Adding upstream version 5.1.upstream/5.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 248 insertions, 0 deletions

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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. 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.
+
+  9. The POSIX startup files are executed ('$ENV') rather than the
+     normal Bash files.
+
+  10. Tilde expansion is only performed on assignments preceding a
+     command name, rather than on all assignment statements on the line.
+
+  11. The default history file is '~/.sh_history' (this is the default
+     value of '$HISTFILE').
+
+  12. Redirection operators do not perform filename expansion on the
+     word in the redirection unless the shell is interactive.
+
+  13. Redirection operators do not perform word splitting on the word in
+     the redirection.
+
+  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. Function names may not be the same as one of the POSIX special
+     builtins.
+
+  16. POSIX special builtins are found before shell functions during
+     command lookup.
+
+  17. When printing shell function definitions (e.g., by 'type'), Bash
+     does not print the 'function' keyword.
+
+  18. Literal tildes that appear as the first character in elements of
+     the 'PATH' variable are not expanded as described above under *note
+     Tilde Expansion::.
+
+  19. 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.
+
+  20. 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.
+
+  21. The parser does not recognize 'time' as a reserved word if the
+     next token begins with a '-'.
+
+  22. The '!' character does not introduce history expansion within a
+     double-quoted string, even if the 'histexpand' option is enabled.
+
+  23. 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.
+
+  24. 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.
+
+  25. 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.
+
+  26. 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.
+
+  27. Non-interactive shells exit if FILENAME in '.'  FILENAME is not
+     found.
+
+  28. Non-interactive shells exit if a syntax error in an arithmetic
+     expansion results in an invalid expression.
+
+  29. Non-interactive shells exit if a parameter expansion error occurs.
+
+  30. 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.
+
+  31. While variable indirection is available, it may not be applied to
+     the '#' and '?' special parameters.
+
+  32. When expanding the '*' special parameter in a pattern context
+     where the expansion is double-quoted does not treat the '$*' as if
+     it were double-quoted.
+
+  33. Assignment statements preceding POSIX special builtins persist in
+     the shell environment after the builtin completes.
+
+  34. 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'.
+
+  35. 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.
+
+  36. The output of 'kill -l' prints all the signal names on a single
+     line, separated by spaces, without the 'SIG' prefix.
+
+  37. The 'kill' builtin does not accept signal names with a 'SIG'
+     prefix.
+
+  38. The 'export' and 'readonly' builtin commands display their output
+     in the format required by POSIX.
+
+  39. The 'trap' builtin displays signal names without the leading
+     'SIG'.
+
+  40. 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.
+
+  41. 'trap -p' displays signals whose dispositions are set to SIG_DFL
+     and those that were ignored when the shell started.
+
+  42. The '.' and 'source' builtins do not search the current directory
+     for the filename argument if it is not found by searching 'PATH'.
+
+  43. 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.
+
+  44. 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.
+
+  45. When the 'alias' builtin displays alias definitions, it does not
+     display them with a leading 'alias ' unless the '-p' option is
+     supplied.
+
+  46. When the 'set' builtin is invoked without options, it does not
+     display shell function names and definitions.
+
+  47. 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.
+
+  48. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
+     constructed from '$PWD' and the directory name supplied as an
+     argument does not refer to an existing directory, 'cd' will fail
+     instead of falling back to PHYSICAL mode.
+
+  49. When the 'cd' builtin cannot change a directory because the length
+     of the pathname constructed from '$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.
+
+  50. 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.
+
+  51. When listing the history, the 'fc' builtin does not include an
+     indication of whether or not a history entry has been modified.
+
+  52. The default editor used by 'fc' is 'ed'.
+
+  53. 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 '$PATH'.
+
+  54. The 'vi' editing mode will invoke the 'vi' editor directly when
+     the 'v' command is run, instead of checking '$VISUAL' and
+     '$EDITOR'.
+
+  55. When the 'xpg_echo' option is enabled, Bash does not attempt to
+     interpret any arguments to 'echo' as options.  Each argument is
+     displayed, after escape characters are converted.
+
+  56. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
+     and '-f' options.
+
+  57. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
+     interrupt the 'wait' builtin and cause it to return immediately.
+     The trap command is run once for each child that exits.
+
+  58. The 'read' builtin may be interrupted by a signal for which a trap
+     has been set.  If Bash receives a trapped signal while executing
+     'read', the trap handler executes and 'read' returns an exit status
+     greater than 128.
+
+  59. Bash removes an exited background process's status from the list
+     of such statuses after the 'wait' builtin is used to obtain it.
+
+There is other POSIX behavior that Bash does not implement by default
+even when in POSIX mode.  Specifically:
+
+  1. The 'fc' builtin checks '$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::).
+