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POSIX
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POSIX
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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::).