This documents significant changes in the 93u+m branch of AT&T ksh93.
For full details, see the git log at: https://github.com/ksh93/ksh

Any uppercase BUG_* names are modernish shell bug IDs.

2021-05-10:

- Release 1.0.0-beta.1.

2021-05-07:

- Backported three ksh 93v- math.tab changes, allowing for an exp10()
  arithmetic function if one exists in the C library, a new float()
  function, and lastly an updated int() function that rounds to zero
  instead of being an alias to floor().

2021-05-05:

- Fixed: a preceding variable assignment like foo=bar in 'foo=bar command'
  (with no command arguments after 'command') incorrectly survived the
  'command' regular built-in command invocation.

- Fixed: 'command -p some_utility' intermittently failed to find the utility
  under certain conditions due to a memory corruption issue.

2021-05-03:

- 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).

- Fixed program flow corruption that occurred in scripts on executing a
  background job in a nested subshell, as in ( ( simple_command & ) ).

- Completed the 2021-04-30 fix for ${var<OP>'{}'} where <OP> is '-', '+',
  ':-' or ':+' by fixing a bug that caused an extra '}' to be output.

- Following the resolution of Austin Group bug 1393[*] that is set to be
  included in the next version of the POSIX standard, the 'command' prefix
  in POSIX mode (set -o posix) no longer disables the declaration properties
  of declaration built-ins. This reverts a change introduced on 2020-09-11.
  [*] https://austingroupbugs.net/view.php?id=1393

- Fixed arithmetic assignment operations for multidimensional indexed arrays

2021-04-30:

- The emacs 'ESC .' (M-.) and vi '_' commands now take shell quoting into
  account when repeating a word from the previous command line. For example, if
  the previous command is 'ls Stairway\ To\ Heaven.mp3', then they now insert
  'Stairway\ To\ Heaven.mp3' instead of 'Heaven.mp3'. Thanks to Govind Kamat.

- Fixed a bug introduced on 2020-09-05 that caused "echo ${var:+'{}'}"
  to be misparsed.

- Fixed: the effects of 'builtin', 'exec' and 'ulimit' leaked out of a parent
  virtual subshell if run from a ${ shared-state; } command substitution.

2021-04-26:

- Fixed a bug introduced on 2021-02-20 in which a shared-state command
  substitution stopped sharing its state with the calling shell environment
  if it executed a command that locally redirected standard outpuut.

2021-04-22:

- shcomp (the shell bytecode compiler) was fixed to correctly compile process
  substitutions used as the file name to a redirection, as in 'cmd < <(cmd)'.

- Fixed a bug introduced on 2020-07-13 that set LINENO to the wrong line
  number after leaving a virtual subshell in which LINENO had been unset.

2021-04-21:

- Fixed a bug introduced on 2020-09-28 that caused an interactive ksh to exit
  if a profile script (such as ~/.kshrc) contains a syntax error.

2021-04-20:

- Fixed three problems with the /opt/ast/bin/getconf built-in command:
  1. The -l/--lowercase option did not change all variable names to lower case.
  2. The -q/--quote option now quotes all string values. Previously, it only
     quoted string values that had a space or other non-shellsafe character.
  3. The -c/--call, -n/--name and -s/--standard options matched all variable
     names provided by 'getconf -a', even if none were actual matches.

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

2021-04-16:

- Fixed a bug in emacs mode: after using tab completion to complete the name
  of a directory, it was not possible to type numbers after the slash.

- Fixed an optimization bug that caused the <>; redirection operator to fail
  when used with the last command in a -c script.

2021-04-14:

- 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 as expected:
	$ /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
  Non-canonical paths such as /opt/ast/./bin/cat will not find the built-ins.

- Path-bound built-ins will now also be found on a PATH set locally using an
  assignment preceding the command, so the following will now work as expected:
	$ PATH=/opt/ast/bin cat --version
	  version         cat (AT&T Research) 2012-05-31

2021-04-13:

- Fixed a few bugs that could cause ksh to show the wrong error message and/or
  return the wrong exit status if a command couldn't be executed. In
  scenarios where the command was found in the PATH but it was not executable,
  ksh now returns with exit status 126. Otherwise, ksh will return with exit
  status 127 (such as if the command isn't found or if the command name is
  too long).

2021-04-12:

- Corrected a memory fault when an attempt was made to unset the default
  nameref KSH_VERSION from the shell environment prior to any other name
  reference variable creation or modification.

2021-04-11:

- Fixed two related regressions introduced on 2020-06-16:
  1. The += assignment failed to append the value of variables when used
     in an invocation-local scope. The following should print '5', but
     the regression resulted in '3' being printed instead:
       $ integer foo=2; foo+=3 command eval 'echo $foo'
       3
  2. Any += assignment used in an invocation-local scope could modify
     readonly variables.

2021-04-10:

- Fixed: the internal count of the recursion level for arithmetic expressions
  was not reset when certain errors occurred in a virtual subshell. This could
  cause an erroneous "recursion to deep" error when a loop executed many
  subshells containing arithmetic expressions with errors, e.g. for testing.

2021-04-09:

- Fixed a bug that caused ksh to enable -c during the shell's initialization
  if the only argument passed was --posix.

- Fixed a related bug that caused 'set --posix' to leave the braceexpand and
  letoctal shell options unchanged.

- Fixed a bug that caused 'set --default' to unset the restricted option
  in restricted shells.

2021-04-08:

- Path-bound builtins will now be used by restricted shells if /opt/ast/bin
  is in the $PATH upon invoking the shell or before setting it to restricted.

- Fixed a bug that caused "printf '%T\n' now" to ignore $LC_ALL and $LC_TIME
  if the current locale was previously set, unset then set again.

2021-04-07:

- The $LC_TIME variable is now recognized by ksh and if set to an invalid
  locale will show an error.

- Fixed BUG_CSUBSTDO: If standard output is closed before running a command
  substitution, redirecting any other file descriptor no longer closes standard
  output inside of the command substitution.

2021-04-05:

- Fixed a regression, introduced in ksh 93t+ 2009-07-31, that caused a command
  like 'unset arr[3]' to unset not just element 3 of the array but all elements
  starting from 3, if a range expansion like ${arr[5..10]} was previously used.

- Several fixes for arrays of a type created by 'enum' were backported from ksh
  93v-, further to the two enum array fixes already applied on 2021-02-01:
  1. The array[@]} expansion was fixed for associative arrays of an enum type.
  2. Assignments now work correctly for all enum values for both indexed and
     associative arrays.
  3. 'unset' will now completely unset an associative array of an enum type.

2021-04-04:

- A bug was fixed that caused a broken prompt display upon redrawing the
  command line if the last line of the prompt includes an xterm escape
  sequence that is terminated by $'\a' (the bell character).

- Harden readonly variables. Readonly variables or arrays no longer allow
  attribute changes which would otherwise allow their value to be altered.
  Expanded support for readonly variables within multidimensional arrays.

2021-04-03:

- Fixed a bug that caused the uname builtin's -d option to change the output
  of the -o option.

- Fixed a possible crash that could occur when showing the domain name
  with the uname builtin's -d option.

2021-03-31:

- Fixed a bug that caused 'cd -' to ignore the current value of $OLDPWD
  when it's set to a different directory in a new scope.

- Fixed a related bug that caused ksh to use the wrong value for $PWD
  when in a new scope.

2021-03-29:

- Fixed an intermittent crash that could occur in vi mode when using the 'b'
  or 'B' commands to go back one word.

2021-03-27:

- The 'test' builtin will now show an error message when given the invalid ']]'
  or '=~' operators; it also properly returns with exit status 2 now (instead
  of exit status 1). If the invalid operator is supported by [[ ... ]] (such
  as '=~'), test will now suggest the usage of [[ ... ]] instead.

2021-03-22:

- A new --globcasedetect shell option is added to ksh on OSs where we can check
  for a case-insensitive file system (currently macOS, Windows/Cygwin, Linux
  and QNX 7.0+). When this option is turned on, file name generation
  (globbing), as well as file name tab completion on interactive shells,
  automatically become case-insensitive on file systems where the difference
  between upper- and lowercase is ignored for file names. This is transparently
  determined for each directory, so a path pattern that spans multiple file
  systems can be part case-sensitive and part case-insensitive.
      The option is not compiled into ksh on systems where we do not know of a
  method to check for file system case insensitivity. The shell option can be
  force-compiled by setting SHOPT_GLOBCASEDET to 1 in src/cmd/ksh93/SHOPT.sh,
  but it won't have any effect on non-supported systems, so this is not
  recommended. It can be removed from ksh by setting SHOPT_GLOBCASEDET to 0.

2021-03-17:

- Fixed a bug with file name completion on the interactive shell in multibyte
  locales. Upon encountering two filenames with multibyte characters starting
  with the same byte, a partial multibyte character was autocompleted.

2021-03-16:

- Tilde expansion can now be extended or modified by defining a .sh.tilde.get
  or .sh.tilde.set discipline function. This replaces a 2004 undocumented
  attempt to add this functionality via a .sh.tilde built-in, which never
  worked and crashed the shell. See the manual for details on the new method.

- Fixed a bug in interactive shells: if a variable used by the shell called
  a discipline function (such as PS1.get() or COLUMNS.set()), the value of $?
  was set to the exit status of the discipline function instead of the last
  command run.

2021-03-15:

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

- Tighten up potential invalid typeset attribute combos when more than
  one numeric type has been requested. In particular, -F and -i are no
  longer masked over by previously given float types.

2021-03-13:

- Fixed a file descriptor leak that occurred when ksh used /dev/fd for
  process substitutions passed to functions.

- Fixed a separate file descriptor leak that happened when a process
  substitution was passed to a nonexistent command.

2021-03-11:

- Fixed an intermittent bug that caused process substitutions to infinitely
  loop in Linux virtual machines that use systemd.

- Fixed a bug that caused process substitutions to leave lingering processes
  if the command invoking them never reads from them.

2021-03-09:

- The ${!foo@} and ${!foo*} expansions yield variable names beginning with foo,
  but excluded 'foo' itself. The fix for this is now backported from 93v- beta.

- test -v var, [ -v var ], and [[ -v var ]] did not correctly test if a
  variable is set or unset after it has been given a numeric attribute with
  'typeset' but not yet assigned a value. This has been fixed so that
  [[ -v var ]] is now equivalent to [[ -n ${var+set} ]] as documented.

2021-03-07:

- Fixed the typeset -p display of short integers without an assigned value.
  Also, the last -s or -l attribute option supplied for an integer is used.

- Fixed a bug with -G/--globstar introduced on 2020-08-09: patterns did not
  match anything if any pathname component was '.' or '..', e.g. '**/./glob.c'
  never matched. The 2020-08-09 fix does still apply to patterns like '.*'.

- Enhancement to -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 you would expect.

- Fixed a bug introduced on 2021-02-11 that caused job control on interactive
  ksh sessions to misbehave if the login shell was replaced by ksh using 'exec'.

2021-03-06:

- Fixed an old expansion bug: expansions of type ${var=value} and ${var:=value}
  did not perform an assignment and yielded the value 0 if 'var' was typeset as
  numeric (integer or float) but had not yet been assigned a value.

- Fixed a bug introduced on 2020-08-19: Ctrl+D would break after an
  interactive shell received SIGWINCH.

- Fixed a bug introduced on 2020-05-21: on an interactive shell, command lines
  containing a syntax error were not added to the command history file and
  sometimes corrupted the command history.

2021-03-05:

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

2021-03-04:

- Fixed an arbitrary command execution vulnerability that occurred when
  parsing the subscripts of arrays within arithmetic commands and expansion.

2021-03-01:

- Fixed the retention of size attributes when 'readonly' or 'typeset -r'
  was applied to an existing variable.

2021-02-26:

- Fixed three long-standing bugs with tab completion in the emacs editor:

  1. The editor accepted literal tabs without escaping in certain cases,
     causing buggy and inconsistent completion behaviour. Details:
     https://github.com/ksh93/ksh/issues/71#issuecomment-656970959
     https://github.com/ksh93/ksh/issues/71#issuecomment-657216472
     To enter a literal tab in emacs, you need to escape it with ^V or \.

  2. After completing a filename by choosing from a file completion menu,
     the terminal cursor was placed one position too far to the right,
     corrupting command line display. This happened with multiline active.
     Details: https://github.com/ksh93/ksh/issues/71#issue-655093805

  3. A completion menu was displayed if the file name to be completed was
     at the point where the rest of it started with a number, even if that
     part uniquely identified it so the menu only showed one item. Details:
     https://www.mail-archive.com/ast-users@lists.research.att.com/msg00436.html

- A bug with ${.sh.fun} in combination with the DEBUG trap has been fixed.
  The ${.sh.fun} variable wrongly continued to contain the name of the last
  function executed by the DEBUG trap after the trap action completed.

2021-02-21:

- Fixed: The way that SIGWINCH was handled (i.e. the signal emitted when the
  terminal window size changes) could cause strange emacs/vi editor behaviour.

2021-02-20:

- Fixed a bug introduced on 2021-01-20: if a DEBUG trap action yielded exit
  status 2, the execution of the next command was not skipped as documented.

- Fixed multiple buffer overflows causing crashes in typeset -L/-R-/-Z.

- Fixed typeset -Z zero-filling: if the number was zero, all zeros
  were skipped when changing the initial size value of the -Z attribute,
  leaving an empty string.

2021-02-18:

- A bug was fixed in the 'read' builtin that caused it to fail to process
  multibyte characters properly in Shift-JIS locales.

2021-02-17:

- Emacs mode fixes:
  1. Erasing a backslash while doing a reverse search (^R) no longer deletes
     extra characters.
  2. The backslash now escapes a subsequent interrupt (^C) as documented.

- Fixed a longstanding bug with shared-state command substitutions of the form
  ${ command; }. If these were executed in a subshell, changes made within
  could survive not only the command substitution but also the parent subshell.

2021-02-15:

- Fixed a regression introduced by ksh93 (was not in ksh88): an empty 'case'
  list on a single line ('case x in esac') was a syntax error.

- Fixed a bug in the emacs built-in editor, introduced on 2020-09-17, that
  made the Meta-D and Meta-H keys delete single characters instead of words.

- A new 'backslashctrl' shell option has been added. It is on by default.
  Turning it off (set +o backslashctrl or set --nobackslashctrl) disables the
  special escaping behaviour of the backslash character in the emacs and vi
  built-in editors. Particularly in the emacs editor, this makes it much easier
  to go back, insert a forgotten backslash into a command, and then continue
  editing without having your next cursor key replace your backslash with
  garbage. Note that Ctrl+V (or whatever other character was set using
  'stty lnext') always escapes all control characters in either editing mode.

2021-02-14:

- Due to a deficiency in some UNIX veriants, the 'sleep' built-in command could
  occasionally sleep for slightly less than the time specified. It now performs
  an additional check against the system clock to make sure it sleeps at least
  the given amount of time. Thanks to Lev Kujawski for adding this feature.

- A few bugs were fixed that 93u+m introduced along with the new '-o posix'
  shell option on 2020-09-01:
  1. 'set --posix' now works as the expected equivalent of 'set -o posix'.
  2. As of 2020-09-18, the posix option turns off braceexpand and turns on
     letoctal. Any attempt to override that in a single command such as
     'set -o posix +o letoctal' was quietly ignored. This now works as long
     as the overriding option follows the posix option on the command line.
  3. The --default option to 'set' now stops the 'posix' option, if set or
     unset in the same 'set' command, from changing other options. This allows
     the command output by 'set +o' to correctly restore the current options.

2021-02-11:

- Fixed a bug that caused ksh to lose track of all running background jobs if
  a shared-state command substitution of the form v=${ cmd; } was used twice.

- Job control (the -m/-o monitor option) has been fixed for scripts. Background
  jobs are now correctly assigned their own process group when run from
  subshells (except command substitutions). The 'fg' command now also works for
  scripts as it does on other shells, though 'wait' should be preferred.

2021-02-05:

- Fixed a longstanding bug that caused redirections that store a file
  descriptor > 10 in a variable, such as {var}>file, to stop working if
  brace expansion (the -B or -o braceexpand option) was turned off. (Note
  that '{var}' is not a brace expansion as it does not contain ',' or '..'.)

2021-02-04:

- Fixed ksh crashing if an autoloaded function tried to autoload itself.
  ksh now errors out gracefully with an "autoload loop" error message.

- Fixed crash on trying a very long nonexistent command.

2021-02-01:

- Fixed a bug in 'typeset': the '-s' modifier option for short integer will
  now only be applied if the integer option '-i' is also present, avoiding
  inconsistent results and a crash.
  
- Fixed: scalar arrays (-a) and associative arrays (-A) of a type created by
  'enum' allowed values not specified by the enum type, corrupting results.

- Fixed: the "${array[@]}" expansion for associative arrays of a type created
  by 'enum' expanded to random numbers instead of the array's values.

2021-01-30:

- The -x option to the 'command' built-in now causes it to bypass built-ins
  so that it always runs/queries an external command. See 'command --man'.

- Fixed a bug in 'command -x' that caused the minimum exit status to be 1 if
  a command with many arguments was divided into several command invocations.

- The 2020-08-16 fix is improved with a compile-time feature test that
  detects if the OS requires extra bytes per argument in the arguments list,
  maximising the efficiency of 'command -x' for the system it runs on.

2021-01-24:

- Fixed a bug in 'typeset': combining the -u option with -F or -E caused the
  variable to become a hexadecimal floating point in error.

- Fixed: an unquoted variable expansion evaluated in a DEBUG trap action caused
  IFS field splitting to be deactivated in code executed after the trap action.
  This bug was introduced in ksh 93t+ 2009-11-30.

2021-01-23:

- Fixed: when the DEBUG trap was redefined in a subshell, the DEBUG trap in
  the parent environment was corrupted or the shell crashed.
  When a redirection was used in a DEBUG trap action, the trap was disabled.
  DEBUG traps were also incorrectly inherited by subshells and ksh functions.
  All this was caused by a bug introduced in ksh 93t 2008-07-25.

2021-01-22:

- Compile-time shell options can now be edited in src/cmd/ksh93/SHOPT.sh
  before building.

2021-01-20:

- Fixed: executing a DEBUG trap in a command substitution had side effects
  on the exit status ($?) of non-trap commands.
  This bug was introduced in ksh 93t 2008-11-04.

- The typeset builtin command now gives an informative error message if an
  incompatible combination of options is given.

2021-01-19:

- Fixed a crash when using 'cd' in a virtual/non-forking subshell in a
  situation where the current working directory cannot be determined.

2021-01-08:

- Fixed a crash on exceeding the maximum size of the $PS1 prompt.
  The maximum size is also increased from 160 to 256 bytes.

2021-01-07:

- Fixed a crash that could occur while ksh updated ${.sh.match}.

- Any changes to the hash table (a.k.a. "tracked aliases", i.e. cached $PATH
  searches) in a subshell now no longer affect the parent shell's hash table.

2021-01-05:

- Fixed a bug in 'cd' that caused 'cd ./foo' to search for 'foo' in $CDPATH.

2021-01-03:

- The invocation
	$ ksh +s
  caused an infinite loop and corrupted ~/.sh_history. This is now fixed so
  that the '-s' option is automatically turned on if there are no non-option
  command arguments, as documented in Bolsky & Korn (1995), p. 261.

2020-10-22:

- Fixed: 'typeset -F0', 'typeset -E0', and 'typeset -X0' floating point
  numerics having a precision of 0 with variable assignment.
  'typeset -F0 x; x=4.56' worked but not 'typeset -F0 x=4.56'.

2020-10-21:

- Fixed: More concisely correct the exporting of uppercase and lowercase
  variables when only the export and change case attributes were applied.
  This fix improves upon the previous 2020-09-30 modifications.

2020-10-06:

- The security of virtual/non-forking subshells that locally change the present
  working directory (PWD) using 'cd' has been improved in two ways.
  1. On entering a subshell, if the parent shell's PWD proves inaccessible upon
     saving it, the subshell will now fork into a separate process so the
     parent process never changes its PWD, avoiding the need to restore it.
  2. If some attack renders the parent shell's PWD unrestorable *after* ksh
     enters a virtual subshell, ksh will now error out on exiting it, as
     continuing would mean running arbitrary commands in the wrong PWD.
  Hopefully this is an acceptable compromise between performance and security.
  The proper fix would be to always fork a subshell when changing the working
  directory within it, but the resulting slowdown would likely be unpopular.

2020-09-30:

- Fixed: 'typeset -xu' and 'typeset -xl' (export + change case) failed to
  change the case of a variable's value in certain conditions.

- A ksh 93u+ regression was fixed in the combination of ERR trap handling and
  the 'pipefail' option. A pipeline now triggers the ERR trap correctly again
  if the 'pipefail' option is active and any of the pipeline elements return a
  nonzero exit status. Similarly, if both the 'errexit' and 'pipefail' options
  are active, ksh now correctly exits if any pipeline element returns nonzero.

- Autoloading a function no longer causes the calling script's $LINENO to be
  off by the number of lines in the function definition file that was loaded.
  This also corrects line numbers in warnings and error messages.

2020-09-28:

- While executing a ksh-style function, ksh 93u+ ignored all signals for which
  the function had not set a local trap, except for SIGINT and SIGQUIT. This
  was contrary to the manual, which states that a "trap condition that is not
  caught or ignored by the function causes the function to terminate and the
  condition to be passed on to the caller". This has now been fixed in 93u+m to
  match the documentation, so that e.g. global traps work as expected again.

2020-09-27:

- The shell's lexical analysis of a 'case' statement within a do...done block
  within a command substitution of the form $(...) has been fixed so that code
  like the following no longer throws a spurious syntax error:
	x=$(for i in 1; do case $i in word) true;; esac; done)
  Previously, this required a leading parenthesis before 'word', although the
  syntax error claimed that the ';;' was unexpected.

2020-09-26:

- 'whence -f' now completely ignores the existence of functions, as documented.

- ksh now does not import environment variables whose names are not valid in
  the shell language, as it would be impossible to change or unset them.
  However, they stay in the environment to be passed to child processes.

2020-09-25:

- whence -v/-a now reports the path to the file that an "undefined" (i.e.
  autoloadable) function will be loaded from when invoked, if found in $FPATH.

- When ksh invoked a shell script that does not have a leading
  #!/hashbang/path, 'ps' and /proc/<PID>/cmdline showed corrupted output if
  the new script's command line was shorter than that of the invoking script.
  This has been fixed by wiping the arguments buffer correctly.

2020-09-24:

- An omission made it impossible to turn off brace expansion within command
  substitutions (`...`, $(...) or ${ ...; }) as the code for parsing these
  did not check the -B/braceexpand option. This check has now been added.

2020-09-23:

- Fixed a crash that could occur when running a pipeline containing
  backtick-style command substitutions with job control enabled.

- Fixed a crash that occurred when using 'typeset -u' or 'typeset -l' on a
  special variable such as PATH, ENV or SHELL.

2020-09-21:

- A bug was fixed that caused command substitutions embedded in here-documents
  to lose the output of the commands they ran. This bug occurred when ksh was
  compiled with the SHOPT_SPAWN compile-time option.

- Bugfix: var=$(< file) now reads the file even if the standard inout, standard
  output and/or standard error file descriptors are closed.

2020-09-20:

- Bugfix: when whence -v/-a found an "undefined" (i.e. autoloadable) function
  in $FPATH, it actually loaded the function as a side effect of reporting on
  its existence. Now it only reports, as documented.

- 'whence' will now canonicalise paths properly, resolving '.' and '..'
  elements in paths given to it. It also no longer prefixes a spurious
  double slash when doing something like 'cd / && whence bin/echo'.

2020-09-18:

- Setting the 'posix' option now turns off the 'braceexpand' option, as brace
  expansion is not specified by POSIX and potentially incompatible with sh
  scripts. In addition, 'set -o posix' now turns on the 'letoctal' option
  instead of controlling that behaviour directly. 'set +o posix' does the
  reverse of these.

2020-09-17:

- In the vi and emacs line editors, repeat count parameters can now also be
  used for the arrow keys and the forward-delete key. E.g., in emacs mode,
  <ESC> 7 <left-arrow> will now move the cursor seven positions to the left.
  In vi control mode, this would be entered as: 7 <left-arrow>.

- When a background job on an interactive shell received SIGINT or SIGPIPE, the
  job termination message was empty. It now shows "Interrupt" or "Broken Pipe".

- The -m (-o monitor) option is no longer ignored when specified on the shell
  invocation command line.

- A script that is interrupted with Ctrl+C now terminates its background jobs
  as expected, unless the -m (-o monitor) option was turned on.

2020-09-14:

- Corrected rounding of floating point values by ksh's printf %f formatting
  operator. Fix contributed by @hyenias.

- The forward-delete key now works as expected in emacs and vi editing modes.

2020-09-11:

- The 'command' regular builtin utility (which runs a simple command, removing
  special properties) has been made fully POSIX compliant.
  1. The 'command' name can now result from an expansion (fixing BUG_CMDEXPAN),
     e.g. 'c=command; "$c" ls' and 'set -- command ls; "$@"' now work.
  2. If and only if the POSIX mode (the new -o posix shell option) is active,
     then the 'command' utility now disables not only "special" but also
     "declaration" properties of builtin commands that it invokes, meaning:
     a. arguments that start with a variable name followed by '=' are
	always treated as regular words subject to normal shell syntax;
     b. 'command' can now stop the shell from exiting if a command that it
	invokes tries to modify a readonly variable (fixing BUG_CMDSPEXIT).

- The 'history' (== 'hist -l') and 'r' (== 'hist -s') interactive shell
  history commands have reverted to preset aliases and are now only loaded if
  the shell is interactive and not initialised in POSIX mode. This avoids
  unneeded conflicts with external commands by these names, particularly 'r'.

2020-09-09:

- Fixed BUG_LOOPRET2 and related bugs. The 'exit' and 'return' commands without
  an argument now correctly default to passing down the exit status of the
  last-run command. Tests like the following, in which the last-run command is
  'false', now correctly output 1 instead of 0:
	fn() { return || true; }; false; fn; echo "$?"
	fn() { while return; do true; done; }; false; fn; echo "$?"
	fn() { for i in 1; do return; done; }; false; fn; echo "$?"
	fn() { case 1 in 1) return ;; esac; }; false; fn; echo "$?"
	fn() { { return; } 2>&1; }; false; fn; echo "$?"

2020-09-05:

- Fixed erroneous syntax errors in parameter expansions such as ${var:-wor)d}
  or ${var+w(ord}. The parentheses now correctly lose their normal grammatical
  meaning within the braces. Fix by Eric Scrivner backported from ksh2020.

2020-09-04:

- Fixed a bug that caused a syntax error to be thrown if the special parameter
  expansions ${!} and ${$} (including braces) were used within a here-document.
  Bug reported by @Saikiran-m on GitHub.

2020-09-01:

- The bash-style '&>file' redirection shorthand (for '>file 2>&1') is now
  always recognised and not only when running rc/profile init scripts. It no
  longer issues a warning. This brings ksh93 in line with mksh, bash and zsh.

- A long-form shell option '-o posix' has been added, which implements a
  mode for better compatibility with the POSIX standard. It is automatically
  turned on if ksh is invoked under the name 'sh'.
  For now, it:
  * disables the &> redirection shorthand
  * causes the 'let' arithmetic command to recognise octal numbers by
    leading zeros regardless of the setting of the 'letoctal' option
  * causes file descriptors > 2 to be left open when invoking another program
  * makes the <> redirection operator default to stdin instead of stdout
    (this keeps the 2020-05-13 BUG_REDIRIO fix for the POSIX mode while
    restoring traditional ksh93 behaviour for backwards compatibility)
  * disables a noncompliant 'test -t' == 'test -t 1' compatibility hack
  * disables passing an exported variable's attributes (such as integer or
    readonly) to a new ksh process through the environment

2020-08-19:

- Sped up the 'read' command on most systems by 15-25%. Fixed a hanging bug
  on reading from a FIFO that could occur on macOS.

2020-08-17:

- 'command -p' incorrectly used the hash table entry (a.k.a. tracked alias)
  for a command if its path was previously hashed. It has now been fixed so
  it never consults the hash table.

2020-08-16:

- Fixed 'command -x' on macOS, Linux and Solaris by accounting for a 16-byte
  argument alignment. If execution does fail, it now aborts with an internal
  error message instead of entering an infinite retry loop.

2020-08-13:

- Fixed memory leaks and a crashing bug that occurred when defining and
  running functions in subshells.

2020-08-11:

- Fixed an intermittent crash upon running a large number of subshells.

2020-08-10:

- A number of fixes have been applied to the printf formatting directives
  %H and %#H (as well as the undocumented equivalents %(html)q and %(url)q):
  1. Both formatters have been made multibyte/UTF-8 aware, and no longer
     delete multibyte characters. Invalid UTF-8 byte sequences are rendered
     as ASCII question marks.
  2. %H no longer wrongly changes spaces to non-breaking spaces (&nbsp;).
  3. %H now converts the single quote (') to '%#39;' instead of '&apos;'
     which is not a valid entity in all HTML versions.
  4. %#H failed to encode some reserved characters (e.g. '?') while encoding
     some unreserved ones (e.g. '~'). It now percent-encodes all characters
     except those 'unreserved' as per RFC3986 (ASCII alphanumeric plus -._~).

- Fixed a crash that occurred intermittently after running an external
  command from a command substitution expanded from the $PS1 shell prompt.

2020-08-09:

- File name generation (a.k.a. pathname expansion, 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 files (dotfiles) in the current directory, without
  the harmful inclusion of '.' and '..'.

2020-08-08:

- Argument checking in the 'redirect' builtin command (see 2020-06-11) has
  been improved to error out before executing redirections. For example, an
  error like 'redirect ls >foo.txt' now will not create 'foo.txt' and will
  not leave your standard output permanently redirected to it.

2020-08-06:

- Added the '${.sh.pid}' variable as an alternative to Bash's '$BASHPID'.
  This variable is set to the current shell's PID, unlike '$$' (which is
  set to the parent shell's PID). In virtual subshells '${.sh.pid}' is not
  changed from its previous value, while in forked subshells '${.sh.pid}'
  is set to the subshell's process ID.

2020-08-05:

- Fixed a bug in functions that caused ksh to crash when an array with an
  unset method was turned into a multidimensional array.

- Fixed a bug that caused scripts to continue running after over-shifting
  in a function when the function call had a redirection.

- When generating shellquoted strings (such as with 'printf %q'), the
  hexadecimal value of a quoted unprintable character was not protected with
  square braces, e.g. 0x12 followed by '3' would be quoted as '\x123', which
  is a different value. Such strings are now quoted like '\x[12]3' if the
  next character is a hexadecimal digit.

2020-07-31:

- Fixed a bug that caused multidimensional associative arrays to be created
  with an extra array member.

- Fixed a bug that caused the expansions of positional parameters $1 - $9,
  as well as special parameters such as $? and $-, to corrupt any multibyte
  characters immediately following the expansion if a UTF-8 locale is active.

2020-07-29:

- On a ksh compiled to use fork(2) to run external commands, a bug has been
  fixed that caused signals (such as SIGINT, Ctrl+C) to be ignored within a
  non-forked subshell after running an external command within that subshell.

2020-07-25:

- Fixed BUG_MULTIBIFS: Multibyte characters can now be used as IFS
  delimiters. "$*" was incorrectly joining positional parameters on
  the first byte of a multibyte character. This was due to truncation
  based on the incorrect assumption the IFS would never be larger
  than a single byte.

- Fixed a bug that caused the sleep builtin to continue after being given
  an unrecognized option. 'sleep -: 1' will now show a usage message and
  exit instead of sleep for one second.

- Fixed a bug that caused the 'typeset' variable attributes -a, -A, -l, and
  -u to leak out of a subshell if they were set without assigning a value.

2020-07-23:

- Fixed an infinite loop that could occur when ksh is the system's /bin/sh.

- A command substitution that is run on the same line as a here-document
  will no longer cause a syntax error.

2020-07-22:

- Fixed two race conditions when running external commands on
  interactive shells with job control active.

2020-07-20:

- If a shell function and a built-in command by the same name exist,
  'whence -a' and 'type -a' now report both.

- Fixed a bug that caused file descriptors opened with 'redirect' or 'exec'
  to survive a subshell environment after exiting it.

2020-07-19:

- Fixed a crash that occurred in the '.' command when using kshdb.

- Fixed a crash that occurred when attempting to use redirection with an
  invalid file descriptor.

2020-07-16:

- The 'history' and 'r' default aliases have been made regular built-ins,
  leaving zero default aliases.

- Fixed a bug that caused 'sleep -s' to have no effect with intervals longer
  than 30 seconds.

- The accuracy of the sleep builtin has been improved. It no longer ignores
  microseconds and doesn't add extra milliseconds when the interval is less
  than 31 seconds.

2020-07-15:

- The 'autoload', 'compound', 'float', 'functions', 'integer' and 'nameref'
  default aliases have been converted into regular built-in commands, so
  that 'unalias -a' does not remove them. Shell functions can now use
  these names, which improves compatibility with POSIX shell scripts.

- The End key escape sequence '^[[F' is now handled in the emacs and vi editing
  modes. The End key moves the cursor to the end of the line (in contrast to
  the Home key doing the opposite).

2020-07-14:

- Fixed a bug that caused 'set -b' to have no effect.

- Following the 'time' keyword, the 'times' builtin command now also
  supports millisecond precision.

2020-07-13:

- Fixed a fork bomb that could occur when the vi editor was sent SIGTSTP
  while running in a ksh script.

- Appending a lone percent to the end of a format specifier no longer
  causes a syntax error. The extra percent will be treated as a literal
  '%', like in Bash and zsh.

- The 'time' keyword now has proper support for millisecond precision.
  Although this feature was previously documented, the 'time' keyword
  only supported up to centisecond precision, which caused a command
  like the one below to return '0.000' on certain operating systems:
  $ TIMEFORMAT='%3R'; time sleep .003

- The 'time' keyword now zero-pads seconds less than ten (like mksh).

2020-07-10:

- Fixed a bug that caused types created with 'typeset -T' to throw an error
  when used if the type name started with a lowercase 'a'.

- A potential crash due to memory corruption when using many file
  descriptors has been fixed.

2020-07-09:

- Fixed a crash on syntax error when sourcing/dotting multiple files.

- Fixed a crash when listing indexed arrays.

- Fixed a memory leak when restoring PATH when temporarily setting PATH
  for a command (e.g. PATH=/foo/bar command ...) or in a virtual subshell.

- Combining ((...)) with redirections no longer causes a syntax error
  due to the parser handling '>' incorrectly.

- Fixed a bug that corrupted KIA/CQL cross-reference databases created using
  ksh's -R option; shell warnings were wrongly included in the database file.

- The shell's quoting algorithm (used in xtrace, printf %q, and more) has been
  fixed for UTF-8 (Unicode) locales; it no longer needlessly and inconsistently
  encodes normal printable UTF-8 characters into hexadecimal \u[xxxx] codes.

2020-07-07:

- Four of the date formats accepted by 'printf %()T' have had their
  functionality altered to the common behavior of date(1):
  - '%k' and '%l' print the current hour with blank padding, the former
    based on a 24-hour clock and the latter a twelve hour clock. These
    are common extensions present on Linux and *BSD.
  - '%f' prints a date with the format string '%Y.%m.%d-%H:%M:%S' (BusyBox).
  - '%q' prints the quarter of the year (GNU).

2020-07-06:

- 'notty' is now written to the ksh auditing file instead of '(null)' if
  the user's tty could not be determined.

- Unsetting an associative array no longer causes a memory leak to occur.

2020-07-05:

- In UTF-8 locales, fix corruption of the shell's internal string quoting
  algorithm (as used by xtrace, 'printf %q', and more) that occurred when
  the processing of a multibyte character was interrupted.

2020-07-03:

- Backslashes are no longer escaped in the raw Bourne Shell-like editing
  mode in multibyte locales, i.e. backslashes are no longer treated like
  Control-V if the emacs and vi modes are disabled.

- Deleting a backslash in vi mode with Control-H or Backspace now only
  escapes a backslash if it was the previous input. This means erasing a
  string such as 'ab\\\' will only cause the first backslash to escape a
  Backspace as '^?', like in emacs mode.

- An odd interaction with Backspace when the last character of a separate
  buffer created with Shift-C was '\' has been fixed. '^?' will no longer
  be output repeatedly when attempting to erase a separate buffer with
  a Backspace. Note that buffers created with Shift-C are not meant to be
  erasable:
  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/vi.html#tag_20_152_13_49

- The 'kill' builtin now supports the SIGINFO signal (on operating systems
  with support for SIGINFO).

2020-07-02:

- Fixed a crash that occurred if a directory named '.paths' existed in any
  directory listed in $PATH. The fix was to only read '.paths' if it is a
  regular file or a symlink to a regular file.

2020-06-30:

- 'read -u' will no longer crash with a memory fault when given an out of
  range or negative file descriptor.

- The '=~' operator no longer raises an error if a regular expression
  combines the '{x}' quantifier with a sub-expression.

2020-06-28:

- Variables created with 'typeset -RF' no longer cause a memory fault
  when accessed.

- Unsetting an array that was turned into a compound variable will no
  longer cause silent memory corruption.

- Variables created with 'readonly' in functions are now set to the
  specified value instead of nothing. Note that 'readonly' does not
  create a function-local scope, unlike 'typeset -r' which does.

2020-06-26:

- Changing to a directory that has a name starting with a '.' will no
  longer fail if preceded by '../' (i.e. 'cd ../.local' will now work).

2020-06-24:

- Fixed buggy tab completion of tilde-expanded paths such as
  ~/some in 'vi' mode.

- In the raw/default Bourne Shell-like editing mode that occurs when neither
  the 'emacs' nor the 'vi' shell option is active:
  * tab completion is now correctly disabled, instead of enabled and broken;
  * entering tab characters now moves the cursor the correct amount.

2020-06-23:

- Fixed a bug that caused combining process substitution with redirection
  to create a bizarre file in the user's current working directory.

- Using process substitution while the shell is interactive no longer
  causes the process ID of the asynchronous process to be printed.

2020-06-22:

- The 'stop' and 'suspend' default aliases have been converted into regular
  built-in commands, so that 'unalias -a' does not remove them, 'suspend'
  can do a couple of sanity checks, and something like
	cmd=stop; $cmd $!
  will now work. See 'stop --man' and 'suspend --man' for more information.

- Fixed a bug that caused the kill and stop commands to segfault when given
  a non-existent job.

- Nested functions no longer ignore variable assignments that were prefixed
  to their parent function, i.e. 'VAR=foo func' will now set $VAR to 'foo'
  in the scope of any nested function 'func' runs.

2020-06-20:

- Fixed a bug that caused setting the following variables as readonly in
  a virtual subshell to affect the environment outside of the subshell:
  $_
  ${.sh.name}
  ${.sh.subscript}
  ${.sh.level}
  $RANDOM
  $LINENO

- Fixed two bugs that caused 'unset .sh.lineno' to always produce a memory
  fault and '(unset .sh.level)' to memory fault when run in nested
  functions.

2020-06-18:

- A two decade old bug that caused 'whence -a' to base the path of
  tracked aliases on the user's current working directory has been
  fixed. Now the real path to tracked aliases is shown when '-a' is
  passed to the whence command.

2020-06-17:

- A bug in 'unset -f' was fixed that prevented shell functions from
  unsetting themselves while they were running. A POSIX function no longer
  crashes when doing so, and a KornShell-style function no longer silently
  ignores an 'unset -f' on itself. A function of either form now continues
  running after unsetting itself, and is removed at the end of the run.

2020-06-16:

- Passing the '-d' flag to the read builtin will no longer cause the '-r'
  flag to be discarded when 'read -r -d' is run.

- Fix BUG_CMDSPASGN: preceding a "special builtin"[*] with 'command' now
  prevents preceding invocation-local variable assignments from becoming global.
  [*] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_14

2020-06-15:

- The 'source' alias has been converted into a regular built-in command.

- Functions that set variables in a virtual subshell will no longer affect
  variables of the same name outside of the virtual subshell's environment.

- Terse usage messages written by builtin commands now point the user to
  the --help and --man options for more information.

2020-06-14:

- 'read -S' is now able to correctly handle strings with double quotes
  nested inside of double quotes.

2020-06-13:

- Fixed a timezone name determination bug on FreeBSD that caused the
  output from "LC_ALL=C printf '%T\n' now" to print the wrong time zone name.

2020-06-11:

- Fixed a bug that caused running 'builtin -d' on a special builtin to
  delete it. The man page for the 'builtin' command documents that special
  builtins cannot be deleted.

- POSIX compliance fix: It is now possible to set shell functions named
  'alias' or 'unalias', overriding the commands by the same names. In
  technical terms, they are now regular builtins, not special builtins.

- The redirect='command exec' alias has been converted to a regular
  'redirect' builtin command that only accepts I/O redirections, which
  persist as in 'exec'. This means that:
  * 'unlias -a' no longer removes the 'redirect' command;
  * users no longer accidentally get logged out of their shells if
    they type something intuitive but wrong, like 'redirect ls >file'.

- The undocumented 'login' and 'newgrp' builtin commands have been removed.
  These 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.
  If you do want this behaviour, you can restore it by setting:
	alias login='exec login'
	alias newgrp='exec newgrp'

2020-06-10:

- The 'hash' utility is now a regular builtin instead of an alias to
  'alias -t --'. The functionality of the old command has been removed
  from the alias builtin.

- 'set +r' is no longer able to unset the restricted option. This change
  makes the behavior of 'set +r' identical to 'set +o restricted'.

2020-06-09:

- The 'unalias' builtin will now return a non-zero status if it tries
  to remove a previously set alias that is not currently set.

2020-06-08:

- Fix an issue with the up arrow key in Emacs editing mode.
  Emacs editing mode is bugged in ksh93u+ and ksh2020. Let's
  say you were to run the following commands after starting
  a fresh instance of ksh:
  $ alias foo=true
  $ unalias foo
  If you type 'a' and then press the up arrow on your keyboard,
  ksh will complete 'a' to 'alias foo=true' by doing a reverse
  search for the last command that starts with 'a'.
  Run the alias command again, then type 'u' and press the up
  arrow key again. If ksh is in Vi mode, you will get 'unalias foo',
  but in Emacs mode you will get 'alias foo=true' again.
  All subsequent commands were ignored as ksh was saving the first
  command and only based later searches off of it.

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

- 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.

- Removed support for an obscure early 1990s Bell Labs file system research
  project called 3DFS, which has not existed for decades. This removes:
  - an obnoxious default alias 2d='set -f;_2d' that turned off your file name
    wildcard expansion and then tried to run a nonexistent '_2d' command
  - undocumented builtins 'vmap' and 'vpath' that only printed error messages
  - a non-functional -V unary operator for the test and [[ commands

- If the last program run by a ksh script exits with a signal (e.g. crashed),
  ksh itself now exits normally instead of repeating that same signal.
  In addition, using 'exit x' for x > 256 no longer makes ksh issue a signal.

2020-06-06:

- The 'times' command is now a builtin command that conforms to POSIX
  instead of an alias for the 'time' command. It displays the accumulated
  user and system CPU times, one line with the times used by the shell and
  another with those used by all of the shell's child processes.
  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_27

- The default aliases command='command ' and nohup='nohup ' have been
  removed because they caused breakage in an attempt to circumvent other
  breakage which is being fixed. In the unlikely even that anyone still
  needs alias substitution to continue on the command argument following
  'command' or 'nohup', it's easy to set these aliases yourself.

2020-06-05:

- Fix a bug that caused special variables such as PATH, LANG, LC_ALL,
  etc. to lose their effect after being unset in a subshell. For example:
    (unset PATH; PATH=/dev/null; ls); : wrongly ran 'ls'
    (unset LC_ALL; LC_ALL=badlocale); : failed to print a diagnostic
  This also fixes BUG_KUNSETIFS: unsetting IFS in a subshell failed if IFS
  was set to the empty value in the parent shell.

- Fix crashes on some systems, including at least a crash in 'print -v' on
  macOS, by eliminating an invalid/undefined use of memccpy() on overlapping
  buffers in the commonly used sfputr() function.

- Fix the ${.sh.subshell} level counter; it is no longer reset to zero when a
  non-forked subshell happens to fork into a separate process for some reason
  (an internal implementation detail that should be unnoticeable to scripts).

2020-06-04:

- Fix BUG_KBGPID: the $! special parameter was not set if a background job
  (somecommand &) or co-process (somecommand |&) was launched as the only
  command within a braces block with an attached redirection, for example:
	{
		somecommand &
	} >&2
  With the bug, $! was unchanged; now it contains the PID of somecommand.

2020-05-31:

- Fix a bug in autoloading functions. Directories in the path search list
  which should be skipped (e.g. because they don't exist) did not interact
  correctly with autoloaded functions, so that a function to autoload was
  not always found correctly.
  Details: https://github.com/att/ast/issues/1454

2020-05-30:

- Fix POSIX compliance of 'test'/'[' exit status on error. The command now
  returns status 2 instead of 1 when given an invalid number or arithmetic
  expression, e.g.:
	[ 123 -eq 123x ]; echo $?
  now outputs 2 instead of 1.

2020-05-29:

- Fix BUG_FNSUBSH: functions can now be correctly redefined and unset in
  subshell environments (such as ( ... ), $(command substitutions), etc).
  Before this fix, this was silently ignored, causing the function by the
  same name from the parent shell environment to be executed instead.
	fn() { echo mainsh; }
	(fn() { echo subsh; }; fn); fn
  This now correctly outputs "subsh mainsh" instead of "mainsh mainsh".
	ls() { echo "ls executed"; }
	(unset -f ls; ls); ls
  This now correctly lists your directory and then prints "ls executed",
  instead of printing "ls executed" twice.

- Fix a similar bug with aliases. These can now be correctly unset
  in subshell environments.

2020-05-21:

- Fix truncating of files with the combined redirections '<>;file' and
  '<#pattern'. The bug was caused by out-of-sync streams.
  Details and discussion: https://github.com/att/ast/issues/61

- Patched code injection vulnerability CVE-2019-14868. As a result, you can
  no longer use expressions in imported numeric environment variables; only
  integer literals are allowed.

2020-05-20:

- Fix BUG_ISSETLOOP. Expansions like ${var+set} remained static when used
  within a 'for', 'while' or 'until' loop; the expansions din't change along
  with the state of the variable, so they could not be used to check whether a
  variable is set within a loop if the state of that variable changed in the
  course of the loop.

- Fix BUG_IFSISSET. ${IFS+s} always yielded 's', and [[ -v IFS ]] always
  yielded true, even if IFS is unset. This applied to IFS only.

2020-05-19:

- Fix 'command -p'. The -p option causes the operating system's standard
  utilities path (as output by 'getconf PATH') to be searched instead of $PATH.
  Before this fix, this was broken on non-interactive shells as the internal
  variable holding the default PATH value was not correctly initialised.

2020-05-16:

- Fix 'test -t 1', '[ -t 1 ]', '[[ -t 1 ]]' in command substitutions.
  Standard output (file descriptor 1) tested as being on a terminal within a
  command substitution, which makes no sense as the command substitution is
  supposed to be catching standard output.
	v=$(echo begincomsub
	    [ -t 1 ] && echo oops
	    echo endcomsub)
	echo "$v"
  This now does not output "oops".

2020-05-14:

- Fix syncing history when print -s -f is used. For example, the
  following now correctly adds a 'cd' command to the history:
	print -s -f 'cd -- %q\n' "$PWD"
  Ref.:	https://github.com/att/ast/issues/425
	https://github.com/att/ast/pull/442

- Fix BUG_PUTIOERR: Output builtins now correctly detect
  input/output errors. This allows scripts to check for a nonzero exit
  status on the 'print', 'printf' and 'echo' builtins and prevent possible
  infinite loops if SIGPIPE is ignored.

- Add a convenient bin/run_ksh_tests script to the source tree that
  sets up the necessary environment and runs the ksh regression tests.

2020-05-13:

- Fix BUG_CASELIT: an undocumented 'case' pattern matching misbehaviour that
  goes back to the original Bourne shell, but wasn't discovered until 2018.
  If a pattern doesn't match as a pattern, it was tried again as a literal
  string. This broke common validation use cases, e.g.:
      n='[0-9]'
      case $n in
      ( [0-9] )  echo "$n is a number" ;;
      esac
  would output "[0-9] is a number" as the literal string fallback matches the
  pattern. As this misbehaviour was never documented anywhere (not for Bourne,
  ksh88, or ksh93), and it was never replicated in other shells (not even in
  ksh88 clones pdksh and mksh), it is unlikely any scripts rely on it.
     Of course, a literal string fallback, should it be needed, is trivial to
  implement correctly without this breakage:
      case $n in
      ( [0-9] | "[0-9]")  echo "$n is a number or the number pattern" ;;
      esac
  Ref.: https://github.com/att/ast/issues/476

- Fix BUG_REDIRIO: ksh used to redirect standard output by default when no
  file descriptor was specified with the rarely used '<>' reading/writing
  redirection operator. It now redirects standard input by default, as POSIX
  specifies and as all other POSIX shells do. To redirect standard output
  for reading and writing, you now need '1<>'.
  Ref.:	https://github.com/att/ast/issues/75
	http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_07
