bash/NEWS-4.0

o   When using substring expansion on the positional parameters, a starting
    index of 0 now causes $0 to be prefixed to the list.

o   There is a new variable, $BASHPID, which always returns the process id of
    the current shell.

o   There is a new `autocd' option that, when enabled, causes bash to attempt
    to `cd' to a directory name that is supplied as the first word of a
    simple command.

o   There is a new `checkjobs' option that causes the shell to check for and
    report any running or stopped jobs at exit.

o   The programmable completion code exports a new COMP_TYPE variable, set to
    a character describing the type of completion being attempted.

o   The programmable completion code exports a new COMP_KEY variable, set to
    the character that caused the completion to be invoked (e.g., TAB).

o   The programmable completion code now uses the same set of characters as
    readline when breaking the command line into a list of words.

o   The block multiplier for the ulimit -c and -f options is now 512 when in
    Posix mode, as Posix specifies.

o   Changed the behavior of the read builtin to save any partial input received
    in the specified variable when the read builtin times out.  This also
    results in variables specified as arguments to read to be set to the empty
    string when there is no input available.  When the read builtin times out,
    it returns an exit status greater than 128.

o   The shell now has the notion of a `compatibility level', controlled by
    new variables settable by `shopt'.  Setting this variable currently
    restores the bash-3.1 behavior when processing quoted strings on the rhs
    of the `=~' operator to the `[[' command.

o   The `ulimit' builtin now has new -b (socket buffer size) and -T (number
    of threads) options.

o   There is a new `compopt' builtin that allows completion functions to modify
    completion options for existing completions or the completion currently
    being executed.

o   The `read' builtin has a new -i option which inserts text into the reply
    buffer when using readline.

o   A new `-E' option to the complete builtin allows control of the default
    behavior for completion on an empty line.

o   There is now limited support for completing command name words containing
    globbing characters.

o   The `help' builtin now has a new -d option, to display a short description,
    and a -m option, to print help information in a man page-like format.

o   There is a new `mapfile' builtin to populate an array with lines from a
    given file.

o   If a command is not found, the shell attempts to execute a shell function
    named `command_not_found_handle', supplying the command words as the
    function arguments.

o   There is a new shell option: `globstar'.  When enabled, the globbing code
    treats `**' specially -- it matches all directories (and files within
    them, when appropriate) recursively.

o   There is a new shell option: `dirspell'.  When enabled, the filename
    completion code performs spelling correction on directory names during
    completion.

o   The `-t' option to the `read' builtin now supports fractional timeout
    values.

o   Brace expansion now allows zero-padding of expanded numeric values and
    will add the proper number of zeroes to make sure all values contain the
    same number of digits.

o   There is a new bash-specific bindable readline function: `dabbrev-expand'.
    It uses menu completion on a set of words taken from the history list.

o   The command assigned to a key sequence with `bind -x' now sets two new
    variables in the environment of the executed command:  READLINE_LINE_BUFFER
    and READLINE_POINT.  The command can change the current readline line
    and cursor position by modifying READLINE_LINE_BUFFER and READLINE_POINT,
    respectively.

o   There is a new >>& redirection operator, which appends the standard output
    and standard error to the named file.

o   The parser now understands `|&' as a synonym for `2>&1 |', which redirects
    the standard error for a command through a pipe.

o   The new `;&' case statement action list terminator causes execution to
    continue with the action associated with the next pattern in the
    statement rather than terminating the command.

o   The new `;;&' case statement action list terminator causes the shell to
    test the next set of patterns after completing execution of the current
    action, rather than terminating the command.

o   The shell understands a new variable: PROMPT_DIRTRIM.  When set to an
    integer value greater than zero, prompt expansion of \w and \W  will
    retain only that number of trailing pathname components and replace
    the intervening characters with `...'.

o   There are new case-modifying word expansions: uppercase (^[^]) and
    lowercase (,[,]).  They can work on either the first character or
    array element, or globally.  They accept an optional shell pattern
    that determines which characters to modify.  There is an optionally-
    configured feature to include capitalization operators.

o   The shell provides associative array variables, with the appropriate
    support to create, delete, assign values to, and expand them.

o   The `declare' builtin now has new -l (convert value to lowercase upon
    assignment) and -u (convert value to uppercase upon assignment) options.
    There is an optionally-configurable -c option to capitalize a value at
    assignment.

o   There is a new `coproc' reserved word that specifies a coprocess: an
    asynchronous command run with two pipes connected to the creating shell.
    Coprocs can be named.  The input and output file descriptors and the
    PID of the coprocess are available to the calling shell in variables
    with coproc-specific names.

o   A value of 0 for the -t option to `read' now returns success if there is
    input available to be read from the specified file descriptor.

o   CDPATH and GLOBIGNORE are ignored when the shell is running in privileged
    mode.

o   New bindable readline functions shell-forward-word and shell-backward-word,
    which move forward and backward words delimited by shell metacharacters
    and honor shell quoting.

o   New bindable readline functions shell-backward-kill-word and shell-kill-word
    which kill words backward and forward, but use the same word boundaries
    as shell-forward-word and shell-backward-word.