bash: Bash POSIX Mode
1
1 6.11 Bash POSIX Mode
1 ====================
1
1 Starting Bash with the '--posix' command-line option or executing 'set
1 -o posix' while Bash is running will cause Bash to conform more closely
1 to the POSIX standard by changing the behavior to match that specified
1 by POSIX in areas where the Bash default differs.
1
1 When invoked as 'sh', Bash enters POSIX mode after reading the
1 startup files.
1
1 The following list is what's changed when 'POSIX mode' is in effect:
1
1 1. When a command in the hash table no longer exists, Bash will
1 re-search '$PATH' to find the new location. This is also available
1 with 'shopt -s checkhash'.
1
1 2. The message printed by the job control code and builtins when a job
1 exits with a non-zero status is 'Done(status)'.
1
1 3. The message printed by the job control code and builtins when a job
1 is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
1 'SIGTSTP'.
1
1 4. Alias expansion is always enabled, even in non-interactive shells.
1
1 5. Reserved words appearing in a context where reserved words are
1 recognized do not undergo alias expansion.
1
1 6. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number
1 and '!!' to '!' are enabled, and parameter expansion is performed
1 on the values of 'PS1' and 'PS2' regardless of the setting of the
1 'promptvars' option.
1
1 7. The POSIX startup files are executed ('$ENV') rather than the
1 normal Bash files.
1
1 8. Tilde expansion is only performed on assignments preceding a
1 command name, rather than on all assignment statements on the line.
1
1 9. The default history file is '~/.sh_history' (this is the default
1 value of '$HISTFILE').
1
1 10. Redirection operators do not perform filename expansion on the
1 word in the redirection unless the shell is interactive.
1
1 11. Redirection operators do not perform word splitting on the word in
1 the redirection.
1
1 12. Function names must be valid shell 'name's. That is, they may not
1 contain characters other than letters, digits, and underscores, and
1 may not start with a digit. Declaring a function with an invalid
1 name causes a fatal syntax error in non-interactive shells.
1
1 13. Function names may not be the same as one of the POSIX special
1 builtins.
1
1 14. POSIX special builtins are found before shell functions during
1 command lookup.
1
1 15. When printing shell function definitions (e.g., by 'type'), Bash
1 does not print the 'function' keyword.
1
1 16. Literal tildes that appear as the first character in elements of
11 the 'PATH' variable are not expanded as described above under ⇒
Tilde Expansion.
1
1 17. The 'time' reserved word may be used by itself as a command. When
1 used in this way, it displays timing statistics for the shell and
1 its completed children. The 'TIMEFORMAT' variable controls the
1 format of the timing information.
1
1 18. When parsing and expanding a ${...} expansion that appears within
1 double quotes, single quotes are no longer special and cannot be
1 used to quote a closing brace or other special character, unless
1 the operator is one of those defined to perform pattern removal.
1 In this case, they do not have to appear as matched pairs.
1
1 19. The parser does not recognize 'time' as a reserved word if the
1 next token begins with a '-'.
1
1 20. The '!' character does not introduce history expansion within a
1 double-quoted string, even if the 'histexpand' option is enabled.
1
1 21. If a POSIX special builtin returns an error status, a
1 non-interactive shell exits. The fatal errors are those listed in
1 the POSIX standard, and include things like passing incorrect
1 options, redirection errors, variable assignment errors for
1 assignments preceding the command name, and so on.
1
1 22. A non-interactive shell exits with an error status if a variable
1 assignment error occurs when no command name follows the assignment
1 statements. A variable assignment error occurs, for example, when
1 trying to assign a value to a readonly variable.
1
1 23. A non-interactive shell exits with an error status if a variable
1 assignment error occurs in an assignment statement preceding a
1 special builtin, but not with any other simple command.
1
1 24. A non-interactive shell exits with an error status if the
1 iteration variable in a 'for' statement or the selection variable
1 in a 'select' statement is a readonly variable.
1
1 25. Non-interactive shells exit if FILENAME in '.' FILENAME is not
1 found.
1
1 26. Non-interactive shells exit if a syntax error in an arithmetic
1 expansion results in an invalid expression.
1
1 27. Non-interactive shells exit if a parameter expansion error occurs.
1
1 28. Non-interactive shells exit if there is a syntax error in a script
1 read with the '.' or 'source' builtins, or in a string processed by
1 the 'eval' builtin.
1
1 29. Process substitution is not available.
1
1 30. While variable indirection is available, it may not be applied to
1 the '#' and '?' special parameters.
1
1 31. When expanding the '*' special parameter in a pattern context
1 where the expansion is double-quoted does not treat the '$*' as if
1 it were double-quoted.
1
1 32. Assignment statements preceding POSIX special builtins persist in
1 the shell environment after the builtin completes.
1
1 33. Assignment statements preceding shell function calls persist in
1 the shell environment after the function returns, as if a POSIX
1 special builtin command had been executed.
1
1 34. The 'command' builtin does not prevent builtins that take
1 assignment statements as arguments from expanding them as
1 assignment statements; when not in POSIX mode, assignment builtins
1 lose their assignment statement expansion properties when preceded
1 by 'command'.
1
1 35. The 'bg' builtin uses the required format to describe each job
1 placed in the background, which does not include an indication of
1 whether the job is the current or previous job.
1
1 36. The output of 'kill -l' prints all the signal names on a single
1 line, separated by spaces, without the 'SIG' prefix.
1
1 37. The 'kill' builtin does not accept signal names with a 'SIG'
1 prefix.
1
1 38. The 'export' and 'readonly' builtin commands display their output
1 in the format required by POSIX.
1
1 39. The 'trap' builtin displays signal names without the leading
1 'SIG'.
1
1 40. The 'trap' builtin doesn't check the first argument for a possible
1 signal specification and revert the signal handling to the original
1 disposition if it is, unless that argument consists solely of
1 digits and is a valid signal number. If users want to reset the
1 handler for a given signal to the original disposition, they should
1 use '-' as the first argument.
1
1 41. The '.' and 'source' builtins do not search the current directory
1 for the filename argument if it is not found by searching 'PATH'.
1
1 42. Enabling POSIX mode has the effect of setting the
1 'inherit_errexit' option, so subshells spawned to execute command
1 substitutions inherit the value of the '-e' option from the parent
1 shell. When the 'inherit_errexit' option is not enabled, Bash
1 clears the '-e' option in such subshells.
1
1 43. When the 'alias' builtin displays alias definitions, it does not
1 display them with a leading 'alias ' unless the '-p' option is
1 supplied.
1
1 44. When the 'set' builtin is invoked without options, it does not
1 display shell function names and definitions.
1
1 45. When the 'set' builtin is invoked without options, it displays
1 variable values without quotes, unless they contain shell
1 metacharacters, even if the result contains nonprinting characters.
1
1 46. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
1 constructed from '$PWD' and the directory name supplied as an
1 argument does not refer to an existing directory, 'cd' will fail
1 instead of falling back to PHYSICAL mode.
1
1 47. The 'pwd' builtin verifies that the value it prints is the same as
1 the current directory, even if it is not asked to check the file
1 system with the '-P' option.
1
1 48. When listing the history, the 'fc' builtin does not include an
1 indication of whether or not a history entry has been modified.
1
1 49. The default editor used by 'fc' is 'ed'.
1
1 50. The 'type' and 'command' builtins will not report a non-executable
1 file as having been found, though the shell will attempt to execute
1 such a file if it is the only so-named file found in '$PATH'.
1
1 51. The 'vi' editing mode will invoke the 'vi' editor directly when
1 the 'v' command is run, instead of checking '$VISUAL' and
1 '$EDITOR'.
1
1 52. When the 'xpg_echo' option is enabled, Bash does not attempt to
1 interpret any arguments to 'echo' as options. Each argument is
1 displayed, after escape characters are converted.
1
1 53. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
1 and '-f' options.
1
1 54. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
1 interrupt the 'wait' builtin and cause it to return immediately.
1 The trap command is run once for each child that exits.
1
1 55. The 'read' builtin may be interrupted by a signal for which a trap
1 has been set. If Bash receives a trapped signal while executing
1 'read', the trap handler executes and 'read' returns an exit status
1 greater than 128.
1
1 56. Bash removes an exited background process's status from the list
1 of such statuses after the 'wait' builtin is used to obtain it.
1
1 There is other POSIX behavior that Bash does not implement by default
1 even when in POSIX mode. Specifically:
1
1 1. The 'fc' builtin checks '$EDITOR' as a program to edit history
1 entries if 'FCEDIT' is unset, rather than defaulting directly to
1 'ed'. 'fc' uses 'ed' if 'EDITOR' is unset.
1
1 2. As noted above, Bash requires the 'xpg_echo' option to be enabled
1 for the 'echo' builtin to be fully conformant.
1
1 Bash can be configured to be POSIX-conformant by default, by
1 specifying the '--enable-strict-posix-default' to 'configure' when
1 building (⇒Optional Features).
1