# @(#)POSIX 8.1 (Berkeley) 6/6/93 Comments on the IEEE P1003.2 Draft 12 Part 2: Shell and Utilities Section 4.55: sed - Stream editor Diomidis Spinellis <dds@doc.ic.ac.uk> Keith Bostic <bostic@cs.berkeley.edu> In the following paragraphs, "wrong" usually means "inconsistent with historic practice", as most of the following comments refer to undocumented inconsistencies between the historical versions of sed and the POSIX 1003.2 standard. All the comments are notes taken while implementing a POSIX-compatible version of sed, and should not be interpreted as official opinions or criticism towards the POSIX committee. All uses of "POSIX" refer to section 4.55, Draft 12 of POSIX 1003.2. 1. 32V and BSD derived implementations of sed strip the text arguments of the a, c and i commands of their initial blanks, i.e. #!/bin/sed -f a\ foo\ \ indent\ bar produces: foo indent bar POSIX does not specify this behavior as the System V versions of sed do not do this stripping. The argument against stripping is that it is difficult to write sed scripts that have leading blanks if they are stripped. The argument for stripping is that it is difficult to write readable sed scripts unless indentation is allowed and ignored, and leading whitespace is obtainable by entering a backslash in front of it. This implementation follows the BSD historic practice. 2. Historical versions of sed required that the w flag be the last flag to an s command as it takes an additional argument. This is obvious, but not specified in POSIX. 3. Historical versions of sed required that whitespace follow a w flag to an s command. This is not specified in POSIX. This implementation permits whitespace but does not require it. 4. Historical versions of sed permitted any number of whitespace characters to follow the w command. This is not specified in POSIX. This implementation permits whitespace but does not require it. 5. The rule for the l command differs from historic practice. Table 2-15 includes the various ANSI C escape sequences, including \\ for backslash. Some historical versions of sed displayed two digit octal numbers, too, not three as specified by POSIX. POSIX is a cleanup, and is followed by this implementation. 6. The POSIX specification for ! does not specify that for a single command the command must not contain an address specification whereas the command list can contain address specifications. The specification for ! implies that "3!/hello/p" works, and it never has, historically. Note, 3!{ /hello/p } does work. 7. POSIX does not specify what happens with consecutive ! commands (e.g. /foo/!!!p). Historic implementations allow any number of !'s without changing the behaviour. (It seems logical that each one might reverse the behaviour.) This implementation follows historic practice. 8. Historic versions of sed permitted commands to be separated by semi-colons, e.g. 'sed -ne '1p;2p;3q' printed the first three lines of a file. This is not specified by POSIX. Note, the ; command separator is not allowed for the commands a, c, i, w, r, :, b, t, # and at the end of a w flag in the s command. This implementation follows historic practice and implements the ; separator. 9. Historic versions of sed terminated the script if EOF was reached during the execution of the 'n' command, i.e.: sed -e ' n i\ hello ' </dev/null did not produce any output. POSIX does not specify this behavior. This implementation follows historic practice. 10. Deleted. 11. Historical implementations do not output the change text of a c command in the case of an address range whose first line number is greater than the second (e.g. 3,1). POSIX requires that the text be output. Since the historic behavior doesn't seem to have any particular purpose, this implementation follows the POSIX behavior. 12. POSIX does not specify whether address ranges are checked and reset if a command is not executed due to a jump. The following program will behave in different ways depending on whether the 'c' command is triggered at the third line, i.e. will the text be output even though line 3 of the input will never logically encounter that command. 2,4b 1,3c\ text Historic implementations did not output the text in the above example. Therefore it was believed that a range whose second address was never matched extended to the end of the input. However, the current practice adopted by this implementation, as well as by those from GNU and SUN, is as follows: The text from the 'c' command still isn't output because the second address isn't actually matched; but the range is reset after all if its second address is a line number. In the above example, only the first line of the input will be deleted. 13. Historical implementations allow an output suppressing #n at the beginning of -e arguments as well as in a script file. POSIX does not specify this. This implementation follows historical practice. 14. POSIX does not explicitly specify how sed behaves if no script is specified. Since the sed Synopsis permits this form of the command, and the language in the Description section states that the input is output, it seems reasonable that it behave like the cat(1) command. Historic sed implementations behave differently for "ls | sed", where they produce no output, and "ls | sed -e#", where they behave like cat. This implementation behaves like cat in both cases. 15. The POSIX requirement to open all w files at the beginning makes sed behave nonintuitively when the w commands are preceded by addresses or are within conditional blocks. This implementation follows historic practice and POSIX, by default, and provides the -a option which opens the files only when they are needed. 16. POSIX does not specify how escape sequences other than \n and \D (where D is the delimiter character) are to be treated. This is reasonable, however, it also doesn't state that the backslash is to be discarded from the output regardless. A strict reading of POSIX would be that "echo xyz | sed s/./\a" would display "\ayz". As historic sed implementations always discarded the backslash, this implementation does as well. 17. POSIX specifies that an address can be "empty". This implies that constructs like ",d" or "1,d" and ",5d" are allowed. This is not true for historic implementations or this implementation of sed. 18. The b t and : commands are documented in POSIX to ignore leading white space, but no mention is made of trailing white space. Historic implementations of sed assigned different locations to the labels "x" and "x ". This is not useful, and leads to subtle programming errors, but it is historic practice and changing it could theoretically break working scripts. This implementation follows historic practice. 19. Although POSIX specifies that reading from files that do not exist from within the script must not terminate the script, it does not specify what happens if a write command fails. Historic practice is to fail immediately if the file cannot be opened or written. This implementation follows historic practice. 20. Historic practice is that the \n construct can be used for either string1 or string2 of the y command. This is not specified by POSIX. This implementation follows historic practice. 21. Deleted. 22. Historic implementations of sed ignore the RE delimiter characters within character classes. This is not specified in POSIX. This implementation follows historic practice. 23. Historic implementations handle empty RE's in a special way: the empty RE is interpreted as if it were the last RE encountered, whether in an address or elsewhere. POSIX does not document this behavior. For example the command: sed -e /abc/s//XXX/ substitutes XXX for the pattern abc. The semantics of "the last RE" can be defined in two different ways: 1. The last RE encountered when compiling (lexical/static scope). 2. The last RE encountered while running (dynamic scope). While many historical implementations fail on programs depending on scope differences, the SunOS version exhibited dynamic scope behaviour. This implementation does dynamic scoping, as this seems the most useful and in order to remain consistent with historical practice.