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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 with good explanatory value.
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21 This text shall take a look underneath 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 later amended
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26 by a third: The cut program may cut specified characters or bytes
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27 out of the input lines or it may cut out specified fields, which
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28 are defined 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, as shown here with the
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33 rights of a 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 task.)
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60 .PP
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61 However, if it's not about displaying characters, but rather about
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62 storing them, then \f(CW-c\fP is only partly suited. In former times,
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63 when US-ASCII was the omnipresent character encoding, each
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64 character was stored as 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 up to 500 bytes
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69 from the beginning 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 function of cut is important for POSIX, because it provides a
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76 transformation of text files with arbitrary line lenghts to text
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77 files with limited line length
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78 .[[ http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cut.html#tag_20_28_17 .
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79 .PP
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80 The introduction of the new byte mode essentially held the same
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81 functionality as the old character mode. The character mode,
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82 however, required a new, different implementation. In consequence,
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83 the problem was not the support of the byte mode, but rather the
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84 correct support of the new character mode.
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85 .PP
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86 Besides the character and byte modes, cut also offers a field
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87 mode, which is activated by \f(CW-f\fP. It selects fields from
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88 the input. The field-delimiter character for the input as well
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89 as for the output (by default the tab) may be changed using
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90 \f(CW-d\fP.
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91 .PP
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92 The typical example for the use of cut's field mode is the
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93 selection of information from the passwd file. Here, for
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94 instance, the usernames and their uids:
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95 .CS
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96 $ cut -d: -f1,3 /etc/passwd
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97 root:0
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98 bin:1
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99 daemon:2
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100 mail:8
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101 ...
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102 .CE
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103 .LP
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104 (The values to the command line switches may be appended directly
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105 to them or separated by whitespace.)
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106 .PP
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107 The field mode is suited for simple tabulary data, like the
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108 passwd file. Beyond that, it soon reaches its limits. The typical
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109 case of whitespace-separated fields, in particular, is covered
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110 poorly by it. Cut's delimiter is exactly one character,
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111 therefore one may not split at both space and tab characters.
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112 Furthermore, multiple adjacent delimiter characters lead to
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113 empty fields. This is not the expected behavior for
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114 the processing of whitespace-separated fields. Some
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115 implementations, e.g. the one of FreeBSD, have extensions that
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116 handle this case in the expected way. Apart from that, i.e.
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117 if one likes to stay portable, awk comes to rescue.
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118 .PP
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119 Awk provides another functionality that cut lacks: Changing the order
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120 of the fields in the output. For cut, the order of the field
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121 selection specification is irrelevant; it doesn't even matter if
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122 fields occur multiple times. Thus, the invocation
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123 \f(CWcut -c 5-8,1,4-6\fP outputs the characters number
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124 1, 4, 5, 6, 7, and 8 in exactly this order. The
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125 selection specification resembles mathematical set theory: Each
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126 specified field is part of the solution set. The fields in the
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127 solution set are always in the same order as in the input. To
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128 speak with the words of the man page in Version 8 Unix:
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129 ``In data base parlance, it projects a relation.''
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130 .[[ http://man.cat-v.org/unix_8th/1/cut
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131 This means that cut applies the \fIprojection\fP database operation
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132 to the text input. Wikipedia explains it in the following way:
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133 ``In practical terms, it can be roughly thought of as picking a
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134 sub-set of all available columns.''
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135 .[[ https://en.wikipedia.org/wiki/Projection_(relational_algebra)
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136
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137 .SH
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138 Historical Background
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139 .LP
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140 Cut came to public life in 1982 with the release of UNIX System
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141 III. Browsing through the sources of System III, one finds cut.c
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142 with the timestamp 1980-04-11
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143 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=SysIII/usr/src/cmd .
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144 This is the oldest implementation of the program I was able to
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145 discover. However, the SCCS-ID in the source code contains the
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146 version number 1.5. According to Doug McIlroy
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147 .[[ http://minnie.tuhs.org/pipermail/tuhs/2015-May/004083.html ,
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148 the earlier history likely lies in PWB/UNIX, which was the
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149 basis for System III. In the available sources of PWB 1.0 (1977)
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150 .[[ http://minnie.tuhs.org/Archive/PDP-11/Distributions/usdl/ ,
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151 no cut is present. Of PWB 2.0, no sources or useful documentation
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152 seem to be available. PWB 3.0 was later renamed to System III
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153 for marketing purposes only; it is otherwise identical to it. A
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154 branch of PWB was CB UNIX, which was only used in the Bell Labs
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155 internally. The manual of CB UNIX Edition 2.1 of November 1979
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156 contains the earliest mention of cut that my research brought
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157 to light, in the form of a man page
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158 .[[ ftp://sunsite.icm.edu.pl/pub/unix/UnixArchive/PDP-11/Distributions/other/CB_Unix/cbunix_man1_02.pdf .
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159 .PP
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160 A look at BSD: There, my earliest discovery is a cut.c with
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161 the file modification date of 1986-11-07
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162 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-UWisc/src/usr.bin/cut
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163 as part of the special version 4.3BSD-UWisc
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164 .[[ http://gunkies.org/wiki/4.3_BSD_NFS_Wisconsin_Unix ,
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165 which was released in January 1987.
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166 This implementation is mostly identical to the one in System
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167 III. The better known 4.3BSD-Tahoe (1988) does not contain cut.
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168 The subsequent 4.3BSD-Reno (1990) does include cut. It is a freshly
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169 written one by Adam S. Moskowitz and Marciano Pitargue, which was
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170 included in BSD in 1989
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171 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Reno/src/usr.bin/cut .
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172 Its man page
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173 .[[ http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Reno/src/usr.bin/cut/cut.1
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174 already mentions the expected compliance to POSIX.2.
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175 One should note that POSIX.2 was first published in
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176 September 1992, about two years after the man page and the
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177 program were written. Hence, the program must have been
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178 implemented based on a draft version of the standard. A look into
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179 the code confirms the assumption. The function to parse the field
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180 selection includes the following comment:
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181 .QP
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182 This parser is less restrictive than the Draft 9 POSIX spec.
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183 POSIX doesn't allow lists that aren't in increasing order or
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184 overlapping lists.
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185 .LP
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186 Draft 11.2 of POSIX (1991-09) requires this flexibility already:
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187 .QP
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188 The elements in list can be repeated, can overlap, and can
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189 be specified in any order.
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190 .LP
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191 The same draft additionally includes all three operation modes,
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192 whereas this early BSD cut only implemented the original two.
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193 Draft 9 might not have included the byte mode. Without access to
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194 Draft 9 or 10, it wasn't possible to verify this guess.
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195 .PP
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196 The version numbers and change dates of the older BSD
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197 implementations are manifested in the SCCS-IDs, which the
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198 version control system of that time inserted. For instance
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199 in 4.3BSD-Reno: ``5.3 (Berkeley) 6/24/90''.
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200 .PP
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201 The cut implementation of the GNU coreutils contains the
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202 following copyright notice:
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203 .CS
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204 Copyright (C) 1997-2015 Free Software Foundation, Inc.
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205 Copyright (C) 1984 David M. Ihnat
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206 .CE
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207 .LP
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208 The code does have old origins. Further comments show that
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209 the source code was reworked by David MacKenzie first and later
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210 by Jim Meyering, who put it into the version control system in
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211 1992. It is unclear why the years until 1997, at least from
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212 1992 onwards, don't show up in the copyright notice.
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213 .PP
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214 Despite all those year numbers from the 80s, cut is a rather
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215 young tool, at least in relation to the early Unix. Despite
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216 being a decade older than Linux (the kernel), Unix was present
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217 for over ten years by the time cut appeared for the first
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218 time. Most notably, cut wasn't part of Version 7 Unix, which
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219 became the basis for all modern Unix systems. The more complex
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220 tools sed and awk were part of it already. Hence, the
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221 question comes to mind why cut was written at all, as two
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222 programs already existed that were able to cover the use cases of
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223 cut. One reason for cut surely was its compactness and the
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224 resulting speed, in comparison to the then-bulky awk. This lean
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225 shape goes well with the Unix philosopy: Do one job and do it
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226 well! Cut was sufficiently convincing. It found its way to
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227 other Unix variants, it became standardized, and today it is
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228 present everywhere.
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229 .PP
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230 The original variant (without \f(CW-b\fP) was described already
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231 in 1985, by the System V Interface Definition, an important
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232 formal description of UNIX System V. In the following years, it
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233 appeared in all relevant standards. POSIX.2 specified cut for
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234 the first time in its modern form (with \f(CW-b\fP) in 1992.
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235
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236 .SH
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237 Multi-byte support
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238 .LP
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239 The byte mode and thus the multi-byte support of the POSIX
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240 character mode have benn standardized since 1992. But
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241 how about their presence in the available implementations?
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242 Which versions implement POSIX correctly?
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243 .PP
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244 The situation is divided into three parts: There are historic
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245 implementations, which have only \f(CW-c\fP and \f(CW-f\fP.
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246 Then there are implementations that have \f(CW-b\fP, but
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247 treat it as an alias for \f(CW-c\fP only. These
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248 implementations work correctly for single-byte encodings
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249 (e.g. US-ASCII, Latin1) but for multi-byte encodings (e.g.
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250 UTF-8) their \f(CW-c\fP behaves like \f(CW-b\fP (and
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251 \f(CW-n\fP is ignored). Finally, there are implementations
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252 that implement \f(CW-c\fP and \f(CW-b\fP in a POSIX-compliant
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253 way.
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254 .PP
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255 Historic two-mode implementations are the ones of
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256 System III, System V, and the BSD ones until the mid-90s.
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257 .PP
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258 Pseudo multi-byte implementations are provided by GNU,
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259 modern NetBSD, and modern OpenBSD. The level of POSIX compliance
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260 that is presented there is often higher than the level of
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261 compliance that is actually provided. Sometimes it takes a
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262 close look to discover that \f(CW-c\fP and \f(CW-n\fP don't
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263 behave as expected. Some of the implementations take the
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264 easy way by simply being ignorant to any multi-byte
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265 encodings, at least they declare that clearly:
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266 .QP
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267 Since we don't support multi-byte characters, the \f(CW-c\fP
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268 and \f(CW-b\fP options are equivalent, and the \f(CW-n\fP
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269 option is meaningless.
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270 .[[ 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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271 .LP
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272 Standard-adhering implementations, ones that treat
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273 multi-byte characters correctly, are the one of the modern
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274 FreeBSD and the one in the Heirloom toolchest. Tim Robbins
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275 reimplemented the character mode of FreeBSD cut,
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276 conforming to POSIX, in the summer of 2004
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277 .[[ https://svnweb.freebsd.org/base?view=revision&revision=131194 .
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278 The question why the other BSD systems have not
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279 integrated this change is an open one. Maybe the answer an be
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280 found in the above quoted statement.
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281 .PP
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282 How does a user find out if the cut on their own system handles
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283 multi-byte characters correctly? First, one needs to check if
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284 the system itself uses multi-byte characters, because otherwise
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285 characters and bytes are equivalent and the question
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286 is irrelevant. One can check this by looking at the locale
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287 settings, but it is easier to print a typical multi-byte
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288 character, for instance an Umlaut or the Euro currency
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289 symbol, and check if one or more bytes are generated as
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290 output:
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291 .CS
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292 $ echo ä | od -c
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293 0000000 303 244 \\n
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294 0000003
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295 .CE
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296 .LP
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297 In this case it resulted in two bytes: octal 303 and 244. (The
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298 newline character is added by echo.)
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299 .PP
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300 The program iconv converts text to specific encodings. This
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301 is the output for Latin1 and UTF-8, for comparison:
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302 .CS
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303 $ echo ä | iconv -t latin1 | od -c
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304 0000000 344 \\n
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305 0000002
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306 .sp .3
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307 $ echo ä | iconv -t utf8 | od -c
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308 0000000 303 244 \\n
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309 0000003
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310 .CE
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311 .LP
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312 The output (without the iconv conversion) on many European
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313 systems equals one of these two.
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314 .PP
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315 Now for the test of the cut implementation. On a UTF-8 system, a
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316 POSIX-compliant implementation behaves as such:
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317 .CS
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318 $ echo ä | cut -c 1 | od -c
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319 0000000 303 244 \\n
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320 0000003
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321 .sp .3
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322 $ echo ä | cut -b 1 | od -c
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323 0000000 303 \\n
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324 0000002
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325 .sp .3
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326 $ echo ä | cut -b 1 -n | od -c
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327 0000000 \\n
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328 0000001
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329 .CE
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330 .LP
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331 A pseudo-POSIX implementation, in contrast, behaves like the
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332 middle one for all three invocations: Only the first byte is
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333 printed as output.
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334
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335 .SH
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336 Implementations
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337 .LP
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338 Let's take a look at the sources of a selection of
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339 implementations.
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340 .PP
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341 A comparison of the amount of source code is good to get a first
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342 impression. Typically, it grows through time. This can generally
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343 be seen here, but not in all cases. A POSIX-compliant
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344 implementation of the character mode requires more code, thus
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345 these implementations tend to be the larger ones.
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346 .TS
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347 center;
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348 r r r l l l.
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349 SLOC Lines Bytes Belongs to File tyime Category
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350 _
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351 116 123 2966 System III 1980-04-11 historic
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352 118 125 3038 4.3BSD-UWisc 1986-11-07 historic
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353 200 256 5715 4.3BSD-Reno 1990-06-25 historic
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354 200 270 6545 NetBSD 1993-03-21 historic
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355 218 290 6892 OpenBSD 2008-06-27 pseudo-POSIX
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356 224 296 6920 FreeBSD 1994-05-27 historic
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357 232 306 7500 NetBSD 2014-02-03 pseudo-POSIX
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358 340 405 7423 Heirloom 2012-05-20 POSIX
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359 382 586 14175 GNU coreutils 1992-11-08 pseudo-POSIX
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360 391 479 10961 FreeBSD 2012-11-24 POSIX
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361 588 830 23167 GNU coreutils 2015-05-01 pseudo-POSIX
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362 .TE
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363 .LP
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364 Roughly four groups can be seen: (1) The two original
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365 implementations, which are mostly identical, with about 100
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366 SLOC. (2) The five BSD versions, with about 200 SLOC. (3) The
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367 two POSIX-compliant versions and the old GNU one, with a SLOC
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368 count in the 300s. And finally, (4) the modern GNU cut with
|
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369 almost 600 SLOC.
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370 .PP
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371 The variation between the number of logical code
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372 lines (SLOC, measured with SLOCcount) and the number of
|
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373 newlines in the file (\f(CWwc -l\fP) spans between factor
|
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374 1.06 for the oldest versions and factor 1.5 for GNU. The
|
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375 largest influence on it are empty lines, pure comment lines,
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376 and the size of the license block at the beginning of the file.
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377 .PP
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378 Regarding the variation between logical code lines and the
|
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379 file size (\f(CWwc -c\fP), the implementations span between
|
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380 25 and 30 bytes per statement. With only 21 bytes per
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381 statement, the Heirloom implementation marks the lower end;
|
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382 the GNU implementation sets the upper limit at nearly 40 bytes. In
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383 the case of GNU, the reason is mainly their coding style, with
|
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384 special indentation rules and long identifiers. Whether one finds
|
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385 the Heirloom implementation
|
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386 .[[ http://heirloom.cvs.sourceforge.net/viewvc/heirloom/heirloom/cut/cut.c?revision=1.6&view=markup
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387 highly cryptic or exceptionally elegant shall be left
|
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388 to the judgement of the reader. Especially the
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389 comparison to the GNU implementation
|
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390 .[[ http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/cut.c;hb=e981643
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391 is impressive.
|
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392 .PP
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393 The internal structure of the source code (in all cases it is
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394 written in C) is mainly similar. Besides the mandatory main
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395 function, which does the command line argument processing,
|
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396 there usually is a function to convert the field
|
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397 selection specification to an internal data structure.
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398 Furthermore, almost all implementations have separate
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399 functions for each of their operation modes. The POSIX-compliant
|
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400 versions treat the \f(CW-b -n\fP combination as a separate
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401 mode and thus implement it in a separate function. Only the early
|
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402 System III implementation (and its 4.3BSD-UWisc variant) do
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403 everything, apart from error handling, in the main function.
|
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|
404 .PP
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|
405 Implementations of cut typically have two limiting aspects:
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406 One being the maximum number of fields that can be handled,
|
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407 the other being the maximum line length. On System III, both
|
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|
408 numbers are limited to 512. 4.3BSD-Reno and the BSDs of the
|
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|
409 90s have fixed limits as well (\f(CW_BSD_LINE_MAX\fP or
|
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410 \f(CW_POSIX2_LINE_MAX\fP). Modern FreeBSD, modern NetBSD, all GNU
|
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411 implementations, and the Heirloom cut are able to handle
|
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412 arbitrary numbers of fields and line lengths \(en the memory
|
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413 is allocated dynamically. OpenBSD cut is a hybrid: It has a fixed
|
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414 maximum number of fields, but allows arbitrary line lengths.
|
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415 The limited number of fields does not, however, appear to be
|
|
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416 any practical problem, because \f(CW_POSIX2_LINE_MAX\fP is
|
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417 guaranteed to be at least 2048 and is thus probably large enough.
|
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418
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419 .SH
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|
420 Descriptions
|
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421 .LP
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422 Interesting, as well, is a comparison of the short descriptions
|
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423 of cut, as can be found in the headlines of the man
|
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424 pages or at the beginning of the source code files.
|
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425 The following list is roughly grouped by origin:
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426 .TS
|
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427 center;
|
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|
428 l l.
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429 CB UNIX cut out selected fields of each line of a file
|
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|
430 System III cut out selected fields of each line of a file
|
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431 System III \(dg cut and paste columns of a table (projection of a relation)
|
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432 System V cut out selected fields of each line of a file
|
|
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|
433 HP-UX cut out (extract) selected fields of each line of a file
|
|
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|
434 .sp .3
|
|
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|
435 4.3BSD-UWisc \(dg cut and paste columns of a table (projection of a relation)
|
|
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436 4.3BSD-Reno select portions of each line of a file
|
|
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|
437 NetBSD select portions of each line of a file
|
|
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|
438 OpenBSD 4.6 select portions of each line of a file
|
|
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|
439 FreeBSD 1.0 select portions of each line of a file
|
|
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|
440 FreeBSD 10.0 cut out selected portions of each line of a file
|
|
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|
441 SunOS 4.1.3 remove selected fields from each line of a file
|
|
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|
442 SunOS 5.5.1 cut out selected fields of each line of a file
|
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443 .sp .3
|
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444 Heirloom Tools cut out selected fields of each line of a file
|
|
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445 Heirloom Tools \(dg cut out fields of lines of files
|
|
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|
446 .sp .3
|
|
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|
447 GNU coreutils remove sections from each line of files
|
|
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|
448 .sp .3
|
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|
449 Minix select out columns of a file
|
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450 .sp .3
|
|
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|
451 Version 8 Unix rearrange columns of data
|
|
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|
452 ``Unix Reader'' rearrange columns of text
|
|
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|
453 .sp .3
|
|
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454 POSIX cut out selected fields of each line of a file
|
|
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|
455 .TE
|
|
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|
456 .LP
|
|
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457 (The descriptions that are marked with `\(dg' were taken from
|
|
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|
458 source code files. The POSIX entry contains the description
|
|
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|
459 used in the standard. The ``Unix Reader'' is a retrospective
|
|
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|
460 document by Doug McIlroy, which lists the availability of
|
|
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|
461 tools in the Research Unix versions
|
|
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|
462 .[[ http://doc.cat-v.org/unix/unix-reader/contents.pdf .
|
|
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|
463 Its description should actually match the one in Version 8
|
|
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|
464 Unix. The change could be a transfer mistake or a correction.
|
|
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|
465 All other descriptions originate from the various man pages.)
|
|
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|
466 .PP
|
|
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|
467 Over time, the POSIX description was often adopted or it
|
|
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|
468 served as inspiration. One such example is FreeBSD
|
|
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|
469 .[[ https://svnweb.freebsd.org/base?view=revision&revision=167101 .
|
|
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|
470 .PP
|
|
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|
471 It is noteworthy that the GNU coreutils in all versions
|
|
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|
472 describe the performed action as a removal of parts of the
|
|
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|
473 input, although the user clearly selects the parts that then
|
|
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|
474 consistute the output. Probably the words ``cut out'' are too
|
|
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|
475 misleading. HP-UX tried to be more clear.
|
|
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|
476 .PP
|
|
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|
477 Different terms are also used for the part being
|
|
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|
478 selected. Some talk about fields (POSIX), some talk
|
|
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|
479 about portions (BSD) and some call it columns (Research
|
|
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|
480 Unix).
|
|
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|
481 .PP
|
|
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|
482 The seemingly least adequate description, the one of Version
|
|
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|
483 8 Unix (``rearrange columns of data'') is explainable in so
|
|
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|
484 far that the man page covers both cut and paste, and in
|
|
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|
485 their combination, columns can be rearranged. The use of
|
|
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|
486 ``data'' instead of ``text'' might be a lapse, which McIlroy
|
|
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|
487 corrected in his Unix Reader ... but on the other hand, on
|
|
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|
488 Unix, the two words are mostly synonymous, because all data
|
|
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|
489 is text.
|
|
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|
490
|
|
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|
491
|
|
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|
492 .SH
|
|
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|
493 References
|
|
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|
494 .LP
|
|
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|
495 .nf
|
|
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|
496 ._r
|
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497
|
