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| author | meillo@marmaro.de |
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| date | Fri, 19 Feb 2010 12:31:56 +0100 |
| parents | 606f0ffca834 |
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1 .\".if n .pl 1000i |
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| 1 | 22 .\"---------------------------------------- |
| 0 | 23 .TL |
| 6 | 24 Why the Unix Philosophy still matters |
| 0 | 25 .AU |
| 26 markus schnalke <meillo@marmaro.de> | |
| 27 .AB | |
| 1 | 28 .ti \n(.iu |
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29 This paper discusses the importance of the Unix Philosophy in software design. |
| 0 | 30 Today, few software designers are aware of these concepts, |
| 3 | 31 and thus most modern software is limited and does not make use of software leverage. |
| 0 | 32 Knowing and following the tenets of the Unix Philosophy makes software more valuable. |
| 33 .AE | |
| 34 | |
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35 .if t .2C |
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37 .FS |
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38 .ps -1 |
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39 This paper was prepared for the seminar ``Software Analysis'' at University Ulm. |
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40 Mentor was professor Schweiggert. 2010-02-05 |
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41 .br |
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42 You may get this document from my website |
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43 .CW \s-1http://marmaro.de/docs |
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44 .FE |
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45 |
| 0 | 46 .NH 1 |
| 47 Introduction | |
| 48 .LP | |
| 49 Building a software is a process from an idea of the purpose of the software | |
| 3 | 50 to its release. |
| 0 | 51 No matter \fIhow\fP the process is run, two things are common: |
| 52 the initial idea and the release. | |
| 3 | 53 The process inbetween can be of any shape. |
| 54 The the maintainance work after the release is ignored for the moment. | |
| 1 | 55 .PP |
| 0 | 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. | |
| 3 | 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 Philosphy about software design. | |
| 63 .PP | |
| 64 The here discussed ideas can get applied by any development process. | |
| 65 The Unix Philosphy does recommend how the software development process should look like, | |
| 66 but this shall not be of matter here. | |
| 0 | 67 Similar, the question of how to write the code is out of focus. |
| 1 | 68 .PP |
| 3 | 69 The name ``Unix Philosophy'' was already mentioned several times, but it was not explained yet. |
| 1 | 70 The Unix Philosophy is the essence of how the Unix operating system and its toolchest was designed. |
| 3 | 71 It is no limited set of rules, but what people see to be common to typical Unix software. |
| 1 | 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 | |
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102 %O .CW \s-1http://www.faqs.org/docs/artu/ |
| 1 | 103 .] |
| 0 | 104 .LP |
| 1 | 105 These different views on the Unix Philosophy have much in common. |
| 3 | 106 Especially, the main concepts are similar for all of them. |
| 1 | 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 sucessful and unique to Unix. | |
| 6 | 111 .\" really? |
| 1 | 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. | |
| 0 | 116 |
| 117 | |
| 118 .NH 1 | |
| 6 | 119 Importance of software design in general |
| 0 | 120 .LP |
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121 Why should we design software at all? |
| 6 | 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. | |
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131 .PP |
| 6 | 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. | |
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150 .PP |
| 6 | 151 Quality is of few matter when the software gets initially built, |
| 152 but it will be of matter in usage and maintainance 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, rebuiling of parts \(en short: maintainance 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. | |
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171 .PP |
| 6 | 172 The ISO/IEC 9126-1 standard, part 1, |
| 173 .[ | |
| 174 %I International Organization for Standarization | |
| 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 behaviour, resource utilisation) | |
| 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. | |
| 7 | 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. | |
| 0 | 213 |
| 214 | |
| 215 .NH 1 | |
| 216 The Unix Philosophy | |
| 4 | 217 .LP |
| 218 The origins of the Unix Philosophy were already introduced. | |
| 8 | 219 This chapter explains the philosophy, oriented on Gancarz, |
| 220 and shows concrete examples of its application. | |
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222 .SH |
| 4 | 223 Examples |
| 0 | 224 .LP |
| 4 | 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 | |
| 230 .CW | |
| 231 ls | wc -l | |
| 232 .DE | |
| 233 The | |
| 234 .CW ls | |
| 235 command lists all files in the current directory, one per line, | |
| 236 and | |
| 237 .CW "wc -l | |
| 8 | 238 counts the number of lines. |
| 4 | 239 .PP |
| 8 | 240 Counting the number of files that do not contain ``foo'' in their name: |
| 4 | 241 .DS |
| 242 .CW | |
| 243 ls | grep -v foo | wc -l | |
| 244 .DE | |
| 245 Here, the list of files is filtered by | |
| 246 .CW grep | |
| 247 to remove all that contain ``foo''. | |
| 248 The rest is the same as in the previous example. | |
| 249 .PP | |
| 250 Finding the five largest entries in the current directory. | |
| 251 .DS | |
| 252 .CW | |
| 253 du -s * | sort -nr | sed 5q | |
| 254 .DE | |
| 255 .CW "du -s * | |
| 256 returns the recursively summed sizes of all files | |
| 8 | 257 \(en no matter if they are regular files or directories. |
| 4 | 258 .CW "sort -nr |
| 259 sorts the list numerically in reverse order. | |
| 260 Finally, | |
| 261 .CW "sed 5q | |
| 262 quits after it has printed the fifth line. | |
| 263 .PP | |
| 264 The presented command lines are examples of what Unix people would use | |
| 265 to get the desired output. | |
| 266 There are also other ways to get the same output. | |
| 267 It's a user's decision which way to go. | |
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269 .SH |
| 4 | 270 Pipes |
| 0 | 271 .LP |
| 8 | 272 The examples show that many tasks on a Unix system |
| 4 | 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 | |
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286 %O .CW \s-1http://www.princeton.edu/~hos/frs122/unixhist/finalhis.htm |
| 4 | 287 .] |
| 288 .PP | |
| 289 Being able to specify pipelines in an easy way is, | |
| 290 however, not enough by itself. | |
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291 It is only one half. |
| 4 | 292 The other is the design of the programs that are used in the pipeline. |
| 8 | 293 They have to interfaces that allows them to be used in such a way. |
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294 |
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295 .SH |
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296 Interface architecture |
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297 .LP |
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298 Unix is, first of all, simple: Everything is a file. |
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299 Files are sequences of bytes, without any special structure. |
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300 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin) |
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301 and write a stream of bytes to ``standard output'' (stdout). |
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302 .PP |
| 8 | 303 If the files \fIare\fP sequences of bytes, |
| 304 and the programs \fIare\fP filters on byte streams, | |
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305 then there is exactly one standardized interface. |
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306 Thus it is possible to combine them in any desired way. |
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307 .PP |
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308 Even a handful of small programs will yield a large set of combinations, |
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309 and thus a large set of different functions. |
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310 This is leverage! |
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311 If the programs are orthogonal to each other \(en the best case \(en |
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312 then the set of different functions is greatest. |
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313 .PP |
| 8 | 314 The Unix toolchest \fIis\fP a set of small programs that |
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315 are filters on byte streams. |
| 8 | 316 They are, to a large extend, unrelated in their function. |
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317 Hence, the Unix toolchest provides a large set of functions |
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318 that can be accessed by combining the programs in the desired way. |
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319 |
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320 .SH |
| 8 | 321 The toolchest approach |
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322 .LP |
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323 A toolchest is a set of tools. |
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324 Instead of having one big tool for all tasks, one has many small tools, |
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325 each for one task. |
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326 Difficult tasks are solved by combining several of the small, simple tools. |
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327 .PP |
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328 It is easier and less error-prone to write small programs. |
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329 It is also easier and less error-prone to write a large set of small programs, |
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330 than to write one large program with all the functionality included. |
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331 If the small programs are combinable, then they offer even a larger set |
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332 of functions than the single large program. |
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333 Hence, one gets two advantages out of writing small, combinable programs. |
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334 .PP |
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335 There are two drawbacks of the toolchest approach. |
| 8 | 336 First, one simple, standardized, unidirectional interface has to be sufficient. |
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337 If one feels the need for more ``logic'' than a stream of bytes, |
| 8 | 338 then a different approach might be of need. |
| 339 But it is also possbile, that he just can not imaging a design where | |
| 340 a stream of bytes is sufficient. | |
| 341 By becoming more familiar with the ``Unix style of thinking'', | |
| 342 developers will more often and easier find simple designs where | |
| 343 a stream of bytes is a sufficient interface. | |
| 344 .PP | |
| 345 The second drawback of a toolchest affects the users. | |
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346 A toolchest is often more difficult to use for novices. |
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347 It is neccessary to become familiar with each of the tools, |
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348 to be able to use the right one in a given situation. |
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349 Additinally, one needs to combine the tools in a senseful way on its own. |
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350 This is like a sharp knive \(en it is a powerful tool in the hand of a master, |
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351 but of no good value in the hand of an unskilled. |
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352 .PP |
| 8 | 353 However, learning single, small tool of the toolchest is easier than |
| 354 learning a complex tool. | |
| 355 The user will have a basic understanding of a yet unknown tool, | |
| 356 if the several tools of the toolchest have a common style. | |
| 357 He will be able to transfer knowledge over one tool to another. | |
| 358 .PP | |
| 359 Moreover, the second drawback can be removed easily by adding wrappers | |
| 360 around the single tools. | |
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361 Novice users do not need to learn several tools if a professional wraps |
| 8 | 362 the single commands into a more high-level script. |
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363 Note that the wrapper script still calls the small tools; |
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364 the wrapper script is just like a skin around. |
| 8 | 365 No complexity is added this way, |
| 366 but new programs can get created out of existing one with very low effort. | |
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367 .PP |
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368 A wrapper script for finding the five largest entries in the current directory |
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369 could look like this: |
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370 .DS |
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371 .CW |
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372 #!/bin/sh |
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373 du -s * | sort -nr | sed 5q |
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374 .DE |
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375 The script itself is just a text file that calls the command line |
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376 a professional user would type in directly. |
| 8 | 377 Making the program flexible on the number of entries it prints, |
| 378 is easily possible: | |
| 379 .DS | |
| 380 .CW | |
| 381 #!/bin/sh | |
| 382 num=5 | |
| 383 [ $# -eq 1 ] && num="$1" | |
| 384 du -sh * | sort -nr | sed "${num}q" | |
| 385 .DE | |
| 386 This script acts like the one before, when called without an argument. | |
| 387 But one can also specify a numerical argument to define the number of lines to print. | |
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388 |
| 8 | 389 .SH |
| 390 A powerful shell | |
| 391 .LP | |
| 392 The Unix shell provides the possibility to combine small programs into | |
| 393 large ones easily. | |
| 394 But the powerful shell is great feature in other ways, too. | |
| 395 .PP | |
| 396 It encourages rapid prototyping. | |
| 397 It includes a scripting language with built in control statements. | |
| 398 The functions, however, are the normal programs, everyone can use on the system. | |
| 399 Thus, the programs are known and learning to program in the shell is easy. | |
| 400 Using normal programs as functions in the shell programming language | |
| 401 is only possible because they are small, combinable tools in a toolchest style. | |
| 402 .PP | |
| 403 The Unix shell encourages to write small scripts out of other programs, | |
| 404 because it is so easy to do. | |
| 405 This is a great step towards automation. | |
| 406 It is wonderful if the effort to automate a task equals the effort | |
| 407 it takes to do it the second time by hand. | |
| 408 If it is so, then the user will be happy to automate everything he does more than once. | |
| 409 .PP | |
| 410 Small programs that do one job well, standardized interfaces between them, | |
| 411 a mechanism to combine parts to larger parts, and an easy way to automate tasks, | |
| 412 this will inevitably produce software leverage. | |
| 413 Getting multiple times the benefit of an investment is a great offer. | |
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414 |
| 4 | 415 |
| 416 | |
| 0 | 417 |
| 418 | |
| 419 .NH 2 | |
| 420 Results | |
| 421 .LP | |
| 422 The unix phil is an answer to the sw design question | |
| 423 .LP | |
| 424 tool chains empower the uses of sw | |
| 425 | |
| 8 | 426 |
| 427 | |
| 0 | 428 .NH 1 |
| 429 Case study: nmh | |
| 430 | |
| 431 .NH 2 | |
| 432 History | |
| 433 .LP | |
| 434 MH, nmh. | |
| 435 They are old. | |
| 436 | |
| 437 .NH 2 | |
| 438 Contrasts to similar sw | |
| 439 .LP | |
| 440 vs. Thunderbird, mutt, mailx, pine | |
| 441 .LP | |
| 442 flexibility, no redundancy, use the shell | |
| 443 | |
| 444 .NH 2 | |
| 445 Gains of the design | |
| 446 .LP | |
| 447 | |
| 448 .NH 2 | |
| 449 Problems | |
| 450 .LP | |
| 451 | |
| 8 | 452 |
| 453 | |
| 0 | 454 .NH 1 |
| 455 Case study: uzbl | |
| 456 | |
| 457 .NH 2 | |
| 458 History | |
| 459 .LP | |
| 460 uzbl is young | |
| 461 | |
| 462 .NH 2 | |
| 463 Contrasts to similar sw | |
| 464 .LP | |
| 465 like with nmh | |
| 466 .LP | |
| 467 addons, plugins, modules | |
| 468 | |
| 469 .NH 2 | |
| 470 Gains of the design | |
| 471 .LP | |
| 472 | |
| 473 .NH 2 | |
| 474 Problems | |
| 475 .LP | |
| 476 broken web | |
| 477 | |
| 8 | 478 |
| 479 | |
| 0 | 480 .NH 1 |
| 481 Final thoughts | |
| 482 | |
| 483 .NH 2 | |
| 484 Quick summary | |
| 485 .LP | |
| 486 good design | |
| 487 .LP | |
| 488 unix phil | |
| 489 .LP | |
| 490 case studies | |
| 491 | |
| 492 .NH 2 | |
| 493 Why people should choose | |
| 494 .LP | |
| 495 Make the right choice! | |
| 496 | |
| 497 .nr PI .5i | |
| 498 .rm ]< | |
| 499 .de ]< | |
| 500 .LP | |
| 501 .de FP | |
| 502 .IP \\\\$1. | |
| 503 \\.. | |
| 504 .rm FS FE | |
| 505 .. | |
| 506 .SH | |
| 507 References | |
| 508 .[ | |
| 509 $LIST$ | |
| 510 .] | |
| 511 .wh -1p |