1 # $NetBSD: POSIX,v 1.5 2014/06/06 00:13:13 christos Exp $ 2 # @(#)POSIX 8.1 (Berkeley) 6/6/93 3 # $FreeBSD: head/usr.bin/sed/POSIX 168417 2007-04-06 08:43:30Z yar $ 4 5 Comments on the IEEE P1003.2 Draft 12 6 Part 2: Shell and Utilities 7 Section 4.55: sed - Stream editor 8 9 Diomidis Spinellis <dds (a] doc.ic.ac.uk> 10 Keith Bostic <bostic (a] cs.berkeley.edu> 11 12 In the following paragraphs, "wrong" usually means "inconsistent with 13 historic practice", as most of the following comments refer to 14 undocumented inconsistencies between the historical versions of sed and 15 the POSIX 1003.2 standard. All the comments are notes taken while 16 implementing a POSIX-compatible version of sed, and should not be 17 interpreted as official opinions or criticism towards the POSIX committee. 18 All uses of "POSIX" refer to section 4.55, Draft 12 of POSIX 1003.2. 19 20 1. 32V and BSD derived implementations of sed strip the text 21 arguments of the a, c and i commands of their initial blanks, 22 i.e. 23 24 #!/bin/sed -f 25 a\ 26 foo\ 27 \ indent\ 28 bar 29 30 produces: 31 32 foo 33 indent 34 bar 35 36 POSIX does not specify this behavior as the System V versions of 37 sed do not do this stripping. The argument against stripping is 38 that it is difficult to write sed scripts that have leading blanks 39 if they are stripped. The argument for stripping is that it is 40 difficult to write readable sed scripts unless indentation is allowed 41 and ignored, and leading whitespace is obtainable by entering a 42 backslash in front of it. This implementation follows the BSD 43 historic practice. 44 45 2. Historical versions of sed required that the w flag be the last 46 flag to an s command as it takes an additional argument. This 47 is obvious, but not specified in POSIX. 48 49 3. Historical versions of sed required that whitespace follow a w 50 flag to an s command. This is not specified in POSIX. This 51 implementation permits whitespace but does not require it. 52 53 4. Historical versions of sed permitted any number of whitespace 54 characters to follow the w command. This is not specified in 55 POSIX. This implementation permits whitespace but does not 56 require it. 57 58 5. The rule for the l command differs from historic practice. Table 59 2-15 includes the various ANSI C escape sequences, including \\ 60 for backslash. Some historical versions of sed displayed two 61 digit octal numbers, too, not three as specified by POSIX. POSIX 62 is a cleanup, and is followed by this implementation. 63 64 6. The POSIX specification for ! does not specify that for a single 65 command the command must not contain an address specification 66 whereas the command list can contain address specifications. The 67 specification for ! implies that "3!/hello/p" works, and it never 68 has, historically. Note, 69 70 3!{ 71 /hello/p 72 } 73 74 does work. 75 76 7. POSIX does not specify what happens with consecutive ! commands 77 (e.g. /foo/!!!p). Historic implementations allow any number of 78 !'s without changing the behaviour. (It seems logical that each 79 one might reverse the behaviour.) This implementation follows 80 historic practice. 81 82 8. Historic versions of sed permitted commands to be separated 83 by semi-colons, e.g. 'sed -ne '1p;2p;3q' printed the first 84 three lines of a file. This is not specified by POSIX. 85 Note, the ; command separator is not allowed for the commands 86 a, c, i, w, r, :, b, t, # and at the end of a w flag in the s 87 command. This implementation follows historic practice and 88 implements the ; separator. 89 90 9. Historic versions of sed terminated the script if EOF was reached 91 during the execution of the 'n' command, i.e.: 92 93 sed -e ' 94 n 95 i\ 96 hello 97 ' </dev/null 98 99 did not produce any output. POSIX does not specify this behavior. 100 This implementation follows historic practice. 101 102 10. Deleted. 103 104 11. Historical implementations do not output the change text of a c 105 command in the case of an address range whose first line number 106 is greater than the second (e.g. 3,1). POSIX requires that the 107 text be output. Since the historic behavior doesn't seem to have 108 any particular purpose, this implementation follows the POSIX 109 behavior. 110 111 12. POSIX does not specify whether address ranges are checked and 112 reset if a command is not executed due to a jump. The following 113 program will behave in different ways depending on whether the 114 'c' command is triggered at the third line, i.e. will the text 115 be output even though line 3 of the input will never logically 116 encounter that command. 117 118 2,4b 119 1,3c\ 120 text 121 122 Historic implementations did not output the text in the above 123 example. Therefore it was believed that a range whose second 124 address was never matched extended to the end of the input. 125 However, the current practice adopted by this implementation, 126 as well as by those from GNU and SUN, is as follows: The text 127 from the 'c' command still isn't output because the second address 128 isn't actually matched; but the range is reset after all if its 129 second address is a line number. In the above example, only the 130 first line of the input will be deleted. 131 132 13. Historical implementations allow an output suppressing #n at the 133 beginning of -e arguments as well as in a script file. POSIX 134 does not specify this. This implementation follows historical 135 practice. 136 137 14. POSIX does not explicitly specify how sed behaves if no script is 138 specified. Since the sed Synopsis permits this form of the command, 139 and the language in the Description section states that the input 140 is output, it seems reasonable that it behave like the cat(1) 141 command. Historic sed implementations behave differently for "ls | 142 sed", where they produce no output, and "ls | sed -e#", where they 143 behave like cat. This implementation behaves like cat in both cases. 144 145 15. The POSIX requirement to open all w files at the beginning makes 146 sed behave nonintuitively when the w commands are preceded by 147 addresses or are within conditional blocks. This implementation 148 follows historic practice and POSIX, by default, and provides the 149 -a option which opens the files only when they are needed. 150 151 16. POSIX does not specify how escape sequences other than \n and \D 152 (where D is the delimiter character) are to be treated. This is 153 reasonable, however, it also doesn't state that the backslash is 154 to be discarded from the output regardless. A strict reading of 155 POSIX would be that "echo xyz | sed s/./\a" would display "\ayz". 156 As historic sed implementations always discarded the backslash, 157 this implementation does as well. 158 159 17. POSIX specifies that an address can be "empty". This implies 160 that constructs like ",d" or "1,d" and ",5d" are allowed. This 161 is not true for historic implementations or this implementation 162 of sed. 163 164 18. The b t and : commands are documented in POSIX to ignore leading 165 white space, but no mention is made of trailing white space. 166 Historic implementations of sed assigned different locations to 167 the labels "x" and "x ". This is not useful, and leads to subtle 168 programming errors, but it is historic practice and changing it 169 could theoretically break working scripts. This implementation 170 follows historic practice. 171 172 19. Although POSIX specifies that reading from files that do not exist 173 from within the script must not terminate the script, it does not 174 specify what happens if a write command fails. Historic practice 175 is to fail immediately if the file cannot be opened or written. 176 This implementation follows historic practice. 177 178 20. Historic practice is that the \n construct can be used for either 179 string1 or string2 of the y command. This is not specified by 180 POSIX. This implementation follows historic practice. 181 182 21. Deleted. 183 184 22. Historic implementations of sed ignore the RE delimiter characters 185 within character classes. This is not specified in POSIX. This 186 implementation follows historic practice. 187 188 23. Historic implementations handle empty RE's in a special way: the 189 empty RE is interpreted as if it were the last RE encountered, 190 whether in an address or elsewhere. POSIX does not document this 191 behavior. For example the command: 192 193 sed -e /abc/s//XXX/ 194 195 substitutes XXX for the pattern abc. The semantics of "the last 196 RE" can be defined in two different ways: 197 198 1. The last RE encountered when compiling (lexical/static scope). 199 2. The last RE encountered while running (dynamic scope). 200 201 While many historical implementations fail on programs depending 202 on scope differences, the SunOS version exhibited dynamic scope 203 behaviour. This implementation does dynamic scoping, as this seems 204 the most useful and in order to remain consistent with historical 205 practice. 206
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