Bash POSIX Mode
===============

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 `!' to the history
     number and `!!' to `!' are enabled, and parameter expansion is
     performed on the values of `PS1' and `PS2' regardless of the
     setting of the `promptvars' option.

  6. Interactive comments are enabled by default.  (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' (this is the 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 FILENAME in `.' FILENAME is not
     found.

 12. Non-interactive shells exit if a syntax error in an arithmetic
     expansion results in an invalid expression.

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

 14. Function names must be valid shell `name's.  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 invalid
     name causes a fatal syntax error in non-interactive shells.

 15. POSIX.2 `special' builtins are found before shell functions during
     command lookup.

 16. 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.

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

 18. If `$CDPATH' is set, the `cd' builtin will not implicitly append
     the current directory to it.  This means that `cd' will fail if no
     valid directory name can be constructed from any of the entries in
     `$CDPATH', even if the a directory with the same name as the name
     given as an argument to `cd' exists in the current directory.

 19. 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 readonly variable.

 20. 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 readonly variable.

 21. Process substitution is not available.

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

 23. The `export' and `readonly' builtin commands display their output
     in the format required by POSIX.2.


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.

