+Compatibility with previous versions
+====================================
+
This document details the incompatibilities between this version of bash,
-bash-4.0, and the previous widely-available versions, bash-1.14 (which is
-still the `standard' version for a few Linux distributions) and bash-2.x.
-These were discovered by users of bash-2.x and 3.x, so this list is not
-comprehensive. Some of these incompatibilities occur between the current
-version and versions 2.0 and above. (The differences between bash-1.14 and
-bash-2.0 were significant.)
+bash-5.1, 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, 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)
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'.
+ 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
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
specifies, so the SIGCHLD trap is no longer always invoked once per
- exiting child if you are using `wait' to wait for all children.
+ 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
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.
+ 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.
+
+
+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
+
+
+-------------------------------------------------------------------------------
+
+Copying and distribution of this file, with or without modification,
+are permitted in any medium without royalty provided the copyright
+notice and this notice are preserved. This file is offered as-is,
+without any warranty.
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