+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-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
+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
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)').
+
+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
+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.
+
+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
+
+compat32 set
+ - the < and > operators to the [[ command do not consider the current
+ locale when comparing strings; they use ASCII ordering
+
+compat40 set
+ - 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)
+
+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
+ - the replacement string in double-quoted pattern substitution is not
+ run through quote removal, as in previous versions
+
+compat43 set
+ - 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)')
+ - 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.
+
+-------------------------------------------------------------------------------
+
+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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