Starting bash with the `--posix' command-line option or executing
`set -o posix' while bash is running will cause bash to conform more
closely to the Posix.2 standard by changing the behavior to match that
specified by Posix.2 in areas where the bash default differs.

The following list is what's changed when `posix mode' is in effect:

1.  When a command in the hash table no longer exists, bash will re-search
    $PATH to find the new location.  This is also available with
    `shopt -s checkhash'.

2.  The >& redirection does not redirect stdout and stderr.

3.  The message printed by the job control code and builtins when a job
    exits with a non-zero status is `Done(status)'.

4.  Reserved words may not be aliased.

5.  The Posix.2 PS1 and PS2 expansions of `!' -> history number and
    `!!' -> `!' are enabled, and parameter expansion is performed on
    the value regardless of the setting of the `promptvars' option.

6.  Interactive comments are enabled by default.  (Note that bash has
    them on by default anyway.)

7.  The Posix.2 startup files are executed ($ENV) rather than the normal
    bash files.

8.  Tilde expansion is only performed on assignments preceding a command
    name, rather than on all assignment statements on the line.

9.  The default history file is ~/.sh_history (default value of $HISTFILE).

10. The output of `kill -l' prints all the signal names on a single line,
    separated by spaces.

11. Non-interactive shells exit if `file' in `. file' is not found.

12. Redirection operators do not perform pathname expansion on the word
    in the redirection unless the shell is interactive

13. Function names must be valid shell identifiers.  That is, they may not
    contain characters other than letters, digits, and underscores, and
    may not start with a digit.  Declaring a function with an illegal name
    causes a fatal syntax error in non-interactive shells.

14. Posix.2 `special' builtins are found before shell functions during command
    lookup.

15. If a Posix.2 special builtin returns an error status, a non-interactive
    shell exits.  The fatal errors are those listed in the POSIX.2 standard,
    and include things like passing incorrect options, redirection errors,
    variable assignment errors for assignments preceding the command name,
    and so on.

16. The environment passed to executed commands is not sorted.  Neither is
    the output of `set'.  This is not strictly Posix.2 behavior, but sh
    does it this way.  Ksh does not.  It's not necessary to sort the
    environment; no program should rely on it being sorted.

17. If the `cd' builtin finds a directory to change to using $CDPATH, the
    value it assigns to $PWD does not contain any symbolic links, as if
    `cd -P' had been executed.

18. A non-interactive shell exits with an error status if a variable
    assignment error occurs when no command name follows the assignment
    statements.  A variable assignment error occurs, for example, when
    trying to assign a value to a read-only variable.

19. A non-interactive shell exits with an error status if the iteration
    variable in a for statement or the selection variable in a select
    statement is a read-only variable.

20. Process substitution is not available.

21. Assignment statements preceding POSIX.2 `special' builtins persist in
    the shell environment after the builtin completes.

There is other Posix.2 behavior that bash does not implement.  Specifically:

1.  Assignment statements affect the execution environment of all builtins,
    not just special ones.