source: Daodan/MSYS2/usr/share/doc/bash/COMPAT@ 1173

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[1166]1Compatibility with previous versions
2====================================
3
4This document details the incompatibilities between this version of bash,
5bash-5.1, and the previous widely-available versions, bash-3.2 (which is
6still the `standard' version for Mac OS X), 4.2/4.3 (which are still
7standard on a few Linux distributions), and bash-4.4/bash-5.0, the current
8widely-available versions. These were discovered by users of bash-2.x
9through 5.x, so this list is not comprehensive. Some of these
10incompatibilities occur between the current version and versions 2.0 and
11above.
12
131. Bash uses a new quoting syntax, $"...", to do locale-specific
14 string translation. Users who have relied on the (undocumented)
15 behavior of bash-1.14 will have to change their scripts. For
16 instance, if you are doing something like this to get the value of
17 a variable whose name is the value of a second variable:
18
19 eval var2=$"$var1"
20
21 you will have to change to a different syntax.
22
23 This capability is directly supported by bash-2.0:
24
25 var2=${!var1}
26
27 This alternate syntax will work portably between bash-1.14 and bash-2.0:
28
29 eval var2=\$${var1}
30
312. One of the bugs fixed in the YACC grammar tightens up the rules
32 concerning group commands ( {...} ). The `list' that composes the
33 body of the group command must be terminated by a newline or
34 semicolon. That's because the braces are reserved words, and are
35 recognized as such only when a reserved word is legal. This means
36 that while bash-1.14 accepted shell function definitions like this:
37
38 foo() { : }
39
40 bash-2.0 requires this:
41
42 foo() { :; }
43
44 This is also an issue for commands like this:
45
46 mkdir dir || { echo 'could not mkdir' ; exit 1; }
47
48 The syntax required by bash-2.0 is also accepted by bash-1.14.
49
503. The options to `bind' have changed to make them more consistent with
51 the rest of the bash builtins. If you are using `bind -d' to list
52 the readline key bindings in a form that can be re-read, use `bind -p'
53 instead. If you were using `bind -v' to list the key bindings, use
54 `bind -P' instead.
55
564. The `long' invocation options must now be prefixed by `--' instead
57 of `-'. (The old form is still accepted, for the time being.)
58
595. There was a bug in the version of readline distributed with bash-1.14
60 that caused it to write badly-formatted key bindings when using
61 `bind -d'. The only key sequences that were affected are C-\ (which
62 should appear as \C-\\ in a key binding) and C-" (which should appear
63 as \C-\"). If these key sequences appear in your inputrc, as, for
64 example,
65
66 "\C-\": self-insert
67
68 they will need to be changed to something like the following:
69
70 "\C-\\": self-insert
71
726. A number of people complained about having to use ESC to terminate an
73 incremental search, and asked for an alternate mechanism. Bash-2.03
74 uses the value of the settable readline variable `isearch-terminators'
75 to decide which characters should terminate an incremental search. If
76 that variable has not been set, ESC and Control-J will terminate a
77 search.
78
797. Some variables have been removed: MAIL_WARNING, notify, history_control,
80 command_oriented_history, glob_dot_filenames, allow_null_glob_expansion,
81 nolinks, hostname_completion_file, noclobber, no_exit_on_failed_exec, and
82 cdable_vars. Most of them are now implemented with the new `shopt'
83 builtin; others were already implemented by `set'. Here is a list of
84 correspondences:
85
86 MAIL_WARNING shopt mailwarn
87 notify set -o notify
88 history_control HISTCONTROL
89 command_oriented_history shopt cmdhist
90 glob_dot_filenames shopt dotglob
91 allow_null_glob_expansion shopt nullglob
92 nolinks set -o physical
93 hostname_completion_file HOSTFILE
94 noclobber set -o noclobber
95 no_exit_on_failed_exec shopt execfail
96 cdable_vars shopt cdable_vars
97
988. `ulimit' now sets both hard and soft limits and reports the soft limit
99 by default (when neither -H nor -S is specified). This is compatible
100 with versions of sh and ksh that implement `ulimit'. The bash-1.14
101 behavior of, for example,
102
103 ulimit -c 0
104
105 can be obtained with
106
107 ulimit -S -c 0
108
109 It may be useful to define an alias:
110
111 alias ulimit="ulimit -S"
112
1139. Bash-2.01 uses a new quoting syntax, $'...' to do ANSI-C string
114 translation. Backslash-escaped characters in ... are expanded and
115 replaced as specified by the ANSI C standard.
116
11710. The sourcing of startup files has changed somewhat. This is explained
118 more completely in the INVOCATION section of the manual page.
119
120 A non-interactive shell not named `sh' and not in posix mode reads
121 and executes commands from the file named by $BASH_ENV. A
122 non-interactive shell started by `su' and not in posix mode will read
123 startup files. No other non-interactive shells read any startup files.
124
125 An interactive shell started in posix mode reads and executes commands
126 from the file named by $ENV.
127
12811. The <> redirection operator was changed to conform to the POSIX.2 spec.
129 In the absence of any file descriptor specification preceding the `<>',
130 file descriptor 0 is used. In bash-1.14, this was the behavior only
131 when in POSIX mode. The bash-1.14 behavior may be obtained with
132
133 <>filename 1>&0
134
13512. The `alias' builtin now checks for invalid options and takes a `-p'
136 option to display output in POSIX mode. If you have old aliases beginning
137 with `-' or `+', you will have to add the `--' to the alias command
138 that declares them:
139
140 alias -x='chmod a-x' --> alias -- -x='chmod a-x'
141
14213. The behavior of range specificiers within bracket matching expressions
143 in the pattern matcher (e.g., [A-Z]) depends on the current locale,
144 specifically the value of the LC_COLLATE environment variable. Setting
145 this variable to C or POSIX will result in the traditional ASCII behavior
146 for range comparisons. If the locale is set to something else, e.g.,
147 en_US (specified by the LANG or LC_ALL variables), collation order is
148 locale-dependent. For example, the en_US locale sorts the upper and
149 lower case letters like this:
150
151 AaBb...Zz
152
153 so a range specification like [A-Z] will match every letter except `z'.
154 Other locales collate like
155
156 aAbBcC...zZ
157
158 which means that [A-Z] matches every letter except `a'.
159
160 The portable way to specify upper case letters is [:upper:] instead of
161 A-Z; lower case may be specified as [:lower:] instead of a-z.
162
163 Look at the manual pages for setlocale(3), strcoll(3), and, if it is
164 present, locale(1).
165
166 You can find your current locale information by running locale(1):
167
168 caleb.ins.cwru.edu(2)$ locale
169 LANG=en_US
170 LC_CTYPE="en_US"
171 LC_NUMERIC="en_US"
172 LC_TIME="en_US"
173 LC_COLLATE="en_US"
174 LC_MONETARY="en_US"
175 LC_MESSAGES="en_US"
176 LC_ALL=en_US
177
178 My advice is to put
179
180 export LC_COLLATE=C
181
182 into /etc/profile and inspect any shell scripts run from cron for
183 constructs like [A-Z]. This will prevent things like
184
185 rm [A-Z]*
186
187 from removing every file in the current directory except those beginning
188 with `z' and still allow individual users to change the collation order.
189 Users may put the above command into their own profiles as well, of course.
190
19114. Bash versions up to 1.14.7 included an undocumented `-l' operator to
192 the `test/[' builtin. It was a unary operator that expanded to the
193 length of its string argument. This let you do things like
194
195 test -l $variable -lt 20
196
197 for example.
198
199 This was included for backwards compatibility with old versions of the
200 Bourne shell, which did not provide an easy way to obtain the length of
201 the value of a shell variable.
202
203 This operator is not part of the POSIX standard, because one can (and
204 should) use ${#variable} to get the length of a variable's value.
205 Bash-2.x does not support it.
206
20715. Bash no longer auto-exports the HOME, PATH, SHELL, TERM, HOSTNAME,
208 HOSTTYPE, MACHTYPE, or OSTYPE variables. If they appear in the initial
209 environment, the export attribute will be set, but if bash provides a
210 default value, they will remain local to the current shell.
211
21216. Bash no longer initializes the FUNCNAME, GROUPS, or DIRSTACK variables
213 to have special behavior if they appear in the initial environment.
214
21517. Bash no longer removes the export attribute from the SSH_CLIENT or
216 SSH2_CLIENT variables, and no longer attempts to discover whether or
217 not it has been invoked by sshd in order to run the startup files.
218
21918. Bash no longer requires that the body of a function be a group command;
220 any compound command is accepted.
221
22219. As of bash-3.0, the pattern substitution operators no longer perform
223 quote removal on the pattern before attempting the match. This is the
224 way the pattern removal functions behave, and is more consistent.
225
22620. After bash-3.0 was released, I reimplemented tilde expansion, incorporating
227 it into the mainline word expansion code. This fixes the bug that caused
228 the results of tilde expansion to be re-expanded. There is one
229 incompatibility: a ${paramOPword} expansion within double quotes will not
230 perform tilde expansion on WORD. This is consistent with the other
231 expansions, and what POSIX specifies.
232
23321. A number of variables have the integer attribute by default, so the +=
234 assignment operator returns expected results: RANDOM, LINENO, MAILCHECK,
235 HISTCMD, OPTIND.
236
23722. Bash-3.x is much stricter about $LINENO correctly reflecting the line
238 number in a script; assignments to LINENO have little effect.
239
24023. By default, readline binds the terminal special characters to their
241 readline equivalents. As of bash-3.1/readline-5.1, this is optional and
242 controlled by the bind-tty-special-chars readline variable.
243
24424. The \W prompt string expansion abbreviates $HOME as `~'. The previous
245 behavior is available with ${PWD##/*/}.
246
24725. The arithmetic exponentiation operator is right-associative as of bash-3.1.
248
24926. The rules concerning valid alias names are stricter, as per POSIX.2.
250
25127. The Readline key binding functions now obey the convert-meta setting active
252 when the binding takes place, as the dispatch code does when characters
253 are read and processed.
254
25528. The historical behavior of `trap' reverting signal disposition to the
256 original handling in the absence of a valid first argument is implemented
257 only if the first argument is a valid signal number.
258
25929. In versions of bash after 3.1, the ${parameter//pattern/replacement}
260 expansion does not interpret `%' or `#' specially. Those anchors don't
261 have any real meaning when replacing every match.
262
26330. Beginning with bash-3.1, the combination of posix mode and enabling the
264 `xpg_echo' option causes echo to ignore all options, not looking for `-n'
265
26631. Beginning with bash-3.2, bash follows the Bourne-shell-style (and POSIX-
267 style) rules for parsing the contents of old-style backquoted command
268 substitutions. Previous versions of bash attempted to recursively parse
269 embedded quoted strings and shell constructs; bash-3.2 uses strict POSIX
270 rules to find the closing backquote and simply passes the contents of the
271 command substitution to a subshell for parsing and execution.
272
27332. Beginning with bash-3.2, bash uses access(2) when executing primaries for
274 the test builtin and the [[ compound command, rather than looking at the
275 file permission bits obtained with stat(2). This obeys restrictions of
276 the file system (e.g., read-only or noexec mounts) not available via stat.
277
27833. Bash-3.2 adopts the convention used by other string and pattern matching
279 operators for the `[[' compound command, and matches any quoted portion
280 of the right-hand-side argument to the =~ operator as a string rather
281 than a regular expression.
282
28334. Bash-4.0 allows the behavior in the previous item to be modified using
284 the notion of a shell `compatibility level'. If the compat31 shopt
285 option is set, quoting the pattern has no special effect.
286
28735. Bash-3.2 (patched) and Bash-4.0 fix a bug that leaves the shell in an
288 inconsistent internal state following an assignment error. One of the
289 changes means that compound commands or { ... } grouping commands are
290 aborted under some circumstances in which they previously were not.
291 This is what Posix specifies.
292
29336. Bash-4.0 now allows process substitution constructs to pass unchanged
294 through brace expansion, so any expansion of the contents will have to be
295 separately specified, and each process substitution will have to be
296 separately entered.
297
29837. Bash-4.0 now allows SIGCHLD to interrupt the wait builtin, as Posix
299 specifies, so the SIGCHLD trap is no longer always invoked once per
300 exiting child if you are using `wait' to wait for all children. As
301 of bash-4.2, this is the status quo only when in posix mode.
302
30338. Since bash-4.0 now follows Posix rules for finding the closing delimiter
304 of a $() command substitution, it will not behave as previous versions
305 did, but will catch more syntax and parsing errors before spawning a
306 subshell to evaluate the command substitution.
307
30839. The programmable completion code uses the same set of delimiting characters
309 as readline when breaking the command line into words, rather than the
310 set of shell metacharacters, so programmable completion and readline
311 should be more consistent.
312
31340. When the read builtin times out, it attempts to assign any input read to
314 specified variables, which also causes variables to be set to the empty
315 string if there is not enough input. Previous versions discarded the
316 characters read.
317
31841. Beginning with bash-4.0, when one of the commands in a pipeline is killed
319 by a SIGINT while executing a command list, the shell acts as if it
320 received the interrupt. This can be disabled by setting the compat31 or
321 compat32 shell options.
322
32342. Bash-4.0 changes the handling of the set -e option so that the shell exits
324 if a pipeline fails (and not just if the last command in the failing
325 pipeline is a simple command). This is not as Posix specifies. There is
326 work underway to update this portion of the standard; the bash-4.0
327 behavior attempts to capture the consensus at the time of release.
328
32943. Bash-4.0 fixes a Posix mode bug that caused the . (source) builtin to
330 search the current directory for its filename argument, even if "." is
331 not in $PATH. Posix says that the shell shouldn't look in $PWD in this
332 case.
333
33444. Bash-4.1 uses the current locale when comparing strings using the < and
335 > operators to the `[[' command. This can be reverted to the previous
336 behavior (ASCII collating and strcmp(3)) by setting one of the
337 `compatNN' shopt options, where NN is less than 41.
338
33945. Bash-4.1 conforms to the current Posix specification for `set -u':
340 expansions of $@ and $* when there are no positional parameters do not
341 cause the shell to exit.
342
34346. Bash-4.1 implements the current Posix specification for `set -e' and
344 exits when any command fails, not just a simple command or pipeline.
345
34647. Command substitutions now remove the caller's trap strings when trap is
347 run to set a new trap in the subshell. Previous to bash-4.2, the old
348 trap strings persisted even though the actual signal handlers were reset.
349
35048. When in Posix mode, a single quote is not treated specially in a
351 double-quoted ${...} expansion, unless the expansion operator is
352 # or % or the new `//', `^', or `,' expansions. In particular, it
353 does not define a new quoting context. This is from Posix interpretation
354 221.
355
35649. Posix mode shells no longer exit if a variable assignment error occurs
357 with an assignment preceding a command that is not a special builtin.
358
35950. Bash-4.2 attempts to preserve what the user typed when performing word
360 completion, instead of, for instance, expanding shell variable
361 references to their value.
362
36351. When in Posix mode, bash-4.2 exits if the filename supplied as an argument
364 to `.' is not found and the shell is not interactive.
365
36652. When compiled for strict Posix compatibility, bash-4.3 does not enable
367 history expansion by default in interactive shells, since it results in
368 a non-conforming environment.
369
37053. Bash-4.3 runs the replacement string in the pattern substitution word
371 expansion through quote removal. The code already treats quote
372 characters in the replacement string as special; if it treats them as
373 special, then quote removal should remove them.
374
37554. Bash-4.4 no longer considers a reference to ${a[@]} or ${a[*]}, where `a'
376 is an array without any elements set, to be a reference to an unset
377 variable. This means that such a reference will not cause the shell to
378 exit when the `-u' option is enabled.
379
38055. Bash-4.4 allows double quotes to quote the history expansion character (!)
381 when in Posix mode, since Posix specifies the effects of double quotes.
382
38356. Bash-4.4 does not inherit $PS4 from the environment if running as root.
384
38557. Bash-4.4 doesn't allow a `break' or `continue' in a function to affect
386 loop execution in the calling context.
387
38858. Bash-4.4 no longer expands tildes in $PATH elements when in Posix mode.
389
39059. Bash-4.4 does not attempt to perform a compound array assignment if an
391 argument to `declare' or a similar builtin expands to a word that looks
392 like a compound array assignment (e.g. declare w=$x where x='(foo)').
393
39460. Bash-5.0 only sets up BASH_ARGV and BASH_ARGC at startup if extended
395 debugging mode is active. The old behavior of unconditionally setting
396 BASH_ARGC and BASH_ARGV is available at compatibility levels less than
397 or equal to 44.
398
39961. Bash-5.0 doesn't allow a `break' or `continue' in a subshell to attempt
400 to break or continue loop execution inherited from the calling context.
401
40262. Bash-5.0 doesn't allow variable assignments preceding builtins like
403 export and readonly to modify variables with the same name in preceding
404 contexts (including the global context) unless the shell is in posix
405 mode, since export and readonly are special builtins.
406
40763. Bash-5.1 changes the way posix-mode shells handle assignment statements
408 preceding shell function calls. Previous versions of POSIX specified that
409 such assignments would persist after the function returned; subsequent
410 versions of the standard removed that requirement (interpretation #654).
411 Bash-5.1 posix mode assignment statements preceding shell function calls
412 do not persist after the function returns.
413
41464. Bash-5.1 reverts to the bash-4.4 treatment of pathname expansion of words
415 containing backslashes but no other special globbing characters. This comes
416 after a protracted discussion and a POSIX interpretation (#1234).
417
41865. In bash-5.1, disabling posix mode attempts to restore the state of several
419 options that posix mode modifies to the state they had before enabling
420 posix mode. Previous versions restored these options to default values.
421
422
423Shell Compatibility Level
424=========================
425
426Bash-4.0 introduced the concept of a `shell compatibility level', specified
427as a set of options to the shopt builtin (compat31, compat32, compat40,
428compat41, and so on). There is only one current compatibility level --
429each option is mutually exclusive. The compatibility level is intended to
430allow users to select behavior from previous versions that is incompatible
431with newer versions while they migrate scripts to use current features and
432behavior. It's intended to be a temporary solution.
433
434This section does not mention behavior that is standard for a particular
435version (e.g., setting compat32 means that quoting the rhs of the regexp
436matching operator quotes special regexp characters in the word, which is
437default behavior in bash-3.2 and above).
438
439If a user enables, say, compat32, it may affect the behavior of other
440compatibility levels up to and including the current compatibility level.
441The idea is that each compatibility level controls behavior that changed in
442that version of bash, but that behavior may have been present in earlier
443versions. For instance, the change to use locale-based comparisons with
444the `[[' command came in bash-4.1, and earlier versions used ASCII-based
445comparisons, so enabling compat32 will enable ASCII-based comparisons as
446well. That granularity may not be sufficient for all uses, and as a result
447users should employ compatibility levels carefully. Read the documentation
448for a particular feature to find out the current behavior.
449
450Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value assigned
451to this variable (a decimal version number like 4.2, or an integer
452corresponding to the compatNN option, like 42) determines the compatibility
453level.
454
455Starting with bash-4.4, bash has begun deprecating older compatibility
456levels. Eventually, the options will be removed in favor of the
457BASH_COMPAT variable.
458
459Bash-5.0 is the final version for which there will be an individual shopt
460option for the previous version. Users should use the BASH_COMPAT variable
461on bash-5.0 and later versions.
462
463The following table describes the behavior changes controlled by each
464compatibility level setting. The `compatNN' tag is used as shorthand for
465setting the compatibility level to NN using one of the following
466mechanisms. For versions prior to bash-5.0, the compatibility level may be
467set using the corresponding compatNN shopt option. For bash-4.3 and later
468versions, the BASH_COMPAT variable is preferred, and it is required for
469bash-5.1 and later versions.
470
471compat31
472 - the < and > operators to the [[ command do not consider the current
473 locale when comparing strings; they use ASCII ordering
474 - quoting the rhs of the [[ command's regexp matching operator (=~)
475 has no special effect
476
477compat32
478 - the < and > operators to the [[ command do not consider the current
479 locale when comparing strings; they use ASCII ordering
480 - interrupting a command list such as "a ; b ; c" causes the execution
481 of the next command in the list (in bash-4.0 and later versions,
482 the shell acts as if it received the interrupt, so interrupting
483 one command in a list aborts the execution of the entire list)
484
485compat40
486 - the < and > operators to the [[ command do not consider the current
487 locale when comparing strings; they use ASCII ordering.
488 Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
489 bash-4.1 and later use the current locale's collation sequence and
490 strcoll(3).
491
492compat41
493 - in posix mode, `time' may be followed by options and still be
494 recognized as a reserved word (this is POSIX interpretation 267)
495 - in posix mode, the parser requires that an even number of single
496 quotes occur in the `word' portion of a double-quoted ${...}
497 parameter expansion and treats them specially, so that characters
498 within the single quotes are considered quoted (this is POSIX
499 interpretation 221)
500
501compat42
502 - the replacement string in double-quoted pattern substitution is not
503 run through quote removal, as it is in versions after bash-4.2
504 - in posix mode, single quotes are considered special when expanding
505 the `word' portion of a double-quoted ${...} parameter expansion
506 and can be used to quote a closing brace or other special character
507 (this is part of POSIX interpretation 221); in later versions,
508 single quotes are not special within double-quoted word expansions
509
510compat43
511 - the shell does not print a warning message if an attempt is made to
512 use a quoted compound assignment as an argument to declare
513 (declare -a foo='(1 2)'). Later versions warn that this usage is
514 deprecated.
515 - word expansion errors are considered non-fatal errors that cause the
516 current command to fail, even in posix mode (the default behavior is
517 to make them fatal errors that cause the shell to exit)
518 - when executing a shell function, the loop state (while/until/etc.)
519 is not reset, so `break' or `continue' in that function will break
520 or continue loops in the calling context. Bash-4.4 and later reset
521 the loop state to prevent this
522
523compat44
524 - the shell sets up the values used by BASH_ARGV and BASH_ARGC so
525 they can expand to the shell's positional parameters even if extended
526 debug mode is not enabled
527 - a subshell inherits loops from its parent context, so `break'
528 or `continue' will cause the subshell to exit. Bash-5.0 and later
529 reset the loop state to prevent the exit
530 - variable assignments preceding builtins like export and readonly
531 that set attributes continue to affect variables with the same
532 name in the calling environment even if the shell is not in posix
533 mode
534
535compat50 (set using BASH_COMPAT)
536 - Bash-5.1 changed the way $RANDOM is generated to introduce slightly
537 more randomness. If the shell compatibility level is set to 50 or
538 lower, it reverts to the method from bash-5.0 and previous versions,
539 so seeding the random number generator by assigning a value to
540 RANDOM will produce the same sequence as in bash-5.0
541 - If the command hash table is empty, bash versions prior to bash-5.1
542 printed an informational message to that effect even when writing
543 output in a format that can be reused as input (-l). Bash-5.1
544 suppresses that message if -l is supplied
545
546
547-------------------------------------------------------------------------------
548
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