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Added first version of the translation to English
author markus schnalke <meillo@marmaro.de>
date Tue, 04 Aug 2015 21:04:10 +0200
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children 0d7329867dd1
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1 .so macros
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2 .lc_ctype en_US.utf8
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3 .pl -4v
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4
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5 .TL
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6 Cut out selected fields of each line of a file
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7 .AU
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8 markus schnalke <meillo@marmaro.de>
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9 ..
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10 .FS
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11 2015-05.
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12 This text is in the public domain (CC0).
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13 It is available online:
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14 .I http://marmaro.de/docs/
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15 .FE
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16
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17 .LP
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18 Cut is a classic program in the Unix toolchest.
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19 It is present in most tutorials on shell programming, because it
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20 is such a nice and useful tool which good explanationary value.
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21 This text shall take a look behind its surface.
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22 .SH
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23 Usage
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24 .LP
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25 Initially, cut had two operation modes, which were amended by a
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26 third one, later. Cut may cut specified characters out of the
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27 input lines or it may cut out specified fields, which are defined
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28 by a delimiting character.
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29 .PP
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30 The character mode is well suited to slice fixed-width input
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31 formats into parts. One might, for instance, extract the access
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32 rights from the output of \f(CWls -l\fP, here the rights of the
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33 file's owner:
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34 .CS
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35 $ ls -l foo
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36 -rw-rw-r-- 1 meillo users 0 May 12 07:32 foo
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37 .sp .3
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38 $ ls -l foo | cut -c 2-4
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39 rw-
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40 .CE
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41 .LP
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42 Or the write permission for the owner, the group and the
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43 world:
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44 .CS
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45 $ ls -l foo | cut -c 3,6,9
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46 ww-
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47 .CE
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48 .LP
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49 Cut can also be used to shorten strings:
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50 .CS
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51 $ long=12345678901234567890
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52 .sp .3
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53 $ echo "$long" | cut -c -10
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54 1234567890
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55 .CE
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56 .LP
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57 This command outputs no more than the first 10 characters of
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58 \f(CW$long\fP. (Alternatively, on could use \f(CWprintf
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59 "%.10s\\n" "$long"\fP for this job.)
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60 .PP
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61 However, if it's not about displaying characters but about their
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62 storing, then \f(CW-c\fP is only partly suited. In former times,
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63 when US-ASCII had been the omnipresent character encoding, each
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64 character was stored with exactly one byte. Therefore, \f(CWcut
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65 -c\fP selected both, output characters and bytes, equally. With
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66 the uprise of multi-byte encodings (like UTF-8), this assumption
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67 became obsolete. Consequently, a byte mode (option \f(CW-b\fP)
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68 was added to cut, with POSIX.2-1992. To select the first up to
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69 500 bytes of each line (and ignore the rest), one can use:
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70 .CS
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71 $ cut -b -500
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72 .CE
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73 .LP
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74 The remainder can be caught with \f(CWcut -b 501-\fP. This
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75 possibility is important for POSIX, because it allows to create
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76 text files with limited line length
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77 .[[ http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cut.html#tag_20_28_17 .
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78 .PP
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79 Although the byte mode was newly introduced, it was meant to
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80 behave exactly as the old character mode. The character mode,
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81 however, had to be implemented differently. In consequence,
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82 the problem wasn't to support the byte mode, but to support the
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83 new character mode correctly.
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84 .PP
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85 Besides the character and byte modes, cut has the field mode,
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86 which is activated by \f(CW-f\fP. It selects fields from the
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87 input. The delimiting character (by default, the tab) may be
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88 changed using \f(CW-d\fP. It applies to the input as well as to
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89 the output.
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90 .PP
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91 The typical example for the use of cut's field mode is the
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92 selection of information from the passwd file. Here, for
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93 instance, the username and its uid:
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94 .CS
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95 $ cut -d: -f1,3 /etc/passwd
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96 root:0
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97 bin:1
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98 daemon:2
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99 mail:8
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100 ...
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101 .CE
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102 .LP
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103 (The values to the command line switches may be appended directly
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104 to them or separated by whitespace.)
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105 .PP
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106 The field mode is suited for simple tabulary data, like the
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107 passwd file. Beyond that, it soon reaches its limits. Especially,
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108 the typical case of whitespace-separated fields is covered poorly
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109 by it. Cut's delimiter is exactly one character,
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110 therefore one may not split at both, space and tab characters.
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111 Furthermore, multiple adjacent delimiter characters lead to
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112 empty fields. This is not the expected behavior for
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113 the processing of whitespace-separated fields. Some
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114 implementations, e.g. the one of FreeBSD, have extensions that
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115 handle this case in the expected way. Apart from that, i.e.
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116 if one likes to stay portable, awk comes to rescue.
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117 .PP
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118 Awk provides another function that cut misses: Changing the order
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119 of the fields in the output. For cut, the order of the field
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120 selection specification is irrelevant; it doesn't even matter if
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121 fields are given multiple times. Thus, the invocation
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122 \f(CWcut -c 5-8,1,4-6\fP outputs the characters number
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123 1, 4, 5, 6, 7 and 8 in exactly this order. The
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124 selection is like in the mathematical set theory: Each
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125 specified field is part of the solution set. The fields in the
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126 solution set are always in the same order as in the input. To
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127 speak with the words of the man page in Version 8 Unix:
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128 ``In data base parlance, it projects a relation.''
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129 .[[ http://man.cat-v.org/unix_8th/1/cut
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130 This means, cut applies the database operation \fIprojection\fP
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131 to the text input. Wikipedia explains it in the following way:
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132 ``In practical terms, it can be roughly thought of as picking a
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133 sub-set of all available columns.''
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134 .[[ https://en.wikipedia.org/wiki/Projection_(relational_algebra)
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135
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136 .SH
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137 Historical Background
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138 .LP
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139 Cut came to public life in 1982 with the release of UNIX System
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140 III. Browsing through the sources of System III, one finds cut.c
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141 with the timestamp 1980-04-11
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142 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=SysIII/usr/src/cmd .
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143 This is the oldest implementation of the program, I was able to
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144 discover. However, the SCCS-ID in the source code speaks of
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145 version 1.5. According to Doug McIlroy
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146 .[[ http://minnie.tuhs.org/pipermail/tuhs/2015-May/004083.html ,
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147 the earlier history likely lays in PWB/UNIX, which was the
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148 basis for System III. In the available sources of PWB 1.0 (1977)
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149 .[[ http://minnie.tuhs.org/Archive/PDP-11/Distributions/usdl/ ,
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150 no cut is present. Of PWB 2.0, no sources or useful documentation
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151 seem to be available. PWB 3.0 was later renamed to System III
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152 for marketing purposes, hence it is identical to it. A side line
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153 of PWB was CB UNIX, which was only used in the Bell Labs
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154 internally. The manual of CB UNIX Edition 2.1 of November 1979
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155 contains the earliest mentioning of cut, that my research brought
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156 to light: A man page for it
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157 .[[ ftp://sunsite.icm.edu.pl/pub/unix/UnixArchive/PDP-11/Distributions/other/CB_Unix/cbunix_man1_02.pdf .
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158 .PP
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159 Now a look on BSD: There, my earliest discovery is a cut.c with
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160 the file modification date of 1986-11-07
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161 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-UWisc/src/usr.bin/cut
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162 as part of the special version 4.3BSD-UWisc
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163 .[[ http://gunkies.org/wiki/4.3_BSD_NFS_Wisconsin_Unix ,
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164 which was released in January 1987.
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165 This implementation is mostly identical to the one in System
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166 III. The better known 4.3BSD-Tahoe (1988) does not contain cut.
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167 The following 4.3BSD-Reno (1990) does include cut. It is a freshly
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168 written one by Adam S. Moskowitz and Marciano Pitargue, which was
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169 included in BSD in 1989
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170 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Reno/src/usr.bin/cut .
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171 Its man page
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172 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Reno/src/usr.bin/cut/cut.1
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173 already mentions the expected compliance to POSIX.2.
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174 One should note that POSIX.2 was first published in
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175 September 1992, about two years after the man page and the
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176 program were written. Hence, the program must have been
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177 implemented based on a draft version of the standard. A look into
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178 the code confirms the assumption. The function to parse the field
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179 selection includes the following comment:
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180 .QP
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181 This parser is less restrictive than the Draft 9 POSIX spec.
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182 POSIX doesn't allow lists that aren't in increasing order or
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183 overlapping lists.
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184 .LP
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185 Draft 11.2 of POSIX (1991-09) requires this flexibility already:
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186 .QP
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187 The elements in list can be repeated, can overlap, and can
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188 be specified in any order.
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189 .LP
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190 The same draft additionally includes all three operation modes,
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191 whereas this early BSD cut only implemented the original two.
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192 Draft 9 might not have included the byte mode. Without access to
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193 Draft 9 or 10, it wasn't possible to verify this guess.
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194 .PP
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195 The version numbers and change dates of the older BSD
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196 implementations are manifested in the SCCS-IDs, which the
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197 version control system of that time inserted. For instance
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198 in 4.3BSD-Reno: ``5.3 (Berkeley) 6/24/90''.
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199 .PP
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200 The cut implementation of the GNU coreutils contains the
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201 following copyright notice:
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202 .CS
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203 Copyright (C) 1997-2015 Free Software Foundation, Inc.
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204 Copyright (C) 1984 David M. Ihnat
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205 .CE
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206 .LP
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207 The code does have pretty old origins. Further comments show that
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208 the source code was reworked by David MacKenzie first and later
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209 by Jim Meyering, who put it into the version control system in
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210 1992. It is unclear, why the years until 1997, at least from
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211 1992 on, don't show up in the copyright notice.
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212 .PP
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213 Despite all those year numbers from the 80s, cut is a rather
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214 young tool, at least in relation to the early Unix. Despite
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215 being a decade older than Linux, the kernel, Unix had been
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216 present for over ten years until cut appeared for the first
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217 time. Most notably, cut wasn't part of Version 7 Unix, which
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218 became the basis for all modern Unix systems. The more complex
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219 tools sed and awk had been part of it already. Hence, the
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220 question comes to mind, why cut was written at all, as there
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221 existed two programs that were able to cover the use cases of
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222 cut. On reason for cut surely was its compactness and the
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223 resulting speed, in comparison to the then bulky awk. This lean
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224 shape goes well with the Unix philosopy: Do one job and do it
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225 well! Cut convinced. It found it's way to other Unix variants,
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226 it became standardized and today it is present everywhere.
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227 .PP
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228 The original variant (without \f(CW-b\fP) was described by the
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229 System V Interface Defintion, an important formal description
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230 of UNIX System V, already in 1985. In the following years, it
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231 appeared in all relevant standards. POSIX.2 in 1992 specified
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232 cut for the first time in its modern form (with \f(CW-b\fP).
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233
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234 .SH
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235 Multi-byte support
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236 .LP
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237 The byte mode and thus the multi-byte support of
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238 the POSIX character mode are standardized since 1992. But
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239 how about their presence in the available implementations?
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240 Which versions do implement POSIX correctly?
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241 .PP
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242 The situation is divided in three parts: There are historic
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243 implementations, which have only \f(CW-c\fP and \f(CW-f\fP.
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244 Then there are implementations, which have \f(CW-b\fP but
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245 treat it as an alias for \f(CW-c\fP only. These
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246 implementations work correctly for single-byte encodings
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247 (e.g. US-ASCII, Latin1) but for multi-byte encodings (e.g.
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248 UTF-8) their \f(CW-c\fP behaves like \f(CW-b\fP (and
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249 \f(CW-n\fP is ignored). Finally, there are implementations
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250 that implement \f(CW-b\fP and \f(CW-c\fP POSIX-compliant.
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251 .PP
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252 Historic two-mode implementations are the ones of
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253 System III, System V and the BSD ones until the mid-90s.
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254 .PP
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255 Pseudo multi-byte implementations are provided by GNU and
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256 modern NetBSD and OpenBSD. The level of POSIX compliance
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257 that is presented there is often higher than the level of
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258 compliance that is actually provided. Sometimes it takes a
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259 close look to discover that \f(CW-c\fP and \f(CW-n\fP don't
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260 behave as expected. Some of the implementations take the
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261 easy way by simply being ignorant to any multi-byte
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262 encodings, at least they tell that clearly:
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263 .QP
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264 Since we don't support multi-byte characters, the \f(CW-c\fP and \f(CW-b\fP
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265 options are equivalent, and the \f(CW-n\fP option is meaningless.
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266 .[[ http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/cut/cut.c?rev=1.18&content-type=text/x-cvsweb-markup
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267 .LP
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268 Standard-adhering implementations, ones that treat
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269 multi-byte characters correctly, are the one of the modern
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270 FreeBSD and the one in the Heirloom toolchest. Tim Robbins
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271 reimplemented the character mode of FreeBSD cut,
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272 conforming to POSIX, in summer 2004
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273 .[[ https://svnweb.freebsd.org/base?view=revision&revision=131194 .
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274 The question, why the other BSD systems have not
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275 integrated this change, is an open one. Maybe the answer an be
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276 found in the above quoted statement.
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277 .PP
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278 How does a user find out if the cut on the own system handles
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279 multi-byte characters correclty? First, one needs to check if
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280 the system itself uses multi-byte characters, because otherwise
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281 characters and bytes are equivalent and the question
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282 is irrelevant. One can check this by looking at the locale
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283 settings, but it is easier to print a typical multi-byte
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284 character, for instance an Umlaut or the Euro currency
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285 symbol, and check if one or more bytes are output:
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286 .CS
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287 $ echo ä | od -c
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288 0000000 303 244 \\n
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289 0000003
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290 .CE
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291 .LP
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292 In this case it were two bytes: octal 303 and 244. (The
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293 Newline character is added by echo.)
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294 .PP
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295 The program iconv converts text to specific encodings. This
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296 is the output for Latin1 and UTF-8, for comparison:
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297 .CS
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298 $ echo ä | iconv -t latin1 | od -c
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299 0000000 344 \\n
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300 0000002
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301 .sp .3
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302 $ echo ä | iconv -t utf8 | od -c
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303 0000000 303 244 \\n
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304 0000003
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305 .CE
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306 .LP
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307 The output (without the iconv conversion) on many European
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308 systems equals one of these two.
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309 .PP
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310 Now the test of the cut implementation. On a UTF-8 system, a
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311 POSIX compliant implementation behaves as such:
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312 .CS
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313 $ echo ä | cut -c 1 | od -c
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314 0000000 303 244 \\n
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315 0000003
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316 .sp .3
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317 $ echo ä | cut -b 1 | od -c
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318 0000000 303 \\n
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319 0000002
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320 .sp .3
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321 $ echo ä | cut -b 1 -n | od -c
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322 0000000 \\n
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323 0000001
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324 .CE
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325 .LP
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326 A pseudo POSIX implementation, in contrast, behaves like the
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327 middle one, for all three invocations: Only the first byte is
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328 output.
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329
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330 .SH
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331 Implementations
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332 .LP
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333 Let's take a look at the sources of a selection of
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334 implementations.
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335 .PP
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336 A comparison of the amount of source code is good to get a first
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337 impression. Typically, it grows through time. This can be seen
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338 here, in general but not in all cases. A POSIX-compliant
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339 implementation of the character mode requires more code, thus
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340 these implementations are rather the larger ones.
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341 .TS
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342 center;
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343 r r r l l l.
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344 SLOC Lines Bytes Belongs to File tyime Category
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345 _
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346 116 123 2966 System III 1980-04-11 historic
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347 118 125 3038 4.3BSD-UWisc 1986-11-07 historic
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348 200 256 5715 4.3BSD-Reno 1990-06-25 historic
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349 200 270 6545 NetBSD 1993-03-21 historic
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350 218 290 6892 OpenBSD 2008-06-27 pseudo-POSIX
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351 224 296 6920 FreeBSD 1994-05-27 historic
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352 232 306 7500 NetBSD 2014-02-03 pseudo-POSIX
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353 340 405 7423 Heirloom 2012-05-20 POSIX
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354 382 586 14175 GNU coreutils 1992-11-08 pseudo-POSIX
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355 391 479 10961 FreeBSD 2012-11-24 POSIX
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356 588 830 23167 GNU coreutils 2015-05-01 pseudo-POSIX
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357 .TE
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358 .LP
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359 Roughly four groups can be seen: (1) The two original
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360 implementaions, which are mostly identical, with about 100
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361 SLOC. (2) The five BSD versions, with about 200 SLOC. (3) The
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362 two POSIX-compliant versions and the old GNU one, with a SLOC
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363 count in the 300s. And finally (4) the modern GNU cut with
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364 almost 600 SLOC.
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365 .PP
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366 The variation between the number of logical code
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367 lines (SLOC, meassured with SLOCcount) and the number of
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368 Newlines in the file (\f(CWwc -l\fP) spans between factor
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369 1.06 for the oldest versions and factor 1.5 for GNU. The
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370 largest influence on it are empty lines, pure comment lines
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371 and the size of the license block at the beginning of the file.
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372 .PP
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373 Regarding the variation between logical code lines and the
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374 file size (\f(CWwc -c\fP), the implementations span between
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375 25 and 30 bytes per statement. With only 21 bytes per
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376 statement, the Heirloom implementation marks the lower end;
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377 the GNU implementation sets the upper limit at nearly 40. In
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378 the case of GNU, the reason is mainly their coding style, with
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379 special indent rules and long identifiers. Whether one finds
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380 the Heirloom implementation
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381 .[[ http://heirloom.cvs.sourceforge.net/viewvc/heirloom/heirloom/cut/cut.c?revision=1.6&view=markup
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382 highly cryptic or exceptionally elegant, shall be left
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383 open to the judgement of the reader. Especially the
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384 comparison to the GNU implementation
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385 .[[ http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/cut.c;hb=e981643
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386 is impressive.
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387 .PP
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388 The internal structure of the source code (in all cases it is
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389 written in C) is mainly similar. Besides the mandatory main
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390 function, which does the command line argument processing,
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391 there usually exists a function to convert the field
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392 selection specification to an internal data structure.
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393 Further more, almost all implementations have separate
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394 functions for each of their operation modes. The POSIX-compliant
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395 versions treat the \f(CW-b -n\fP combination as a separate
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396 mode and thus implement it in an own function. Only the early
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397 System III implementation (and its 4.3BSD-UWisc variant) do
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398 everything, apart from error handling, in the main function.
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399 .PP
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400 Implementations of cut typically have two limiting aspects:
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401 One being the maximum number of fields that can be handled,
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402 the other being the maximum line length. On System III, both
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403 numbers are limited to 512. 4.3BSD-Reno and the BSDs of the
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404 90s have fixed limits as well (\f(CW_BSD_LINE_MAX\fP or
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405 \f(CW_POSIX2_LINE_MAX\fP). Modern FreeBSD, NetBSD, all GNU
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406 implementations and the Heirloom cut is able to handle
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407 arbitrary numbers of fields and line lengths \(en the memory
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408 is allocated dynamically. OpenBSD cut is a hybrid: It has a fixed
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409 maximum number of fields, but allows arbitrary line lengths.
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410 The limited number of fields does, however, not appear to be
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411 any practical problem, because \f(CW_POSIX2_LINE_MAX\fP is
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412 guaranteed to be at least 2048 and is thus probably large enough.
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413
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414 .SH
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415 Descriptions
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416 .LP
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417 Interesting, as well, is a comparison of the short descriptions
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418 of cut, as can be found in the headlines of the man
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419 pages or at the beginning of the source code files.
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420 The following list is roughly sorted by time and grouped by
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421 decent:
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422 .TS
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423 center;
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424 l l.
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425 CB UNIX cut out selected fields of each line of a file
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426 System III cut out selected fields of each line of a file
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427 System III \(dg cut and paste columns of a table (projection of a relation)
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428 System V cut out selected fields of each line of a file
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429 HP-UX cut out (extract) selected fields of each line of a file
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430 .sp .3
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431 4.3BSD-UWisc \(dg cut and paste columns of a table (projection of a relation)
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432 4.3BSD-Reno select portions of each line of a file
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433 NetBSD select portions of each line of a file
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434 OpenBSD 4.6 select portions of each line of a file
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435 FreeBSD 1.0 select portions of each line of a file
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436 FreeBSD 10.0 cut out selected portions of each line of a file
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437 SunOS 4.1.3 remove selected fields from each line of a file
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438 SunOS 5.5.1 cut out selected fields of each line of a file
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439 .sp .3
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440 Heirloom Tools cut out selected fields of each line of a file
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441 Heirloom Tools \(dg cut out fields of lines of files
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442 .sp .3
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443 GNU coreutils remove sections from each line of files
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444 .sp .3
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445 Minix select out columns of a file
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446 .sp .3
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447 Version 8 Unix rearrange columns of data
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448 ``Unix Reader'' rearrange columns of text
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449 .sp .3
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450 POSIX cut out selected fields of each line of a file
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451 .TE
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452 .LP
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453 (The descriptions that are marked with `\(dg' were taken from
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454 source code files. The POSIX entry contains the description
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455 used in the standard. The ``Unix Reader'' is a retrospective
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456 document by Doug McIlroy, which lists the availability of
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457 tools in the Research Unix versions
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458 .[[ http://doc.cat-v.org/unix/unix-reader/contents.pdf .
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459 Its description should actually match the one in Version 8
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460 Unix. The change could be a transfer mistake or a correction.
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461 All other descriptions originate from the various man pages.)
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462 .PP
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463 Over time, the POSIX description was often adopted or it
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464 served as inspiration. One such example is FreeBSD
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465 .[[ https://svnweb.freebsd.org/base?view=revision&revision=167101 .
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466 .PP
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467 It is noteworthy that the GNU coreutils in all versions
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468 describe the performed action as a removal of parts of the
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469 input, although the user clearly selects the parts that are
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470 output. Probably the words ``cut out'' are too misleading.
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471 HP-UX concretized them.
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472 .PP
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473 There are also different terms used for the thing being
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474 selected. Some talk about fields (POSIX), some talk
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475 about portions (BSD) and some call it columns (Research
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476 Unix).
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477 .PP
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478 The seemingly least adequate description, the one of Version
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479 8 Unix (``rearrange columns of data'') is explainable in so
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480 far that the man page covers both, cut and paste, and in
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481 their combination, columns can be rearranged. The use of
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482 ``data'' instead of ``text'' might be a lapse, which McIlroy
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483 corrected in his Unix Reader ... but, on the other hand, on
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484 Unix, the two words are mostly synonymous, because all data
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485 is text.
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486
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487
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488 .SH
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489 Referenzen
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490 .LP
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491 .nf
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492 ._r
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493