COMPAT

Compatibility with previous versions
====================================

This document details the incompatibilities between this version of bash,
bash-5.2, and the previous widely-available versions, bash-3.2 (which is
still the `standard' version for Mac OS X), 4.2/4.3 (which are still
standard on a few Linux distributions), and bash-4.4/bash-5.0/bash-5.1,
the current widely-available versions.  These were discovered by users of
bash-2.x through 5.x, so this list is not comprehensive.  Some of these
incompatibilities occur between the current version and versions 2.0 and
above. 

1.  Bash uses a new quoting syntax, $"...", to do locale-specific
    string translation.  Users who have relied on the (undocumented)
    behavior of bash-1.14 will have to change their scripts.  For
    instance, if you are doing something like this to get the value of
    a variable whose name is the value of a second variable:

	eval var2=$"$var1"

    you will have to change to a different syntax.

    This capability is directly supported by bash-2.0:

	var2=${!var1}

    This alternate syntax will work portably between bash-1.14 and bash-2.0:

	eval var2=\$${var1}

2.  One of the bugs fixed in the YACC grammar tightens up the rules
    concerning group commands ( {...} ).  The `list' that composes the
    body of the group command must be terminated by a newline or
    semicolon.  That's because the braces are reserved words, and are
    recognized as such only when a reserved word is legal.  This means
    that while bash-1.14 accepted shell function definitions like this:

	foo() { : }

    bash-2.0 requires this:

	foo() { :; }

    This is also an issue for commands like this:

	mkdir dir || { echo 'could not mkdir' ; exit 1; }

    The syntax required by bash-2.0 is also accepted by bash-1.14.

3.  The options to `bind' have changed to make them more consistent with
    the rest of the bash builtins.  If you are using `bind -d' to list
    the readline key bindings in a form that can be re-read, use `bind -p'
    instead.  If you were using `bind -v' to list the key bindings, use
    `bind -P' instead.

4.  The `long' invocation options must now be prefixed by `--' instead
    of `-'.  (The old form is still accepted, for the time being.)

5.  There was a bug in the version of readline distributed with bash-1.14
    that caused it to write badly-formatted key bindings when using 
    `bind -d'.  The only key sequences that were affected are C-\ (which
    should appear as \C-\\ in a key binding) and C-" (which should appear
    as \C-\").  If these key sequences appear in your inputrc, as, for
    example,

	"\C-\": self-insert

    they will need to be changed to something like the following:

	"\C-\\": self-insert

6.  A number of people complained about having to use ESC to terminate an
    incremental search, and asked for an alternate mechanism.  Bash-2.03
    uses the value of the settable readline variable `isearch-terminators'
    to decide which characters should terminate an incremental search.  If
    that variable has not been set, ESC and Control-J will terminate a
    search.

7.  Some variables have been removed:  MAIL_WARNING, notify, history_control,
    command_oriented_history, glob_dot_filenames, allow_null_glob_expansion,
    nolinks, hostname_completion_file, noclobber, no_exit_on_failed_exec, and
    cdable_vars.  Most of them are now implemented with the new `shopt'
    builtin; others were already implemented by `set'.  Here is a list of
    correspondences:

	MAIL_WARNING			shopt mailwarn
	notify				set -o notify
	history_control			HISTCONTROL
	command_oriented_history	shopt cmdhist
	glob_dot_filenames		shopt dotglob
	allow_null_glob_expansion	shopt nullglob
	nolinks				set -o physical
	hostname_completion_file	HOSTFILE
	noclobber			set -o noclobber
	no_exit_on_failed_exec		shopt execfail
	cdable_vars			shopt cdable_vars

8. `ulimit' now sets both hard and soft limits and reports the soft limit
    by default (when neither -H nor -S is specified).  This is compatible
    with versions of sh and ksh that implement `ulimit'.  The bash-1.14
    behavior of, for example,

		ulimit -c 0

    can be obtained with

		ulimit -S -c 0

    It may be useful to define an alias:

		alias ulimit="ulimit -S"

9.  Bash-2.01 uses a new quoting syntax, $'...' to do ANSI-C string
    translation.  Backslash-escaped characters in ... are expanded and
    replaced as specified by the ANSI C standard.

10. The sourcing of startup files has changed somewhat.  This is explained
    more completely in the INVOCATION section of the manual page.

    A non-interactive shell not named `sh' and not in posix mode reads
    and executes commands from the file named by $BASH_ENV.  A
    non-interactive shell started by `su' and not in posix mode will read
    startup files.  No other non-interactive shells read any startup files.

    An interactive shell started in posix mode reads and executes commands
    from the file named by $ENV.

11. The <> redirection operator was changed to conform to the POSIX.2 spec.
    In the absence of any file descriptor specification preceding the `<>',
    file descriptor 0 is used.  In bash-1.14, this was the behavior only
    when in POSIX mode.  The bash-1.14 behavior may be obtained with

	<>filename 1>&0

12. The `alias' builtin now checks for invalid options and takes a `-p'
    option to display output in POSIX mode.  If you have old aliases beginning
    with `-' or `+', you will have to add the `--' to the alias command
    that declares them:

	alias -x='chmod a-x' --> alias -- -x='chmod a-x'

13. The behavior of range specificiers within bracket matching expressions
    in the pattern matcher (e.g., [A-Z]) depends on the current locale,
    specifically the value of the LC_COLLATE environment variable.  Setting
    this variable to C or POSIX will result in the traditional ASCII behavior
    for range comparisons.  If the locale is set to something else, e.g.,
    en_US (specified by the LANG or LC_ALL variables), collation order is
    locale-dependent.  For example, the en_US locale sorts the upper and
    lower case letters like this:

	AaBb...Zz

    so a range specification like [A-Z] will match every letter except `z'.
    Other locales collate like

        aAbBcC...zZ

    which means that [A-Z] matches every letter except `a'.

    The portable way to specify upper case letters is [:upper:] instead of
    A-Z; lower case may be specified as [:lower:] instead of a-z.

    Look at the manual pages for setlocale(3), strcoll(3), and, if it is
    present, locale(1).

    You can find your current locale information by running locale(1):

	caleb.ins.cwru.edu(2)$ locale
	LANG=en_US
	LC_CTYPE="en_US"
	LC_NUMERIC="en_US"
	LC_TIME="en_US"
	LC_COLLATE="en_US"
	LC_MONETARY="en_US"
	LC_MESSAGES="en_US"
	LC_ALL=en_US

    My advice is to put

	export LC_COLLATE=C

    into /etc/profile and inspect any shell scripts run from cron for
    constructs like [A-Z].  This will prevent things like

	rm [A-Z]*

    from removing every file in the current directory except those beginning
    with `z' and still allow individual users to change the collation order.
    Users may put the above command into their own profiles as well, of course.

14. Bash versions up to 1.14.7 included an undocumented `-l' operator to
    the `test/[' builtin.  It was a unary operator that expanded to the
    length of its string argument.  This let you do things like

	test -l $variable -lt 20

    for example.

    This was included for backwards compatibility with old versions of the
    Bourne shell, which did not provide an easy way to obtain the length of
    the value of a shell variable.

    This operator is not part of the POSIX standard, because one can (and
    should) use ${#variable} to get the length of a variable's value.
    Bash-2.x does not support it.

15. Bash no longer auto-exports the HOME, PATH, SHELL, TERM, HOSTNAME,
    HOSTTYPE, MACHTYPE, or OSTYPE variables.  If they appear in the initial
    environment, the export attribute will be set, but if bash provides a
    default value, they will remain local to the current shell.

16. Bash no longer initializes the FUNCNAME, GROUPS, or DIRSTACK variables
    to have special behavior if they appear in the initial environment.

17. Bash no longer removes the export attribute from the SSH_CLIENT or
    SSH2_CLIENT variables, and no longer attempts to discover whether or
    not it has been invoked by sshd in order to run the startup files.

18. Bash no longer requires that the body of a function be a group command;
    any compound command is accepted.

19. As of bash-3.0, the pattern substitution operators no longer perform
    quote removal on the pattern before attempting the match.  This is the
    way the pattern removal functions behave, and is more consistent.

20. After bash-3.0 was released, I reimplemented tilde expansion, incorporating
    it into the mainline word expansion code.  This fixes the bug that caused
    the results of tilde expansion to be re-expanded.  There is one
    incompatibility:  a ${paramOPword} expansion within double quotes will not
    perform tilde expansion on WORD.  This is consistent with the other
    expansions, and what POSIX specifies.

21. A number of variables have the integer attribute by default, so the +=
    assignment operator returns expected results: RANDOM, LINENO, MAILCHECK,
    HISTCMD, OPTIND.

22. Bash-3.x is much stricter about $LINENO correctly reflecting the line
    number in a script; assignments to LINENO have little effect.

23. By default, readline binds the terminal special characters to their
    readline equivalents.  As of bash-3.1/readline-5.1, this is optional and
    controlled by the bind-tty-special-chars readline variable.

24. The \W prompt string expansion abbreviates $HOME as `~'.  The previous
    behavior is available with ${PWD##/*/}.

25. The arithmetic exponentiation operator is right-associative as of bash-3.1.

26. The rules concerning valid alias names are stricter, as per POSIX.2.

27. The Readline key binding functions now obey the convert-meta setting active
    when the binding takes place, as the dispatch code does when characters
    are read and processed.

28. The historical behavior of `trap' reverting signal disposition to the
    original handling in the absence of a valid first argument is implemented
    only if the first argument is a valid signal number.

29. In versions of bash after 3.1, the ${parameter//pattern/replacement}
    expansion does not interpret `%' or `#' specially.  Those anchors don't
    have any real meaning when replacing every match.

30. Beginning with bash-3.1, the combination of posix mode and enabling the
    `xpg_echo' option causes echo to ignore all options, not looking for `-n'

31. Beginning with bash-3.2, bash follows the Bourne-shell-style (and POSIX-
    style) rules for parsing the contents of old-style backquoted command
    substitutions.  Previous versions of bash attempted to recursively parse
    embedded quoted strings and shell constructs; bash-3.2 uses strict POSIX
    rules to find the closing backquote and simply passes the contents of the
    command substitution to a subshell for parsing and execution.

32. Beginning with bash-3.2, bash uses access(2) when executing primaries for
    the test builtin and the [[ compound command, rather than looking at the
    file permission bits obtained with stat(2).  This obeys restrictions of
    the file system (e.g., read-only or noexec mounts) not available via stat.

33. Bash-3.2 adopts the convention used by other string and pattern matching
    operators for the `[[' compound command, and matches any quoted portion
    of the right-hand-side argument to the =~ operator as a string rather
    than a regular expression.

34. Bash-4.0 allows the behavior in the previous item to be modified using
    the notion of a shell `compatibility level'.  If the compat31 shopt
    option is set, quoting the pattern has no special effect.

35. Bash-3.2 (patched) and Bash-4.0 fix a bug that leaves the shell in an
    inconsistent internal state following an assignment error.  One of the
    changes means that compound commands or { ... } grouping commands are
    aborted under some circumstances in which they previously were not.
    This is what Posix specifies.

36. Bash-4.0 now allows process substitution constructs to pass unchanged
    through brace expansion, so any expansion of the contents will have to be
    separately specified, and each process substitution will have to be
    separately entered.

37. Bash-4.0 now allows SIGCHLD to interrupt the wait builtin, as Posix
    specifies, so the SIGCHLD trap is no longer always invoked once per
    exiting child if you are using `wait' to wait for all children.  As
    of bash-4.2, this is the status quo only when in posix mode.

38. Since bash-4.0 now follows Posix rules for finding the closing delimiter
    of a $() command substitution, it will not behave as previous versions
    did, but will catch more syntax and parsing errors before spawning a
    subshell to evaluate the command substitution.

39. The programmable completion code uses the same set of delimiting characters
    as readline when breaking the command line into words, rather than the
    set of shell metacharacters, so programmable completion and readline
    should be more consistent.

40. When the read builtin times out, it attempts to assign any input read to
    specified variables, which also causes variables to be set to the empty
    string if there is not enough input.  Previous versions discarded the
    characters read.

41. Beginning with bash-4.0, when one of the commands in a pipeline is killed
    by a SIGINT while executing a command list, the shell acts as if it
    received the interrupt.  This can be disabled by setting the compat31 or
    compat32 shell options.

42. Bash-4.0 changes the handling of the set -e option so that the shell exits
    if a pipeline fails (and not just if the last command in the failing
    pipeline is a simple command).  This is not as Posix specifies.  There is
    work underway to update this portion of the standard; the bash-4.0
    behavior attempts to capture the consensus at the time of release.

43. Bash-4.0 fixes a Posix mode bug that caused the . (source) builtin to
    search the current directory for its filename argument, even if "." is
    not in $PATH.  Posix says that the shell shouldn't look in $PWD in this
    case.

44. Bash-4.1 uses the current locale when comparing strings using the < and
    > operators to the `[[' command.  This can be reverted to the previous
    behavior (ASCII collating and strcmp(3)) by setting one of the
    `compatNN' shopt options, where NN is less than 41.

45. Bash-4.1 conforms to the current Posix specification for `set -u':
    expansions of $@ and $* when there are no positional parameters do not
    cause the shell to exit.

46. Bash-4.1 implements the current Posix specification for `set -e' and
    exits when any command fails, not just a simple command or pipeline.

47. Command substitutions now remove the caller's trap strings when trap is
    run to set a new trap in the subshell.  Previous to bash-4.2, the old
    trap strings persisted even though the actual signal handlers were reset.

48. When in Posix mode, a single quote is not treated specially in a
    double-quoted ${...} expansion, unless the expansion operator is
    # or % or the new `//', `^', or `,' expansions.  In particular, it
    does not define a new quoting context.  This is from Posix interpretation
    221.

49. Posix mode shells no longer exit if a variable assignment error occurs
    with an assignment preceding a command that is not a special builtin.

50. Bash-4.2 attempts to preserve what the user typed when performing word
    completion, instead of, for instance, expanding shell variable
    references to their value.

51. When in Posix mode, bash-4.2 exits if the filename supplied as an argument
    to `.' is not found and the shell is not interactive.

52. When compiled for strict Posix compatibility, bash-4.3 does not enable
    history expansion by default in interactive shells, since it results in
    a non-conforming environment.

53. Bash-4.3 runs the replacement string in the pattern substitution word
    expansion through quote removal.  The code already treats quote
    characters in the replacement string as special; if it treats them as
    special, then quote removal should remove them.

54. Bash-4.4 no longer considers a reference to ${a[@]} or ${a[*]}, where `a'
    is an array without any elements set, to be a reference to an unset
    variable.  This means that such a reference will not cause the shell to
    exit when the `-u' option is enabled.

55. Bash-4.4 allows double quotes to quote the history expansion character (!)
    when in Posix mode, since Posix specifies the effects of double quotes.

56. Bash-4.4 does not inherit $PS4 from the environment if running as root.

57. Bash-4.4 doesn't allow a `break' or `continue' in a function to affect
    loop execution in the calling context.

58. Bash-4.4 no longer expands tildes in $PATH elements when in Posix mode.

59. Bash-4.4 does not attempt to perform a compound array assignment if an
    argument to `declare' or a similar builtin expands to a word that looks
    like a compound array assignment (e.g. declare w=$x where x='(foo)').

60. Bash-5.0 only sets up BASH_ARGV and BASH_ARGC at startup if extended
    debugging mode is active. The old behavior of unconditionally setting
    BASH_ARGC and BASH_ARGV is available at compatibility levels less than
    or equal to 44.

61. Bash-5.0 doesn't allow a `break' or `continue' in a subshell to attempt
    to break or continue loop execution inherited from the calling context.

62. Bash-5.0 doesn't allow variable assignments preceding builtins like
    export and readonly to modify variables with the same name in preceding
    contexts (including the global context) unless the shell is in posix
    mode, since export and readonly are special builtins.

63. Bash-5.1 changes the way posix-mode shells handle assignment statements
    preceding shell function calls. Previous versions of POSIX specified that
    such assignments would persist after the function returned; subsequent
    versions of the standard removed that requirement (interpretation #654).
    Bash-5.1 posix mode assignment statements preceding shell function calls
    do not persist after the function returns.

64. Bash-5.1 reverts to the bash-4.4 treatment of pathname expansion of words
    containing backslashes but no other special globbing characters. This comes
    after a protracted discussion and a POSIX interpretation (#1234).

65. In bash-5.1, disabling posix mode attempts to restore the state of several
    options that posix mode modifies to the state they had before enabling
    posix mode. Previous versions restored these options to default values.

66. Bash-5.2 attempts to prevent double-expansion of array subscripts under
    certain circumstances, especially arithmetic evaluation, by acting as if
    the `assoc_expand_once' shell option were set.

67. The `unset' builtin in bash-5.2 treats array subscripts `@' and `*'
    differently than previous versions, and differently depending on whether
    the array is indexed or associative.


Shell Compatibility Level
=========================

Bash-4.0 introduced the concept of a `shell compatibility level', specified
as a set of options to the shopt builtin (compat31, compat32, compat40,
compat41, and so on).  There is only one current compatibility level --
each option is mutually exclusive.  The compatibility level is intended to
allow users to select behavior from previous versions that is incompatible
with newer versions while they migrate scripts to use current features and
behavior.  It's intended to be a temporary solution. 

This section does not mention behavior that is standard for a particular
version (e.g., setting compat32 means that quoting the rhs of the regexp
matching operator quotes special regexp characters in the word, which is
default behavior in bash-3.2 and above). 

If a user enables, say, compat32, it may affect the behavior of other
compatibility levels up to and including the current compatibility level. 
The idea is that each compatibility level controls behavior that changed in
that version of bash, but that behavior may have been present in earlier
versions.  For instance, the change to use locale-based comparisons with
the `[[' command came in bash-4.1, and earlier versions used ASCII-based
comparisons, so enabling compat32 will enable ASCII-based comparisons as
well.  That granularity may not be sufficient for all uses, and as a result
users should employ compatibility levels carefully.  Read the documentation
for a particular feature to find out the current behavior. 

Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value assigned
to this variable (a decimal version number like 4.2, or an integer
corresponding to the compatNN option, like 42) determines the compatibility
level.

Starting with bash-4.4, bash has begun deprecating older compatibility
levels.  Eventually, the options will be removed in favor of the
BASH_COMPAT variable. 

Bash-5.0 is the final version for which there will be an individual shopt
option for the previous version.  Users should use the BASH_COMPAT variable
on bash-5.0 and later versions. 

The following table describes the behavior changes controlled by each
compatibility level setting.  The `compatNN' tag is used as shorthand for
setting the compatibility level to NN using one of the following
mechanisms.  For versions prior to bash-5.0, the compatibility level may be
set using the corresponding compatNN shopt option.  For bash-4.3 and later
versions, the BASH_COMPAT variable is preferred, and it is required for
bash-5.1 and later versions. 

compat31
	- the < and > operators to the [[ command do not consider the current
	  locale when comparing strings; they use ASCII ordering
	- quoting the rhs of the [[ command's regexp matching operator (=~)
	  has no special effect

compat32
	- the < and > operators to the [[ command do not consider the current
	  locale when comparing strings; they use ASCII ordering
	- interrupting a command list such as "a ; b ; c" causes the execution
	  of the next command in the list (in bash-4.0 and later versions,
	  the shell acts as if it received the interrupt, so interrupting
	  one command in a list aborts the execution of the entire list)

compat40
	- the < and > operators to the [[ command do not consider the current
	  locale when comparing strings; they use ASCII ordering.
	  Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
	  bash-4.1 and later use the current locale's collation sequence and
	  strcoll(3).

compat41
	- in posix mode, `time' may be followed by options and still be
	  recognized as a reserved word (this is POSIX interpretation 267)
	- in posix mode, the parser requires that an even number of single
	  quotes occur in the `word' portion of a double-quoted ${...}
	  parameter expansion and treats them specially, so that characters
	  within the single quotes are considered quoted (this is POSIX
	  interpretation 221)

compat42
	- the replacement string in double-quoted pattern substitution is not
	  run through quote removal, as it is in versions after bash-4.2
	- in posix mode, single quotes are considered special when expanding
	  the `word' portion of a double-quoted ${...} parameter expansion
	  and can be used to quote a closing brace or other special character
	  (this is part of POSIX interpretation 221); in later versions,
	  single quotes are not special within double-quoted word expansions

compat43
	- the shell does not print a warning message if an attempt is made to
	  use a quoted compound assignment as an argument to declare
	  (declare -a foo='(1 2)'). Later versions warn that this usage is
	  deprecated.
	- word expansion errors are considered non-fatal errors that cause the
	  current command to fail, even in posix mode (the default behavior is
	  to make them fatal errors that cause the shell to exit)
	- when executing a shell function, the loop state (while/until/etc.)
	  is not reset, so `break' or `continue' in that function will break
	  or continue loops in the calling context. Bash-4.4 and later reset
	  the loop state to prevent this

compat44
	- the shell sets up the values used by BASH_ARGV and BASH_ARGC so
	  they can expand to the shell's positional parameters even if extended
	  debug mode is not enabled
	- a subshell inherits loops from its parent context, so `break'
	  or `continue' will cause the subshell to exit. Bash-5.0 and later
	  reset the loop state to prevent the exit
	- variable assignments preceding builtins like export and readonly
	  that set attributes continue to affect variables with the same
	  name in the calling environment even if the shell is not in posix
	  mode

compat50 (set using BASH_COMPAT)
	- Bash-5.1 changed the way $RANDOM is generated to introduce slightly
	  more randomness. If the shell compatibility level is set to 50 or
	  lower, it reverts to the method from bash-5.0 and previous versions,
	  so seeding the random number generator by assigning a value to
	  RANDOM will produce the same sequence as in bash-5.0
	- If the command hash table is empty, bash versions prior to bash-5.1
	  printed an informational message to that effect even when writing
	  output in a format that can be reused as input (-l). Bash-5.1
	  suppresses that message if -l is supplied
	- Bash-5.1 and later use pipes for here-documents and here-strings if
	  they are smaller than the pipe capacity. If the shell compatibility
	  level is set to 50 or lower, it reverts to using temporary files.

compat51 (set using BASH_COMPAT)
	- The `unset' builtin will unset the array a given an argument like
	  `a[@]'. Bash-5.2 will unset an element with key `@' (associative
	  arrays) or remove all the elements without unsetting the array
	  (indexed arrays)
	- arithmetic commands ( ((...)) ) and the expressions in an arithmetic
	  for statement can be expanded more than once
	- expressions used as arguments to arithmetic operators in the [[
	  conditional command can be expanded more than once
	- indexed and associative array subscripts used as arguments to the
	  operators in the [[ conditional command (e.g., `[[ -v') can be
	  expanded more than once. Bash-5.2 behaves as if the
	  `assoc_expand_once' option were enabled.
	- the expressions in substring parameter brace expansion can be
	  expanded more than once
	- the expressions in the $(( ... )) word expansion can be expanded
	  more than once
	- arithmetic expressions used as indexed array subscripts can be
	  expanded more than once;
	- `test -v', when given an argument of A[@], where A is an existing
	  associative array, will return true if the array has any set
	  elements. Bash-5.2 will look for a key named `@';
	- the ${param[:]=value} word expansion will return VALUE, before any
	  variable-specific transformations have been performed (e.g.,
	  converting to lowercase). Bash-5.2 will return the final value
	  assigned to the variable, as POSIX specifies;
	- Parsing command substitutions will act as if extended glob is
	  enabled, so that parsing a command substitution containing an extglob
	  pattern (say, as part of a shell function) will not fail. This
	  assumes the intent is to enable extglob before the command is
	  executed and word expansions are performed. It will fail at word
	  expansion time if extglob hasn't been enabled by the time the
	  command is executed.

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