compat

android-w.song.android.widget

    This was included for backwards compatibility with old versions of the
    Bourne shell, which did not provide an easy way to obtain the length of
    the value of a shell variable.

    This operator is not part of the POSIX standard, because one can (and
    should) use ${#variable} to get the length of a variable's value.
    Bash-2.x does not support it.

15. Bash no longer auto-exports the HOME, PATH, SHELL, TERM, HOSTNAME,
    HOSTTYPE, MACHTYPE, or OSTYPE variables.  If they appear in the initial
    environment, the export attribute will be set, but if bash provides a
    default value, they will remain local to the current shell.

16. Bash no longer initializes the FUNCNAME, GROUPS, or DIRSTACK variables
    to have special behavior if they appear in the initial environment.

17. Bash no longer removes the export attribute from the SSH_CLIENT or
    SSH2_CLIENT variables, and no longer attempts to discover whether or
    not it has been invoked by sshd in order to run the startup files.

18. Bash no longer requires that the body of a function be a group command;
    any compound command is accepted.

19. As of bash-3.0, the pattern substitution operators no longer perform
    quote removal on the pattern before attempting the match.  This is the
    way the pattern removal functions behave, and is more consistent.

20. After bash-3.0 was released, I reimplemented tilde expansion, incorporating
    it into the mainline word expansion code.  This fixes the bug that caused
    the results of tilde expansion to be re-expanded.  There is one
    incompatibility:  a ${paramOPword} expansion within double quotes will not
    perform tilde expansion on WORD.  This is consistent with the other
    expansions, and what POSIX specifies.

21. A number of variables have the integer attribute by default, so the +=
    assignment operator returns expected results: RANDOM, LINENO, MAILCHECK,
    HISTCMD, OPTIND.

22. Bash-3.x is much stricter about $LINENO correctly reflecting the line
    number in a script; assignments to LINENO have little effect.

23. By default, readline binds the terminal special characters to their
    readline equivalents.  As of bash-3.1/readline-5.1, this is optional and
    controlled by the bind-tty-special-chars readline variable.

24. The \W prompt string expansion abbreviates $HOME as `~'.  The previous
    behavior is available with ${PWD##/*/}.

25. The arithmetic exponentiation operator is right-associative as of bash-3.1.

26. The rules concerning valid alias names are stricter, as per POSIX.2.

27. The Readline key binding functions now obey the convert-meta setting active
    when the binding takes place, as the dispatch code does when characters
    are read and processed.

28. The historical behavior of `trap' reverting signal disposition to the
    original handling in the absence of a valid first argument is implemented
    only if the first argument is a valid signal number.

29. In versions of bash after 3.1, the ${parameter//pattern/replacement}
    expansion does not interpret `%' or `#' specially.  Those anchors don't
    have any real meaning when replacing every match.

30. Beginning with bash-3.1, the combination of posix mode and enabling the
    `xpg_echo' option causes echo to ignore all options, not looking for `-n'

31. Beginning with bash-3.2, bash follows the Bourne-shell-style (and POSIX-
    style) rules for parsing the contents of old-style backquoted command
    substitutions.  Previous versions of bash attempted to recursively parse
    embedded quoted strings and shell constructs; bash-3.2 uses strict POSIX
    rules to find the closing backquote and simply passes the contents of the
    command substitution to a subshell for parsing and execution.

32. Beginning with bash-3.2, bash uses access(2) when executing primaries for
    the test builtin and the [[ compound command, rather than looking at the
    file permission bits obtained with stat(2).  This obeys restrictions of
    the file system (e.g., read-only or noexec mounts) not available via stat.

33. Bash-3.2 adopts the convention used by other string and pattern matching
    operators for the `[[' compound command, and matches any quoted portion
    of the right-hand-side argument to the =~ operator as a string rather
    than a regular expression.

34. Bash-4.0 allows the behavior in the previous item to be modified using
    the notion of a shell `compatibility level'.  If the compat31 shopt
    option is set, quoting the pattern has no special effect.

35. Bash-3.2 (patched) and Bash-4.0 fix a bug that leaves the shell in an
    inconsistent internal state following an assignment error.  One of the
    changes means that compound commands or { ... } grouping commands are
    aborted under some circumstances in which they previously were not.
    This is what Posix specifies.

36. Bash-4.0 now allows process substitution constructs to pass unchanged
    through brace expansion, so any expansion of the contents will have to be
    separately specified, and each process subsitution will have to be
    separately entered.

37. Bash-4.0 now allows SIGCHLD to interrupt the wait builtin, as Posix
    specifies, so the SIGCHLD trap is no longer always invoked once per
    exiting child if you are using `wait' to wait for all children.  As
    of bash-4.2, this is the status quo only when in posix mode.

38. Since bash-4.0 now follows Posix rules for finding the closing delimiter
    of a $() command substitution, it will not behave as previous versions
    did, but will catch more syntax and parsing errors before spawning a
    subshell to evaluate the command substitution.

39. The programmable completion code uses the same set of delimiting characters
    as readline when breaking the command line into words, rather than the
    set of shell metacharacters, so programmable completion and readline
    should be more consistent.

40. When the read builtin times out, it attempts to assign any input read to
    specified variables, which also causes variables to be set to the empty
    string if there is not enough input.  Previous versions discarded the
    characters read.

41. Beginning with bash-4.0, when one of the commands in a pipeline is killed
    by a SIGINT while executing a command list, the shell acts as if it
    received the interrupt.  This can be disabled by setting the compat31 or
    compat32 shell options.

42. Bash-4.0 changes the handling of the set -e option so that the shell exits
    if a pipeline fails (and not just if the last command in the failing
    pipeline is a simple command).  This is not as Posix specifies.  There is
    work underway to update this portion of the standard; the bash-4.0
    behavior attempts to capture the consensus at the time of release.

43. Bash-4.0 fixes a Posix mode bug that caused the . (source) builtin to
    search the current directory for its filename argument, even if "." is
    not in $PATH.  Posix says that the shell shouldn't look in $PWD in this
    case.

44. Bash-4.1 uses the current locale when comparing strings using the < and
    > operators to the `[[' command.  This can be reverted to the previous
    behavior (ASCII collating and strcmp(3)) by setting one of the
    `compatNN' shopt options, where NN is less than 41.

45. Command substitutions now remove the caller's trap strings when trap is
    run to set a new trap in the subshell.  Previous to bash-4.2, the old
    trap strings persisted even though the actual signal handlers were reset.

46. When in Posix mode, a single quote is not treated specially in a
    double-quoted ${...} expansion, unless the expansion operator is
    # or % or the new `//', `^', or `,' expansions.  In particular, it
    does not define a new quoting context.  This is from Posix interpretation
    221.

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


Shell Compatibility Level
=========================

Bash-4.0 introduced the concept of a `shell compatibility level', specified
as a set of options to the shopt builtin (compat31, compat32, compat40, and
compat41 at this writing).  There is only one current compatibility level --
each option is mutually exclusive.  This list does not mention behavior
that is standard for a particular version (e.g., setting compat32 means that
quoting the rhs of the regexp matching operator quotes special regexp
characters in the word, which is default behavior in bash-3.2 and above).

compat31 set
	- the < and > operators to the [[ command do not consider the current
	  locale when comparing strings; they use ASCII ordering
	- quoting the rhs of the regexp matching operator (=~) has no
	  special effect

compat32 set
	- the < and > operators to the [[ command do not consider the current
	  locale when comparing strings; they use ASCII ordering

compat40 set
	- the < and > operators to the [[ command do not consider the current
	  locale when comparing strings; they use ASCII ordering
	- interrupting a command list such as "a ; b ; c" causes the execution
	  of the entire list to be aborted (in versions before bash-4.0,
	  interrupting one command in a list caused the next to be executed)

compat41 set
	- interrupting a command list such as "a ; b ; c" causes the execution
	  of the entire list to be aborted (in versions before bash-4.0,
	  interrupting one command in a list caused the next to be executed)
	- when in posix mode, single quotes in the `word' portion of a
	  double-quoted parameter expansion define a new quoting context and
	  are treated specially
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