerance, recoverability)
186 .IP \(bu
187 .I Usability
188 (understandability, learnability, operability, attractiveness)
189 .IP \(bu
190 .I Efficiency
191 (time behavior, resource utilization)
192 .IP \(bu
193 .I Maintainability
194 (analysability, changeability, stability, testability)
195 .IP \(bu
196 .I Portability
197 (adaptability, installability, co-existence, replaceability)
198 .LP
199 These goals are parts of a software's design.
200 Good design can give these properties to a software,
201 bad designed software will miss them.
202 .PP
203 One further goal of software design is consistency.
204 Consistency eases understanding, working on, and using things.
205 Consistent internals and consistent interfaces to the outside can be provided by good design.
206 .PP
207 We should design software because good design avoids many problems during a software's lifetime.
208 And we should design software because good design can offer much gain,
209 that can be unrelated to the software main intend.
210 Indeed, we should spend much effort into good design to make the software more valuable.
211 The Unix Philosophy shows how to design software well.
212 It offers guidelines to achieve good quality and high gain for the effort spent.
215 .NH 1
216 The Unix Philosophy
217 .LP
218 The origins of the Unix Philosophy were already introduced.
219 This chapter explains the philosophy, oriented on Gancarz,
220 and shows concrete examples of its application.
222 .NH 2
223 Pipes
224 .LP
225 Following are some examples to demonstrate how applied Unix Philosophy feels like.
226 Knowledge of using the Unix shell is assumed.
227 .PP
228 Counting the number of files in the current directory:
229 .DS I 2n
230 .CW
231 .ps -1
232 ls | wc -l
233 .DE
234 The
235 .CW ls
236 command lists all files in the current directory, one per line,
237 and
238 .CW "wc -l
239 counts the number of lines.
240 .PP
241 Counting the number of files that do not contain ``foo'' in their name:
242 .DS I 2n
243 .CW
244 .ps -1
245 ls | grep -v foo | wc -l
246 .DE
247 Here, the list of files is filtered by
248 .CW grep
249 to remove all that contain ``foo''.
250 The rest is the same as in the previous example.
251 .PP
252 Finding the five largest entries in the current directory.
253 .DS I 2n
254 .CW
255 .ps -1
256 du -s * | sort -nr | sed 5q
257 .DE
258 .CW "du -s *
259 returns the recursively summed sizes of all files
260 \(en no matter if they are regular files or directories.
261 .CW "sort -nr
262 sorts the list numerically in reverse order.
263 Finally,
264 .CW "sed 5q
265 quits after it has printed the fifth line.
266 .PP
267 The presented command lines are examples of what Unix people would use
268 to get the desired output.
269 There are also other ways to get the same output.
270 It's a user's decision which way to go.
271 .PP
272 The examples show that many tasks on a Unix system
273 are accomplished by combining several small programs.
274 The connection between the single programs is denoted by the pipe operator `|'.
275 .PP
276 Pipes, and their extensive and easy use, are one of the great
277 achievements of the Unix system.
278 Pipes between programs have been possible in earlier operating systems,
279 but it has never been a so central part of the concept.
280 When, in the early seventies, Doug McIlroy introduced pipes for the
281 Unix system,
282 ``it was this concept and notation for linking several programs together
283 that transformed Unix from a basic file-sharing system to an entirely new way of computing.''
284 .[
285 %T Unix: An Oral History
286 %O .CW \s-1http://www.princeton.edu/~hos/frs122/unixhist/finalhis.htm
287 .]
288 .PP
289 Being able to specify pipelines in an easy way is,
290 however, not enough by itself.
291 It is only one half.
292 The other is the design of the programs that are used in the pipeline.
293 They have to interfaces that allows them to be used in such a way.
295 .NH 2
296 Interface design
297 .LP
298 Unix is, first of all, simple \(en Everything is a file.
299 Files are sequences of bytes, without any special structure.
300 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin)
301 and write a stream of bytes to ``standard output'' (stdout).
302 .PP
303 If the files \fIare\fP sequences of bytes,
304 and the programs \fIare\fP filters on byte streams,
305 then there is exactly one standardized data interface.
306 Thus it is possible to combine them in any desired way.
307 .PP
308 Even a handful of small programs will yield a large set of combinations,
309 and thus a large set of different functions.
310 This is leverage!
311 If the programs are orthogonal to each other \(en the best case \(en
312 then the set of different functions is greatest.
313 .PP
314 Programs might also have a separate control interface,
315 besides their data interface.
316 The control interface is often called ``user interface'',
317 because it is usually designed to be used by humans.
318 The Unix Philosophy discourages to assume the user to be human.
319 Interactive use of software is slow use of software,
320 because the program waits for user input most of the time.
321 Interactive software requires the user to be in front of the computer
322 all the time.
323 Interactive software occupy the user's attention while they are running.
324 .PP
325 Now we come back to the idea of using several small programs, combined,
326 to have a more specific function.
327 If these single tools would all be interactive,
328 how would the user control them?
329 It is not only a problem to control several programs at once if they run at the same time,
330 it also very inefficient to have to control each of the single programs
331 that are intended to work as one large program.
332 Hence, the Unix Philosophy discourages programs to demand interactive use.
333 The behavior of programs should be defined at invocation.
334 This is done by specifying arguments (``command line switches'') to the program call.
335 Gancarz discusses this topic as ``avoid captive user interfaces''.
336 .[
337 %A Mike Gancarz
338 %T The UNIX Philosophy
339 %I Digital Press
340 %D 1995
341 %P 88 ff.
342 .]
343 .PP
344 Non-interactive use is, during development, also an advantage for testing.
345 Testing of interactive programs is much more complicated,
346 than testing of non-interactive programs.
348 .NH 2
349 The toolchest approach
350 .LP
351 A toolchest is a set of tools.
352 Instead of having one big tool for all tasks, one has many small tools,
353 each for one task.
354 Difficult tasks are solved by combining several of the small, simple tools.
355 .PP
356 The Unix toolchest \fIis\fP a set of small, (mostly) non-interactive programs
357 that are filters on byte streams.
358 They are, to a large extend, unrelated in their function.
359 Hence, the Unix toolchest provides a large set of functions
360 that can be accessed by combining the programs in the desired way.
361 .PP
362 There are also advantages for developing small toolchest programs.
363 It is easier and less error-prone to write small programs.
364 It is also easier and less error-prone to write a large set of small programs,
365 than to write one large program with all the functionality included.
366 If the small programs are combinable, then they offer even a larger set
367 of functions than the single large program.
368 Hence, one gets two advantages out of writing small, combinable programs.
369 .PP
370 There are two drawbacks of the toolchest approach.
371 First, one simple, standardized, unidirectional interface has to be sufficient.
372 If one feels the need for more ``logic'' than a stream of bytes,
373 then a different approach might be of need.
374 But it is also possible, that he just can not imagine a design where
375 a stream of bytes is sufficient.
376 By becoming more familiar with the ``Unix style of thinking'',
377 developers will more often and easier find simple designs where
378 a stream of bytes is a sufficient interface.
379 .PP
380 The second drawback of a toolchest affects the users.
381 A toolchest is often more difficult to use for novices.
382 It is necessary to become familiar with each of the tools,
383 to be able to use the right one in a given situation.
384 Additionally, one needs to combine the tools in a senseful way on its own.
385 This is like a sharp knife \(en it is a powerful tool in the hand of a master,
386 but of no good value in the hand of an unskilled.
387 .PP
388 However, learning single, small tool of the toolchest is easier than
389 learning a complex tool.
390 The user will have a basic understanding of a yet unknown tool,
391 if the several tools of the toolchest have a common style.
392 He will be able to transfer knowledge over one tool to another.
393 .PP
394 Moreover, the second drawback can be removed easily by adding wrappers
395 around the single tools.
396 Novice users do not need to learn several tools if a professional wraps
397 the single commands into a more high-level script.
398 Note that the wrapper script still calls the small tools;
399 the wrapper script is just like a skin around.
400 No complexity is added this way,
401 but new programs can get created out of existing one with very low effort.
402 .PP
403 A wrapper script for finding the five largest entries in the current directory
404 could look like this:
405 .DS I 2n
406 .CW
407 .ps -1
408 #!/bin/sh
409 du -s * | sort -nr | sed 5q
410 .DE
411 The script itself is just a text file that calls the command line
412 a professional user would type in directly.
413 Making the program flexible on the number of entries it prints,
414 is easily possible:
415 .DS I 2n
416 .CW
417 .ps -1
418 #!/bin/sh
419 num=5
420 [ $# -eq 1 ] && num="$1"
421 du -sh * | sort -nr | sed "${num}q"
422 .DE
423 This script acts like the one before, when called without an argument.
424 But one can also specify a numerical argument to define the number of lines to print.
426 .NH 2
427 A powerful shell
428 .LP
429 It was already said, that the Unix shell provides the possibility to
430 combine small programs into large ones easily.
431 A powerful shell is a great feature in other ways, too.
432 .PP
433 For instance by including a scripting language.
434 The control statements are build into the shell.
435 The functions, however, are the normal programs, everyone can use on the system.
436 Thus, the programs are known, so learning to program in the shell is easy.
437 Using normal programs as functions in the shell programming language
438 is only possible because they are small and combinable tools in a toolchest style.
439 .PP
440 The Unix shell encourages to write small scripts out of other programs,
441 because it is so easy to do.
442 This is a great step towards automation.
443 It is wonderful if the effort to automate a task equals the effort
444 it takes to do it the second time by hand.
445 If it is so, then the user will be happy to automate everything he does more than once.
446 .PP
447 Small programs that do one job well, standardized interfaces between them,
448 a mechanism to combine parts to larger parts, and an easy way to automate tasks,
449 this will inevitably produce software leverage.
450 Getting multiple times the benefit of an investment is a great offer.
451 .PP
452 The shell also encourages rapid prototyping.
453 Many well known programs started as quickly hacked shell scripts,
454 and turned into ``real'' programs, written in C, later.
455 Building a prototype first is a way to avoid the biggest problems
456 in application development.
457 Fred Brooks writes in ``No Silver Bullet'':
458 .[
459 %A Frederick P. Brooks, Jr.
460 %T No Silver Bullet: Essence and Accidents of Software Engineering
461 %B Information Processing 1986, the Proceedings of the IFIP Tenth World Computing Conference
462 %E H.-J. Kugler
463 %D 1986
464 %P 1069\(en1076
465 %I Elsevier Science B.V.
466 %C Amsterdam, The Netherlands
467 .]
468 .QP
469 The hardest single part of building a software system is deciding precisely what to build.
470 No other part of the conceptual work is so difficult as establishing the detailed
471 technical requirements, [...].
472 No other part of the work so cripples the resulting system if done wrong.
473 No other part is more difficult to rectify later.
474 .PP
475 Writing a prototype is a great method to become familiar with the requirements
476 and to actually run into real problems.
477 Today, prototyping is often seen as a first step in building a software.
478 This is, of course, good.
479 However, the Unix Philosophy has an \fIadditional\fP perspective on prototyping:
480 After having built the prototype, one might notice, that the prototype is already
481 \fIgood enough\fP.
482 Hence, no reimplementation, in a more sophisticated programming language, might be of need,
483 for the moment.
484 Maybe later, it might be neccessary to rewrite the software, but not now.
485 .PP
486 By delaying further work, one keeps the flexibility to react easily on
487 changing requirements.
488 Software parts that are not written will not miss the requirements.
490 .NH 2
491 Worse is better
492 .LP
493 The Unix Philosophy aims for the 80% solution;
494 others call it the ``Worse is better'' approach.
495 .PP
496 First, practical experience shows, that it is almost never possible to define the
497 requirements completely and correctly the first time.
498 Hence one should not try to; it will fail anyway.
499 Second, practical experience shows, that requirements change during time.
500 Hence it is best to delay requirement-based design decisions as long as possible.
501 Also, the software should be small and flexible as long as possible
502 to react on changing requirements.
503 Shell scripts, for example, are more easily adjusted as C programs.
504 Third, practical experience shows, that maintenance is hard work.
505 Hence, one should keep the amount of software as small as possible;
506 it should just fulfill the \fIcurrent\fP requirements.
507 Software parts that will be written later, do not need maintenance now.
508 .PP
509 Starting with a prototype in a scripting language has several advantages:
510 .IP \(bu
511 As the initial effort is low, one will likely start right away.
512 .IP \(bu
513 As working parts are available soon, the real requirements can get identified soon.
514 .IP \(bu
515 When a software is usable, it gets used, and thus tested.
516 Hence problems will be found at early stages of the development.
517 .IP \(bu
518 The prototype might be enough for the moment,
519 thus further work on the software can be delayed to a time
520 when one knows better about the requirements and problems,
521 than now.
522 .IP \(bu
523 Implementing now only the parts that are actually needed now,
524 requires fewer maintenance work.
525 .IP \(bu
526 If the global situation changes so that the software is not needed anymore,
527 then less effort was spent into the project, than it would have be
528 when a different approach had been used.
530 .NH 2
531 Upgrowth and survival of software
532 .LP
533 So far it was talked about \fIwriting\fP or \fIbuilding\fP software.
534 Although these are just verbs, they do imply a specific view on the work process
535 they describe.
536 The better verb, however, is to \fIgrow\fP.
537 .PP
538 Creating software in the sense of the Unix Philosophy is an incremental process.
539 It starts with a first prototype, which evolves as requirements change.
540 A quickly hacked shell script might become a large, sophisticated,
541 compiled program this way.
542 Its lifetime begins with the initial prototype and ends when the software is not used anymore.
543 While being alive it will get extended, rearranged, rebuilt (from scratch).
544 Growing software matches the view that ``software is never finished. It is only released.''
545 .[
546 %O FIXME
547 %A Mike Gancarz
548 %T The UNIX Philosophy
549 %P 26
550 .]
551 .PP
552 Software can be seen as being controlled by evolutionary processes.
553 Successful software is software that is used by many for a long time.
554 This implies that the software is needed, useful, and better than alternatives.
555 Darwin talks about: ``The survival of the fittest.''
556 .[
557 %O FIXME
558 %A Charles Darwin
559 .]
560 Transferred to software: The most successful software, is the fittest,
561 is the one that survives.
562 (This may be at the level of one creature, or at the level of one species.)
563 The fitness of software is affected mainly by four properties:
564 portability of code, portability of data, range of usability, and reusability of parts.
565 .\" .IP \(bu
566 .\" portability of code
567 .\" .IP \(bu
568 .\" portability of data
569 .\" .IP \(bu
570 .\" range of usability
571 .\" .IP \(bu
572 .\" reuseability of parts
573 .PP
574 (1)
575 .I "Portability of code
576 means, using high-level programming languages,
577 sticking to the standard,
578 and avoiding optimizations that introduce dependencies on specific hardware.
579 Hardware has a much lower lifetime than software.
580 By chaining software to a specific hardware,
581 the software's lifetime gets shortened to that of this hardware.
582 In contrast, software should be easy to port \(en
583 adaption is the key to success.
584 .\" cf. practice of prog: ch08
585 .PP
586 (2)
587 .I "Portability of data
588 is best achieved by avoiding binary representations
589 to store data, because binary representations differ from machine to machine.
590 Textual represenation is favored.
591 Historically, ASCII was the charset of choice.
592 In the future, UTF-8 might be the better choice, however.
593 Important is that it is a plain text representation in a
594 very common charset encoding.
595 Apart from being able to transfer data between machines,
596 readable data has the great advantage, that humans are able
597 to directly edit it with text editors and other tools from the Unix toolchest.
598 .\" gancarz tenet 5
599 .PP
600 (3)
601 A large
602 .I "range of usability
603 ensures good adaption, and thus good survival.
604 It is a special distinction if a software becomes used in fields of action,
605 the original authors did never imagine.
606 Software that solves problems in a general way will likely be used
607 for all kinds of similar problems.
608 Being too specific limits the range of uses.
609 Requirements change through time, thus use cases change or even vanish.
610 A good example in this point is Allman's sendmail.
611 Allman identifies flexibility to be one major reason for sendmail's success:
612 .[
613 %O FIXME
614 %A Allman
615 %T sendmail
616 .]
617 .QP
618 Second, I limited myself to the routing function [...].
619 This was a departure from the dominant thought of the time, [...].
620 .QP
621 Third, the sendmail configuration file was flexible enough to adopt
622 to a rapidly changing world [...].
623 .LP
624 Successful software adopts itself to the changing world.
625 .PP
626 (4)
627 .I "Reuse of parts
628 is even one step further.
629 A software may completely lose its field of action,
630 but parts of which the software is build may be general and independent enough
631 to survive this death.
632 If software is build by combining small independent programs,
633 then there are parts readily available for reuse.
634 Who cares if the large program is a failure,
635 but parts of it become successful instead?
637 .NH 2
638 Summary
639 .LP
640 This chapter explained the central ideas of the Unix Philosophy.
641 For each of the ideas, it was exposed what advantages they introduce.
642 The Unix Philosophy are guidelines that help to write valuable software.
643 From the view point of a software developer or software designer,
644 the Unix Philosophy provides answers to many software design problem.
645 .PP
646 The various ideas of the Unix Philosophy are very interweaved
647 and can hardly be applied independently.
648 However, the probably most important messages are:
649 .I "``Do one thing well!''" ,
650 .I "``Keep it simple!''" ,
651 and
652 .I "``Use software leverage!''
656 .NH 1
657 Case study: nmh
659 .NH 2
660 History
661 .LP
662 MH, nmh.
663 They are old.
665 .NH 2
666 Contrasts to similar sw
667 .LP
668 vs. Thunderbird, mutt, mailx, pine
669 .LP
670 flexibility, no redundancy, use the shell
672 .NH 2
673 Gains of the design
674 .LP
676 .NH 2
677 Problems
678 .LP
682 .NH 1
683 Case study: uzbl
685 .NH 2
686 History
687 .LP
688 uzbl is young
690 .NH 2
691 Contrasts to similar sw
692 .LP
693 like with nmh
694 .LP
695 addons, plugins, modules
697 .NH 2
698 Gains of the design
699 .LP
701 .NH 2
702 Problems
703 .LP
704 broken web
708 .NH 1
709 Final thoughts
711 .NH 2
712 Quick summary
713 .LP
714 good design
715 .LP
716 unix phil
717 .LP
718 case studies
720 .NH 2
721 Why people should choose
722 .LP
723 Make the right choice!
725 .nr PI .5i
726 .rm ]<
727 .de ]<
728 .LP
729 .de FP
730 .IP \\\\$1.
731 \\..
732 .rm FS FE
733 ..
734 .SH
735 References
736 .[
737 $LIST$
738 .]
739 .wh -1p