faq

android-w.song.android.widget

	unset BASH_ENV

A8) I just changed my login shell to bash, and now I can't FTP into my
   machine.  Why not?

You must add the full pathname to bash to the file /etc/shells.  As
noted in the answer to the previous question, many systems require
this before you can make bash your login shell. 

Most versions of ftpd use this file to prohibit `special' users
such as `uucp' and `news' from using FTP. 

A9)  What's the `POSIX Shell and Utilities standard'?

POSIX is a name originally coined by Richard Stallman for a
family of open system standards based on UNIX.  There are a
number of aspects of UNIX under consideration for
standardization, from the basic system services at the system
call and C library level to applications and tools to system
administration and management.  Each area of standardization is
assigned to a working group in the 1003 series. 

The POSIX Shell and Utilities standard was originally developed by
IEEE Working Group 1003.2 (POSIX.2).  Today it has been merged with
the original 1003.1 Working Group and is maintained by the Austin
Group (a joint working group of the IEEE, The Open Group and
ISO/IEC SC22/WG15).  Today the Shell and Utilities are a volume
within the set of documents that make up IEEE Std 1003.1-2001, and
thus now the former POSIX.2 (from 1992) is now part of the current
POSIX.1 standard (POSIX 1003.1-2001). 

The Shell and Utilities volume concentrates on the command
interpreter interface and utility programs commonly executed from
the command line or by other programs.  The standard is freely
available on the web at http://www.UNIX-systems.org/version3/ . 
Work continues at the Austin Group on maintenance issues; see
http://www.opengroup.org/austin/ to join the discussions. 

Bash is concerned with the aspects of the shell's behavior defined
by the POSIX Shell and Utilities volume.  The shell command
language has of course been standardized, including the basic flow
control and program execution constructs, I/O redirection and
pipelining, argument handling, variable expansion, and quoting. 

The `special' builtins, which must be implemented as part of the
shell to provide the desired functionality, are specified as
being part of the shell; examples of these are `eval' and
`export'.  Other utilities appear in the sections of POSIX not
devoted to the shell which are commonly (and in some cases must
be) implemented as builtin commands, such as `read' and `test'. 
POSIX also specifies aspects of the shell's interactive
behavior as part of the UPE, including job control and command
line editing.  Only vi-style line editing commands have been
standardized; emacs editing commands were left out due to
objections.

The latest version of the POSIX Shell and Utilities standard is
available (now updated to the 2004 Edition) as part of the Single
UNIX Specification Version 3 at

http://www.UNIX-systems.org/version3/

A10)  What is the bash `posix mode'?

Although bash is an implementation of the POSIX shell
specification, there are areas where the bash default behavior
differs from that spec.  The bash `posix mode' changes the bash
behavior in these areas so that it obeys the spec more closely. 

Posix mode is entered by starting bash with the --posix or
'-o posix' option or executing `set -o posix' after bash is running.

The specific aspects of bash which change when posix mode is
active are listed in the file POSIX in the bash distribution.
They are also listed in a section in the Bash Reference Manual
(from which that file is generated).

Section B:  The latest version

B1) What's new in version 4.2?

Bash-4.2 is the second revision to the fourth major release of bash.

Bash-4.2 contains the following new features (see the manual page for
complete descriptions and the CHANGES and NEWS files in the bash-4.2
distribution):

o   `exec -a foo' now sets $0 to `foo' in an executable shell script without a
    leading #!.

o   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.

o   `trap -p' will now show signals ignored at shell startup, though their
    disposition still cannot be modified.

o   $'...', echo, and printf understand \uXXXX and \UXXXXXXXX escape sequences.

o   declare/typeset has a new `-g' option, which creates variables in the
    global scope even when run in a shell function.

o   test/[/[[ have a new -v variable unary operator, which returns success if
    `variable' has been set.

o   Posix parsing changes to allow `! time command' and multiple consecutive
    instances of `!' (which toggle) and `time' (which have no cumulative
    effect).

o   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.

o   $((...)) is always parsed as an arithmetic expansion first, instead of as
    a potential nested command substitution, as Posix requires.

o   A new FUNCNEST variable to allow the user to control the maximum shell
    function nesting (recursive execution) level.

o   The mapfile builtin now supplies a third argument to the callback command:
    the line about to be assigned to the supplied array index.

o   The printf builtin has as new %(fmt)T specifier, which allows time values
    to use strftime-like formatting.

o   There is a new `compat41' shell option.

o   The cd builtin has a new Posix-mandated `-e' option.

o   Negative subscripts to indexed arrays, previously errors, now are treated
    as offsets from the maximum assigned index + 1.

o   Negative length specifications in the ${var:offset:length} expansion,
    previously errors, are now treated as offsets from the end of the variable.

o   Parsing change to allow `time -p --'.

o   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.

o   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.

o   History expansion no longer expands the `$!' variable expansion.

o   Posix mode shells no longer exit if a variable assignment error occurs
    with an assignment preceding a command that is not a special builtin.

o   Non-interactive mode shells exit if -u is enabled an an attempt is made
    to use an unset variable with the % or # expansions, the `//', `^', or
    `,' expansions, or the parameter length expansion.

o   Posix-mode shells use the argument passed to `.' as-is if a $PATH search
    fails, effectively searching the current directory.  Posix-2008 change.

A short feature history dating back to Bash-2.0:

Bash-4.1 contained the following new features:

o   Here-documents within $(...) command substitutions may once more be
    delimited by the closing right paren, instead of requiring a newline.

o   Bash's file status checks (executable, readable, etc.) now take file
    system ACLs into account on file systems that support them.

o   Bash now passes environment variables with names that are not valid
    shell variable names through into the environment passed to child
    processes.

o   The `execute-unix-command' readline function now attempts to clear and
    reuse the current line rather than move to a new one after the command
    executes.

o   `printf -v' can now assign values to array indices.

o   New `complete -E' and `compopt -E' options that work on the "empty"
    completion: completion attempted on an empty command line.

o   New complete/compgen/compopt -D option to define a `default' completion:
    a completion to be invoked on command for which no completion has been
    defined.  If this function returns 124, programmable completion is
    attempted again, allowing a user to dynamically build a set of completions
    as completion is attempted by having the default completion function
    install individual completion functions each time it is invoked.

o   When displaying associative arrays, subscripts are now quoted.

o   Changes to dabbrev-expand to make it more `emacs-like': no space appended
    after matches, completions are not sorted, and most recent history entries
    are presented first.

o   The [[ and (( commands are now subject to the setting of `set -e' and the
    ERR trap.

o   The source/. builtin now removes NUL bytes from the file before attempting
    to parse commands.

o   There is a new configuration option (in config-top.h) that forces bash to
    forward all history entries to syslog.

o   A new variable $BASHOPTS to export shell options settable using `shopt' to
    child processes.

o   There is a new confgure option that forces the extglob option to be
    enabled by default.

o   New variable $BASH_XTRACEFD; when set to an integer bash will write xtrace
    output to that file descriptor.

o   If the optional left-hand-side of a redirection is of the form {var}, the
    shell assigns the file descriptor used to $var or uses $var as the file
    descriptor to move or close, depending on the redirection operator.

o   The < and > operators to the [[ conditional command now do string
    comparison according to the current locale.

o   Programmable completion now uses the completion for `b' instead of `a'
    when completion is attempted on a line like: a $(b c.

o   Force extglob on temporarily when parsing the pattern argument to
    the == and != operators to the [[ command, for compatibility.

o   Changed the behavior of interrupting the wait builtin when a SIGCHLD is
    received and a trap on SIGCHLD is set to be Posix-mode only.

o   The read builtin has a new `-N nchars' option, which reads exactly NCHARS
    characters, ignoring delimiters like newline.

o   The mapfile/readarray builtin no longer stores the commands it invokes via
    callbacks in the history list.

o   There is a new `compat40' shopt option.

o   The < and > operators to [[ do string comparisons using the current locale
    only if the compatibility level is greater than 40 (set to 41 by default).

o   New bindable readline function: menu-complete-backward.

o   In the readline vi-mode insertion keymap, C-n is now bound to menu-complete
    by default, and C-p to menu-complete-backward.

o   When in readline vi command mode, repeatedly hitting ESC now does nothing,
    even when ESC introduces a bound key sequence.  This is closer to how
    historical vi behaves.

o   New bindable readline function: skip-csi-sequence.  Can be used as a
    default to consume key sequences generated by keys like Home and End
    without having to bind all keys.

o   New bindable readline variable: skip-completed-text, active when
    completing in the middle of a word.  If enabled, it means that characters
    in the completion that match characters in the remainder of the word are
    "skipped" rather than inserted into the line.

o   The pre-readline-6.0 version of menu completion is available as
    "old-menu-complete" for users who do not like the readline-6.0 version.

o   New bindable readline variable: echo-control-characters.  If enabled, and
    the tty ECHOCTL bit is set, controls the echoing of characters
    corresponding to keyboard-generated signals.

o   New bindable readline variable: enable-meta-key.  Controls whether or not
    readline sends the smm/rmm sequences if the terminal indicates it has a
    meta key that enables eight-bit characters.

Bash-4.0 contained the following new features:

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