Mercurial > docs > unix-phil
annotate unix-phil.ms @ 44:46e34e433231
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| author | meillo@marmaro.de |
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| date | Sat, 10 Apr 2010 15:00:38 +0200 |
| parents | 303e8f449e77 |
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2 .nr VS 13 |
| 0 | 3 .nr lu 0 |
| 4 .de CW | |
| 5 .nr PQ \\n(.f | |
| 6 .if t .ft CW | |
| 17 | 7 .ie ^\\$1^^ .if n .ul 999 |
| 0 | 8 .el .if n .ul 1 |
| 17 | 9 .if t .if !^\\$1^^ \&\\$1\f\\n(PQ\\$2 |
| 0 | 10 .if n .if \\n(.$=1 \&\\$1 |
| 11 .if n .if \\n(.$>1 \&\\$1\c | |
| 12 .if n .if \\n(.$>1 \&\\$2 | |
| 13 .. | |
| 14 .ds [. \ [ | |
| 15 .ds .] ] | |
| 42 | 16 |
| 17 .rn NH _N | |
| 18 .de NH | |
| 19 .if '\\$1'1' .sp 2v | |
| 20 .if '\\$1'1' .nr PS +2 | |
| 21 ._N \\$1 | |
| 22 .if '\\$1'1' .nr PS -2 | |
| 23 .. | |
| 24 | |
| 1 | 25 .\"---------------------------------------- |
| 42 | 26 |
| 0 | 27 .TL |
| 42 | 28 .ps +4 |
| 6 | 29 Why the Unix Philosophy still matters |
| 0 | 30 .AU |
| 31 markus schnalke <meillo@marmaro.de> | |
| 32 .AB | |
| 1 | 33 .ti \n(.iu |
| 39 | 34 This paper explains the importance of the Unix Philosophy for software design. |
| 0 | 35 Today, few software designers are aware of these concepts, |
| 39 | 36 and thus a lot of modern software is more limited than necessary |
| 37 and makes less use of software leverage than possible. | |
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38 Knowing and following the guidelines of the Unix Philosophy makes software more valuable. |
| 0 | 39 .AE |
| 40 | |
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42 .ps -1 |
| 39 | 43 This paper was prepared for the ``Software Analysis'' seminar at University Ulm. |
| 44 Mentor was professor Schweiggert. 2010-04-05 | |
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45 .br |
| 39 | 46 You may retrieve this document from |
| 47 .CW \s-1http://marmaro.de/docs \ . | |
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48 .FE |
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49 |
| 0 | 50 .NH 1 |
| 51 Introduction | |
| 42 | 52 .XS |
| 53 \*(SN Introduction | |
| 54 .XE | |
| 0 | 55 .LP |
| 40 | 56 The Unix Philosophy is the essence of how the Unix operating system, |
| 57 especially its toolchest, was designed. | |
| 58 It is no limited set of fixed rules, | |
| 59 but a loose set of guidelines which tell how to write software that | |
| 60 suites well into Unix. | |
| 61 Actually, the Unix Philosophy describes what is common to typical Unix software. | |
| 62 The Wikipedia has an accurate definition: | |
| 63 .[ | |
| 44 | 64 wikipedia |
| 65 unix philosophy | |
| 40 | 66 .] |
| 67 .QP | |
| 41 | 68 .ps -1 |
| 40 | 69 The \fIUnix philosophy\fP is a set of cultural norms and philosophical |
| 70 approaches to developing software based on the experience of leading | |
| 71 developers of the Unix operating system. | |
| 3 | 72 .PP |
| 40 | 73 As there is no single definition of the Unix Philosophy, |
| 74 several people have stated their view on what it comprises. | |
| 1 | 75 Best known are: |
| 76 .IP \(bu | |
| 77 Doug McIlroy's summary: ``Write programs that do one thing and do it well.'' | |
| 78 .[ | |
| 44 | 79 mahoney |
| 80 oral history | |
| 1 | 81 .] |
| 82 .IP \(bu | |
| 83 Mike Gancarz' book ``The UNIX Philosophy''. | |
| 84 .[ | |
| 44 | 85 gancarz |
| 86 unix philosophy | |
| 1 | 87 .] |
| 88 .IP \(bu | |
| 89 Eric S. Raymond's book ``The Art of UNIX Programming''. | |
| 90 .[ | |
| 44 | 91 raymond |
| 92 art of unix programming | |
| 1 | 93 .] |
| 0 | 94 .LP |
| 1 | 95 These different views on the Unix Philosophy have much in common. |
| 40 | 96 Especially, the main concepts are similar in all of them. |
| 97 McIlroy's definition can surely be called the core of the Unix Philosophy, | |
| 98 but the fundamental idea behind it all, is ``small is beautiful''. | |
| 99 | |
| 100 .PP | |
| 101 The Unix Philosophy tells how to design and write good software for Unix. | |
| 102 Many concepts described here base on facilities of Unix. | |
| 103 Other operating systems may not offer such facilities, | |
| 41 | 104 hence it may not be possible to design software in the way of the |
| 105 Unix Philosophy for them. | |
| 106 .PP | |
| 107 The Unix Philosophy has an idea of how the process of software development | |
| 108 should look like, but large parts of the philosophy are quite independent | |
| 109 from the development process used. | |
| 110 However, one will soon recognize that some development processes work well | |
| 111 with the ideas of the Unix Philosophy and support them, while others are | |
| 112 at cross-purposes. | |
| 113 Kent Beck's books about Extreme Programming are valuable supplimental | |
| 114 resources. | |
| 40 | 115 .PP |
| 41 | 116 The question of how to actually write code and how the code should looks |
| 117 like internally, are out of focus here. | |
| 118 ``The Practice of Programming'' by Kernighan and Pike, | |
| 119 .[ | |
| 44 | 120 kernighan pike |
| 121 practice of programming | |
| 41 | 122 .] |
| 123 is a good book that covers this topic. | |
| 124 Its point of view matches to the one of this paper. | |
| 0 | 125 |
| 126 .NH 1 | |
| 6 | 127 Importance of software design in general |
| 42 | 128 .XS |
| 129 .sp .5v | |
| 130 \*(SN Importance of software design in general | |
| 131 .XE | |
| 0 | 132 .LP |
| 40 | 133 Software design is the planning of how the internal structure |
| 134 and external interfaces of a software should look like. | |
| 39 | 135 It has nothing to do with visual appearance. |
| 136 If we take a program as a car, then its color is of no matter. | |
| 137 Its design would be the car's size, its shape, the locations of doors, | |
| 138 the passenger/space ratio, the luggage capacity, and so forth. | |
| 139 .PP | |
| 140 Why should software get designed at all? | |
| 6 | 141 It is general knowledge, that even a bad plan is better than no plan. |
| 39 | 142 Not designing software means programming without plan. |
| 143 This will pretty sure lead to horrible results. | |
| 144 Horrible to use and horrible to maintain. | |
| 145 These two aspects are the visible ones. | |
| 146 Often invisible are the wasted possible gains. | |
| 147 Good software design can make these gains available. | |
| 6 | 148 .PP |
| 39 | 149 A software's design deals with quality properties. |
| 150 Good design leads to good quality, and quality is important. | |
| 151 Any car may be able to drive from A to B, | |
| 152 but it depends on the car's properties whether it is a good choice | |
| 153 for passenger transport or not. | |
| 154 It depends on its properties if it is a good choice | |
| 155 for a rough mountain area. | |
| 156 And it depends on its properties if the ride will be fun. | |
| 157 | |
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158 .PP |
| 39 | 159 Requirements for a software are twofold: |
| 160 functional and non-functional. | |
| 161 .IP \(bu | |
| 162 Functional requirements define directly the software's functions. | |
| 163 They are the reason why software gets written. | |
| 164 Someone has a problem and needs a tool to solve it. | |
| 165 Being able to solve the problem is the main functional goal. | |
| 166 It is the driving force behind all programming effort. | |
| 6 | 167 Functional requirements are easier to define and to verify. |
| 39 | 168 .IP \(bu |
| 169 Non-functional requirements are also called \fIquality\fP requirements. | |
| 170 The quality of a software are the properties that are not directly related to | |
| 171 the software's basic functions. | |
| 172 Tools of bad quality often solve the problems they were written for, | |
| 173 but introduce problems and difficulties for usage and development, later on. | |
| 174 Quality aspects are often overlooked at first sight, | |
| 175 and they are often difficult to define clearly and to verify. | |
| 6 | 176 .PP |
| 39 | 177 Quality is of few matter when the software gets built initially, |
| 178 but it is of matter for usage and maintenance of the software. | |
| 6 | 179 A short-sighted might see in developing a software mainly building something up. |
| 39 | 180 But experience shows, that building the software the first time is |
| 181 only a small amount of the overall work. | |
| 182 Bug fixing, extending, rebuilding of parts | |
| 183 \(en maintenance work, for short \(en | |
| 6 | 184 does soon take over the major part of the time spent on a software. |
| 185 Not to forget the usage of the software. | |
| 186 These processes are highly influenced by the software's quality. | |
| 39 | 187 Thus, quality must not be neglected. |
| 188 The problem with quality is that you hardly ``stumble over'' | |
| 189 bad quality during the first build, | |
| 6 | 190 but this is the time when you should care about good quality most. |
| 191 .PP | |
| 39 | 192 Software design is less the basic function of a software \(en |
| 193 this requirement will get satisfied anyway. | |
| 194 Software design is more about quality aspects of the software. | |
| 195 Good design leads to good quality, bad design to bad quality. | |
| 6 | 196 The primary functions of the software will be affected modestly by bad quality, |
| 39 | 197 but good quality can provide a lot of additional gain, |
| 6 | 198 even at places where one never expected it. |
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199 .PP |
| 6 | 200 The ISO/IEC 9126-1 standard, part 1, |
| 201 .[ | |
| 44 | 202 iso product quality |
| 6 | 203 .] |
| 204 defines the quality model as consisting out of: | |
| 205 .IP \(bu | |
| 206 .I Functionality | |
| 207 (suitability, accuracy, inter\%operability, security) | |
| 208 .IP \(bu | |
| 209 .I Reliability | |
| 210 (maturity, fault tolerance, recoverability) | |
| 211 .IP \(bu | |
| 212 .I Usability | |
| 213 (understandability, learnability, operability, attractiveness) | |
| 214 .IP \(bu | |
| 215 .I Efficiency | |
| 9 | 216 (time behavior, resource utilization) |
| 6 | 217 .IP \(bu |
| 218 .I Maintainability | |
| 23 | 219 (analyzability, changeability, stability, testability) |
| 6 | 220 .IP \(bu |
| 221 .I Portability | |
| 222 (adaptability, installability, co-existence, replaceability) | |
| 223 .LP | |
| 39 | 224 Good design can improve these properties of a software, |
| 225 bad designed software probably suffers from not having them. | |
| 7 | 226 .PP |
| 227 One further goal of software design is consistency. | |
| 228 Consistency eases understanding, working on, and using things. | |
| 39 | 229 Consistent internal structure and consistent interfaces to the outside |
| 230 can be provided by good design. | |
| 7 | 231 .PP |
| 39 | 232 Software should be well designed because good design avoids many |
| 233 problems during the software's lifetime. | |
| 234 And software should be well designed because good design can offer | |
| 235 much additional gain. | |
| 236 Indeed, much effort should be spent into good design to make software more valuable. | |
| 237 The Unix Philosophy shows a way of how to design software well. | |
| 7 | 238 It offers guidelines to achieve good quality and high gain for the effort spent. |
| 0 | 239 |
| 240 | |
| 241 .NH 1 | |
| 242 The Unix Philosophy | |
| 42 | 243 .XS |
| 244 .sp .5v | |
| 245 \*(SN The Unix Philosophy | |
| 246 .XE | |
| 4 | 247 .LP |
| 248 The origins of the Unix Philosophy were already introduced. | |
| 8 | 249 This chapter explains the philosophy, oriented on Gancarz, |
| 250 and shows concrete examples of its application. | |
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251 |
| 16 | 252 .NH 2 |
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253 Pipes |
| 42 | 254 .XS |
| 255 \*(SN Pipes | |
| 256 .XE | |
| 0 | 257 .LP |
| 4 | 258 Following are some examples to demonstrate how applied Unix Philosophy feels like. |
| 259 Knowledge of using the Unix shell is assumed. | |
| 260 .PP | |
| 261 Counting the number of files in the current directory: | |
| 41 | 262 .DS |
| 4 | 263 .CW |
| 9 | 264 .ps -1 |
| 4 | 265 ls | wc -l |
| 266 .DE | |
| 267 The | |
| 268 .CW ls | |
| 269 command lists all files in the current directory, one per line, | |
| 270 and | |
| 271 .CW "wc -l | |
| 8 | 272 counts the number of lines. |
| 4 | 273 .PP |
| 8 | 274 Counting the number of files that do not contain ``foo'' in their name: |
| 41 | 275 .DS |
| 4 | 276 .CW |
| 9 | 277 .ps -1 |
| 4 | 278 ls | grep -v foo | wc -l |
| 279 .DE | |
| 280 Here, the list of files is filtered by | |
| 281 .CW grep | |
| 282 to remove all that contain ``foo''. | |
| 283 The rest is the same as in the previous example. | |
| 284 .PP | |
| 285 Finding the five largest entries in the current directory. | |
| 41 | 286 .DS |
| 4 | 287 .CW |
| 9 | 288 .ps -1 |
| 4 | 289 du -s * | sort -nr | sed 5q |
| 290 .DE | |
| 291 .CW "du -s * | |
| 292 returns the recursively summed sizes of all files | |
| 8 | 293 \(en no matter if they are regular files or directories. |
| 4 | 294 .CW "sort -nr |
| 295 sorts the list numerically in reverse order. | |
| 296 Finally, | |
| 297 .CW "sed 5q | |
| 298 quits after it has printed the fifth line. | |
| 299 .PP | |
| 300 The presented command lines are examples of what Unix people would use | |
| 301 to get the desired output. | |
| 302 There are also other ways to get the same output. | |
| 303 It's a user's decision which way to go. | |
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304 .PP |
| 8 | 305 The examples show that many tasks on a Unix system |
| 4 | 306 are accomplished by combining several small programs. |
| 307 The connection between the single programs is denoted by the pipe operator `|'. | |
| 308 .PP | |
| 309 Pipes, and their extensive and easy use, are one of the great | |
| 310 achievements of the Unix system. | |
| 311 Pipes between programs have been possible in earlier operating systems, | |
| 312 but it has never been a so central part of the concept. | |
| 313 When, in the early seventies, Doug McIlroy introduced pipes for the | |
| 314 Unix system, | |
| 315 ``it was this concept and notation for linking several programs together | |
| 316 that transformed Unix from a basic file-sharing system to an entirely new way of computing.'' | |
| 317 .[ | |
| 44 | 318 aughenbaugh |
| 319 unix oral history | |
| 4 | 320 .] |
| 321 .PP | |
| 322 Being able to specify pipelines in an easy way is, | |
| 323 however, not enough by itself. | |
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324 It is only one half. |
| 4 | 325 The other is the design of the programs that are used in the pipeline. |
| 8 | 326 They have to interfaces that allows them to be used in such a way. |
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327 |
| 16 | 328 .NH 2 |
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329 Interface design |
| 42 | 330 .XS |
| 331 \*(SN Interface design | |
| 332 .XE | |
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333 .LP |
| 11 | 334 Unix is, first of all, simple \(en Everything is a file. |
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335 Files are sequences of bytes, without any special structure. |
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336 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin) |
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337 and write a stream of bytes to ``standard output'' (stdout). |
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338 .PP |
| 8 | 339 If the files \fIare\fP sequences of bytes, |
| 340 and the programs \fIare\fP filters on byte streams, | |
| 11 | 341 then there is exactly one standardized data interface. |
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342 Thus it is possible to combine them in any desired way. |
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343 .PP |
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344 Even a handful of small programs will yield a large set of combinations, |
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345 and thus a large set of different functions. |
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346 This is leverage! |
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347 If the programs are orthogonal to each other \(en the best case \(en |
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348 then the set of different functions is greatest. |
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349 .PP |
| 11 | 350 Programs might also have a separate control interface, |
| 351 besides their data interface. | |
| 352 The control interface is often called ``user interface'', | |
| 353 because it is usually designed to be used by humans. | |
| 354 The Unix Philosophy discourages to assume the user to be human. | |
| 355 Interactive use of software is slow use of software, | |
| 356 because the program waits for user input most of the time. | |
| 357 Interactive software requires the user to be in front of the computer | |
| 358 all the time. | |
| 359 Interactive software occupy the user's attention while they are running. | |
| 360 .PP | |
| 361 Now we come back to the idea of using several small programs, combined, | |
| 362 to have a more specific function. | |
| 363 If these single tools would all be interactive, | |
| 364 how would the user control them? | |
| 365 It is not only a problem to control several programs at once if they run at the same time, | |
| 366 it also very inefficient to have to control each of the single programs | |
| 367 that are intended to work as one large program. | |
| 368 Hence, the Unix Philosophy discourages programs to demand interactive use. | |
| 369 The behavior of programs should be defined at invocation. | |
| 370 This is done by specifying arguments (``command line switches'') to the program call. | |
| 371 Gancarz discusses this topic as ``avoid captive user interfaces''. | |
| 372 .[ | |
| 44 | 373 gancarz unix philosophy |
| 11 | 374 %P 88 ff. |
| 375 .] | |
| 44 | 376 .ds _p ", p. 88 ff. |
| 11 | 377 .PP |
| 378 Non-interactive use is, during development, also an advantage for testing. | |
| 379 Testing of interactive programs is much more complicated, | |
| 380 than testing of non-interactive programs. | |
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381 |
| 16 | 382 .NH 2 |
| 8 | 383 The toolchest approach |
| 42 | 384 .XS |
| 385 \*(SN The toolchest approach | |
| 386 .XE | |
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387 .LP |
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388 A toolchest is a set of tools. |
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389 Instead of having one big tool for all tasks, one has many small tools, |
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390 each for one task. |
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391 Difficult tasks are solved by combining several of the small, simple tools. |
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392 .PP |
| 11 | 393 The Unix toolchest \fIis\fP a set of small, (mostly) non-interactive programs |
| 394 that are filters on byte streams. | |
| 395 They are, to a large extend, unrelated in their function. | |
| 396 Hence, the Unix toolchest provides a large set of functions | |
| 397 that can be accessed by combining the programs in the desired way. | |
| 398 .PP | |
| 399 There are also advantages for developing small toolchest programs. | |
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400 It is easier and less error-prone to write small programs. |
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401 It is also easier and less error-prone to write a large set of small programs, |
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402 than to write one large program with all the functionality included. |
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403 If the small programs are combinable, then they offer even a larger set |
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404 of functions than the single large program. |
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405 Hence, one gets two advantages out of writing small, combinable programs. |
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406 .PP |
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407 There are two drawbacks of the toolchest approach. |
| 8 | 408 First, one simple, standardized, unidirectional interface has to be sufficient. |
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409 If one feels the need for more ``logic'' than a stream of bytes, |
| 8 | 410 then a different approach might be of need. |
| 13 | 411 But it is also possible, that he just can not imagine a design where |
| 8 | 412 a stream of bytes is sufficient. |
| 413 By becoming more familiar with the ``Unix style of thinking'', | |
| 414 developers will more often and easier find simple designs where | |
| 415 a stream of bytes is a sufficient interface. | |
| 416 .PP | |
| 417 The second drawback of a toolchest affects the users. | |
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418 A toolchest is often more difficult to use for novices. |
| 9 | 419 It is necessary to become familiar with each of the tools, |
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420 to be able to use the right one in a given situation. |
| 9 | 421 Additionally, one needs to combine the tools in a senseful way on its own. |
| 422 This is like a sharp knife \(en it is a powerful tool in the hand of a master, | |
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423 but of no good value in the hand of an unskilled. |
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424 .PP |
| 8 | 425 However, learning single, small tool of the toolchest is easier than |
| 426 learning a complex tool. | |
| 427 The user will have a basic understanding of a yet unknown tool, | |
| 428 if the several tools of the toolchest have a common style. | |
| 429 He will be able to transfer knowledge over one tool to another. | |
| 430 .PP | |
| 431 Moreover, the second drawback can be removed easily by adding wrappers | |
| 432 around the single tools. | |
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433 Novice users do not need to learn several tools if a professional wraps |
| 8 | 434 the single commands into a more high-level script. |
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435 Note that the wrapper script still calls the small tools; |
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436 the wrapper script is just like a skin around. |
| 8 | 437 No complexity is added this way, |
| 438 but new programs can get created out of existing one with very low effort. | |
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439 .PP |
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440 A wrapper script for finding the five largest entries in the current directory |
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441 could look like this: |
| 41 | 442 .DS |
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443 .CW |
| 9 | 444 .ps -1 |
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445 #!/bin/sh |
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446 du -s * | sort -nr | sed 5q |
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447 .DE |
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448 The script itself is just a text file that calls the command line |
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449 a professional user would type in directly. |
| 8 | 450 Making the program flexible on the number of entries it prints, |
| 451 is easily possible: | |
| 41 | 452 .DS |
| 8 | 453 .CW |
| 9 | 454 .ps -1 |
| 8 | 455 #!/bin/sh |
| 456 num=5 | |
| 457 [ $# -eq 1 ] && num="$1" | |
| 458 du -sh * | sort -nr | sed "${num}q" | |
| 459 .DE | |
| 460 This script acts like the one before, when called without an argument. | |
| 461 But one can also specify a numerical argument to define the number of lines to print. | |
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462 |
| 16 | 463 .NH 2 |
| 8 | 464 A powerful shell |
| 42 | 465 .XS |
| 466 \*(SN A powerful shell | |
| 467 .XE | |
| 8 | 468 .LP |
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469 It was already said, that the Unix shell provides the possibility to |
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470 combine small programs into large ones easily. |
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471 A powerful shell is a great feature in other ways, too. |
| 8 | 472 .PP |
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473 For instance by including a scripting language. |
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474 The control statements are build into the shell. |
| 8 | 475 The functions, however, are the normal programs, everyone can use on the system. |
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476 Thus, the programs are known, so learning to program in the shell is easy. |
| 8 | 477 Using normal programs as functions in the shell programming language |
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478 is only possible because they are small and combinable tools in a toolchest style. |
| 8 | 479 .PP |
| 480 The Unix shell encourages to write small scripts out of other programs, | |
| 481 because it is so easy to do. | |
| 482 This is a great step towards automation. | |
| 483 It is wonderful if the effort to automate a task equals the effort | |
| 484 it takes to do it the second time by hand. | |
| 485 If it is so, then the user will be happy to automate everything he does more than once. | |
| 486 .PP | |
| 487 Small programs that do one job well, standardized interfaces between them, | |
| 488 a mechanism to combine parts to larger parts, and an easy way to automate tasks, | |
| 489 this will inevitably produce software leverage. | |
| 490 Getting multiple times the benefit of an investment is a great offer. | |
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491 .PP |
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492 The shell also encourages rapid prototyping. |
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493 Many well known programs started as quickly hacked shell scripts, |
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494 and turned into ``real'' programs, written in C, later. |
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495 Building a prototype first is a way to avoid the biggest problems |
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496 in application development. |
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497 Fred Brooks writes in ``No Silver Bullet'': |
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498 .[ |
| 44 | 499 brooks |
| 500 no silver bullet | |
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501 .] |
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502 .QP |
| 41 | 503 .ps -1 |
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504 The hardest single part of building a software system is deciding precisely what to build. |
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505 No other part of the conceptual work is so difficult as establishing the detailed |
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506 technical requirements, [...]. |
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507 No other part of the work so cripples the resulting system if done wrong. |
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508 No other part is more difficult to rectify later. |
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509 .PP |
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510 Writing a prototype is a great method to become familiar with the requirements |
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511 and to actually run into real problems. |
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512 Today, prototyping is often seen as a first step in building a software. |
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513 This is, of course, good. |
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514 However, the Unix Philosophy has an \fIadditional\fP perspective on prototyping: |
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515 After having built the prototype, one might notice, that the prototype is already |
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516 \fIgood enough\fP. |
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517 Hence, no reimplementation, in a more sophisticated programming language, might be of need, |
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518 for the moment. |
| 23 | 519 Maybe later, it might be necessary to rewrite the software, but not now. |
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520 .PP |
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521 By delaying further work, one keeps the flexibility to react easily on |
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522 changing requirements. |
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523 Software parts that are not written will not miss the requirements. |
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524 |
| 16 | 525 .NH 2 |
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526 Worse is better |
| 42 | 527 .XS |
| 528 \*(SN Worse is better | |
| 529 .XE | |
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530 .LP |
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531 The Unix Philosophy aims for the 80% solution; |
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532 others call it the ``Worse is better'' approach. |
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533 .PP |
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534 First, practical experience shows, that it is almost never possible to define the |
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535 requirements completely and correctly the first time. |
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536 Hence one should not try to; it will fail anyway. |
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537 Second, practical experience shows, that requirements change during time. |
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538 Hence it is best to delay requirement-based design decisions as long as possible. |
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539 Also, the software should be small and flexible as long as possible |
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540 to react on changing requirements. |
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541 Shell scripts, for example, are more easily adjusted as C programs. |
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542 Third, practical experience shows, that maintenance is hard work. |
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543 Hence, one should keep the amount of software as small as possible; |
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544 it should just fulfill the \fIcurrent\fP requirements. |
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545 Software parts that will be written later, do not need maintenance now. |
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546 .PP |
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547 Starting with a prototype in a scripting language has several advantages: |
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548 .IP \(bu |
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549 As the initial effort is low, one will likely start right away. |
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550 .IP \(bu |
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551 As working parts are available soon, the real requirements can get identified soon. |
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552 .IP \(bu |
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553 When a software is usable, it gets used, and thus tested. |
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554 Hence problems will be found at early stages of the development. |
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555 .IP \(bu |
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556 The prototype might be enough for the moment, |
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557 thus further work on the software can be delayed to a time |
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558 when one knows better about the requirements and problems, |
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559 than now. |
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560 .IP \(bu |
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561 Implementing now only the parts that are actually needed now, |
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562 requires fewer maintenance work. |
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563 .IP \(bu |
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564 If the global situation changes so that the software is not needed anymore, |
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565 then less effort was spent into the project, than it would have be |
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566 when a different approach had been used. |
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567 |
| 16 | 568 .NH 2 |
| 11 | 569 Upgrowth and survival of software |
| 42 | 570 .XS |
| 571 \*(SN Upgrowth and survival of software | |
| 572 .XE | |
| 11 | 573 .LP |
| 12 | 574 So far it was talked about \fIwriting\fP or \fIbuilding\fP software. |
| 13 | 575 Although these are just verbs, they do imply a specific view on the work process |
| 576 they describe. | |
| 12 | 577 The better verb, however, is to \fIgrow\fP. |
| 578 .PP | |
| 579 Creating software in the sense of the Unix Philosophy is an incremental process. | |
| 580 It starts with a first prototype, which evolves as requirements change. | |
| 581 A quickly hacked shell script might become a large, sophisticated, | |
| 13 | 582 compiled program this way. |
| 583 Its lifetime begins with the initial prototype and ends when the software is not used anymore. | |
| 584 While being alive it will get extended, rearranged, rebuilt (from scratch). | |
| 12 | 585 Growing software matches the view that ``software is never finished. It is only released.'' |
| 586 .[ | |
| 44 | 587 gancarz |
| 588 unix philosophy | |
| 13 | 589 %P 26 |
| 12 | 590 .] |
| 591 .PP | |
| 13 | 592 Software can be seen as being controlled by evolutionary processes. |
| 593 Successful software is software that is used by many for a long time. | |
| 12 | 594 This implies that the software is needed, useful, and better than alternatives. |
| 595 Darwin talks about: ``The survival of the fittest.'' | |
| 596 .[ | |
| 44 | 597 darwin |
| 598 origin of species | |
| 12 | 599 .] |
| 600 Transferred to software: The most successful software, is the fittest, | |
| 601 is the one that survives. | |
| 13 | 602 (This may be at the level of one creature, or at the level of one species.) |
| 603 The fitness of software is affected mainly by four properties: | |
| 15 | 604 portability of code, portability of data, range of usability, and reusability of parts. |
| 605 .\" .IP \(bu | |
| 606 .\" portability of code | |
| 607 .\" .IP \(bu | |
| 608 .\" portability of data | |
| 609 .\" .IP \(bu | |
| 610 .\" range of usability | |
| 611 .\" .IP \(bu | |
| 612 .\" reuseability of parts | |
| 13 | 613 .PP |
| 15 | 614 (1) |
| 615 .I "Portability of code | |
| 616 means, using high-level programming languages, | |
| 13 | 617 sticking to the standard, |
| 618 and avoiding optimizations that introduce dependencies on specific hardware. | |
| 619 Hardware has a much lower lifetime than software. | |
| 620 By chaining software to a specific hardware, | |
| 621 the software's lifetime gets shortened to that of this hardware. | |
| 622 In contrast, software should be easy to port \(en | |
| 23 | 623 adaptation is the key to success. |
| 13 | 624 .\" cf. practice of prog: ch08 |
| 625 .PP | |
| 15 | 626 (2) |
| 627 .I "Portability of data | |
| 628 is best achieved by avoiding binary representations | |
| 13 | 629 to store data, because binary representations differ from machine to machine. |
| 23 | 630 Textual representation is favored. |
| 13 | 631 Historically, ASCII was the charset of choice. |
| 632 In the future, UTF-8 might be the better choice, however. | |
| 633 Important is that it is a plain text representation in a | |
| 634 very common charset encoding. | |
| 635 Apart from being able to transfer data between machines, | |
| 636 readable data has the great advantage, that humans are able | |
| 637 to directly edit it with text editors and other tools from the Unix toolchest. | |
| 638 .\" gancarz tenet 5 | |
| 12 | 639 .PP |
| 15 | 640 (3) |
| 641 A large | |
| 642 .I "range of usability | |
| 23 | 643 ensures good adaptation, and thus good survival. |
| 13 | 644 It is a special distinction if a software becomes used in fields of action, |
| 645 the original authors did never imagine. | |
| 646 Software that solves problems in a general way will likely be used | |
| 647 for all kinds of similar problems. | |
| 648 Being too specific limits the range of uses. | |
| 649 Requirements change through time, thus use cases change or even vanish. | |
| 650 A good example in this point is Allman's sendmail. | |
| 651 Allman identifies flexibility to be one major reason for sendmail's success: | |
| 652 .[ | |
| 44 | 653 allman |
| 654 sendmail | |
| 13 | 655 .] |
| 656 .QP | |
| 41 | 657 .ps -1 |
| 13 | 658 Second, I limited myself to the routing function [...]. |
| 659 This was a departure from the dominant thought of the time, [...]. | |
| 660 .QP | |
| 41 | 661 .ps -1 |
| 13 | 662 Third, the sendmail configuration file was flexible enough to adopt |
| 663 to a rapidly changing world [...]. | |
| 664 .LP | |
| 665 Successful software adopts itself to the changing world. | |
| 12 | 666 .PP |
| 15 | 667 (4) |
| 668 .I "Reuse of parts | |
| 669 is even one step further. | |
| 13 | 670 A software may completely lose its field of action, |
| 671 but parts of which the software is build may be general and independent enough | |
| 672 to survive this death. | |
| 673 If software is build by combining small independent programs, | |
| 674 then there are parts readily available for reuse. | |
| 675 Who cares if the large program is a failure, | |
| 676 but parts of it become successful instead? | |
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677 |
| 16 | 678 .NH 2 |
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679 Summary |
| 42 | 680 .XS |
| 681 \*(SN Summary | |
| 682 .XE | |
| 0 | 683 .LP |
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684 This chapter explained the central ideas of the Unix Philosophy. |
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685 For each of the ideas, it was exposed what advantages they introduce. |
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686 The Unix Philosophy are guidelines that help to write valuable software. |
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687 From the view point of a software developer or software designer, |
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688 the Unix Philosophy provides answers to many software design problem. |
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689 .PP |
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690 The various ideas of the Unix Philosophy are very interweaved |
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691 and can hardly be applied independently. |
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692 However, the probably most important messages are: |
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693 .I "``Do one thing well!''" , |
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694 .I "``Keep it simple!''" , |
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695 and |
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696 .I "``Use software leverage!'' |
| 0 | 697 |
| 8 | 698 |
| 699 | |
| 0 | 700 .NH 1 |
| 19 | 701 Case study: \s-1MH\s0 |
| 42 | 702 .XS |
| 703 .sp .5v | |
| 704 \*(SN Case study: \s-1MH\s0 | |
| 705 .XE | |
| 18 | 706 .LP |
| 30 | 707 The previous chapter introduced and explained the Unix Philosophy |
| 18 | 708 from a general point of view. |
| 30 | 709 The driving force were the guidelines; references to |
| 18 | 710 existing software were given only sparsely. |
| 711 In this and the next chapter, concrete software will be | |
| 712 the driving force in the discussion. | |
| 713 .PP | |
| 23 | 714 This first case study is about the mail user agents (\s-1MUA\s0) |
| 715 \s-1MH\s0 (``mail handler'') and its descendent \fInmh\fP | |
| 716 (``new mail handler''). | |
| 717 \s-1MUA\s0s provide functions to read, compose, and organize mail, | |
| 718 but (ideally) not to transfer. | |
| 19 | 719 In this document, the name \s-1MH\s0 will be used for both of them. |
| 720 A distinction will only be made if differences between | |
| 721 them are described. | |
| 18 | 722 |
| 0 | 723 |
| 724 .NH 2 | |
| 19 | 725 Historical background |
| 42 | 726 .XS |
| 727 \*(SN Historical background | |
| 728 .XE | |
| 0 | 729 .LP |
| 19 | 730 Electronic mail was available in Unix very early. |
| 30 | 731 The first \s-1MUA\s0 on Unix was \f(CWmail\fP, |
| 732 which was already present in the First Edition. | |
| 733 .[ | |
| 44 | 734 salus |
| 735 quarter century of unix | |
| 30 | 736 %P 41 f. |
| 737 .] | |
| 738 It was a small program that either prints the user's mailbox file | |
| 19 | 739 or appends text to someone elses mailbox file, |
| 740 depending on the command line arguments. | |
| 741 .[ | |
| 44 | 742 manual mail(1) |
| 19 | 743 .] |
| 744 It was a program that did one job well. | |
| 23 | 745 This job was emailing, which was very simple then. |
| 19 | 746 .PP |
| 23 | 747 Later, emailing became more powerful, and thus more complex. |
| 19 | 748 The simple \f(CWmail\fP, which knew nothing of subjects, |
| 749 independent handling of single messages, | |
| 750 and long-time storage of them, was not powerful enough anymore. | |
| 751 At Berkeley, Kurt Shoens wrote \fIMail\fP (with capital `M') | |
| 752 in 1978 to provide additional functions for emailing. | |
| 753 Mail was still one program, but now it was large and did | |
| 754 several jobs. | |
| 23 | 755 Its user interface is modeled after the one of \fIed\fP. |
| 19 | 756 It is designed for humans, but is still scriptable. |
| 23 | 757 \fImailx\fP is the adaptation of Berkeley Mail into System V. |
| 19 | 758 .[ |
| 44 | 759 ritter |
| 760 mailx history | |
| 19 | 761 .] |
| 30 | 762 Elm, pine, mutt, and a whole bunch of graphical \s-1MUA\s0s |
| 19 | 763 followed Mail's direction. |
| 764 They are large, monolithic programs which include all emailing functions. | |
| 765 .PP | |
| 23 | 766 A different way was taken by the people of \s-1RAND\s0 Corporation. |
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767 In the beginning, they also had used a monolithic mail system, |
| 30 | 768 called \s-1MS\s0 (for ``mail system''). |
| 19 | 769 But in 1977, Stockton Gaines and Norman Shapiro |
| 770 came up with a proposal of a new email system concept \(en | |
| 771 one that honors the Unix Philosophy. | |
| 772 The concept was implemented by Bruce Borden in 1978 and 1979. | |
| 773 This was the birth of \s-1MH\s0 \(en the ``mail handler''. | |
| 18 | 774 .PP |
| 775 Since then, \s-1RAND\s0, the University of California at Irvine and | |
| 19 | 776 at Berkeley, and several others have contributed to the software. |
| 18 | 777 However, it's core concepts remained the same. |
| 23 | 778 In the late 90s, when development of \s-1MH\s0 slowed down, |
| 19 | 779 Richard Coleman started with \fInmh\fP, the new mail handler. |
| 780 His goal was to improve \s-1MH\s0, especially in regard of | |
| 23 | 781 the requirements of modern emailing. |
| 19 | 782 Today, nmh is developed by various people on the Internet. |
| 18 | 783 .[ |
| 44 | 784 ware |
| 785 rand history | |
| 786 %P 128-137 | |
| 18 | 787 .] |
| 788 .[ | |
| 44 | 789 peek |
| 790 mh | |
| 18 | 791 .] |
| 0 | 792 |
| 793 .NH 2 | |
| 20 | 794 Contrasts to monolithic mail systems |
| 42 | 795 .XS |
| 796 \*(SN Contrasts to monolithic mail systems | |
| 797 .XE | |
| 0 | 798 .LP |
| 19 | 799 All \s-1MUA\s0s are monolithic, except \s-1MH\s0. |
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800 Although there might actually exist further, very little known, |
| 30 | 801 toolchest \s-1MUA\s0s, this statement reflects the situation pretty well. |
| 19 | 802 .PP |
| 30 | 803 Monolithic \s-1MUA\s0s gather all their functions in one program. |
| 804 In contrast, \s-1MH\s0 is a toolchest of many small tools \(en one for each job. | |
| 23 | 805 Following is a list of important programs of \s-1MH\s0's toolchest |
| 30 | 806 and their function. |
| 807 It gives a feeling of how the toolchest looks like. | |
| 19 | 808 .IP \(bu |
| 809 .CW inc : | |
| 30 | 810 incorporate new mail (this is how mail enters the system) |
| 19 | 811 .IP \(bu |
| 812 .CW scan : | |
| 813 list messages in folder | |
| 814 .IP \(bu | |
| 815 .CW show : | |
| 816 show message | |
| 817 .IP \(bu | |
| 818 .CW next\fR/\fPprev : | |
| 819 show next/previous message | |
| 820 .IP \(bu | |
| 821 .CW folder : | |
| 822 change current folder | |
| 823 .IP \(bu | |
| 824 .CW refile : | |
| 825 refile message into folder | |
| 826 .IP \(bu | |
| 827 .CW rmm : | |
| 828 remove message | |
| 829 .IP \(bu | |
| 830 .CW comp : | |
| 831 compose a new message | |
| 832 .IP \(bu | |
| 833 .CW repl : | |
| 834 reply to a message | |
| 835 .IP \(bu | |
| 836 .CW forw : | |
| 837 forward a message | |
| 838 .IP \(bu | |
| 839 .CW send : | |
| 30 | 840 send a prepared message (this is how mail leaves the system) |
| 0 | 841 .LP |
| 19 | 842 \s-1MH\s0 has no special user interface like monolithic \s-1MUA\s0s have. |
| 843 The user does not leave the shell to run \s-1MH\s0, | |
| 30 | 844 but he uses the various \s-1MH\s0 programs within the shell. |
| 23 | 845 Using a monolithic program with a captive user interface |
| 846 means ``entering'' the program, using it, and ``exiting'' the program. | |
| 847 Using toolchests like \s-1MH\s0 means running programs, | |
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848 alone or in combination with others, even from other toolchests, |
| 23 | 849 without leaving the shell. |
| 30 | 850 |
| 851 .NH 2 | |
| 852 Data storage | |
| 42 | 853 .XS |
| 854 \*(SN Data storage | |
| 855 .XE | |
| 30 | 856 .LP |
| 34 | 857 \s-1MH\s0's mail storage is a directory tree under the user's |
| 858 \s-1MH\s0 directory (usually \f(CW$HOME/Mail\fP), | |
| 859 where mail folders are directories and mail messages are text files | |
| 860 within them. | |
| 861 Each mail folder contains a file \f(CW.mh_sequences\fP which lists | |
| 862 the public message sequences of that folder, for instance new messages. | |
| 863 Mail messages are text files located in a mail folder. | |
| 864 The files contain the messages as they were received. | |
| 865 They are numbered in ascending order in each folder. | |
| 19 | 866 .PP |
| 30 | 867 This mailbox format is called ``\s-1MH\s0'' after the \s-1MUA\s0. |
| 868 Alternatives are \fImbox\fP and \fImaildir\fP. | |
| 869 In the mbox format all messages are stored within one file. | |
| 870 This was a good solution in the early days, when messages | |
| 871 were only a few lines of text and were deleted soon. | |
| 872 Today, when single messages often include several megabytes | |
| 873 of attachments, it is a bad solution. | |
| 874 Another disadvantage of the mbox format is that it is | |
| 875 more difficult to write tools that work on mail messages, | |
| 876 because it is always necessary to first find and extract | |
| 877 the relevant message in the mbox file. | |
| 878 With the \s-1MH\s0 mailbox format, | |
| 879 each message is a self-standing item, by definition. | |
| 880 Also, the problem of concurrent access to one mailbox is | |
| 881 reduced to the problem of concurrent access to one message. | |
| 882 Maildir is generally similar to \s-1MH\s0's format, | |
| 883 but modified towards guaranteed reliability. | |
| 884 This involves some complexity, unfortunately. | |
| 34 | 885 .PP |
| 886 Working with \s-1MH\s0's toolchest on mailboxes is much like | |
| 887 working with Unix' toolchest on directory trees: | |
| 888 \f(CWscan\fP is like \f(CWls\fP, | |
| 889 \f(CWshow\fP is like \f(CWcat\fP, | |
| 890 \f(CWfolder\fP is like \f(CWcd\fP and \f(CWpwd\fP, | |
| 891 \f(CWrefile\fP is like \f(CWmv\fP, | |
| 892 and \f(CWrmm\fP is like \f(CWrm\fP. | |
| 893 .PP | |
| 894 The context of tools in Unix consists mainly the current working directory, | |
| 895 the user identification, and the environment variables. | |
| 896 \s-1MH\s0 extends this context by two more items: | |
| 897 .IP \(bu | |
| 898 The current mail folder, which is similar to the current working directory. | |
| 899 For mail folders, \f(CWfolder\fP provides the corresponding functionality | |
| 900 of \f(CWcd\fP and \f(CWpwd\fP for directories. | |
| 901 .IP \(bu | |
| 902 Sequences, which are named sets of messages in a mail folder. | |
| 903 The current message, relative to a mail folder, is a special sequence. | |
| 904 It enables commands like \f(CWnext\fP and \f(CWprev\fP. | |
| 905 .LP | |
| 906 In contrast to Unix' context, which is chained to the shell session, | |
| 907 \s-1MH\s0's context is independent. | |
| 908 Usually there is one context for each user, but a user can have many | |
| 909 contexts. | |
| 910 Public sequences are an exception, as they belong to the mail folder. | |
| 911 .[ | |
| 44 | 912 man page mh-profile mh-sequence |
| 34 | 913 .] |
| 20 | 914 |
| 0 | 915 .NH 2 |
| 20 | 916 Discussion of the design |
| 42 | 917 .XS |
| 918 \*(SN Discussion of the design | |
| 919 .XE | |
| 20 | 920 .LP |
| 921 The following paragraphs discuss \s-1MH\s0 in regard to the tenets | |
| 23 | 922 of the Unix Philosophy which Gancarz identified. |
| 20 | 923 |
| 924 .PP | |
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925 .B "Small is beautiful |
| 20 | 926 and |
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927 .B "do one thing well |
| 20 | 928 are two design goals that are directly visible in \s-1MH\s0. |
| 929 Gancarz actually presents \s-1MH\s0 as example under the headline | |
| 930 ``Making UNIX Do One Thing Well'': | |
| 41 | 931 .[ |
| 44 | 932 gancarz |
| 933 unix philosophy | |
| 41 | 934 %P 125 |
| 935 .] | |
| 20 | 936 .QP |
| 41 | 937 .ps -1 |
| 20 | 938 [\s-1MH\s0] consists of a series of programs which |
| 939 when combined give the user an enormous ability | |
| 940 to manipulate electronic mail messages. | |
| 941 A complex application, it shows that not only is it | |
| 942 possible to build large applications from smaller | |
| 943 components, but also that such designs are actually preferable. | |
| 0 | 944 .LP |
| 20 | 945 The various small programs of \s-1MH\s0 were relatively easy |
| 23 | 946 to write, because each of them is small, limited to one function, |
| 947 and has clear boundaries. | |
| 20 | 948 For the same reasons, they are also good to maintain. |
| 949 Further more, the system can easily get extended. | |
| 950 One only needs to put a new program into the toolchest. | |
| 23 | 951 This was done, for instance, when \s-1MIME\s0 support was added |
| 20 | 952 (e.g. \f(CWmhbuild\fP). |
| 953 Also, different programs can exist to do the basically same job | |
| 954 in different ways (e.g. in nmh: \f(CWshow\fP and \f(CWmhshow\fP). | |
| 955 If someone needs a mail system with some additionally | |
| 23 | 956 functions that are available nowhere yet, he best takes a |
| 20 | 957 toolchest system like \s-1MH\s0 where he can add the |
| 958 functionality with little work. | |
| 959 | |
| 960 .PP | |
| 34 | 961 .B "Store data in flat text files |
| 962 is followed by \s-1MH\s0. | |
| 963 This is not surprising, because email messages are already plain text. | |
| 964 \s-1MH\s0 stores the messages as it receives them, | |
| 965 thus any other tool that works on RFC 2822 mail messages can operate | |
| 966 on the messages in an \s-1MH\s0 mailbox. | |
| 967 All other files \s-1MH\s0 uses are plain text too. | |
| 968 It is therefore possible and encouraged to use the text processing | |
| 969 tools of Unix' toolchest to extend \s-1MH\s0's toolchest. | |
| 20 | 970 |
| 971 .PP | |
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972 .B "Avoid captive user interfaces" . |
| 19 | 973 \s-1MH\s0 is perfectly suited for non-interactive use. |
| 974 It offers all functions directly and without captive user interfaces. | |
| 30 | 975 If, nonetheless, users want a graphical user interface, |
| 20 | 976 they can have it with \fIxmh\fP or \fIexmh\fP, too. |
| 19 | 977 These are graphical frontends for the \s-1MH\s0 toolchest. |
| 978 This means, all email-related work is still done by \s-1MH\s0 tools, | |
| 20 | 979 but the frontend issues the appropriate calls when the user |
| 30 | 980 clicks on buttons. |
| 20 | 981 Providing easy-to-use user interfaces in form of frontends is a good |
| 19 | 982 approach, because it does not limit the power of the backend itself. |
| 20 | 983 The frontend will anyway only be able to make a subset of the |
| 23 | 984 backend's power and flexibility available to the user. |
| 20 | 985 But if it is a separate program, |
| 986 then the missing parts can still be accessed at the backend directly. | |
| 19 | 987 If it is integrated, then this will hardly be possible. |
| 30 | 988 Further more, it is possible to have different frontends to the same |
| 989 backend. | |
| 19 | 990 |
| 991 .PP | |
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992 .B "Choose portability over efficiency |
| 20 | 993 and |
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994 .B "use shell scripts to increase leverage and portability" . |
| 20 | 995 These two tenets are indirectly, but nicely, demonstrated by |
| 30 | 996 Bolsky and Korn in their book about the Korn Shell. |
| 20 | 997 .[ |
| 44 | 998 bolsky korn |
| 999 korn shell | |
| 20 | 1000 .] |
| 30 | 1001 They demonstrated, in chapter 18 of the book, a basic implementation |
| 20 | 1002 of a subset of \s-1MH\s0 in ksh scripts. |
| 1003 Of course, this was just a demonstration, but a brilliant one. | |
| 1004 It shows how quickly one can implement such a prototype with shell scripts, | |
| 1005 and how readable they are. | |
| 1006 The implementation in the scripting language may not be very fast, | |
| 1007 but it can be fast enough though, and this is all that matters. | |
| 1008 By having the code in an interpreted language, like the shell, | |
| 1009 portability becomes a minor issue, if we assume the interpreter | |
| 1010 to be widespread. | |
| 1011 This demonstration also shows how easy it is to create single programs | |
| 1012 of a toolchest software. | |
| 30 | 1013 There are eight tools (two of them have multiple names) and 16 functions |
| 1014 with supporting code. | |
| 1015 Each tool comprises between 12 and 38 lines of ksh, | |
| 1016 in total about 200 lines. | |
| 1017 The functions comprise between 3 and 78 lines of ksh, | |
| 1018 in total about 450 lines. | |
| 20 | 1019 Such small software is easy to write, easy to understand, |
| 1020 and thus easy to maintain. | |
| 23 | 1021 A toolchest improves the possibility to only write some parts |
| 20 | 1022 and though create a working result. |
| 1023 Expanding the toolchest without global changes will likely be | |
| 1024 possible, too. | |
| 1025 | |
| 1026 .PP | |
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1027 .B "Use software leverage to your advantage |
| 20 | 1028 and the lesser tenet |
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1029 .B "allow the user to tailor the environment |
| 20 | 1030 are ideally followed in the design of \s-1MH\s0. |
| 21 | 1031 Tailoring the environment is heavily encouraged by the ability to |
| 30 | 1032 directly define default options to programs. |
| 1033 It is even possible to define different default options | |
| 21 | 1034 depending on the name under which the program was called. |
| 1035 Software leverage is heavily encouraged by the ease it is to | |
| 1036 create shell scripts that run a specific command line, | |
| 30 | 1037 built of several \s-1MH\s0 programs. |
| 21 | 1038 There is few software that so much wants users to tailor their |
| 1039 environment and to leverage the use of the software, like \s-1MH\s0. | |
| 1040 Just to make one example: | |
| 23 | 1041 One might prefer a different listing format for the \f(CWscan\fP |
| 21 | 1042 program. |
| 30 | 1043 It is possible to take one of the distributed format files |
| 21 | 1044 or to write one yourself. |
| 1045 To use the format as default for \f(CWscan\fP, a single line, | |
| 1046 reading | |
| 1047 .DS | |
| 1048 .CW | |
| 1049 scan: -form FORMATFILE | |
| 1050 .DE | |
| 1051 must be added to \f(CW.mh_profile\fP. | |
| 1052 If one wants this different format as an additional command, | |
| 23 | 1053 instead of changing the default, he needs to create a link to |
| 1054 \f(CWscan\fP, for instance titled \f(CWscan2\fP. | |
| 21 | 1055 The line in \f(CW.mh_profile\fP would then start with \f(CWscan2\fP, |
| 30 | 1056 as the option should only be in effect when scan is called as |
| 21 | 1057 \f(CWscan2\fP. |
| 20 | 1058 |
| 1059 .PP | |
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1060 .B "Make every program a filter |
| 21 | 1061 is hard to find in \s-1MH\s0. |
| 1062 The reason therefore is that most of \s-1MH\s0's tools provide | |
| 1063 basic file system operations for the mailboxes. | |
| 30 | 1064 The reason is the same because of which |
| 21 | 1065 \f(CWls\fP, \f(CWcp\fP, \f(CWmv\fP, and \f(CWrm\fP |
| 1066 aren't filters neither. | |
| 23 | 1067 However, they build a basis on which filters can operate. |
| 1068 \s-1MH\s0 does not provide many filters itself, but it is a basis | |
| 1069 to write filters for. | |
| 30 | 1070 An example would be a mail message text highlighter, |
| 1071 that means a program that makes use of a color terminal to display | |
| 1072 header lines, quotations, and signatures in distinct colors. | |
| 1073 The author's version of this program, for instance, | |
| 1074 is a 25 line awk script. | |
| 21 | 1075 |
| 1076 .PP | |
|
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1077 .B "Build a prototype as soon as possible |
| 21 | 1078 was again well followed by \s-1MH\s0. |
| 1079 This tenet, of course, focuses on early development, which is | |
| 1080 long time ago for \s-1MH\s0. | |
| 1081 But without following this guideline at the very beginning, | |
| 23 | 1082 Bruce Borden may have not convinced the management of \s-1RAND\s0 |
| 1083 to ever create \s-1MH\s0. | |
| 1084 In Bruce' own words: | |
| 41 | 1085 .[ |
| 44 | 1086 ware rand history |
| 1087 %P 132 | |
| 41 | 1088 .] |
| 21 | 1089 .QP |
| 41 | 1090 .ps -1 |
| 30 | 1091 [...] but they [Stockton Gaines and Norm Shapiro] were not able |
| 23 | 1092 to convince anyone that such a system would be fast enough to be usable. |
| 21 | 1093 I proposed a very short project to prove the basic concepts, |
| 1094 and my management agreed. | |
| 1095 Looking back, I realize that I had been very lucky with my first design. | |
| 1096 Without nearly enough design work, | |
| 1097 I built a working environment and some header files | |
| 1098 with key structures and wrote the first few \s-1MH\s0 commands: | |
| 1099 inc, show/next/prev, and comp. | |
| 1100 [...] | |
| 1101 With these three, I was able to convince people that the structure was viable. | |
| 1102 This took about three weeks. | |
| 0 | 1103 |
| 1104 .NH 2 | |
| 1105 Problems | |
| 42 | 1106 .XS |
| 1107 \*(SN Problems | |
| 1108 .XE | |
| 0 | 1109 .LP |
| 22 | 1110 \s-1MH\s0, for sure is not without problems. |
| 30 | 1111 There are two main problems: one is technical, the other is about human behavior. |
| 22 | 1112 .PP |
| 1113 \s-1MH\s0 is old and email today is very different to email in the time | |
| 1114 when \s-1MH\s0 was designed. | |
| 1115 \s-1MH\s0 adopted to the changes pretty well, but it is limited. | |
| 1116 For example in development resources. | |
| 1117 \s-1MIME\s0 support and support for different character encodings | |
| 1118 is available, but only on a moderate level. | |
| 1119 More active developers could quickly improve there. | |
| 1120 It is also limited by design, which is the larger problem. | |
| 1121 \s-1IMAP\s0, for example, conflicts with \s-1MH\s0's design to a large extend. | |
| 1122 These design conflicts are not easily solvable. | |
| 1123 Possibly, they require a redesign. | |
| 30 | 1124 Maybe \s-1IMAP\s0 is too different to the classic mail model which \s-1MH\s0 covers, |
| 1125 hence \s-1MH\s0 may never work well with \s-1IMAP\s0. | |
| 22 | 1126 .PP |
| 1127 The other kind of problem is human habits. | |
| 1128 When in this world almost all \s-1MUA\s0s are monolithic, | |
| 1129 it is very difficult to convince people to use a toolbox style \s-1MUA\s0 | |
| 1130 like \s-1MH\s0. | |
| 1131 The habits are so strong, that even people who understood the concept | |
| 30 | 1132 and advantages of \s-1MH\s0 do not like to switch, |
| 1133 simply because \s-1MH\s0 is different. | |
| 1134 Unfortunately, the frontends to \s-1MH\s0, which could provide familiar look'n'feel, | |
| 1135 are quite outdated and thus not very appealing compared to the modern interfaces | |
| 1136 which monolithic \s-1MUA\s0s offer. | |
| 20 | 1137 |
| 1138 .NH 2 | |
| 1139 Summary \s-1MH\s0 | |
| 42 | 1140 .XS |
| 1141 \*(SN Summary \s-1MH\s0 | |
| 1142 .XE | |
| 20 | 1143 .LP |
| 31 | 1144 \s-1MH\s0 is an \s-1MUA\s0 that follows the Unix Philosophy in its design |
| 1145 and implementation. | |
| 1146 It consists of a toolchest of small tools, each of them does one job well. | |
| 1147 The tools are orthogonal to each other, to a large extend. | |
| 1148 However, for historical reasons, there also exist distinct tools | |
| 1149 that cover the same task. | |
| 1150 .PP | |
| 1151 The toolchest approach offers great flexibility to the user. | |
| 1152 He can use the complete power of the Unix shell with \s-1MH\s0. | |
| 1153 This makes \s-1MH\s0 a very powerful mail system. | |
| 1154 Extending and customizing \s-1MH\s0 is easy and encouraged, too. | |
| 1155 .PP | |
| 1156 Apart from the user's perspective, \s-1MH\s0 is development-friendly. | |
| 1157 Its overall design follows clear rules. | |
| 1158 The single tools do only one job, thus they are easy to understand, | |
| 1159 easy to write, and good to maintain. | |
| 1160 They are all independent and do not interfere with the others. | |
| 1161 Automated testing of their function is a straight forward task. | |
| 1162 .PP | |
| 1163 It is sad, that \s-1MH\s0's differentness is its largest problem, | |
| 1164 as its differentness is also its largest advantage. | |
| 1165 Unfortunately, for most people their habits are stronger | |
| 1166 than the attraction of the clear design and the power, \s-1MH\s0 offers. | |
| 0 | 1167 |
| 8 | 1168 |
| 1169 | |
| 0 | 1170 .NH 1 |
| 1171 Case study: uzbl | |
| 42 | 1172 .XS |
| 1173 .sp .5v | |
| 1174 \*(SN Case study: uzbl | |
| 1175 .XE | |
| 32 | 1176 .LP |
| 1177 The last chapter took a look on the \s-1MUA\s0 \s-1MH\s0, | |
| 1178 this chapter is about uzbl, a web browser that adheres to the Unix Philosophy. | |
| 1179 ``uzbl'' is the \fIlolcat\fP's word for the English adjective ``usable''. | |
| 1180 It is pronounced the identical. | |
| 0 | 1181 |
| 1182 .NH 2 | |
| 32 | 1183 Historical background |
| 42 | 1184 .XS |
| 1185 \*(SN Historical background | |
| 1186 .XE | |
| 0 | 1187 .LP |
| 32 | 1188 Uzbl was started by Dieter Plaetinck in April 2009. |
| 44 | 1189 The idea was born in a thread in the Arch Linux Forums. |
| 32 | 1190 .[ |
| 44 | 1191 arch linux forums |
| 1192 browser | |
| 32 | 1193 .] |
| 1194 After some discussion about failures of well known web browsers, | |
| 1195 Plaetinck (alias Dieter@be) came up with a very sketchy proposal | |
| 1196 of how a better web browser could look like. | |
| 1197 To the question of another member, if Plaetinck would write that program, | |
| 1198 because it would sound fantastic, Plaetinck replied: | |
| 1199 ``Maybe, if I find the time ;-)''. | |
| 1200 .PP | |
| 1201 Fortunately, he found the time. | |
| 1202 One day later, the first prototype was out. | |
| 1203 One week later, uzbl had an own website. | |
| 1204 One month after the first code showed up, | |
| 1205 a mailing list was installed to coordinate and discuss further development. | |
| 1206 A wiki was set up to store documentation and scripts that showed up on the | |
| 1207 mailing list and elsewhere. | |
| 1208 .PP | |
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1209 In the, now, one year of uzbl's existence, it was heavily developed in various branches. |
| 32 | 1210 Plaetinck's task became more and more to only merge the best code from the |
| 1211 different branches into his main branch, and to apply patches. | |
| 1212 About once a month, Plaetinck released a new version. | |
| 1213 In September 2009, he presented several forks of uzbl. | |
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1214 Uzbl, actually, opened the field for a whole family of web browsers with similar shape. |
| 32 | 1215 .PP |
| 1216 In July 2009, \fILinux Weekly News\fP published an interview with Plaetinck about uzbl. | |
| 1217 In September 2009, the uzbl web browser was on \fISlashdot\fP. | |
| 0 | 1218 |
| 1219 .NH 2 | |
| 32 | 1220 Contrasts to other web browsers |
| 42 | 1221 .XS |
| 1222 \*(SN Contrasts to other web browsers | |
| 1223 .XE | |
| 0 | 1224 .LP |
| 32 | 1225 Like most \s-1MUA\s0s are monolithic, but \s-1MH\s0 is a toolchest, |
| 1226 most web browsers are monolithic, but uzbl is a frontend to a toolchest. | |
| 1227 .PP | |
| 1228 Today, uzbl is divided into uzbl-core and uzbl-browser. | |
| 1229 Uzbl-core is, how its name already indicates, the core of uzbl. | |
| 1230 It handles commands and events to interface other programs, | |
| 1231 and also displays webpages by using webkit as render engine. | |
| 1232 Uzbl-browser combines uzbl-core with a bunch of handler scripts, a status bar, | |
| 1233 an event manager, yanking, pasting, page searching, zooming, and more stuff, | |
| 1234 to form a ``complete'' web browser. | |
| 1235 In the following text, the term ``uzbl'' usually stands for uzbl-browser, | |
| 1236 so uzbl-core is included. | |
| 1237 .PP | |
| 1238 Unlike most other web browsers, uzbl is mainly the mediator between the | |
| 1239 various tools that cover single jobs of web browsing. | |
| 35 | 1240 Therefore, uzbl listens for commands on a named pipe (fifo), a Unix socket, |
| 1241 and on stdin, and it writes events to a Unix socket and to stdout. | |
| 1242 The graphical rendering of the webpage is done by webkit, a web content engine. | |
| 1243 Uzbl-core is build around this library. | |
| 1244 Loading a webpage in a running uzbl instance requires only: | |
| 32 | 1245 .DS |
| 1246 .CW | |
| 1247 echo 'uri http://example.org' >/path/to/uzbl-fifo | |
| 1248 .DE | |
| 1249 .PP | |
| 1250 Downloads, browsing history, bookmarks, and thelike are not provided | |
| 1251 by uzbl-core itself, as they are in other web browsers. | |
| 35 | 1252 Uzbl-browser also only provides, so called, handler scripts that wrap |
| 1253 external applications which provide the actual functionality. | |
| 32 | 1254 For instance, \fIwget\fP is used to download files and uzbl-browser |
| 1255 includes a script that calls wget with appropriate options in | |
| 1256 a prepared environment. | |
| 1257 .PP | |
| 1258 Modern web browsers are proud to have addons, plugins, and modules, instead. | |
| 1259 This is their effort to achieve similar goals. | |
| 35 | 1260 But instead of using existing, external programs, modern web browsers |
| 1261 include these functions, although they might be loaded at runtime. | |
| 0 | 1262 |
| 1263 .NH 2 | |
| 32 | 1264 Discussion of the design |
| 42 | 1265 .XS |
| 1266 \*(SN Discussion of the design | |
| 1267 .XE | |
| 0 | 1268 .LP |
| 32 | 1269 This section discusses uzbl in regard of the Unix Philosophy, |
| 1270 as identified by Gancarz. | |
| 1271 | |
| 1272 .PP | |
| 35 | 1273 .B "Make each program do one thing well" . |
| 1274 Uzbl tries to be a web browser and nothing else. | |
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1275 The common definition of a web browser is, of course, highly influenced by |
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1276 existing implementations of web browsers, although they are degenerated. |
| 35 | 1277 Web browsers should be programs to browse the web, and nothing more. |
| 1278 This is the one thing they should do, as demanded by the Unix Philosophy. | |
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1279 .PP |
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1280 Web browsers should, for instance, not manage downloads. |
| 35 | 1281 This is the job download managers exist for. |
| 1282 Download managers do primary care about being good in downloading files. | |
| 1283 Modern web browsers provide download management only as a secondary feature. | |
| 1284 How could they perform this job better, than programs that exist only for | |
| 1285 this very job? | |
| 1286 And how could anyone want less than the best download manager available? | |
| 32 | 1287 .PP |
| 35 | 1288 A web browser's job is to let the user browse the web. |
| 1289 This means, navigating through websites by following links. | |
|
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1290 Rendering the \s-1HTML\s0 sources is a different job, too. |
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1291 It is covered by the webkit render engine, in uzbl's case. |
| 35 | 1292 Audio and video content and files like PostScript, \s-1PDF\s0, and the like, |
|
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1293 are also not the job of a web browser. |
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1294 They should be handled by external applications \(en |
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1295 ones which's job is to handle such data. |
| 35 | 1296 Uzbl strives to do it this way. |
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1297 .PP |
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1298 Remember Doug McIlroy: |
| 35 | 1299 .I |
| 1300 ``Write programs that do one thing and do it well. | |
| 1301 Write programs to work together.'' | |
| 1302 .R | |
| 1303 .PP | |
| 1304 The lesser tenet | |
| 1305 .B "allow the user to tailor the environment | |
| 1306 matches good here. | |
| 1307 There was the question, how anyone could want anything less than the | |
| 1308 best program for the job. | |
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1309 But as personal preferences matter much, |
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1310 it is probably more important to ask: |
| 35 | 1311 How could anyone want something else than his preferred program for the job? |
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1312 .PP |
| 35 | 1313 Usually users want one program for one job. |
| 1314 Hence, whenever the task is, for instance, downloading, | |
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1315 exactly one download manager should be used. |
| 35 | 1316 More advanced users might want to have this download manager in this |
| 1317 situation and that one in that situation. | |
| 1318 They should be able to configure it this way. | |
| 1319 With uzbl, one can use any download manager the user wants. | |
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1320 To switch to a different one, only one line in a small handler script |
| 35 | 1321 needs to be changed. |
|
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1322 Alternatively it would be possible to query an entry in a global file |
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1323 or an environment variable, which specifies the download manager to use, |
| 35 | 1324 in the handler script. |
|
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1325 .PP |
| 35 | 1326 As uzbl does neither have its own download manager nor depends on a |
| 1327 specific one, thus uzbl's browsing abilities will not be lowered by having | |
| 1328 a bad download manager. | |
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1329 Uzbl's download capabilities will just as good as the ones of the best |
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1330 download manager available on the system. |
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1331 Of course, this applies to all of the other supplementary tools, too. |
| 32 | 1332 |
| 1333 .PP | |
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1334 .B "Use software leverage to your advantage" . |
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1335 Shell scripts are a good choice to extend uzbl. |
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1336 Uzbl is designed to be extended by external tools. |
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1337 These external tools are usually wrapped by small handler shell scripts. |
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1338 Shell scripts are the glue in this approach. |
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1339 They make the various parts fit together. |
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1340 .PP |
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1341 As an example, the history mechanism of uzbl shall be presented. |
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1342 Uzbl is configured to spawn a script to append an entry to the history |
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1343 whenever the event of a fully loaded page occurs. |
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1344 The script to append the entry to the history not much more than: |
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1345 .DS |
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1346 .CW |
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1347 #!/bin/sh |
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1348 file=/path/to/uzbl-history |
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1349 echo `date +'%Y-%m-%d %H:%M:%S'`" $6 $7" >> $file |
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1350 .DE |
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1351 \f(CW$6\fP and \f(CW$7\fP expand to the \s-1URL\s0 and the page title. |
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1352 For loading an entry, a key is bound to spawn a load from history script. |
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1353 The script reverses the history to have newer entries first, |
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1354 then displays \fIdmenu\fP to select an item, |
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1355 and afterwards writes the selected \s-1URL\s0 into uzbl's command input pipe. |
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1356 With error checking and corner cases removed, the script looks like this: |
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1357 .DS |
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1358 .CW |
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1359 #!/bin/sh |
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1360 file=/path/to/uzbl-history |
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1361 goto=`tac $file | dmenu | cut -d' ' -f 3` |
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1362 echo "uri $goto" > $4 |
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1363 .DE |
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1364 \f(CW$4\fP expands to the path of the command input pipe of the current |
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1365 uzbl instance. |
| 32 | 1366 |
| 1367 .PP | |
|
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1368 .B "Avoid captive user interfaces" . |
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1369 One could say, that uzbl, to a large extend, actually \fIis\fP |
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1370 a captive user interface. |
| 37 | 1371 But the difference to most other web browsers is, that uzbl is only |
| 1372 the captive user interface frontend and the core of the backend. | |
|
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1373 Many parts of the backend are independent of uzbl. |
| 37 | 1374 Some are distributed with uzbl, for some external programs, handler scripts |
| 1375 are distributed, arbitrary additional functionality can be added if desired. | |
| 1376 .PP | |
| 1377 The frontend is captive \(en that is true. | |
| 1378 This is okay for the task of browsing the web, as this task is only relevant | |
| 1379 for humans. | |
| 1380 Automated programs would \fIcrawl\fP the web. | |
| 1381 That means, they read the source directly. | |
| 1382 The source includes all the semantics. | |
| 1383 The graphical representation is just for humans to transfer the semantics | |
| 1384 more intuitively. | |
| 32 | 1385 |
| 1386 .PP | |
|
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1387 .B "Make every program a filter" . |
| 37 | 1388 Graphical web browsers are almost dead ends in the chain of information flow. |
| 1389 Thus it is difficult to see what graphical web browsers should filter. | |
| 1390 Graphical web browsers exist almost only for interactive use by humans. | |
| 1391 The only case when one might want to automate the rendering function is | |
| 1392 to generate images of rendered webpages. | |
| 1393 | |
| 1394 .PP | |
| 1395 .B "Small is beautiful" | |
|
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1396 is not easy to apply to a web browser, primary because modern web technology |
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1397 is very complex; hence the rendering task is very complex. |
| 37 | 1398 Modern web browsers will always consist of many thousand lines of code, |
| 1399 unfortunately. | |
| 1400 Using the toolchest approach and wrappers can split the browser into | |
| 1401 several small parts, tough. | |
| 1402 .PP | |
| 1403 Uzbl-core consists of about 3\,500 lines of C code. | |
| 1404 The distribution includes another 3\,500 lines of Shell and Python code, | |
| 1405 which are the handler scripts and plugins like a modal interface. | |
|
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1406 Further more, uzbl uses functionality of external tools like |
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1407 \fIwget\fP and \fInetcat\fP. |
| 37 | 1408 Up to this point, uzbl looks pretty neat and small. |
|
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1409 The ugly part of uzbl is the web content renderer, webkit. |
| 37 | 1410 Webkit consists of roughly 400\,000 (!) lines of code. |
|
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1411 Unfortunately, small web render engines are not possible anymore |
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1412 because of the modern web. |
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1413 The problems section will explain this in more detail. |
| 35 | 1414 |
| 1415 .PP | |
| 1416 .B "Build a prototype as soon as possible" . | |
| 1417 Plaetinck made his code public, right from the beginning. | |
|
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1418 Discussion and development was, and still is, open to everyone interested. |
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1419 Development versions of uzbl can be obtained very simply from the code |
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1420 repository. |
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1421 Within the first year of uzbl's existence, a new version was released |
| 35 | 1422 more often than once a month. |
|
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1423 Different forks and branches arose. |
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1424 They introduced new features, which were tested for suitability. |
| 35 | 1425 The experiences of using prototypes influenced further development. |
| 1426 Actually, all development was community driven. | |
|
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1427 Plaetinck says, three months after uzbl's birth: |
| 35 | 1428 ``Right now I hardly code anything myself for Uzbl. |
| 1429 I just merge in other people's code, ponder a lot, and lead the discussions.'' | |
| 1430 .[ | |
| 44 | 1431 lwn |
| 1432 uzbl | |
| 35 | 1433 .] |
| 32 | 1434 |
| 0 | 1435 |
| 1436 .NH 2 | |
| 1437 Problems | |
| 42 | 1438 .XS |
| 1439 \*(SN Problems | |
| 1440 .XE | |
| 0 | 1441 .LP |
|
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1442 Similar to \s-1MH\s0, uzbl, too suffers from being different. |
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1443 It is sad, but people use what they know. |
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1444 Fortunately, uzbl's user interface can look and feel very much the |
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1445 same as the one of the well known web browsers, |
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1446 hiding the internal differences. |
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1447 But uzbl has to provide this similar look and feel to be accepted |
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1448 as a ``normal'' browser by ``normal'' users. |
| 37 | 1449 .PP |
|
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1450 The more important problem is the modern web. |
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1451 The modern web is simply broken. |
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1452 It has state in a state-less protocol, |
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1453 it misuses technologies, |
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1454 and it is helplessly overloaded. |
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1455 The result are web content render engines that must consist |
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1456 of hundreds of thousands lines of code. |
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1457 They also must combine and integrate many different technologies, |
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1458 only to make our modern web usable. |
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1459 Website to image converter are hardly possible to run without |
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1460 human interaction because of state in sessions, impossible |
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1461 deep-linking, and unautomatable technologies. |
| 37 | 1462 .PP |
|
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1463 The web was misused to provide all kinds of imaginable wishes. |
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1464 Now web browsers, and eventually the users, suffer from it. |
| 37 | 1465 |
| 8 | 1466 |
| 32 | 1467 .NH 2 |
| 1468 Summary uzbl | |
| 42 | 1469 .XS |
| 1470 \*(SN Summary uzbl | |
| 1471 .XE | |
| 32 | 1472 .LP |
|
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1473 ``Uzbl is a browser that adheres to the Unix Philosophy'', |
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1474 that is how uzbl is seen by its authors. |
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1475 Indeed, uzbl follows the Unix Philosophy in many ways. |
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1476 It consists of independent parts that work together, |
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1477 its core is mainly a mediator which glues the parts together. |
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1478 .PP |
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1479 Software leverage can excellently be seen in uzbl. |
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1480 It makes use of external tools, separates independent tasks |
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1481 in independent parts, and glues them together with small |
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1482 handler scripts, around uzbl-core. |
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1483 .PP |
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1484 As uzbl, more or less, consists of a set of tools and a bit |
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1485 of glue, anyone can put the parts together and expand it |
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1486 in any desired way. |
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1487 Uzbl is very flexible and customizable. |
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1488 These properties make it valuable for advanced users, |
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1489 but may keep novice users from using it. |
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1490 .PP |
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1491 Uzbl's main problem is the modern web, that makes it hard |
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1492 to design a sane web browser. |
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1493 Despite this bad situation, uzbl does a fairly good job. |
| 32 | 1494 |
| 8 | 1495 |
| 0 | 1496 .NH 1 |
| 1497 Final thoughts | |
| 42 | 1498 .XS |
| 1499 .sp .5v | |
| 1500 \*(SN Final thoughts | |
| 1501 .XE | |
| 0 | 1502 |
| 1503 .NH 2 | |
| 1504 Quick summary | |
| 42 | 1505 .XS |
| 1506 \*(SN Quick summary | |
| 1507 .XE | |
| 0 | 1508 .LP |
| 1509 good design | |
| 1510 .LP | |
| 1511 unix phil | |
| 1512 .LP | |
| 1513 case studies | |
| 1514 | |
| 1515 .NH 2 | |
| 1516 Why people should choose | |
| 42 | 1517 .XS |
| 1518 \*(SN Why people should choose | |
| 1519 .XE | |
| 0 | 1520 .LP |
| 1521 Make the right choice! | |
| 1522 | |
| 42 | 1523 .nr PI .3i |
| 0 | 1524 .rm ]< |
| 1525 .de ]< | |
| 1526 .LP | |
| 1527 .de FP | |
| 1528 .IP \\\\$1. | |
| 1529 \\.. | |
| 1530 .rm FS FE | |
| 1531 .. | |
| 42 | 1532 .ds CH " |
| 1533 .bp | |
| 1534 .rs | |
| 1535 .sp .3i | |
| 1536 .TL | |
| 0 | 1537 References |
| 42 | 1538 .LP |
| 1539 .XS | |
| 1540 .sp .5v | |
| 1541 References | |
| 1542 .XE | |
| 1543 .sp 2v | |
| 1544 .nr PS -1 | |
| 1545 .nr VS -1 | |
| 0 | 1546 .[ |
| 1547 $LIST$ | |
| 1548 .] | |
| 42 | 1549 .\".wh -1p |
| 1550 .bp | |
| 1551 .PX |