====================================
This document details the incompatibilities between this version of bash,
-bash-4.4, and the previous widely-available versions, bash-3.x (which is
-still the `standard' version for Mac OS X), 4.1/4.2 (which are still
-standard on a few Linux distributions), and bash-4.3, the current
-widely-available version. These were discovered by users of bash-2.x
-through 4.x, so this list is not comprehensive. Some of these
+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.
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 subsitution 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
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 compat42 at this writing). There is only one current
-compatibility level -- each option is mutually exclusive. This list 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).
-
-Bash-4.3 introduces a new shell variable: BASH_COMPAT. The value assigned
+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.
-Bash-4.4 has begun deprecating older compatibility levels. Eventually, the
-options will be removed in favor of the BASH_COMPAT variable.
+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.
-compat31 set
- - the < and > operators to the [[ command do not consider the current
- locale when comparing strings; they use ASCII ordering
- - quoting the rhs of the regexp matching operator (=~) has no
- special effect
+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.
-compat32 set
+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
-compat40 set
+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 entire list to be aborted (in versions before bash-4.0,
- interrupting one command in a list caused the next to be executed)
+ 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)
-compat41 set
- - interrupting a command list such as "a ; b ; c" causes the execution
- of the entire list to be aborted (in versions before bash-4.0,
- interrupting one command in a list caused the next to be executed)
- - when in posix mode, single quotes in the `word' portion of a
- double-quoted parameter expansion define a new quoting context and
- are treated specially
-
-compat42 set
+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 in previous versions
-
-compat43 set
+ 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)')
+ (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
- - when executing a shell function, the loop state (while/until/etc.) is
- not reset, so `break' or `continue' in a shell function will break or
- continue loops in the calling context. Bash-4.4 and later reset the
- loop state to prevent this.
+ 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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