POSIX revision 1.1
1 # @(#)POSIX 5.9 (Berkeley) 8/28/92 2 3 Comments on the IEEE P1003.2 Draft 12 4 Part 2: Shell and Utilities 5 Section 4.55: sed - Stream editor 6 7 Diomidis Spinellis <dds (a] doc.ic.ac.uk> 8 Keith Bostic <bostic (a] cs.berkeley.edu> 9 10 In the following paragraphs, "wrong" usually means "inconsistent with 11 historic practice", as most of the following comments refer to 12 undocumented inconsistencies between the historical versions of sed and 13 the POSIX 1003.2 standard. All the comments are notes taken while 14 implementing a POSIX-compatible version of sed, and should not be 15 interpreted as official opinions or criticism towards the POSIX committee. 16 All uses of "POSIX" refer to section 4.55, Draft 12 of POSIX 1003.2. 17 18 1. 32V and BSD derived implementations of sed strip the text 19 arguments of the a, c and i commands of their initial blanks, 20 i.e. 21 22 #!/bin/sed -f 23 a\ 24 foo\ 25 \ indent\ 26 bar 27 28 produces: 29 30 foo 31 indent 32 bar 33 34 POSIX does not specify this behavior as the System V versions of 35 sed do not do this stripping. The argument against stripping is 36 that it is difficult to write sed scripts that have leading blanks 37 if they are stripped. The argument for stripping is that it is 38 difficult to write readable sed scripts unless indentation is allowed 39 and ignored, and leading whitespace is obtainable by entering a 40 backslash in front of it. This implementation follows the BSD 41 historic practice. 42 43 2. Historical versions of sed required that the w flag be the last 44 flag to an s command as it takes an additional argument. This 45 is obvious, but not specified in POSIX. 46 47 3. Historical versions of sed required that whitespace follow a w 48 flag to an s command. This is not specified in POSIX. This 49 implementation permits whitespace but does not require it. 50 51 4. Historical versions of sed permitted any number of whitespace 52 characters to follow the w command. This is not specified in 53 POSIX. This implementation permits whitespace but does not 54 require it. 55 56 5. The rule for the l command differs from historic practice. Table 57 2-15 includes the various ANSI C escape sequences, including \\ 58 for backslash. Some historical versions of sed displayed two 59 digit octal numbers, too, not three as specified by POSIX. POSIX 60 is a cleanup, and is followed by this implementation. 61 62 6. The POSIX specification for ! does not specify that for a single 63 command the command must not contain an address specification 64 whereas the command list can contain address specifications. The 65 specification for ! implies that "3!/hello/p" works, and it never 66 has, historically. Note, 67 68 3!{ 69 /hello/p 70 } 71 72 does work. 73 74 7. POSIX does not specify what happens with consecutive ! commands 75 (e.g. /foo/!!!p). Historic implementations allow any number of 76 !'s without changing the behaviour. (It seems logical that each 77 one might reverse the behaviour.) This implementation follows 78 historic practice. 79 80 8. Historic versions of sed permitted commands to be separated 81 by semi-colons, e.g. 'sed -ne '1p;2p;3q' printed the first 82 three lines of a file. This is not specified by POSIX. 83 Note, the ; command separator is not allowed for the commands 84 a, c, i, w, r, :, b, t, # and at the end of a w flag in the s 85 command. This implementation follows historic practice and 86 implements the ; separator. 87 88 9. Historic versions of sed terminated the script if EOF was reached 89 during the execution of the 'n' command, i.e.: 90 91 sed -e ' 92 n 93 i\ 94 hello 95 ' </dev/null 96 97 did not produce any output. POSIX does not specify this behavior. 98 This implementation follows historic practice. 99 100 10. POSIX does not specify that the q command causes all lines that 101 have been appended to be output and that the pattern space is 102 printed before exiting. This implementation follows historic 103 practice. 104 105 11. Historical implementations do not output the change text of a c 106 command in the case of an address range whose first line number 107 is greater than the second (e.g. 3,1). POSIX requires that the 108 text be output. Since the historic behavior doesn't seem to have 109 any particular purpose, this implementation follows the POSIX 110 behavior. 111 112 12. POSIX does not specify whether address ranges are checked and 113 reset if a command is not executed due to a jump. The following 114 program will behave in different ways depending on whether the 115 'c' command is triggered at the third line, i.e. will the text 116 be output even though line 3 of the input will never logically 117 encounter that command. 118 119 2,4b 120 1,3c\ 121 text 122 123 Historic implementations, and this implementation, do not output 124 the text in the above example. The general rule, therefore, 125 is that a range whose second address is never matched extends to 126 the end of the input. 127 128 13. Historical implementations allow an output suppressing #n at the 129 beginning of -e arguments as well as in a script file. POSIX 130 does not specify this. This implementation follows historical 131 practice. 132 133 14. POSIX does not explicitly specify how sed behaves if no script is 134 specified. Since the sed Synopsis permits this form of the command, 135 and the language in the Description section states that the input 136 is output, it seems reasonable that it behave like the cat(1) 137 command. Historic sed implementations behave differently for "ls | 138 sed", where they produce no output, and "ls | sed -e#", where they 139 behave like cat. This implementation behaves like cat in both cases. 140 141 15. The POSIX requirement to open all w files at the beginning makes 142 sed behave nonintuitively when the w commands are preceded by 143 addresses or are within conditional blocks. This implementation 144 follows historic practice and POSIX, by default, and provides the 145 -a option which opens the files only when they are needed. 146 147 16. POSIX does not specify how escape sequences other than \n and \D 148 (where D is the delimiter character) are to be treated. This is 149 reasonable, however, it also doesn't state that the backslash is 150 to be discarded from the output regardless. A strict reading of 151 POSIX would be that "echo xyz | sed s/./\a" would display "\ayz". 152 As historic sed implementations always discarded the backslash, 153 this implementation does as well. 154 155 17. POSIX specifies that an address can be "empty". This implies 156 that constructs like ",d" or "1,d" and ",5d" are allowed. This 157 is not true for historic implementations or this implementation 158 of sed. 159 160 18. The b t and : commands are documented in POSIX to ignore leading 161 white space, but no mention is made of trailing white space. 162 Historic implementations of sed assigned different locations to 163 the labels "x" and "x ". This is not useful, and leads to subtle 164 programming errors, but it is historic practice and changing it 165 could theoretically break working scripts. This implementation 166 follows historic practice. 167 168 19. Although POSIX specifies that reading from files that do not exist 169 from within the script must not terminate the script, it does not 170 specify what happens if a write command fails. Historic practice 171 is to fail immediately if the file cannot be opened or written. 172 This implementation follows historic practice. 173 174 20. Historic practice is that the \n construct can be used for either 175 string1 or string2 of the y command. This is not specified by 176 POSIX. This implementation follows historic practice. 177 178 21. POSIX does not specify if the "Nth occurrence" of an RE in a 179 substitute command is an overlapping or a non-overlapping one, 180 i.e. what is the result of s/a*/A/2 on the pattern "aaaaa aaaaa". 181 Historical practice is to drop core or only do non-overlapping 182 RE's. This implementation only does non-overlapping RE's. 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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