#	@(#)POSIX	8.1 (Berkeley) 6/6/93Comments on the IEEE P1003.2 Draft 12     Part 2: Shell and Utilities  Section 4.55: sed - Stream editorDiomidis Spinellis <dds@doc.ic.ac.uk>Keith Bostic <bostic@cs.berkeley.edu>In the following paragraphs, "wrong" usually means "inconsistent withhistoric practice", as most of the following comments refer toundocumented inconsistencies between the historical versions of sed andthe POSIX 1003.2 standard.  All the comments are notes taken whileimplementing a POSIX-compatible version of sed, and should not beinterpreted 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, and this implementation, do not output	the text in the above example.  The general rule, therefore,	is that a range whose second address is never matched extends to	the end of the input.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.