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@@ -0,0 +1,264 @@ +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::). + |