1. New Features in Bash
a. `exec -a foo' now sets $0 to `foo' in an executable shell script without a
leading #!.
b. Subshells begun to execute command substitutions or run shell functions or
builtins in subshells do not reset trap strings until a new trap is
specified. This allows $(trap) to display the caller's traps and the
trap strings to persist until a new trap is set.
c. `trap -p' will now show signals ignored at shell startup, though their
disposition still cannot be modified.
d. $'...', echo, and printf understand \uXXXX and \UXXXXXXXX escape sequences.
e. declare/typeset has a new `-g' option, which creates variables in the
global scope even when run in a shell function.
f. test/[/[[ have a new -v variable unary operator, which returns success if
`variable' has been set.
g. Posix parsing changes to allow `! time command' and multiple consecutive
instances of `!' (which toggle) and `time' (which have no cumulative
effect).
h. Posix change to allow `time' as a command by itself to print the elapsed
user, system, and real times for the shell and its children.
j. $((...)) is always parsed as an arithmetic expansion first, instead of as
a potential nested command substitution, as Posix requires.
k. A new FUNCNEST variable to allow the user to control the maximum shell
function nesting (recursive execution) level.
l. The mapfile builtin now supplies a third argument to the callback command:
the line about to be assigned to the supplied array index.
m. The printf builtin has a new %(fmt)T specifier, which allows time values
to use strftime-like formatting.
n. There is a new `compat41' shell option.
o. The cd builtin has a new Posix-mandated `-e' option.
p. Negative subscripts to indexed arrays, previously errors, now are treated
as offsets from the maximum assigned index + 1.
q. Negative length specifications in the ${var:offset:length} expansion,
previously errors, are now treated as offsets from the end of the variable.
r. Parsing change to allow `time -p --'.
s. Posix-mode parsing change to not recognize `time' as a keyword if the
following token begins with a `-'. This means no more Posix-mode
`time -p'. Posix interpretation 267.
t. There is a new `lastpipe' shell option that runs the last command of a
pipeline in the current shell context. The lastpipe option has no
effect if job control is enabled.
u. History expansion no longer expands the `$!' variable expansion.
v. Posix mode shells no longer exit if a variable assignment error occurs
with an assignment preceding a command that is not a special builtin.
w. Non-interactive mode shells exit if -u is enabled and an attempt is made
to use an unset variable with the % or # expansions, the `//', `^', or
`,' expansions, or the parameter length expansion.
x. Posix-mode shells use the argument passed to `.' as-is if a $PATH search
fails, effectively searching the current directory. Posix-2008 change.
2. New Features in Readline
a. The history library does not try to write the history filename in the
current directory if $HOME is unset. This closes a potential security
problem if the application does not specify a history filename.
b. New bindable variable `completion-display-width' to set the number of
columns used when displaying completions.
c. New bindable variable `completion-case-map' to cause case-insensitive
completion to treat `-' and `_' as identical.
d. There are new bindable vi-mode command names to avoid readline's case-
insensitive matching not allowing them to be bound separately.
e. New bindable variable `menu-complete-display-prefix' causes the menu
completion code to display the common prefix of the possible completions
before cycling through the list, instead of after.
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