-This document details the incompatibilites between this version of bash,
-bash-2.05, and the previous widely-available version, bash-1.14 (which
-is still the `standard' version for many Linux distributions). These
-were discovered by users of bash-2.x, so this list is not comprehensive.
-
-1. Bash now uses a new quoting syntax, $"...", to do locale-specific
+Compatibility with previous versions
+====================================
+
+This document details the incompatibilities between this version of bash,
+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)
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
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 keybindings in a form that can be re-read, use `bind -p'
- instead. If you were using `bind -v' to list the keybindings, use
+ 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
"\C-\\": self-insert
-6. A number of people complained above having to use ESC to terminate an
+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
alias -x='chmod a-x' --> alias -- -x='chmod a-x'
-13. There was a bug in bash-1.14 and previous versions that caused it to
- accept as valid syntax for loops of the form
-
- for f in ; do ... ; done
-
- This should be a syntax error, and bash-2.x treats it as such.
-
-14. The behavior of range specificiers within bracket matching expressions
+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
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.
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.
-15. 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
+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.
+ 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.
- 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.
+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.
- 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.
+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.
+
+
+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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