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author meillo@marmaro.de
date Fri, 19 Feb 2010 23:11:09 +0100
parents 924b2ac2d477
children 355ed69a34a8
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meillo@2 1 .\".if n .pl 1000i
meillo@0 2 .de XX
meillo@0 3 .pl 1v
meillo@0 4 ..
meillo@0 5 .em XX
meillo@1 6 .\".nr PI 0
meillo@1 7 .\".if t .nr PD .5v
meillo@1 8 .\".if n .nr PD 1v
meillo@0 9 .nr lu 0
meillo@0 10 .de CW
meillo@0 11 .nr PQ \\n(.f
meillo@0 12 .if t .ft CW
meillo@0 13 .ie \\$1 .if n .ul 999
meillo@0 14 .el .if n .ul 1
meillo@0 15 .if t .if !\\$1 \&\\$1\f\\n(PQ\\$2
meillo@0 16 .if n .if \\n(.$=1 \&\\$1
meillo@0 17 .if n .if \\n(.$>1 \&\\$1\c
meillo@0 18 .if n .if \\n(.$>1 \&\\$2
meillo@0 19 ..
meillo@0 20 .ds [. \ [
meillo@0 21 .ds .] ]
meillo@1 22 .\"----------------------------------------
meillo@0 23 .TL
meillo@6 24 Why the Unix Philosophy still matters
meillo@0 25 .AU
meillo@0 26 markus schnalke <meillo@marmaro.de>
meillo@0 27 .AB
meillo@1 28 .ti \n(.iu
meillo@2 29 This paper discusses the importance of the Unix Philosophy in software design.
meillo@0 30 Today, few software designers are aware of these concepts,
meillo@3 31 and thus most modern software is limited and does not make use of software leverage.
meillo@0 32 Knowing and following the tenets of the Unix Philosophy makes software more valuable.
meillo@0 33 .AE
meillo@0 34
meillo@2 35 .if t .2C
meillo@2 36
meillo@2 37 .FS
meillo@2 38 .ps -1
meillo@2 39 This paper was prepared for the seminar ``Software Analysis'' at University Ulm.
meillo@2 40 Mentor was professor Schweiggert. 2010-02-05
meillo@2 41 .br
meillo@2 42 You may get this document from my website
meillo@2 43 .CW \s-1http://marmaro.de/docs
meillo@2 44 .FE
meillo@2 45
meillo@0 46 .NH 1
meillo@0 47 Introduction
meillo@0 48 .LP
meillo@0 49 Building a software is a process from an idea of the purpose of the software
meillo@3 50 to its release.
meillo@0 51 No matter \fIhow\fP the process is run, two things are common:
meillo@0 52 the initial idea and the release.
meillo@9 53 The process in between can be of any shape.
meillo@9 54 The the maintenance work after the release is ignored for the moment.
meillo@1 55 .PP
meillo@0 56 The process of building splits mainly in two parts:
meillo@0 57 the planning of what and how to build, and implementing the plan by writing code.
meillo@3 58 This paper focuses on the planning part \(en the designing of the software.
meillo@3 59 .PP
meillo@3 60 Software design is the plan of how the internals and externals of the software should look like,
meillo@3 61 based on the requirements.
meillo@9 62 This paper discusses the recommendations of the Unix Philosophy about software design.
meillo@3 63 .PP
meillo@3 64 The here discussed ideas can get applied by any development process.
meillo@9 65 The Unix Philosophy does recommend how the software development process should look like,
meillo@3 66 but this shall not be of matter here.
meillo@0 67 Similar, the question of how to write the code is out of focus.
meillo@1 68 .PP
meillo@3 69 The name ``Unix Philosophy'' was already mentioned several times, but it was not explained yet.
meillo@1 70 The Unix Philosophy is the essence of how the Unix operating system and its toolchest was designed.
meillo@3 71 It is no limited set of rules, but what people see to be common to typical Unix software.
meillo@1 72 Several people stated their view on the Unix Philosophy.
meillo@1 73 Best known are:
meillo@1 74 .IP \(bu
meillo@1 75 Doug McIlroy's summary: ``Write programs that do one thing and do it well.''
meillo@1 76 .[
meillo@1 77 %A M. D. McIlroy
meillo@1 78 %A E. N. Pinson
meillo@1 79 %A B. A. Taque
meillo@1 80 %T UNIX Time-Sharing System Forward
meillo@1 81 %J The Bell System Technical Journal
meillo@1 82 %D 1978
meillo@1 83 %V 57
meillo@1 84 %N 6
meillo@1 85 %P 1902
meillo@1 86 .]
meillo@1 87 .IP \(bu
meillo@1 88 Mike Gancarz' book ``The UNIX Philosophy''.
meillo@1 89 .[
meillo@1 90 %A Mike Gancarz
meillo@1 91 %T The UNIX Philosophy
meillo@1 92 %D 1995
meillo@1 93 %I Digital Press
meillo@1 94 .]
meillo@1 95 .IP \(bu
meillo@1 96 Eric S. Raymond's book ``The Art of UNIX Programming''.
meillo@1 97 .[
meillo@1 98 %A Eric S. Raymond
meillo@1 99 %T The Art of UNIX Programming
meillo@1 100 %D 2003
meillo@1 101 %I Addison-Wesley
meillo@2 102 %O .CW \s-1http://www.faqs.org/docs/artu/
meillo@1 103 .]
meillo@0 104 .LP
meillo@1 105 These different views on the Unix Philosophy have much in common.
meillo@3 106 Especially, the main concepts are similar for all of them.
meillo@1 107 But there are also points on which they differ.
meillo@1 108 This only underlines what the Unix Philosophy is:
meillo@1 109 A retrospective view on the main concepts of Unix software;
meillo@9 110 especially those that were successful and unique to Unix.
meillo@6 111 .\" really?
meillo@1 112 .PP
meillo@1 113 Before we will have a look at concrete concepts,
meillo@1 114 we discuss why software design is important
meillo@1 115 and what problems bad design introduces.
meillo@0 116
meillo@0 117
meillo@0 118 .NH 1
meillo@6 119 Importance of software design in general
meillo@0 120 .LP
meillo@2 121 Why should we design software at all?
meillo@6 122 It is general knowledge, that even a bad plan is better than no plan.
meillo@6 123 Ignoring software design is programming without a plan.
meillo@6 124 This will lead pretty sure to horrible results.
meillo@2 125 .PP
meillo@6 126 The design of a software is its internal and external shape.
meillo@6 127 The design talked about here has nothing to do with visual appearance.
meillo@6 128 If we see a program as a car, then its color is of no matter.
meillo@6 129 Its design would be the car's size, its shape, the number and position of doors,
meillo@6 130 the ratio of passenger and cargo transport, and so forth.
meillo@2 131 .PP
meillo@6 132 A software's design is about quality properties.
meillo@6 133 Each of the cars may be able to drive from A to B,
meillo@6 134 but it depends on its properties whether it is a good car for passenger transport or not.
meillo@6 135 It also depends on its properties if it is a good choice for a rough mountain area.
meillo@2 136 .PP
meillo@6 137 Requirements to a software are twofold: functional and non-functional.
meillo@6 138 Functional requirements are easier to define and to verify.
meillo@6 139 They are directly the software's functions.
meillo@6 140 Functional requirements are the reason why software gets written.
meillo@6 141 Someone has a problem and needs a tool to solve it.
meillo@6 142 Being able to solve the problem is the main functional requirement.
meillo@6 143 It is the driving force behind all programming effort.
meillo@2 144 .PP
meillo@6 145 On the other hand, there are also non-functional requirements.
meillo@6 146 They are called \fIquality\fP requirements, too.
meillo@6 147 The quality of a software is about properties that are not directly related to
meillo@6 148 the software's basic functions.
meillo@6 149 Quality aspects are about the properties that are overlooked at first sight.
meillo@2 150 .PP
meillo@6 151 Quality is of few matter when the software gets initially built,
meillo@9 152 but it will be of matter in usage and maintenance of the software.
meillo@6 153 A short-sighted might see in developing a software mainly building something up.
meillo@6 154 Reality shows, that building the software the first time is only a small amount
meillo@6 155 of the overall work.
meillo@9 156 Bug fixing, extending, rebuilding of parts \(en short: maintenance work \(en
meillo@6 157 does soon take over the major part of the time spent on a software.
meillo@6 158 Not to forget the usage of the software.
meillo@6 159 These processes are highly influenced by the software's quality.
meillo@6 160 Thus, quality should never be neglected.
meillo@6 161 The problem is that you hardly ``stumble over'' bad quality during the first build,
meillo@6 162 but this is the time when you should care about good quality most.
meillo@6 163 .PP
meillo@6 164 Software design is not about the basic function of a software;
meillo@6 165 this requirement will get satisfied anyway, as it is the main driving force behind the development.
meillo@6 166 Software design is about quality aspects of the software.
meillo@6 167 Good design will lead to good quality, bad design to bad quality.
meillo@6 168 The primary functions of the software will be affected modestly by bad quality,
meillo@6 169 but good quality can provide a lot of additional gain from the software,
meillo@6 170 even at places where one never expected it.
meillo@6 171 .PP
meillo@6 172 The ISO/IEC 9126-1 standard, part 1,
meillo@6 173 .[
meillo@9 174 %I International Organization for Standardization
meillo@6 175 %T ISO Standard 9126: Software Engineering \(en Product Quality, part 1
meillo@6 176 %C Geneve
meillo@6 177 %D 2001
meillo@6 178 .]
meillo@6 179 defines the quality model as consisting out of:
meillo@6 180 .IP \(bu
meillo@6 181 .I Functionality
meillo@6 182 (suitability, accuracy, inter\%operability, security)
meillo@6 183 .IP \(bu
meillo@6 184 .I Reliability
meillo@6 185 (maturity, fault tolerance, recoverability)
meillo@6 186 .IP \(bu
meillo@6 187 .I Usability
meillo@6 188 (understandability, learnability, operability, attractiveness)
meillo@6 189 .IP \(bu
meillo@6 190 .I Efficiency
meillo@9 191 (time behavior, resource utilization)
meillo@6 192 .IP \(bu
meillo@6 193 .I Maintainability
meillo@6 194 (analysability, changeability, stability, testability)
meillo@6 195 .IP \(bu
meillo@6 196 .I Portability
meillo@6 197 (adaptability, installability, co-existence, replaceability)
meillo@6 198 .LP
meillo@6 199 These goals are parts of a software's design.
meillo@6 200 Good design can give these properties to a software,
meillo@6 201 bad designed software will miss them.
meillo@7 202 .PP
meillo@7 203 One further goal of software design is consistency.
meillo@7 204 Consistency eases understanding, working on, and using things.
meillo@7 205 Consistent internals and consistent interfaces to the outside can be provided by good design.
meillo@7 206 .PP
meillo@7 207 We should design software because good design avoids many problems during a software's lifetime.
meillo@7 208 And we should design software because good design can offer much gain,
meillo@7 209 that can be unrelated to the software main intend.
meillo@7 210 Indeed, we should spend much effort into good design to make the software more valuable.
meillo@7 211 The Unix Philosophy shows how to design software well.
meillo@7 212 It offers guidelines to achieve good quality and high gain for the effort spent.
meillo@0 213
meillo@0 214
meillo@0 215 .NH 1
meillo@0 216 The Unix Philosophy
meillo@4 217 .LP
meillo@4 218 The origins of the Unix Philosophy were already introduced.
meillo@8 219 This chapter explains the philosophy, oriented on Gancarz,
meillo@8 220 and shows concrete examples of its application.
meillo@5 221
meillo@5 222 .SH
meillo@4 223 Examples
meillo@4 224 .LP
meillo@4 225 Following are some examples to demonstrate how applied Unix Philosophy feels like.
meillo@4 226 Knowledge of using the Unix shell is assumed.
meillo@4 227 .PP
meillo@4 228 Counting the number of files in the current directory:
meillo@9 229 .DS I 2n
meillo@4 230 .CW
meillo@9 231 .ps -1
meillo@4 232 ls | wc -l
meillo@4 233 .DE
meillo@4 234 The
meillo@4 235 .CW ls
meillo@4 236 command lists all files in the current directory, one per line,
meillo@4 237 and
meillo@4 238 .CW "wc -l
meillo@8 239 counts the number of lines.
meillo@4 240 .PP
meillo@8 241 Counting the number of files that do not contain ``foo'' in their name:
meillo@9 242 .DS I 2n
meillo@4 243 .CW
meillo@9 244 .ps -1
meillo@4 245 ls | grep -v foo | wc -l
meillo@4 246 .DE
meillo@4 247 Here, the list of files is filtered by
meillo@4 248 .CW grep
meillo@4 249 to remove all that contain ``foo''.
meillo@4 250 The rest is the same as in the previous example.
meillo@4 251 .PP
meillo@4 252 Finding the five largest entries in the current directory.
meillo@9 253 .DS I 2n
meillo@4 254 .CW
meillo@9 255 .ps -1
meillo@4 256 du -s * | sort -nr | sed 5q
meillo@4 257 .DE
meillo@4 258 .CW "du -s *
meillo@4 259 returns the recursively summed sizes of all files
meillo@8 260 \(en no matter if they are regular files or directories.
meillo@4 261 .CW "sort -nr
meillo@4 262 sorts the list numerically in reverse order.
meillo@4 263 Finally,
meillo@4 264 .CW "sed 5q
meillo@4 265 quits after it has printed the fifth line.
meillo@4 266 .PP
meillo@4 267 The presented command lines are examples of what Unix people would use
meillo@4 268 to get the desired output.
meillo@4 269 There are also other ways to get the same output.
meillo@4 270 It's a user's decision which way to go.
meillo@5 271
meillo@5 272 .SH
meillo@4 273 Pipes
meillo@4 274 .LP
meillo@8 275 The examples show that many tasks on a Unix system
meillo@4 276 are accomplished by combining several small programs.
meillo@4 277 The connection between the single programs is denoted by the pipe operator `|'.
meillo@4 278 .PP
meillo@4 279 Pipes, and their extensive and easy use, are one of the great
meillo@4 280 achievements of the Unix system.
meillo@4 281 Pipes between programs have been possible in earlier operating systems,
meillo@4 282 but it has never been a so central part of the concept.
meillo@4 283 When, in the early seventies, Doug McIlroy introduced pipes for the
meillo@4 284 Unix system,
meillo@4 285 ``it was this concept and notation for linking several programs together
meillo@4 286 that transformed Unix from a basic file-sharing system to an entirely new way of computing.''
meillo@4 287 .[
meillo@4 288 %T Unix: An Oral History
meillo@5 289 %O .CW \s-1http://www.princeton.edu/~hos/frs122/unixhist/finalhis.htm
meillo@4 290 .]
meillo@4 291 .PP
meillo@4 292 Being able to specify pipelines in an easy way is,
meillo@4 293 however, not enough by itself.
meillo@5 294 It is only one half.
meillo@4 295 The other is the design of the programs that are used in the pipeline.
meillo@8 296 They have to interfaces that allows them to be used in such a way.
meillo@5 297
meillo@5 298 .SH
meillo@5 299 Interface architecture
meillo@5 300 .LP
meillo@5 301 Unix is, first of all, simple: Everything is a file.
meillo@5 302 Files are sequences of bytes, without any special structure.
meillo@5 303 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin)
meillo@5 304 and write a stream of bytes to ``standard output'' (stdout).
meillo@5 305 .PP
meillo@8 306 If the files \fIare\fP sequences of bytes,
meillo@8 307 and the programs \fIare\fP filters on byte streams,
meillo@5 308 then there is exactly one standardized interface.
meillo@5 309 Thus it is possible to combine them in any desired way.
meillo@5 310 .PP
meillo@5 311 Even a handful of small programs will yield a large set of combinations,
meillo@5 312 and thus a large set of different functions.
meillo@5 313 This is leverage!
meillo@5 314 If the programs are orthogonal to each other \(en the best case \(en
meillo@5 315 then the set of different functions is greatest.
meillo@5 316 .PP
meillo@8 317 The Unix toolchest \fIis\fP a set of small programs that
meillo@5 318 are filters on byte streams.
meillo@8 319 They are, to a large extend, unrelated in their function.
meillo@5 320 Hence, the Unix toolchest provides a large set of functions
meillo@5 321 that can be accessed by combining the programs in the desired way.
meillo@5 322
meillo@5 323 .SH
meillo@8 324 The toolchest approach
meillo@5 325 .LP
meillo@5 326 A toolchest is a set of tools.
meillo@5 327 Instead of having one big tool for all tasks, one has many small tools,
meillo@5 328 each for one task.
meillo@5 329 Difficult tasks are solved by combining several of the small, simple tools.
meillo@5 330 .PP
meillo@5 331 It is easier and less error-prone to write small programs.
meillo@5 332 It is also easier and less error-prone to write a large set of small programs,
meillo@5 333 than to write one large program with all the functionality included.
meillo@5 334 If the small programs are combinable, then they offer even a larger set
meillo@5 335 of functions than the single large program.
meillo@5 336 Hence, one gets two advantages out of writing small, combinable programs.
meillo@5 337 .PP
meillo@5 338 There are two drawbacks of the toolchest approach.
meillo@8 339 First, one simple, standardized, unidirectional interface has to be sufficient.
meillo@5 340 If one feels the need for more ``logic'' than a stream of bytes,
meillo@8 341 then a different approach might be of need.
meillo@9 342 But it is also possible, that he just can not imaging a design where
meillo@8 343 a stream of bytes is sufficient.
meillo@8 344 By becoming more familiar with the ``Unix style of thinking'',
meillo@8 345 developers will more often and easier find simple designs where
meillo@8 346 a stream of bytes is a sufficient interface.
meillo@8 347 .PP
meillo@8 348 The second drawback of a toolchest affects the users.
meillo@5 349 A toolchest is often more difficult to use for novices.
meillo@9 350 It is necessary to become familiar with each of the tools,
meillo@5 351 to be able to use the right one in a given situation.
meillo@9 352 Additionally, one needs to combine the tools in a senseful way on its own.
meillo@9 353 This is like a sharp knife \(en it is a powerful tool in the hand of a master,
meillo@5 354 but of no good value in the hand of an unskilled.
meillo@5 355 .PP
meillo@8 356 However, learning single, small tool of the toolchest is easier than
meillo@8 357 learning a complex tool.
meillo@8 358 The user will have a basic understanding of a yet unknown tool,
meillo@8 359 if the several tools of the toolchest have a common style.
meillo@8 360 He will be able to transfer knowledge over one tool to another.
meillo@8 361 .PP
meillo@8 362 Moreover, the second drawback can be removed easily by adding wrappers
meillo@8 363 around the single tools.
meillo@5 364 Novice users do not need to learn several tools if a professional wraps
meillo@8 365 the single commands into a more high-level script.
meillo@5 366 Note that the wrapper script still calls the small tools;
meillo@5 367 the wrapper script is just like a skin around.
meillo@8 368 No complexity is added this way,
meillo@8 369 but new programs can get created out of existing one with very low effort.
meillo@5 370 .PP
meillo@5 371 A wrapper script for finding the five largest entries in the current directory
meillo@5 372 could look like this:
meillo@9 373 .DS I 2n
meillo@5 374 .CW
meillo@9 375 .ps -1
meillo@5 376 #!/bin/sh
meillo@5 377 du -s * | sort -nr | sed 5q
meillo@5 378 .DE
meillo@5 379 The script itself is just a text file that calls the command line
meillo@5 380 a professional user would type in directly.
meillo@8 381 Making the program flexible on the number of entries it prints,
meillo@8 382 is easily possible:
meillo@9 383 .DS I 2n
meillo@8 384 .CW
meillo@9 385 .ps -1
meillo@8 386 #!/bin/sh
meillo@8 387 num=5
meillo@8 388 [ $# -eq 1 ] && num="$1"
meillo@8 389 du -sh * | sort -nr | sed "${num}q"
meillo@8 390 .DE
meillo@8 391 This script acts like the one before, when called without an argument.
meillo@8 392 But one can also specify a numerical argument to define the number of lines to print.
meillo@5 393
meillo@8 394 .SH
meillo@8 395 A powerful shell
meillo@8 396 .LP
meillo@8 397 The Unix shell provides the possibility to combine small programs into
meillo@8 398 large ones easily.
meillo@8 399 But the powerful shell is great feature in other ways, too.
meillo@8 400 .PP
meillo@8 401 It encourages rapid prototyping.
meillo@8 402 It includes a scripting language with built in control statements.
meillo@8 403 The functions, however, are the normal programs, everyone can use on the system.
meillo@8 404 Thus, the programs are known and learning to program in the shell is easy.
meillo@8 405 Using normal programs as functions in the shell programming language
meillo@8 406 is only possible because they are small, combinable tools in a toolchest style.
meillo@8 407 .PP
meillo@8 408 The Unix shell encourages to write small scripts out of other programs,
meillo@8 409 because it is so easy to do.
meillo@8 410 This is a great step towards automation.
meillo@8 411 It is wonderful if the effort to automate a task equals the effort
meillo@8 412 it takes to do it the second time by hand.
meillo@8 413 If it is so, then the user will be happy to automate everything he does more than once.
meillo@8 414 .PP
meillo@8 415 Small programs that do one job well, standardized interfaces between them,
meillo@8 416 a mechanism to combine parts to larger parts, and an easy way to automate tasks,
meillo@8 417 this will inevitably produce software leverage.
meillo@8 418 Getting multiple times the benefit of an investment is a great offer.
meillo@5 419
meillo@5 420
meillo@0 421
meillo@4 422
meillo@0 423
meillo@0 424 .NH 2
meillo@0 425 Results
meillo@0 426 .LP
meillo@0 427 The unix phil is an answer to the sw design question
meillo@0 428 .LP
meillo@0 429 tool chains empower the uses of sw
meillo@0 430
meillo@8 431
meillo@8 432
meillo@0 433 .NH 1
meillo@0 434 Case study: nmh
meillo@0 435
meillo@0 436 .NH 2
meillo@0 437 History
meillo@0 438 .LP
meillo@0 439 MH, nmh.
meillo@0 440 They are old.
meillo@0 441
meillo@0 442 .NH 2
meillo@0 443 Contrasts to similar sw
meillo@0 444 .LP
meillo@0 445 vs. Thunderbird, mutt, mailx, pine
meillo@0 446 .LP
meillo@0 447 flexibility, no redundancy, use the shell
meillo@0 448
meillo@0 449 .NH 2
meillo@0 450 Gains of the design
meillo@0 451 .LP
meillo@0 452
meillo@0 453 .NH 2
meillo@0 454 Problems
meillo@0 455 .LP
meillo@0 456
meillo@8 457
meillo@8 458
meillo@0 459 .NH 1
meillo@0 460 Case study: uzbl
meillo@0 461
meillo@0 462 .NH 2
meillo@0 463 History
meillo@0 464 .LP
meillo@0 465 uzbl is young
meillo@0 466
meillo@0 467 .NH 2
meillo@0 468 Contrasts to similar sw
meillo@0 469 .LP
meillo@0 470 like with nmh
meillo@0 471 .LP
meillo@0 472 addons, plugins, modules
meillo@0 473
meillo@0 474 .NH 2
meillo@0 475 Gains of the design
meillo@0 476 .LP
meillo@0 477
meillo@0 478 .NH 2
meillo@0 479 Problems
meillo@0 480 .LP
meillo@0 481 broken web
meillo@0 482
meillo@8 483
meillo@8 484
meillo@0 485 .NH 1
meillo@0 486 Final thoughts
meillo@0 487
meillo@0 488 .NH 2
meillo@0 489 Quick summary
meillo@0 490 .LP
meillo@0 491 good design
meillo@0 492 .LP
meillo@0 493 unix phil
meillo@0 494 .LP
meillo@0 495 case studies
meillo@0 496
meillo@0 497 .NH 2
meillo@0 498 Why people should choose
meillo@0 499 .LP
meillo@0 500 Make the right choice!
meillo@0 501
meillo@0 502 .nr PI .5i
meillo@0 503 .rm ]<
meillo@0 504 .de ]<
meillo@0 505 .LP
meillo@0 506 .de FP
meillo@0 507 .IP \\\\$1.
meillo@0 508 \\..
meillo@0 509 .rm FS FE
meillo@0 510 ..
meillo@0 511 .SH
meillo@0 512 References
meillo@0 513 .[
meillo@0 514 $LIST$
meillo@0 515 .]
meillo@0 516 .wh -1p