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rewrote ch02
date Thu, 18 Feb 2010 11:24:36 +0100
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meillo@1 6 .\".nr PI 0
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meillo@0 10 .de CW
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meillo@0 19 ..
meillo@0 20 .ds [. \ [
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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@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-1
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@3 53 The process inbetween can be of any shape.
meillo@3 54 The the maintainance 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@3 62 This paper discusses the recommendations of the Unix Philosphy about software design.
meillo@3 63 .PP
meillo@3 64 The here discussed ideas can get applied by any development process.
meillo@3 65 The Unix Philosphy 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-1
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@1 110 especially those that were sucessful 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@6 152 but it will be of matter in usage and maintainance 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@6 156 Bug fixing, extending, rebuiling of parts \(en short: maintainance 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@6 174 %I International Organization for Standarization
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@6 191 (time behaviour, resource utilisation)
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@0 202
meillo@0 203
meillo@0 204 .NH 1
meillo@0 205 The Unix Philosophy
meillo@4 206 .LP
meillo@4 207 The origins of the Unix Philosophy were already introduced.
meillo@4 208 This chapter explains the philosophy and shows concrete examples of its application.
meillo@5 209
meillo@5 210 .SH
meillo@4 211 Examples
meillo@4 212 .LP
meillo@4 213 Following are some examples to demonstrate how applied Unix Philosophy feels like.
meillo@4 214 Knowledge of using the Unix shell is assumed.
meillo@4 215 .PP
meillo@4 216 Counting the number of files in the current directory:
meillo@4 217 .DS
meillo@4 218 .CW
meillo@4 219 ls | wc -l
meillo@4 220 .DE
meillo@4 221 The
meillo@4 222 .CW ls
meillo@4 223 command lists all files in the current directory, one per line,
meillo@4 224 and
meillo@4 225 .CW "wc -l
meillo@4 226 counts how many lines they are.
meillo@4 227 .PP
meillo@4 228 Counting all files that do not contain ``foo'' in their name:
meillo@4 229 .DS
meillo@4 230 .CW
meillo@4 231 ls | grep -v foo | wc -l
meillo@4 232 .DE
meillo@4 233 Here, the list of files is filtered by
meillo@4 234 .CW grep
meillo@4 235 to remove all that contain ``foo''.
meillo@4 236 The rest is the same as in the previous example.
meillo@4 237 .PP
meillo@4 238 Finding the five largest entries in the current directory.
meillo@4 239 .DS
meillo@4 240 .CW
meillo@4 241 du -s * | sort -nr | sed 5q
meillo@4 242 .DE
meillo@4 243 .CW "du -s *
meillo@4 244 returns the recursively summed sizes of all files
meillo@4 245 -- no matter if they are regular files or directories.
meillo@4 246 .CW "sort -nr
meillo@4 247 sorts the list numerically in reverse order.
meillo@4 248 Finally,
meillo@4 249 .CW "sed 5q
meillo@4 250 quits after it has printed the fifth line.
meillo@4 251 .PP
meillo@4 252 The presented command lines are examples of what Unix people would use
meillo@4 253 to get the desired output.
meillo@4 254 There are also other ways to get the same output.
meillo@4 255 It's a user's decision which way to go.
meillo@5 256
meillo@5 257 .SH
meillo@4 258 Pipes
meillo@4 259 .LP
meillo@4 260 The examples show that a lot of tasks on a Unix system
meillo@4 261 are accomplished by combining several small programs.
meillo@4 262 The connection between the single programs is denoted by the pipe operator `|'.
meillo@4 263 .PP
meillo@4 264 Pipes, and their extensive and easy use, are one of the great
meillo@4 265 achievements of the Unix system.
meillo@4 266 Pipes between programs have been possible in earlier operating systems,
meillo@4 267 but it has never been a so central part of the concept.
meillo@4 268 When, in the early seventies, Doug McIlroy introduced pipes for the
meillo@4 269 Unix system,
meillo@4 270 ``it was this concept and notation for linking several programs together
meillo@4 271 that transformed Unix from a basic file-sharing system to an entirely new way of computing.''
meillo@4 272 .[
meillo@4 273 %T Unix: An Oral History
meillo@5 274 %O .CW \s-1
meillo@4 275 .]
meillo@4 276 .PP
meillo@4 277 Being able to specify pipelines in an easy way is,
meillo@4 278 however, not enough by itself.
meillo@5 279 It is only one half.
meillo@4 280 The other is the design of the programs that are used in the pipeline.
meillo@5 281 They have to be of an external shape that allows them to be be used in such a way.
meillo@5 282
meillo@5 283 .SH
meillo@5 284 Interface architecture
meillo@5 285 .LP
meillo@5 286 Unix is, first of all, simple: Everything is a file.
meillo@5 287 Files are sequences of bytes, without any special structure.
meillo@5 288 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin)
meillo@5 289 and write a stream of bytes to ``standard output'' (stdout).
meillo@5 290 .PP
meillo@5 291 If our files \fIare\fP sequences of bytes,
meillo@5 292 and our programs \fIare\fP filters on byte streams,
meillo@5 293 then there is exactly one standardized interface.
meillo@5 294 Thus it is possible to combine them in any desired way.
meillo@5 295 .PP
meillo@5 296 Even a handful of small programs will yield a large set of combinations,
meillo@5 297 and thus a large set of different functions.
meillo@5 298 This is leverage!
meillo@5 299 .PP
meillo@5 300 If the programs are orthogonal to each other \(en the best case \(en
meillo@5 301 then the set of different functions is greatest.
meillo@5 302 .PP
meillo@5 303 Now, the Unix toolchest is a set of small programs that
meillo@5 304 are filters on byte streams.
meillo@5 305 They are to a large extend unrelated in their function.
meillo@5 306 Hence, the Unix toolchest provides a large set of functions
meillo@5 307 that can be accessed by combining the programs in the desired way.
meillo@5 308
meillo@5 309 .SH
meillo@5 310 Advantages of toolchests
meillo@5 311 .LP
meillo@5 312 A toolchest is a set of tools.
meillo@5 313 Instead of having one big tool for all tasks, one has many small tools,
meillo@5 314 each for one task.
meillo@5 315 Difficult tasks are solved by combining several of the small, simple tools.
meillo@5 316 .PP
meillo@5 317 It is easier and less error-prone to write small programs.
meillo@5 318 It is also easier and less error-prone to write a large set of small programs,
meillo@5 319 than to write one large program with all the functionality included.
meillo@5 320 If the small programs are combinable, then they offer even a larger set
meillo@5 321 of functions than the single large program.
meillo@5 322 Hence, one gets two advantages out of writing small, combinable programs.
meillo@5 323 .PP
meillo@5 324 There are two drawbacks of the toolchest approach.
meillo@5 325 First, one simple, standardized, unidirectional Interface has to be sufficient.
meillo@5 326 If one feels the need for more ``logic'' than a stream of bytes,
meillo@5 327 then a different approach might be of need, or, more likely,
meillo@5 328 he just did not came to a design where a stream of bytes is sufficient.
meillo@5 329 The other drawback of a toolchest affects the users.
meillo@5 330 A toolchest is often more difficult to use for novices.
meillo@5 331 It is neccessary to become familiar with each of the tools,
meillo@5 332 to be able to use the right one in a given situation.
meillo@5 333 Additinally, one needs to combine the tools in a senseful way on its own.
meillo@5 334 This is like a sharp knive \(en it is a powerful tool in the hand of a master,
meillo@5 335 but of no good value in the hand of an unskilled.
meillo@5 336 .PP
meillo@5 337 Luckily, the second drawback can be solved easily by adding wrappers around the single tools.
meillo@5 338 Novice users do not need to learn several tools if a professional wraps
meillo@5 339 the single commands into a single script.
meillo@5 340 Note that the wrapper script still calls the small tools;
meillo@5 341 the wrapper script is just like a skin around.
meillo@5 342 No complexity is added this way.
meillo@5 343 .PP
meillo@5 344 A wrapper script for finding the five largest entries in the current directory
meillo@5 345 could look like this:
meillo@5 346 .DS
meillo@5 347 .CW
meillo@5 348 #!/bin/sh
meillo@5 349 du -s * | sort -nr | sed 5q
meillo@5 350 .DE
meillo@5 351 The script itself is just a text file that calls the command line
meillo@5 352 a professional user would type in directly.
meillo@5 353
meillo@5 354
meillo@5 355
meillo@0 356
meillo@4 357
meillo@0 358
meillo@0 359 .NH 2
meillo@5 360 foo
meillo@0 361 .LP
meillo@0 362 standalone vs. tool chain
meillo@0 363 .LP
meillo@0 364 software leverage
meillo@0 365 .LP
meillo@0 366 possiblities
meillo@0 367
meillo@0 368 .NH 2
meillo@0 369 Results
meillo@0 370 .LP
meillo@0 371 The unix phil is an answer to the sw design question
meillo@0 372 .LP
meillo@0 373 tool chains empower the uses of sw
meillo@0 374
meillo@0 375 .NH 1
meillo@0 376 Case study: nmh
meillo@0 377
meillo@0 378 .NH 2
meillo@0 379 History
meillo@0 380 .LP
meillo@0 381 MH, nmh.
meillo@0 382 They are old.
meillo@0 383
meillo@0 384 .NH 2
meillo@0 385 Contrasts to similar sw
meillo@0 386 .LP
meillo@0 387 vs. Thunderbird, mutt, mailx, pine
meillo@0 388 .LP
meillo@0 389 flexibility, no redundancy, use the shell
meillo@0 390
meillo@0 391 .NH 2
meillo@0 392 Gains of the design
meillo@0 393 .LP
meillo@0 394
meillo@0 395 .NH 2
meillo@0 396 Problems
meillo@0 397 .LP
meillo@0 398
meillo@0 399 .NH 1
meillo@0 400 Case study: uzbl
meillo@0 401
meillo@0 402 .NH 2
meillo@0 403 History
meillo@0 404 .LP
meillo@0 405 uzbl is young
meillo@0 406
meillo@0 407 .NH 2
meillo@0 408 Contrasts to similar sw
meillo@0 409 .LP
meillo@0 410 like with nmh
meillo@0 411 .LP
meillo@0 412 addons, plugins, modules
meillo@0 413
meillo@0 414 .NH 2
meillo@0 415 Gains of the design
meillo@0 416 .LP
meillo@0 417
meillo@0 418 .NH 2
meillo@0 419 Problems
meillo@0 420 .LP
meillo@0 421 broken web
meillo@0 422
meillo@0 423 .NH 1
meillo@0 424 Final thoughts
meillo@0 425
meillo@0 426 .NH 2
meillo@0 427 Quick summary
meillo@0 428 .LP
meillo@0 429 good design
meillo@0 430 .LP
meillo@0 431 unix phil
meillo@0 432 .LP
meillo@0 433 case studies
meillo@0 434
meillo@0 435 .NH 2
meillo@0 436 Why people should choose
meillo@0 437 .LP
meillo@0 438 Make the right choice!
meillo@0 439
meillo@0 440 .nr PI .5i
meillo@0 441 .rm ]<
meillo@0 442 .de ]<
meillo@0 443 .LP
meillo@0 444 .de FP
meillo@0 445 .IP \\\\$1.
meillo@0 446 \\..
meillo@0 447 .rm FS FE
meillo@0 448 ..
meillo@0 449 .SH
meillo@0 450 References
meillo@0 451 .[
meillo@0 452 $LIST$
meillo@0 453 .]
meillo@0 454 .wh -1p