Mercurial > docs > unix-phil
annotate unix-phil.ms @ 7:606f0ffca834
added two more paragraphs to ch02
| author | meillo@marmaro.de |
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| date | Thu, 18 Feb 2010 12:15:39 +0100 |
| parents | a6b837d822b7 |
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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. | |
| 219 This chapter explains the philosophy and shows concrete examples of its application. | |
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221 .SH |
| 4 | 222 Examples |
| 0 | 223 .LP |
| 4 | 224 Following are some examples to demonstrate how applied Unix Philosophy feels like. |
| 225 Knowledge of using the Unix shell is assumed. | |
| 226 .PP | |
| 227 Counting the number of files in the current directory: | |
| 228 .DS | |
| 229 .CW | |
| 230 ls | wc -l | |
| 231 .DE | |
| 232 The | |
| 233 .CW ls | |
| 234 command lists all files in the current directory, one per line, | |
| 235 and | |
| 236 .CW "wc -l | |
| 237 counts how many lines they are. | |
| 238 .PP | |
| 239 Counting all files that do not contain ``foo'' in their name: | |
| 240 .DS | |
| 241 .CW | |
| 242 ls | grep -v foo | wc -l | |
| 243 .DE | |
| 244 Here, the list of files is filtered by | |
| 245 .CW grep | |
| 246 to remove all that contain ``foo''. | |
| 247 The rest is the same as in the previous example. | |
| 248 .PP | |
| 249 Finding the five largest entries in the current directory. | |
| 250 .DS | |
| 251 .CW | |
| 252 du -s * | sort -nr | sed 5q | |
| 253 .DE | |
| 254 .CW "du -s * | |
| 255 returns the recursively summed sizes of all files | |
| 256 -- no matter if they are regular files or directories. | |
| 257 .CW "sort -nr | |
| 258 sorts the list numerically in reverse order. | |
| 259 Finally, | |
| 260 .CW "sed 5q | |
| 261 quits after it has printed the fifth line. | |
| 262 .PP | |
| 263 The presented command lines are examples of what Unix people would use | |
| 264 to get the desired output. | |
| 265 There are also other ways to get the same output. | |
| 266 It's a user's decision which way to go. | |
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268 .SH |
| 4 | 269 Pipes |
| 0 | 270 .LP |
| 4 | 271 The examples show that a lot of tasks on a Unix system |
| 272 are accomplished by combining several small programs. | |
| 273 The connection between the single programs is denoted by the pipe operator `|'. | |
| 274 .PP | |
| 275 Pipes, and their extensive and easy use, are one of the great | |
| 276 achievements of the Unix system. | |
| 277 Pipes between programs have been possible in earlier operating systems, | |
| 278 but it has never been a so central part of the concept. | |
| 279 When, in the early seventies, Doug McIlroy introduced pipes for the | |
| 280 Unix system, | |
| 281 ``it was this concept and notation for linking several programs together | |
| 282 that transformed Unix from a basic file-sharing system to an entirely new way of computing.'' | |
| 283 .[ | |
| 284 %T Unix: An Oral History | |
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285 %O .CW \s-1http://www.princeton.edu/~hos/frs122/unixhist/finalhis.htm |
| 4 | 286 .] |
| 287 .PP | |
| 288 Being able to specify pipelines in an easy way is, | |
| 289 however, not enough by itself. | |
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290 It is only one half. |
| 4 | 291 The other is the design of the programs that are used in the pipeline. |
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292 They have to be of an external shape that allows them to be be used in such a way. |
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293 |
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294 .SH |
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295 Interface architecture |
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296 .LP |
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297 Unix is, first of all, simple: Everything is a file. |
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298 Files are sequences of bytes, without any special structure. |
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299 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin) |
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300 and write a stream of bytes to ``standard output'' (stdout). |
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301 .PP |
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302 If our files \fIare\fP sequences of bytes, |
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303 and our programs \fIare\fP filters on byte streams, |
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304 then there is exactly one standardized interface. |
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305 Thus it is possible to combine them in any desired way. |
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306 .PP |
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307 Even a handful of small programs will yield a large set of combinations, |
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308 and thus a large set of different functions. |
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309 This is leverage! |
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310 .PP |
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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 |
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314 Now, the Unix toolchest is a set of small programs that |
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315 are filters on byte streams. |
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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 |
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321 Advantages of toolchests |
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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. |
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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, |
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338 then a different approach might be of need, or, more likely, |
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339 he just did not came to a design where a stream of bytes is sufficient. |
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340 The other drawback of a toolchest affects the users. |
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341 A toolchest is often more difficult to use for novices. |
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342 It is neccessary to become familiar with each of the tools, |
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343 to be able to use the right one in a given situation. |
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344 Additinally, one needs to combine the tools in a senseful way on its own. |
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345 This is like a sharp knive \(en it is a powerful tool in the hand of a master, |
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346 but of no good value in the hand of an unskilled. |
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347 .PP |
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348 Luckily, the second drawback can be solved easily by adding wrappers around the single tools. |
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349 Novice users do not need to learn several tools if a professional wraps |
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350 the single commands into a single script. |
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351 Note that the wrapper script still calls the small tools; |
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352 the wrapper script is just like a skin around. |
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353 No complexity is added this way. |
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354 .PP |
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355 A wrapper script for finding the five largest entries in the current directory |
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356 could look like this: |
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357 .DS |
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358 .CW |
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359 #!/bin/sh |
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360 du -s * | sort -nr | sed 5q |
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361 .DE |
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362 The script itself is just a text file that calls the command line |
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363 a professional user would type in directly. |
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364 |
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365 |
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366 |
| 4 | 367 |
| 368 | |
| 0 | 369 |
| 370 .NH 2 | |
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371 foo |
| 0 | 372 .LP |
| 373 standalone vs. tool chain | |
| 374 .LP | |
| 375 software leverage | |
| 376 .LP | |
| 377 possiblities | |
| 378 | |
| 379 .NH 2 | |
| 380 Results | |
| 381 .LP | |
| 382 The unix phil is an answer to the sw design question | |
| 383 .LP | |
| 384 tool chains empower the uses of sw | |
| 385 | |
| 386 .NH 1 | |
| 387 Case study: nmh | |
| 388 | |
| 389 .NH 2 | |
| 390 History | |
| 391 .LP | |
| 392 MH, nmh. | |
| 393 They are old. | |
| 394 | |
| 395 .NH 2 | |
| 396 Contrasts to similar sw | |
| 397 .LP | |
| 398 vs. Thunderbird, mutt, mailx, pine | |
| 399 .LP | |
| 400 flexibility, no redundancy, use the shell | |
| 401 | |
| 402 .NH 2 | |
| 403 Gains of the design | |
| 404 .LP | |
| 405 | |
| 406 .NH 2 | |
| 407 Problems | |
| 408 .LP | |
| 409 | |
| 410 .NH 1 | |
| 411 Case study: uzbl | |
| 412 | |
| 413 .NH 2 | |
| 414 History | |
| 415 .LP | |
| 416 uzbl is young | |
| 417 | |
| 418 .NH 2 | |
| 419 Contrasts to similar sw | |
| 420 .LP | |
| 421 like with nmh | |
| 422 .LP | |
| 423 addons, plugins, modules | |
| 424 | |
| 425 .NH 2 | |
| 426 Gains of the design | |
| 427 .LP | |
| 428 | |
| 429 .NH 2 | |
| 430 Problems | |
| 431 .LP | |
| 432 broken web | |
| 433 | |
| 434 .NH 1 | |
| 435 Final thoughts | |
| 436 | |
| 437 .NH 2 | |
| 438 Quick summary | |
| 439 .LP | |
| 440 good design | |
| 441 .LP | |
| 442 unix phil | |
| 443 .LP | |
| 444 case studies | |
| 445 | |
| 446 .NH 2 | |
| 447 Why people should choose | |
| 448 .LP | |
| 449 Make the right choice! | |
| 450 | |
| 451 .nr PI .5i | |
| 452 .rm ]< | |
| 453 .de ]< | |
| 454 .LP | |
| 455 .de FP | |
| 456 .IP \\\\$1. | |
| 457 \\.. | |
| 458 .rm FS FE | |
| 459 .. | |
| 460 .SH | |
| 461 References | |
| 462 .[ | |
| 463 $LIST$ | |
| 464 .] | |
| 465 .wh -1p |