		ksh 93u+m vs. ksh 93u+

The following is a list of changes between ksh 93u+ 2012-08-01 and the new
ksh 93u+m reboot that could cause incompatibilities in rare corner cases.
Fixes of clear bugs in ksh 93u+ are not included here, even though any bugfix
could potentially cause an incompatibility in a script that relies on the bug.
For more details, see the NEWS file and for complete details, see the git log.

0.	A new '-o posix' shell option has been added to ksh 93u+m that makes
	the ksh language more compatible with other shells by following the
	POSIX standard more closely. See the manual page for details. It is
	enabled by default if ksh is invoked as sh.

1.	Bytecode compiled by shcomp 93u+m will not run on older ksh versions.
	(However, bytecode compiled by older shcomp will run on ksh 93u+m.)

2.	Pathname expansion (a.k.a. filename generation, a.k.a. globbing) now
	never matches the special navigational names '.' (current directory)
	and '..' (parent directory). This change makes a pattern like .*
	useful; it now matches all hidden 'dotfiles' in the current directory.

3.	The bash-style &>foo redirection operator (shorthand for >foo 2>&1) can
	now always be used if -o posix is off, and not only in profile scripts.

4.	Redirections that store a file descriptor > 9 in a variable, such as
	{var}>file, now continue to work if brace expansion is turned off.

5.	Most predefined aliases have been converted to regular built-in
	commands that work the same way. 'unalias' no longer removes these.
	To remove a built-in command, use 'builtin -d'. The 'history' and 'r'
	predefined aliases remain, but are now only set on interactive shells.
	There are some minor changes in behavior in some former aliases:
	-  'redirect' now checks if all arguments are valid redirections
	   before performing them. If an error occurs, it issues an error
	   message instead of terminating the shell.
	-  'suspend' now refuses to suspend a login shell, as there is probably
	   no parent shell to return to and the login session would freeze.
	   If you really want to suspend a login shell, use 'kill -s STOP $$'.
	-  'times' now gives high precision output in a POSIX compliant format.

6.	'command' and 'nohup' no longer expand aliases in their first argument,
	as this is no longer required after the foregoing change. In the
	unlikely event that you still need this behavior, you can set:
		alias command='command '
		alias nohup='nohup '

7.	The 'login' and 'newgrp' special built-in commands have been removed,
	so it is no longer an error to define shell functions by these names.
	These built-ins replaced your shell session with the external commands
	by the same name, as in 'exec'. If an error occurred (e.g. due to a
	typo), you would end up immediately logged out, except on a few
	commercial Unix systems whose 'login' and 'newgrp' cope with this
	by starting a new shell session upon error. If you do want the old
	behavior, you can restore it by setting:
		alias login='exec login'
		alias newgrp='exec newgrp'

8.	'case' no longer retries to match patterns as literal strings if they
	fail to match as patterns. This undocumented behaviour broke validation
	use cases that are expected to work. For example:
		n='[0-9]'
		case $n in
		[0-9])	echo "$n is a digit" ;;
		esac
	would output "[0-9] is a digit". In the unlikely event that a script
	does rely on this behavior, it can be fixed like this:
		case $n in
		[0-9] | "[0-9]")
			echo "$n is a digit or the digit pattern" ;;
		esac

9.	If 'set -u'/'set -o nounset' is active, then the shell now errors out
	if a nonexistent positional parameter such as $1, $2, ... is accessed.
	(This does *not* apply to "$@" and "$*".)

10.	If 'set -u'/'set -o nounset' is active, then the shell now errors out
	if $! is accessed before the shell has launched any background process.

11.	The 'print', 'printf' and 'echo' built-in commands now return a nonzero
	exit status if an input/output error occurs.

12.	Four obsolete date format specifiers for 'printf %(format)T' were
	changed to make them compatible with modern date(1) commands:
	- %k and %l now return a blank-padded hour (24-hour and 12-hour clock).
	- %f now returns a date with the format '%Y.%m.%d-%H:%M:%S'.
	- %q now returns the quarter of the current year.

13.	The 'typeset' built-in now properly detects and reports options that
	cannot be used together if they are given as part of the same command.

14.	The DEBUG trap has reverted to pre-93t behavior. It is now once again
	reset like other traps upon entering a subshell or ksh-style function,
	as documented, and it is no longer prone to crash or get corrupted.

15.	'command -x' now always runs an external command, bypassing built-ins.

16.	Unbalanced quotes and backticks now correctly produce a syntax error
	in -c scripts, 'eval', and backtick-style command substitutions.

17.	-G/--globstar: Symbolic links to directories are now followed if they
	match a normal (non-**) glob pattern. For example, if '/lnk' is a
	symlink to a directory, '/lnk/**' and '/l?k/**' now work as expected.

18.	The variable name search expansions ${!prefix@} and ${!prefix*} now
	also include the variable 'prefix' itself in the possible results.

19.	[[ -v var ]] is now properly equivalent to [[ -n ${var+set} ]].
	Undocumented special-casing for numeric types has been removed.
	For example, the following no longer produces an unexpected error:
	    $ ksh -o nounset -c 'float n; [[ -v n ]] && echo $n'

20.	If the HOME variable is unset, the bare tilde ~ now expands to the
	current user's system home directory instead of merely the username.

21.	On Windows/Cygwin, globbing is no longer case-insensitive by default.
	Turning on the new --globcasedetect shell option restores
	case-insensitive globbing for case-insensitive file systems.

22.	If $PWD or $OLDPWD are passed as invocation-local assignments to cd,
	their values are no longer altered in the outer scope when cd finishes.
	For example:
		ksh -c 'OLDPWD=/bin; OLDPWD=/tmp cd - > /dev/null; echo $OLDPWD'
		ksh -c 'cd /var; PWD=/tmp cd /usr; echo $PWD'
	now prints '/bin' followed by '/var'.

23.	Path-bound built-ins (such as /opt/ast/bin/cat) can now be executed
	by invoking the canonical path, so the following will now work:
		$ /opt/ast/bin/cat --version
		  version         cat (AT&T Research) 2012-05-31
		$ (PATH=/opt/ast/bin:$PATH; "$(whence -p cat)" --version)
		  version         cat (AT&T Research) 2012-05-31
	In the event an external command by that path exists, the path-bound
	built-in will now override it when invoked using the canonical path.
	To invoke a possible external command at that path, you can still use
	a non-canonical path, e.g.: /opt//ast/bin/cat or /opt/ast/./bin/cat

24.	The readonly attribute of ksh variables is no longer imported from
	or exported to other ksh shell instances through the environment.

25.	Subshells (even if non-forked) now keep a properly separated state
	of the pseudorandom generator used for $RANDOM, so that using
	$RANDOM in a non-forked subshell no longer influences a reproducible
	$RANDOM sequence in the parent environment. In addition, upon
	invoking a subshell, $RANDOM is now reseeded (as mksh and bash do).

26.	The built-in arithmetic function int() has changed to round towards
	zero instead of negative infinity. Previously, int() was an alias to
	floor(), but now it behaves like trunc().

27.	The '!' logical negation operator in the '[[' compound command now
	correctly negates another '!', e.g., [[ ! ! 1 -eq 1 ]] now returns
	0/true. Note that this has always been the case for 'test'/'['.

28.	By default, arithmetic expressions in ksh no longer interpret a number
	with a leading zero as octal in any context. Use 8#octalnumber instead.
	Before, ksh would arbitrarily recognize the leading octal zero in some
	contexts but not others. One of several examples is:
		x=010; echo "$((x)), $(($x))"
	would output '10, 8'. This now outputs '10, 10'. Arithmetic
	expressions now also behave identically within and outside ((...))
	and $((...)). Setting the --posix compliance option turns on the
	recognition of the leading octal zero for all arithmetic contexts.

29.	It is now an error for arithmetic expressions to assign an out-of-range
	index value to a variable of an enumeration type created with 'enum'.

30.	For the 'return' built-in command, you can now freely specify any
	return value that fits in a signed integer, typically a 32-bit value.
	Note that $? is truncated to 8 bits when the current (sub)shell exits.

____________________________________________________________________________

		KSH-93 VS. KSH-88

(Written by David Korn for ksh 93u+ 2012-08-01)

The following is a list of known incompatibilities between ksh-93 and ksh-88.
I have not included cases that are clearly bugs in ksh-88.  I also have
omitted features that are completely upward compatible.

1.	Functions, defined with name() with ksh-93 are compatible with
	the POSIX standard, not with ksh-88.  No local variables are
	permitted, and there is no separate scope.  Functions defined
	with the function name syntax have local variables as in ksh-88,
	but are statically scoped as in C so that a function does not
	automatically have access to local variables of the caller.
	This change also affects function traces.

2.	! is now a reserved word.  As a result, any command by that
	name will no longer work with ksh-93.

3.	The -x attribute of alias and typeset -f is no longer
	effective and the ENV file is only read for interactive
	shells.  You need to use FPATH to make function definitions
	visible to scripts.

4.	A built-in command named command has been added which is
	always found before the PATH search.  Any script which uses
	this name as the name of a command (or function) will not
	be compatible.

5.	The output format for some built-ins has changed.  In particular
	the output format for set, typeset and alias now have single
	quotes around values that have special characters.  The output
	for trap without arguments has a format that can be used as input.

6.	With ksh-88, a dollar sign ($') followed by a single quote was
	interpreted literally.  Now it is an ANSI C string.  You
	must quote the dollar sign to get the previous behavior.
	Also, a $ in front of a " indicates that the string needs
	to be translated for locales other than C or POSIX.  The $
	is ignored in the C and POSIX locale.

7.	With ksh-88, tilde expansion did not take place inside ${...}.
	with ksh-93, ${foo-~} will cause tilde expansion if foo is
	not set.  You need to escape the ~ for the previous behavior.

8.      Some changes in the tokenizing rules were made that might
	cause some scripts with previously ambiguous use of quoting
	to produce syntax errors.

9.	Programs that rely on specific exit values for the shell,
	(rather than 0 or non-zero) may not be compatible.  The
	exit status for many shell failures has been changed.

10.	Built-ins in ksh-88 were always executed before looking for
	the command in the PATH variable.  This is no longer true.
	Thus, with ksh-93, if you have the current directory first
	in your PATH, and you have a program named test in your
	directory, it will be executed when you type test; the
	built-in version will be run at the point /bin is found
	in your PATH.

11.	Some undocumented combinations of argument passing to ksh
	builtins no longer works since ksh-93 is getopts conforming
	with respect to its built-ins.  For example, typeset -8i
	previously would work as a synonym for typeset -i8.

12.	Command substitution and arithmetic expansion are now performed
	on PS1, PS3, and ENV when they are expanded.  Thus,  ` and $(
	as part of the value of these variables must be preceded by a \
	to preserve their previous behavior.

13.	The ERRNO variable has been dropped.

14.	If the file name following a redirection symbol contain pattern
	characters they will only be expanded for interactive shells.

15.	The arguments to a dot script will be restored when it completes.

16.	The list of tracked aliases is not displayed with alias unless
	the -t option is specified.

17.	The POSIX standard requires that test "$arg" have exit status
	of 0, if and only if $arg is null.  However, since this breaks
	programs that use test -t, ksh-93 treats an explicit test -t
	as if the user had entered test -t 1.

18.	The ^T directive of emacs mode has been changed to work the
	way it does in gnu-emacs.

19.	ksh-88 allowed unbalanced parentheses within ${name op val} whereas
	ksh-93 does not.  Thus, ${foo-(} needs to be written as ${foo-\(}
	which works with both versions.
	[2021 UPDATE: This is now once again allowed in ksh 93u+m. Note that
	balanced parentheses ${foo-()} were also broken and are now fixed.]

20.     kill -l in ksh-93 lists only the signal names, not their numerical
	values.

21.	Local variables defined by typeset are statically scoped in
	ksh-93.  In ksh-88 they were dynamically scoped although this
	behavior was never documented.

22.	The value of the variable given to getopts is set to ? when
	the end-of-options is reached to conform to the POSIX standard.

23.	Since the POSIX standard requires that octal constants be
	recognized, doing arithmetic on typeset -Z variables can
	yield different results that with ksh-88.  Most of these
	differences were eliminated in ksh-93o.  Starting in ksh-93u+, the
	let command no longer recognizes octal constants starting with 0
	for compatibility with ksh-88 unless the option letoctal is on.

24.	Starting after ksh-93l, If you run ksh name, where name does
	not contain a /, the current directory will be searched
	before doing a path search on name as required by the POSIX
	shell standard.

25.	In ksh-93, cd - will output the directory that it changes
	to on standard output as required by X/Open.  With ksh-88,
	this only happened for interactive shells.

26.	As an undocumented feature of ksh-88, a leading 0 to an
	assignment of an integer variable caused that variable
	to be treated as unsigned.  This behavior was removed
	starting in ksh-93p.

27.	The getopts builtin in ksh-93 requires that optstring contain
	a leading + to allow options to begin with a +.

28.	In emacs/gmacs mode, control-v will not display the version when
	the stty lnext character is set to control-v or is unset.
	The sequence escape control-v will display the shell version.

29.	In ksh-88, DEBUG traps were executed after each command.  In ksh-93
	DEBUG traps are executed before each command.

30.	In ksh-88, a redirection to a file name given by an empty string was
	ignored.  In ksh-93, this is an error.

I am interested in expanding this list so please let me know if you
uncover any others.
