annotate unix-phil.ms @ 6:a6b837d822b7

rewrote ch02
author meillo@marmaro.de
date Thu, 18 Feb 2010 11:24:36 +0100
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children 606f0ffca834
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19 ..
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20 .ds [. \ [
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21 .ds .] ]
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22 .\"----------------------------------------
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23 .TL
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24 Why the Unix Philosophy still matters
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25 .AU
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26 markus schnalke <meillo@marmaro.de>
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27 .AB
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28 .ti \n(.iu
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29 This paper discusses the importance of the Unix Philosophy in software design.
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30 Today, few software designers are aware of these concepts,
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31 and thus most modern software is limited and does not make use of software leverage.
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32 Knowing and following the tenets of the Unix Philosophy makes software more valuable.
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33 .AE
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35 .if t .2C
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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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46 .NH 1
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47 Introduction
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48 .LP
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49 Building a software is a process from an idea of the purpose of the software
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50 to its release.
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51 No matter \fIhow\fP the process is run, two things are common:
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52 the initial idea and the release.
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53 The process inbetween can be of any shape.
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54 The the maintainance work after the release is ignored for the moment.
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55 .PP
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56 The process of building splits mainly in two parts:
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57 the planning of what and how to build, and implementing the plan by writing code.
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58 This paper focuses on the planning part \(en the designing of the software.
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59 .PP
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60 Software design is the plan of how the internals and externals of the software should look like,
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61 based on the requirements.
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62 This paper discusses the recommendations of the Unix Philosphy about software design.
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63 .PP
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64 The here discussed ideas can get applied by any development process.
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65 The Unix Philosphy does recommend how the software development process should look like,
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66 but this shall not be of matter here.
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67 Similar, the question of how to write the code is out of focus.
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68 .PP
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69 The name ``Unix Philosophy'' was already mentioned several times, but it was not explained yet.
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70 The Unix Philosophy is the essence of how the Unix operating system and its toolchest was designed.
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71 It is no limited set of rules, but what people see to be common to typical Unix software.
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72 Several people stated their view on the Unix Philosophy.
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73 Best known are:
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74 .IP \(bu
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75 Doug McIlroy's summary: ``Write programs that do one thing and do it well.''
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76 .[
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77 %A M. D. McIlroy
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78 %A E. N. Pinson
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79 %A B. A. Taque
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80 %T UNIX Time-Sharing System Forward
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81 %J The Bell System Technical Journal
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82 %D 1978
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83 %V 57
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84 %N 6
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85 %P 1902
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86 .]
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87 .IP \(bu
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88 Mike Gancarz' book ``The UNIX Philosophy''.
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89 .[
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90 %A Mike Gancarz
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91 %T The UNIX Philosophy
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92 %D 1995
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93 %I Digital Press
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94 .]
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95 .IP \(bu
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96 Eric S. Raymond's book ``The Art of UNIX Programming''.
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97 .[
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98 %A Eric S. Raymond
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99 %T The Art of UNIX Programming
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100 %D 2003
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101 %I Addison-Wesley
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102 %O .CW \s-1http://www.faqs.org/docs/artu/
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103 .]
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104 .LP
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105 These different views on the Unix Philosophy have much in common.
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106 Especially, the main concepts are similar for all of them.
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107 But there are also points on which they differ.
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108 This only underlines what the Unix Philosophy is:
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109 A retrospective view on the main concepts of Unix software;
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110 especially those that were sucessful and unique to Unix.
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111 .\" really?
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112 .PP
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113 Before we will have a look at concrete concepts,
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114 we discuss why software design is important
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115 and what problems bad design introduces.
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116
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117
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118 .NH 1
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119 Importance of software design in general
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120 .LP
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121 Why should we design software at all?
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122 It is general knowledge, that even a bad plan is better than no plan.
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123 Ignoring software design is programming without a plan.
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124 This will lead pretty sure to horrible results.
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125 .PP
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126 The design of a software is its internal and external shape.
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127 The design talked about here has nothing to do with visual appearance.
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128 If we see a program as a car, then its color is of no matter.
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129 Its design would be the car's size, its shape, the number and position of doors,
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130 the ratio of passenger and cargo transport, and so forth.
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131 .PP
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132 A software's design is about quality properties.
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133 Each of the cars may be able to drive from A to B,
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134 but it depends on its properties whether it is a good car for passenger transport or not.
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135 It also depends on its properties if it is a good choice for a rough mountain area.
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136 .PP
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137 Requirements to a software are twofold: functional and non-functional.
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138 Functional requirements are easier to define and to verify.
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139 They are directly the software's functions.
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140 Functional requirements are the reason why software gets written.
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141 Someone has a problem and needs a tool to solve it.
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142 Being able to solve the problem is the main functional requirement.
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143 It is the driving force behind all programming effort.
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144 .PP
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145 On the other hand, there are also non-functional requirements.
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146 They are called \fIquality\fP requirements, too.
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147 The quality of a software is about properties that are not directly related to
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148 the software's basic functions.
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149 Quality aspects are about the properties that are overlooked at first sight.
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150 .PP
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151 Quality is of few matter when the software gets initially built,
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152 but it will be of matter in usage and maintainance of the software.
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153 A short-sighted might see in developing a software mainly building something up.
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154 Reality shows, that building the software the first time is only a small amount
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155 of the overall work.
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156 Bug fixing, extending, rebuiling of parts \(en short: maintainance work \(en
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157 does soon take over the major part of the time spent on a software.
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158 Not to forget the usage of the software.
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159 These processes are highly influenced by the software's quality.
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160 Thus, quality should never be neglected.
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161 The problem is that you hardly ``stumble over'' bad quality during the first build,
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162 but this is the time when you should care about good quality most.
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163 .PP
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164 Software design is not about the basic function of a software;
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165 this requirement will get satisfied anyway, as it is the main driving force behind the development.
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166 Software design is about quality aspects of the software.
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167 Good design will lead to good quality, bad design to bad quality.
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168 The primary functions of the software will be affected modestly by bad quality,
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169 but good quality can provide a lot of additional gain from the software,
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170 even at places where one never expected it.
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171 .PP
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172 The ISO/IEC 9126-1 standard, part 1,
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173 .[
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174 %I International Organization for Standarization
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175 %T ISO Standard 9126: Software Engineering \(en Product Quality, part 1
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176 %C Geneve
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177 %D 2001
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178 .]
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179 defines the quality model as consisting out of:
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180 .IP \(bu
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181 .I Functionality
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182 (suitability, accuracy, inter\%operability, security)
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183 .IP \(bu
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184 .I Reliability
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185 (maturity, fault tolerance, recoverability)
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186 .IP \(bu
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187 .I Usability
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188 (understandability, learnability, operability, attractiveness)
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189 .IP \(bu
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190 .I Efficiency
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191 (time behaviour, resource utilisation)
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192 .IP \(bu
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193 .I Maintainability
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194 (analysability, changeability, stability, testability)
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195 .IP \(bu
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196 .I Portability
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197 (adaptability, installability, co-existence, replaceability)
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198 .LP
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199 These goals are parts of a software's design.
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200 Good design can give these properties to a software,
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201 bad designed software will miss them.
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202
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203
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204 .NH 1
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205 The Unix Philosophy
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206 .LP
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207 The origins of the Unix Philosophy were already introduced.
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208 This chapter explains the philosophy and shows concrete examples of its application.
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209
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210 .SH
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211 Examples
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212 .LP
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213 Following are some examples to demonstrate how applied Unix Philosophy feels like.
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214 Knowledge of using the Unix shell is assumed.
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215 .PP
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216 Counting the number of files in the current directory:
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217 .DS
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218 .CW
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219 ls | wc -l
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220 .DE
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221 The
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222 .CW ls
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223 command lists all files in the current directory, one per line,
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224 and
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225 .CW "wc -l
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226 counts how many lines they are.
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227 .PP
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228 Counting all files that do not contain ``foo'' in their name:
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229 .DS
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230 .CW
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231 ls | grep -v foo | wc -l
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232 .DE
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233 Here, the list of files is filtered by
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234 .CW grep
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235 to remove all that contain ``foo''.
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236 The rest is the same as in the previous example.
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237 .PP
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238 Finding the five largest entries in the current directory.
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239 .DS
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240 .CW
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241 du -s * | sort -nr | sed 5q
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242 .DE
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243 .CW "du -s *
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244 returns the recursively summed sizes of all files
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245 -- no matter if they are regular files or directories.
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246 .CW "sort -nr
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247 sorts the list numerically in reverse order.
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248 Finally,
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249 .CW "sed 5q
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250 quits after it has printed the fifth line.
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251 .PP
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252 The presented command lines are examples of what Unix people would use
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253 to get the desired output.
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254 There are also other ways to get the same output.
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255 It's a user's decision which way to go.
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256
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257 .SH
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258 Pipes
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259 .LP
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260 The examples show that a lot of tasks on a Unix system
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261 are accomplished by combining several small programs.
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262 The connection between the single programs is denoted by the pipe operator `|'.
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263 .PP
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264 Pipes, and their extensive and easy use, are one of the great
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265 achievements of the Unix system.
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266 Pipes between programs have been possible in earlier operating systems,
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267 but it has never been a so central part of the concept.
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268 When, in the early seventies, Doug McIlroy introduced pipes for the
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269 Unix system,
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270 ``it was this concept and notation for linking several programs together
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271 that transformed Unix from a basic file-sharing system to an entirely new way of computing.''
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272 .[
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273 %T Unix: An Oral History
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274 %O .CW \s-1http://www.princeton.edu/~hos/frs122/unixhist/finalhis.htm
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275 .]
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276 .PP
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277 Being able to specify pipelines in an easy way is,
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278 however, not enough by itself.
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279 It is only one half.
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280 The other is the design of the programs that are used in the pipeline.
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281 They have to be of an external shape that allows them to be be used in such a way.
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282
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283 .SH
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284 Interface architecture
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285 .LP
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286 Unix is, first of all, simple: Everything is a file.
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287 Files are sequences of bytes, without any special structure.
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288 Programs should be filters, which read a stream of bytes from ``standard input'' (stdin)
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289 and write a stream of bytes to ``standard output'' (stdout).
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290 .PP
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291 If our files \fIare\fP sequences of bytes,
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292 and our programs \fIare\fP filters on byte streams,
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293 then there is exactly one standardized interface.
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294 Thus it is possible to combine them in any desired way.
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295 .PP
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296 Even a handful of small programs will yield a large set of combinations,
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297 and thus a large set of different functions.
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298 This is leverage!
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299 .PP
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300 If the programs are orthogonal to each other \(en the best case \(en
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301 then the set of different functions is greatest.
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302 .PP
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303 Now, the Unix toolchest is a set of small programs that
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304 are filters on byte streams.
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305 They are to a large extend unrelated in their function.
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306 Hence, the Unix toolchest provides a large set of functions
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307 that can be accessed by combining the programs in the desired way.
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308
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309 .SH
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310 Advantages of toolchests
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311 .LP
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312 A toolchest is a set of tools.
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313 Instead of having one big tool for all tasks, one has many small tools,
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314 each for one task.
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315 Difficult tasks are solved by combining several of the small, simple tools.
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316 .PP
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317 It is easier and less error-prone to write small programs.
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318 It is also easier and less error-prone to write a large set of small programs,
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319 than to write one large program with all the functionality included.
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320 If the small programs are combinable, then they offer even a larger set
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321 of functions than the single large program.
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322 Hence, one gets two advantages out of writing small, combinable programs.
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323 .PP
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324 There are two drawbacks of the toolchest approach.
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325 First, one simple, standardized, unidirectional Interface has to be sufficient.
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326 If one feels the need for more ``logic'' than a stream of bytes,
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327 then a different approach might be of need, or, more likely,
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328 he just did not came to a design where a stream of bytes is sufficient.
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329 The other drawback of a toolchest affects the users.
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330 A toolchest is often more difficult to use for novices.
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331 It is neccessary to become familiar with each of the tools,
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332 to be able to use the right one in a given situation.
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333 Additinally, one needs to combine the tools in a senseful way on its own.
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334 This is like a sharp knive \(en it is a powerful tool in the hand of a master,
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335 but of no good value in the hand of an unskilled.
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336 .PP
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337 Luckily, the second drawback can be solved easily by adding wrappers around the single tools.
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338 Novice users do not need to learn several tools if a professional wraps
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339 the single commands into a single script.
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340 Note that the wrapper script still calls the small tools;
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341 the wrapper script is just like a skin around.
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342 No complexity is added this way.
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343 .PP
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344 A wrapper script for finding the five largest entries in the current directory
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345 could look like this:
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346 .DS
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347 .CW
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348 #!/bin/sh
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349 du -s * | sort -nr | sed 5q
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350 .DE
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351 The script itself is just a text file that calls the command line
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352 a professional user would type in directly.
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353
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354
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355
4
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356
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357
0
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358
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359 .NH 2
5
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360 foo
0
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361 .LP
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362 standalone vs. tool chain
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363 .LP
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364 software leverage
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365 .LP
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366 possiblities
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367
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368 .NH 2
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369 Results
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370 .LP
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371 The unix phil is an answer to the sw design question
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372 .LP
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373 tool chains empower the uses of sw
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374
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375 .NH 1
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376 Case study: nmh
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377
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378 .NH 2
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379 History
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380 .LP
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381 MH, nmh.
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382 They are old.
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383
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384 .NH 2
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385 Contrasts to similar sw
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386 .LP
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387 vs. Thunderbird, mutt, mailx, pine
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388 .LP
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389 flexibility, no redundancy, use the shell
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390
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391 .NH 2
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392 Gains of the design
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393 .LP
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394
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395 .NH 2
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396 Problems
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397 .LP
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398
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399 .NH 1
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400 Case study: uzbl
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401
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402 .NH 2
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403 History
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404 .LP
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405 uzbl is young
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406
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407 .NH 2
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408 Contrasts to similar sw
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409 .LP
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410 like with nmh
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411 .LP
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412 addons, plugins, modules
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413
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414 .NH 2
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415 Gains of the design
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416 .LP
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417
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418 .NH 2
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419 Problems
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420 .LP
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421 broken web
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422
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423 .NH 1
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424 Final thoughts
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425
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426 .NH 2
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427 Quick summary
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428 .LP
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429 good design
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430 .LP
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431 unix phil
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432 .LP
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433 case studies
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434
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435 .NH 2
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436 Why people should choose
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437 .LP
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438 Make the right choice!
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439
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440 .nr PI .5i
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441 .rm ]<
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442 .de ]<
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443 .LP
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444 .de FP
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445 .IP \\\\$1.
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446 \\..
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447 .rm FS FE
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448 ..
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449 .SH
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450 References
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451 .[
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452 $LIST$
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453 .]
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454 .wh -1p