1# @(#)POSIX 8.1 (Berkeley) 6/6/93 2# $FreeBSD$ 3 4Comments on the IEEE P1003.2 Draft 12 5 Part 2: Shell and Utilities 6 Section 4.55: sed - Stream editor 7 8Diomidis Spinellis <dds@doc.ic.ac.uk> 9Keith Bostic <bostic@cs.berkeley.edu> 10 11In the following paragraphs, "wrong" usually means "inconsistent with 12historic practice", as most of the following comments refer to 13undocumented inconsistencies between the historical versions of sed and 14the POSIX 1003.2 standard. All the comments are notes taken while 15implementing a POSIX-compatible version of sed, and should not be 16interpreted as official opinions or criticism towards the POSIX committee. 17All uses of "POSIX" refer to section 4.55, Draft 12 of POSIX 1003.2. 18 19 1. 32V and BSD derived implementations of sed strip the text 20 arguments of the a, c and i commands of their initial blanks, 21 i.e. 22 23 #!/bin/sed -f 24 a\ 25 foo\ 26 \ indent\ 27 bar 28 29 produces: 30 31 foo 32 indent 33 bar 34 35 POSIX does not specify this behavior as the System V versions of 36 sed do not do this stripping. The argument against stripping is 37 that it is difficult to write sed scripts that have leading blanks 38 if they are stripped. The argument for stripping is that it is 39 difficult to write readable sed scripts unless indentation is allowed 40 and ignored, and leading whitespace is obtainable by entering a 41 backslash in front of it. This implementation follows the BSD 42 historic practice. 43 44 2. Historical versions of sed required that the w flag be the last 45 flag to an s command as it takes an additional argument. This 46 is obvious, but not specified in POSIX. 47 48 3. Historical versions of sed required that whitespace follow a w 49 flag to an s command. This is not specified in POSIX. This 50 implementation permits whitespace but does not require it. 51 52 4. Historical versions of sed permitted any number of whitespace 53 characters to follow the w command. This is not specified in 54 POSIX. This implementation permits whitespace but does not 55 require it. 56 57 5. The rule for the l command differs from historic practice. Table 58 2-15 includes the various ANSI C escape sequences, including \\ 59 for backslash. Some historical versions of sed displayed two 60 digit octal numbers, too, not three as specified by POSIX. POSIX 61 is a cleanup, and is followed by this implementation. 62 63 6. The POSIX specification for ! does not specify that for a single 64 command the command must not contain an address specification 65 whereas the command list can contain address specifications. The 66 specification for ! implies that "3!/hello/p" works, and it never 67 has, historically. Note, 68 69 3!{ 70 /hello/p 71 } 72 73 does work. 74 75 7. POSIX does not specify what happens with consecutive ! commands 76 (e.g. /foo/!!!p). Historic implementations allow any number of 77 !'s without changing the behaviour. (It seems logical that each 78 one might reverse the behaviour.) This implementation follows 79 historic practice. 80 81 8. Historic versions of sed permitted commands to be separated 82 by semi-colons, e.g. 'sed -ne '1p;2p;3q' printed the first 83 three lines of a file. This is not specified by POSIX. 84 Note, the ; command separator is not allowed for the commands 85 a, c, i, w, r, :, b, t, # and at the end of a w flag in the s 86 command. This implementation follows historic practice and 87 implements the ; separator. 88 89 9. Historic versions of sed terminated the script if EOF was reached 90 during the execution of the 'n' command, i.e.: 91 92 sed -e ' 93 n 94 i\ 95 hello 96 ' </dev/null 97 98 did not produce any output. POSIX does not specify this behavior. 99 This implementation follows historic practice. 100 10110. Deleted. 102 10311. Historical implementations do not output the change text of a c 104 command in the case of an address range whose first line number 105 is greater than the second (e.g. 3,1). POSIX requires that the 106 text be output. Since the historic behavior doesn't seem to have 107 any particular purpose, this implementation follows the POSIX 108 behavior. 109 11012. POSIX does not specify whether address ranges are checked and 111 reset if a command is not executed due to a jump. The following 112 program will behave in different ways depending on whether the 113 'c' command is triggered at the third line, i.e. will the text 114 be output even though line 3 of the input will never logically 115 encounter that command. 116 117 2,4b 118 1,3c\ 119 text 120 121 Historic implementations did not output the text in the above 122 example. Therefore it was believed that a range whose second 123 address was never matched extended to the end of the input. 124 However, the current practice adopted by this implementation, 125 as well as by those from GNU and SUN, is as follows: The text 126 from the 'c' command still isn't output because the second address 127 isn't actually matched; but the range is reset after all if its 128 second address is a line number. In the above example, only the 129 first line of the input will be deleted. 130 13113. Historical implementations allow an output suppressing #n at the 132 beginning of -e arguments as well as in a script file. POSIX 133 does not specify this. This implementation follows historical 134 practice. 135 13614. POSIX does not explicitly specify how sed behaves if no script is 137 specified. Since the sed Synopsis permits this form of the command, 138 and the language in the Description section states that the input 139 is output, it seems reasonable that it behave like the cat(1) 140 command. Historic sed implementations behave differently for "ls | 141 sed", where they produce no output, and "ls | sed -e#", where they 142 behave like cat. This implementation behaves like cat in both cases. 143 14415. The POSIX requirement to open all w files at the beginning makes 145 sed behave nonintuitively when the w commands are preceded by 146 addresses or are within conditional blocks. This implementation 147 follows historic practice and POSIX, by default, and provides the 148 -a option which opens the files only when they are needed. 149 15016. POSIX does not specify how escape sequences other than \n and \D 151 (where D is the delimiter character) are to be treated. This is 152 reasonable, however, it also doesn't state that the backslash is 153 to be discarded from the output regardless. A strict reading of 154 POSIX would be that "echo xyz | sed s/./\a" would display "\ayz". 155 As historic sed implementations always discarded the backslash, 156 this implementation does as well. 157 15817. POSIX specifies that an address can be "empty". This implies 159 that constructs like ",d" or "1,d" and ",5d" are allowed. This 160 is not true for historic implementations or this implementation 161 of sed. 162 16318. The b t and : commands are documented in POSIX to ignore leading 164 white space, but no mention is made of trailing white space. 165 Historic implementations of sed assigned different locations to 166 the labels "x" and "x ". This is not useful, and leads to subtle 167 programming errors, but it is historic practice and changing it 168 could theoretically break working scripts. This implementation 169 follows historic practice. 170 17119. Although POSIX specifies that reading from files that do not exist 172 from within the script must not terminate the script, it does not 173 specify what happens if a write command fails. Historic practice 174 is to fail immediately if the file cannot be opened or written. 175 This implementation follows historic practice. 176 17720. Historic practice is that the \n construct can be used for either 178 string1 or string2 of the y command. This is not specified by 179 POSIX. This implementation follows historic practice. 180 18121. Deleted. 182 18322. Historic implementations of sed ignore the RE delimiter characters 184 within character classes. This is not specified in POSIX. This 185 implementation follows historic practice. 186 18723. Historic implementations handle empty RE's in a special way: the 188 empty RE is interpreted as if it were the last RE encountered, 189 whether in an address or elsewhere. POSIX does not document this 190 behavior. For example the command: 191 192 sed -e /abc/s//XXX/ 193 194 substitutes XXX for the pattern abc. The semantics of "the last 195 RE" can be defined in two different ways: 196 197 1. The last RE encountered when compiling (lexical/static scope). 198 2. The last RE encountered while running (dynamic scope). 199 200 While many historical implementations fail on programs depending 201 on scope differences, the SunOS version exhibited dynamic scope 202 behaviour. This implementation does dynamic scoping, as this seems 203 the most useful and in order to remain consistent with historical 204 practice. 205