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Compatibility with previous versions
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====================================
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This document details the incompatibilities between this version of bash,
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bash-4.3, and the previous widely-available versions, bash-3.x (which is
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still the `standard' version for Mac OS X), 4.0/4.1 (which are still
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standard on a few Linux distributions), and bash-4.2, the current
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widely-available version. These were discovered by users of bash-2.x
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through 4.x, so this list is not comprehensive. Some of these
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incompatibilities occur between the current version and versions 2.0 and
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1. Bash uses a new quoting syntax, $"...", to do locale-specific
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string translation. Users who have relied on the (undocumented)
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behavior of bash-1.14 will have to change their scripts. For
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instance, if you are doing something like this to get the value of
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a variable whose name is the value of a second variable:
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you will have to change to a different syntax.
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This capability is directly supported by bash-2.0:
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This alternate syntax will work portably between bash-1.14 and bash-2.0:
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2. One of the bugs fixed in the YACC grammar tightens up the rules
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concerning group commands ( {...} ). The `list' that composes the
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body of the group command must be terminated by a newline or
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semicolon. That's because the braces are reserved words, and are
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recognized as such only when a reserved word is legal. This means
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that while bash-1.14 accepted shell function definitions like this:
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bash-2.0 requires this:
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This is also an issue for commands like this:
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mkdir dir || { echo 'could not mkdir' ; exit 1; }
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The syntax required by bash-2.0 is also accepted by bash-1.14.
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3. The options to `bind' have changed to make them more consistent with
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the rest of the bash builtins. If you are using `bind -d' to list
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the readline key bindings in a form that can be re-read, use `bind -p'
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instead. If you were using `bind -v' to list the key bindings, use
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4. The `long' invocation options must now be prefixed by `--' instead
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of `-'. (The old form is still accepted, for the time being.)
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5. There was a bug in the version of readline distributed with bash-1.14
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that caused it to write badly-formatted key bindings when using
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`bind -d'. The only key sequences that were affected are C-\ (which
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should appear as \C-\\ in a key binding) and C-" (which should appear
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as \C-\"). If these key sequences appear in your inputrc, as, for
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they will need to be changed to something like the following:
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6. A number of people complained about having to use ESC to terminate an
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incremental search, and asked for an alternate mechanism. Bash-2.03
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uses the value of the settable readline variable `isearch-terminators'
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to decide which characters should terminate an incremental search. If
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that variable has not been set, ESC and Control-J will terminate a
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7. Some variables have been removed: MAIL_WARNING, notify, history_control,
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command_oriented_history, glob_dot_filenames, allow_null_glob_expansion,
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nolinks, hostname_completion_file, noclobber, no_exit_on_failed_exec, and
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cdable_vars. Most of them are now implemented with the new `shopt'
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builtin; others were already implemented by `set'. Here is a list of
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MAIL_WARNING shopt mailwarn
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history_control HISTCONTROL
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command_oriented_history shopt cmdhist
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glob_dot_filenames shopt dotglob
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allow_null_glob_expansion shopt nullglob
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nolinks set -o physical
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hostname_completion_file HOSTFILE
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noclobber set -o noclobber
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no_exit_on_failed_exec shopt execfail
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cdable_vars shopt cdable_vars
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8. `ulimit' now sets both hard and soft limits and reports the soft limit
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by default (when neither -H nor -S is specified). This is compatible
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with versions of sh and ksh that implement `ulimit'. The bash-1.14
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behavior of, for example,
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It may be useful to define an alias:
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alias ulimit="ulimit -S"
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9. Bash-2.01 uses a new quoting syntax, $'...' to do ANSI-C string
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translation. Backslash-escaped characters in ... are expanded and
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replaced as specified by the ANSI C standard.
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10. The sourcing of startup files has changed somewhat. This is explained
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more completely in the INVOCATION section of the manual page.
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A non-interactive shell not named `sh' and not in posix mode reads
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and executes commands from the file named by $BASH_ENV. A
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non-interactive shell started by `su' and not in posix mode will read
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startup files. No other non-interactive shells read any startup files.
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An interactive shell started in posix mode reads and executes commands
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from the file named by $ENV.
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11. The <> redirection operator was changed to conform to the POSIX.2 spec.
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In the absence of any file descriptor specification preceding the `<>',
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file descriptor 0 is used. In bash-1.14, this was the behavior only
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when in POSIX mode. The bash-1.14 behavior may be obtained with
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12. The `alias' builtin now checks for invalid options and takes a `-p'
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option to display output in POSIX mode. If you have old aliases beginning
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with `-' or `+', you will have to add the `--' to the alias command
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alias -x='chmod a-x' --> alias -- -x='chmod a-x'
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13. The behavior of range specificiers within bracket matching expressions
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in the pattern matcher (e.g., [A-Z]) depends on the current locale,
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specifically the value of the LC_COLLATE environment variable. Setting
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this variable to C or POSIX will result in the traditional ASCII behavior
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for range comparisons. If the locale is set to something else, e.g.,
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en_US (specified by the LANG or LC_ALL variables), collation order is
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locale-dependent. For example, the en_US locale sorts the upper and
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lower case letters like this:
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so a range specification like [A-Z] will match every letter except `z'.
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Other locales collate like
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which means that [A-Z] matches every letter except `a'.
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The portable way to specify upper case letters is [:upper:] instead of
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A-Z; lower case may be specified as [:lower:] instead of a-z.
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Look at the manual pages for setlocale(3), strcoll(3), and, if it is
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You can find your current locale information by running locale(1):
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caleb.ins.cwru.edu(2)$ locale
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into /etc/profile and inspect any shell scripts run from cron for
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constructs like [A-Z]. This will prevent things like
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from removing every file in the current directory except those beginning
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with `z' and still allow individual users to change the collation order.
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Users may put the above command into their own profiles as well, of course.
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14. Bash versions up to 1.14.7 included an undocumented `-l' operator to
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the `test/[' builtin. It was a unary operator that expanded to the
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length of its string argument. This let you do things like
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test -l $variable -lt 20
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This was included for backwards compatibility with old versions of the
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Bourne shell, which did not provide an easy way to obtain the length of
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the value of a shell variable.
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This operator is not part of the POSIX standard, because one can (and
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should) use ${#variable} to get the length of a variable's value.
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Bash-2.x does not support it.
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15. Bash no longer auto-exports the HOME, PATH, SHELL, TERM, HOSTNAME,
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HOSTTYPE, MACHTYPE, or OSTYPE variables. If they appear in the initial
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environment, the export attribute will be set, but if bash provides a
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default value, they will remain local to the current shell.
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16. Bash no longer initializes the FUNCNAME, GROUPS, or DIRSTACK variables
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to have special behavior if they appear in the initial environment.
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17. Bash no longer removes the export attribute from the SSH_CLIENT or
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SSH2_CLIENT variables, and no longer attempts to discover whether or
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not it has been invoked by sshd in order to run the startup files.
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18. Bash no longer requires that the body of a function be a group command;
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any compound command is accepted.
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19. As of bash-3.0, the pattern substitution operators no longer perform
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quote removal on the pattern before attempting the match. This is the
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way the pattern removal functions behave, and is more consistent.
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20. After bash-3.0 was released, I reimplemented tilde expansion, incorporating
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it into the mainline word expansion code. This fixes the bug that caused
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the results of tilde expansion to be re-expanded. There is one
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incompatibility: a ${paramOPword} expansion within double quotes will not
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perform tilde expansion on WORD. This is consistent with the other
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expansions, and what POSIX specifies.
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21. A number of variables have the integer attribute by default, so the +=
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assignment operator returns expected results: RANDOM, LINENO, MAILCHECK,
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22. Bash-3.x is much stricter about $LINENO correctly reflecting the line
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number in a script; assignments to LINENO have little effect.
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23. By default, readline binds the terminal special characters to their
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readline equivalents. As of bash-3.1/readline-5.1, this is optional and
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controlled by the bind-tty-special-chars readline variable.
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24. The \W prompt string expansion abbreviates $HOME as `~'. The previous
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behavior is available with ${PWD##/*/}.
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25. The arithmetic exponentiation operator is right-associative as of bash-3.1.
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26. The rules concerning valid alias names are stricter, as per POSIX.2.
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27. The Readline key binding functions now obey the convert-meta setting active
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when the binding takes place, as the dispatch code does when characters
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are read and processed.
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28. The historical behavior of `trap' reverting signal disposition to the
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original handling in the absence of a valid first argument is implemented
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only if the first argument is a valid signal number.
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29. In versions of bash after 3.1, the ${parameter//pattern/replacement}
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expansion does not interpret `%' or `#' specially. Those anchors don't
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have any real meaning when replacing every match.
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30. Beginning with bash-3.1, the combination of posix mode and enabling the
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`xpg_echo' option causes echo to ignore all options, not looking for `-n'
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31. Beginning with bash-3.2, bash follows the Bourne-shell-style (and POSIX-
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style) rules for parsing the contents of old-style backquoted command
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substitutions. Previous versions of bash attempted to recursively parse
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embedded quoted strings and shell constructs; bash-3.2 uses strict POSIX
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rules to find the closing backquote and simply passes the contents of the
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command substitution to a subshell for parsing and execution.
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32. Beginning with bash-3.2, bash uses access(2) when executing primaries for
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the test builtin and the [[ compound command, rather than looking at the
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file permission bits obtained with stat(2). This obeys restrictions of
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the file system (e.g., read-only or noexec mounts) not available via stat.
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33. Bash-3.2 adopts the convention used by other string and pattern matching
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operators for the `[[' compound command, and matches any quoted portion
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of the right-hand-side argument to the =~ operator as a string rather
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than a regular expression.
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34. Bash-4.0 allows the behavior in the previous item to be modified using
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the notion of a shell `compatibility level'. If the compat31 shopt
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option is set, quoting the pattern has no special effect.
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35. Bash-3.2 (patched) and Bash-4.0 fix a bug that leaves the shell in an
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inconsistent internal state following an assignment error. One of the
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changes means that compound commands or { ... } grouping commands are
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aborted under some circumstances in which they previously were not.
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This is what Posix specifies.
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36. Bash-4.0 now allows process substitution constructs to pass unchanged
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through brace expansion, so any expansion of the contents will have to be
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separately specified, and each process subsitution will have to be
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37. Bash-4.0 now allows SIGCHLD to interrupt the wait builtin, as Posix
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specifies, so the SIGCHLD trap is no longer always invoked once per
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exiting child if you are using `wait' to wait for all children. As
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of bash-4.2, this is the status quo only when in posix mode.
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38. Since bash-4.0 now follows Posix rules for finding the closing delimiter
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of a $() command substitution, it will not behave as previous versions
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did, but will catch more syntax and parsing errors before spawning a
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subshell to evaluate the command substitution.
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39. The programmable completion code uses the same set of delimiting characters
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as readline when breaking the command line into words, rather than the
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set of shell metacharacters, so programmable completion and readline
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should be more consistent.
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40. When the read builtin times out, it attempts to assign any input read to
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specified variables, which also causes variables to be set to the empty
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string if there is not enough input. Previous versions discarded the
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41. Beginning with bash-4.0, when one of the commands in a pipeline is killed
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by a SIGINT while executing a command list, the shell acts as if it
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received the interrupt. This can be disabled by setting the compat31 or
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compat32 shell options.
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42. Bash-4.0 changes the handling of the set -e option so that the shell exits
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if a pipeline fails (and not just if the last command in the failing
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pipeline is a simple command). This is not as Posix specifies. There is
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work underway to update this portion of the standard; the bash-4.0
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behavior attempts to capture the consensus at the time of release.
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43. Bash-4.0 fixes a Posix mode bug that caused the . (source) builtin to
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search the current directory for its filename argument, even if "." is
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not in $PATH. Posix says that the shell shouldn't look in $PWD in this
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44. Bash-4.1 uses the current locale when comparing strings using the < and
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> operators to the `[[' command. This can be reverted to the previous
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behavior (ASCII collating and strcmp(3)) by setting one of the
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`compatNN' shopt options, where NN is less than 41.
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45. Bash-4.1 conforms to the current Posix specification for `set -u':
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expansions of $@ and $* when there are no positional parameters do not
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cause the shell to exit.
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46. Bash-4.1 implements the current Posix specification for `set -e' and
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exits when any command fails, not just a simple command or pipeline.
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47. Command substitutions now remove the caller's trap strings when trap is
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run to set a new trap in the subshell. Previous to bash-4.2, the old
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trap strings persisted even though the actual signal handlers were reset.
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48. When in Posix mode, a single quote is not treated specially in a
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double-quoted ${...} expansion, unless the expansion operator is
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# or % or the new `//', `^', or `,' expansions. In particular, it
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does not define a new quoting context. This is from Posix interpretation
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49. Posix mode shells no longer exit if a variable assignment error occurs
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with an assignment preceding a command that is not a special builtin.
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50. Bash-4.2 attempts to preserve what the user typed when performing word
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completion, instead of, for instance, expanding shell variable
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references to their value.
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51. When in Posix mode, bash-4.2 exits if the filename supplied as an argument
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to `.' is not found and the shell is not interactive.
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52. When compiled for strict Posix compatibility, bash-4.3 does not enable
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history expansion by default in interactive shells, since it results in
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a non-conforming environment.
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53. Bash-4.3 runs the replacement string in the pattern substitution word
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expansion through quote removal. The code already treats quote
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characters in the replacement string as special; if it treats them as
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special, then quote removal should remove them.
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Shell Compatibility Level
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=========================
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Bash-4.0 introduced the concept of a `shell compatibility level', specified
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as a set of options to the shopt builtin (compat31, compat32, compat40,
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compat41, and compat42 at this writing). There is only one current
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compatibility level -- each option is mutually exclusive. This list does not
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mention behavior that is standard for a particular version (e.g., setting
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compat32 means that quoting the rhs of the regexp matching operator quotes
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special regexp characters in the word, which is default behavior in bash-3.2
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Bash-4.3 introduces a new shell variable: BASH_COMPAT. The value assigned
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to this variable (a decimal version number like 4.2, or an integer
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corresponding to the compatNN option, like 42) determines the compatibility
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- the < and > operators to the [[ command do not consider the current
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locale when comparing strings; they use ASCII ordering
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- quoting the rhs of the regexp matching operator (=~) has no
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- the < and > operators to the [[ command do not consider the current
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locale when comparing strings; they use ASCII ordering
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- the < and > operators to the [[ command do not consider the current
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locale when comparing strings; they use ASCII ordering
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- interrupting a command list such as "a ; b ; c" causes the execution
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of the entire list to be aborted (in versions before bash-4.0,
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interrupting one command in a list caused the next to be executed)
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- interrupting a command list such as "a ; b ; c" causes the execution
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of the entire list to be aborted (in versions before bash-4.1,
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interrupting one command in a list caused the next to be executed)
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- when in posix mode, single quotes in the `word' portion of a
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double-quoted parameter expansion define a new quoting context and
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are treated specially
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- the replacement string in double-quoted pattern substitution is not
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run through quote removal, as in previous versions
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-------------------------------------------------------------------------------
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Copying and distribution of this file, with or without modification,
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are permitted in any medium without royalty provided the copyright
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notice and this notice are preserved. This file is offered as-is,
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without any warranty.
added
removed
COMPAT
