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+6.11 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 standard by changing the behavior to match that specified
+by POSIX in areas where the Bash default differs.
+
+When invoked as 'sh', Bash enters POSIX mode after reading the startup
+files.
+
+The following list is what's changed when 'POSIX mode' is in effect:
+
+ 1. Bash ensures that the 'POSIXLY_CORRECT' variable is set.
+
+ 2. When a command in the hash table no longer exists, Bash will
+ re-search '$PATH' to find the new location. This is also available
+ with 'shopt -s checkhash'.
+
+ 3. Bash will not insert a command without the execute bit set into the
+ command hash table, even if it returns it as a (last-ditch) result
+ from a '$PATH' search.
+
+ 4. The message printed by the job control code and builtins when a job
+ exits with a non-zero status is 'Done(status)'.
+
+ 5. The message printed by the job control code and builtins when a job
+ is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
+ 'SIGTSTP'.
+
+ 6. Alias expansion is always enabled, even in non-interactive shells.
+
+ 7. Reserved words appearing in a context where reserved words are
+ recognized do not undergo alias expansion.
+
+ 8. Alias expansion is performed when initially parsing a command
+ substitution. The default mode generally defers it, when enabled,
+ until the command substitution is executed. This means that
+ command substitution will not expand aliases that are defined after
+ the command substitution is initially parsed (e.g., as part of a
+ function definition).
+
+ 9. The POSIX '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.
+
+ 10. The POSIX startup files are executed ('$ENV') rather than the
+ normal Bash files.
+
+ 11. Tilde expansion is only performed on assignments preceding a
+ command name, rather than on all assignment statements on the line.
+
+ 12. The default history file is '~/.sh_history' (this is the default
+ value of '$HISTFILE').
+
+ 13. Redirection operators do not perform filename expansion on the
+ word in the redirection unless the shell is interactive.
+
+ 14. Redirection operators do not perform word splitting on the word in
+ the redirection.
+
+ 15. 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.
+
+ 16. Function names may not be the same as one of the POSIX special
+ builtins.
+
+ 17. POSIX special builtins are found before shell functions during
+ command lookup.
+
+ 18. When printing shell function definitions (e.g., by 'type'), Bash
+ does not print the 'function' keyword.
+
+ 19. Literal tildes that appear as the first character in elements of
+ the 'PATH' variable are not expanded as described above under *note
+ Tilde Expansion::.
+
+ 20. The 'time' reserved word may be used by itself as a command. When
+ used in this way, it displays timing statistics for the shell and
+ its completed children. The 'TIMEFORMAT' variable controls the
+ format of the timing information.
+
+ 21. When parsing and expanding a ${...} expansion that appears within
+ double quotes, single quotes are no longer special and cannot be
+ used to quote a closing brace or other special character, unless
+ the operator is one of those defined to perform pattern removal.
+ In this case, they do not have to appear as matched pairs.
+
+ 22. The parser does not recognize 'time' as a reserved word if the
+ next token begins with a '-'.
+
+ 23. The '!' character does not introduce history expansion within a
+ double-quoted string, even if the 'histexpand' option is enabled.
+
+ 24. If a POSIX special builtin returns an error status, a
+ non-interactive shell exits. The fatal errors are those listed in
+ the POSIX standard, and include things like passing incorrect
+ options, redirection errors, variable assignment errors for
+ assignments preceding the command name, and so on.
+
+ 25. 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.
+
+ 26. A non-interactive shell exits with an error status if a variable
+ assignment error occurs in an assignment statement preceding a
+ special builtin, but not with any other simple command. For any
+ other simple command, the shell aborts execution of that command,
+ and execution continues at the top level ("the shell shall not
+ perform any further processing of the command in which the error
+ occurred").
+
+ 27. 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.
+
+ 28. Non-interactive shells exit if FILENAME in '.' FILENAME is not
+ found.
+
+ 29. Non-interactive shells exit if a syntax error in an arithmetic
+ expansion results in an invalid expression.
+
+ 30. Non-interactive shells exit if a parameter expansion error occurs.
+
+ 31. Non-interactive shells exit if there is a syntax error in a script
+ read with the '.' or 'source' builtins, or in a string processed by
+ the 'eval' builtin.
+
+ 32. While variable indirection is available, it may not be applied to
+ the '#' and '?' special parameters.
+
+ 33. Expanding the '*' special parameter in a pattern context where the
+ expansion is double-quoted does not treat the '$*' as if it were
+ double-quoted.
+
+ 34. Assignment statements preceding POSIX special builtins persist in
+ the shell environment after the builtin completes.
+
+ 35. The 'command' builtin does not prevent builtins that take
+ assignment statements as arguments from expanding them as
+ assignment statements; when not in POSIX mode, assignment builtins
+ lose their assignment statement expansion properties when preceded
+ by 'command'.
+
+ 36. The 'bg' builtin uses the required format to describe each job
+ placed in the background, which does not include an indication of
+ whether the job is the current or previous job.
+
+ 37. The output of 'kill -l' prints all the signal names on a single
+ line, separated by spaces, without the 'SIG' prefix.
+
+ 38. The 'kill' builtin does not accept signal names with a 'SIG'
+ prefix.
+
+ 39. The 'export' and 'readonly' builtin commands display their output
+ in the format required by POSIX.
+
+ 40. The 'trap' builtin displays signal names without the leading
+ 'SIG'.
+
+ 41. The 'trap' builtin doesn't check the first argument for a possible
+ signal specification and revert the signal handling to the original
+ disposition if it is, unless that argument consists solely of
+ digits and is a valid signal number. If users want to reset the
+ handler for a given signal to the original disposition, they should
+ use '-' as the first argument.
+
+ 42. 'trap -p' displays signals whose dispositions are set to SIG_DFL
+ and those that were ignored when the shell started.
+
+ 43. The '.' and 'source' builtins do not search the current directory
+ for the filename argument if it is not found by searching 'PATH'.
+
+ 44. Enabling POSIX mode has the effect of setting the
+ 'inherit_errexit' option, so subshells spawned to execute command
+ substitutions inherit the value of the '-e' option from the parent
+ shell. When the 'inherit_errexit' option is not enabled, Bash
+ clears the '-e' option in such subshells.
+
+ 45. Enabling POSIX mode has the effect of setting the 'shift_verbose'
+ option, so numeric arguments to 'shift' that exceed the number of
+ positional parameters will result in an error message.
+
+ 46. When the 'alias' builtin displays alias definitions, it does not
+ display them with a leading 'alias ' unless the '-p' option is
+ supplied.
+
+ 47. When the 'set' builtin is invoked without options, it does not
+ display shell function names and definitions.
+
+ 48. When the 'set' builtin is invoked without options, it displays
+ variable values without quotes, unless they contain shell
+ metacharacters, even if the result contains nonprinting characters.
+
+ 49. When the 'cd' builtin is invoked in logical mode, and the pathname
+ constructed from '$PWD' and the directory name supplied as an
+ argument does not refer to an existing directory, 'cd' will fail
+ instead of falling back to physical mode.
+
+ 50. When the 'cd' builtin cannot change a directory because the length
+ of the pathname constructed from '$PWD' and the directory name
+ supplied as an argument exceeds 'PATH_MAX' when all symbolic links
+ are expanded, 'cd' will fail instead of attempting to use only the
+ supplied directory name.
+
+ 51. The 'pwd' builtin verifies that the value it prints is the same as
+ the current directory, even if it is not asked to check the file
+ system with the '-P' option.
+
+ 52. When listing the history, the 'fc' builtin does not include an
+ indication of whether or not a history entry has been modified.
+
+ 53. The default editor used by 'fc' is 'ed'.
+
+ 54. The 'type' and 'command' builtins will not report a non-executable
+ file as having been found, though the shell will attempt to execute
+ such a file if it is the only so-named file found in '$PATH'.
+
+ 55. The 'vi' editing mode will invoke the 'vi' editor directly when
+ the 'v' command is run, instead of checking '$VISUAL' and
+ '$EDITOR'.
+
+ 56. When the 'xpg_echo' option is enabled, Bash does not attempt to
+ interpret any arguments to 'echo' as options. Each argument is
+ displayed, after escape characters are converted.
+
+ 57. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
+ and '-f' options.
+
+ 58. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
+ interrupt the 'wait' builtin and cause it to return immediately.
+ The trap command is run once for each child that exits.
+
+ 59. The 'read' builtin may be interrupted by a signal for which a trap
+ has been set. If Bash receives a trapped signal while executing
+ 'read', the trap handler executes and 'read' returns an exit status
+ greater than 128.
+
+ 60. The 'printf' builtin uses 'double' (via 'strtod') to convert
+ arguments corresponding to floating point conversion specifiers,
+ instead of 'long double' if it's available. The 'L' length
+ modifier forces 'printf' to use 'long double' if it's available.
+
+ 61. Bash removes an exited background process's status from the list
+ of such statuses after the 'wait' builtin is used to obtain it.
+
+There is other POSIX behavior that Bash does not implement by default
+even when in POSIX mode. Specifically:
+
+ 1. The 'fc' builtin checks '$EDITOR' as a program to edit history
+ entries if 'FCEDIT' is unset, rather than defaulting directly to
+ 'ed'. 'fc' uses 'ed' if 'EDITOR' is unset.
+
+ 2. As noted above, Bash requires the 'xpg_echo' option to be enabled
+ for the 'echo' builtin to be fully conformant.
+
+Bash can be configured to be POSIX-conformant by default, by specifying
+the '--enable-strict-posix-default' to 'configure' when building (*note
+Optional Features::).
+