[ViewVC] Contents of: Oni2/Daodan/MSYS2/usr/share/doc/bash/POSIX

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::).

Revision:	1166
Committed:	Tue Oct 26 14:22:36 2021 UTC (4 years ago) by rossy
File size:	10896 byte(s)
Log Message:	Daodan: Replace MinGW build env with an up-to-date MSYS2 env
#	Content
1	6.11 Bash POSIX Mode
2	====================
3
4	Starting Bash with the '--posix' command-line option or executing 'set
5	-o posix' while Bash is running will cause Bash to conform more closely
6	to the POSIX standard by changing the behavior to match that specified
7	by POSIX in areas where the Bash default differs.
8
9	When invoked as 'sh', Bash enters POSIX mode after reading the startup
10	files.
11
12	The following list is what's changed when 'POSIX mode' is in effect:
13
14	1. Bash ensures that the 'POSIXLY_CORRECT' variable is set.
15
16	2. When a command in the hash table no longer exists, Bash will
17	re-search '$PATH' to find the new location. This is also available
18	with 'shopt -s checkhash'.
19
20	3. Bash will not insert a command without the execute bit set into the
21	command hash table, even if it returns it as a (last-ditch) result
22	from a '$PATH' search.
23
24	4. The message printed by the job control code and builtins when a job
25	exits with a non-zero status is 'Done(status)'.
26
27	5. The message printed by the job control code and builtins when a job
28	is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
29	'SIGTSTP'.
30
31	6. Alias expansion is always enabled, even in non-interactive shells.
32
33	7. Reserved words appearing in a context where reserved words are
34	recognized do not undergo alias expansion.
35
36	8. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number
37	and '!!' to '!' are enabled, and parameter expansion is performed
38	on the values of 'PS1' and 'PS2' regardless of the setting of the
39	'promptvars' option.
40
41	9. The POSIX startup files are executed ('$ENV') rather than the
42	normal Bash files.
43
44	10. Tilde expansion is only performed on assignments preceding a
45	command name, rather than on all assignment statements on the line.
46
47	11. The default history file is '~/.sh_history' (this is the default
48	value of '$HISTFILE').
49
50	12. Redirection operators do not perform filename expansion on the
51	word in the redirection unless the shell is interactive.
52
53	13. Redirection operators do not perform word splitting on the word in
54	the redirection.
55
56	14. Function names must be valid shell 'name's. That is, they may not
57	contain characters other than letters, digits, and underscores, and
58	may not start with a digit. Declaring a function with an invalid
59	name causes a fatal syntax error in non-interactive shells.
60
61	15. Function names may not be the same as one of the POSIX special
62	builtins.
63
64	16. POSIX special builtins are found before shell functions during
65	command lookup.
66
67	17. When printing shell function definitions (e.g., by 'type'), Bash
68	does not print the 'function' keyword.
69
70	18. Literal tildes that appear as the first character in elements of
71	the 'PATH' variable are not expanded as described above under *note
72	Tilde Expansion::.
73
74	19. The 'time' reserved word may be used by itself as a command. When
75	used in this way, it displays timing statistics for the shell and
76	its completed children. The 'TIMEFORMAT' variable controls the
77	format of the timing information.
78
79	20. When parsing and expanding a ${...} expansion that appears within
80	double quotes, single quotes are no longer special and cannot be
81	used to quote a closing brace or other special character, unless
82	the operator is one of those defined to perform pattern removal.
83	In this case, they do not have to appear as matched pairs.
84
85	21. The parser does not recognize 'time' as a reserved word if the
86	next token begins with a '-'.
87
88	22. The '!' character does not introduce history expansion within a
89	double-quoted string, even if the 'histexpand' option is enabled.
90
91	23. If a POSIX special builtin returns an error status, a
92	non-interactive shell exits. The fatal errors are those listed in
93	the POSIX standard, and include things like passing incorrect
94	options, redirection errors, variable assignment errors for
95	assignments preceding the command name, and so on.
96
97	24. A non-interactive shell exits with an error status if a variable
98	assignment error occurs when no command name follows the assignment
99	statements. A variable assignment error occurs, for example, when
100	trying to assign a value to a readonly variable.
101
102	25. A non-interactive shell exits with an error status if a variable
103	assignment error occurs in an assignment statement preceding a
104	special builtin, but not with any other simple command.
105
106	26. A non-interactive shell exits with an error status if the
107	iteration variable in a 'for' statement or the selection variable
108	in a 'select' statement is a readonly variable.
109
110	27. Non-interactive shells exit if FILENAME in '.' FILENAME is not
111	found.
112
113	28. Non-interactive shells exit if a syntax error in an arithmetic
114	expansion results in an invalid expression.
115
116	29. Non-interactive shells exit if a parameter expansion error occurs.
117
118	30. Non-interactive shells exit if there is a syntax error in a script
119	read with the '.' or 'source' builtins, or in a string processed by
120	the 'eval' builtin.
121
122	31. While variable indirection is available, it may not be applied to
123	the '#' and '?' special parameters.
124
125	32. When expanding the '*' special parameter in a pattern context
126	where the expansion is double-quoted does not treat the '$*' as if
127	it were double-quoted.
128
129	33. Assignment statements preceding POSIX special builtins persist in
130	the shell environment after the builtin completes.
131
132	34. The 'command' builtin does not prevent builtins that take
133	assignment statements as arguments from expanding them as
134	assignment statements; when not in POSIX mode, assignment builtins
135	lose their assignment statement expansion properties when preceded
136	by 'command'.
137
138	35. The 'bg' builtin uses the required format to describe each job
139	placed in the background, which does not include an indication of
140	whether the job is the current or previous job.
141
142	36. The output of 'kill -l' prints all the signal names on a single
143	line, separated by spaces, without the 'SIG' prefix.
144
145	37. The 'kill' builtin does not accept signal names with a 'SIG'
146	prefix.
147
148	38. The 'export' and 'readonly' builtin commands display their output
149	in the format required by POSIX.
150
151	39. The 'trap' builtin displays signal names without the leading
152	'SIG'.
153
154	40. The 'trap' builtin doesn't check the first argument for a possible
155	signal specification and revert the signal handling to the original
156	disposition if it is, unless that argument consists solely of
157	digits and is a valid signal number. If users want to reset the
158	handler for a given signal to the original disposition, they should
159	use '-' as the first argument.
160
161	41. 'trap -p' displays signals whose dispositions are set to SIG_DFL
162	and those that were ignored when the shell started.
163
164	42. The '.' and 'source' builtins do not search the current directory
165	for the filename argument if it is not found by searching 'PATH'.
166
167	43. Enabling POSIX mode has the effect of setting the
168	'inherit_errexit' option, so subshells spawned to execute command
169	substitutions inherit the value of the '-e' option from the parent
170	shell. When the 'inherit_errexit' option is not enabled, Bash
171	clears the '-e' option in such subshells.
172
173	44. Enabling POSIX mode has the effect of setting the 'shift_verbose'
174	option, so numeric arguments to 'shift' that exceed the number of
175	positional parameters will result in an error message.
176
177	45. When the 'alias' builtin displays alias definitions, it does not
178	display them with a leading 'alias ' unless the '-p' option is
179	supplied.
180
181	46. When the 'set' builtin is invoked without options, it does not
182	display shell function names and definitions.
183
184	47. When the 'set' builtin is invoked without options, it displays
185	variable values without quotes, unless they contain shell
186	metacharacters, even if the result contains nonprinting characters.
187
188	48. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
189	constructed from '$PWD' and the directory name supplied as an
190	argument does not refer to an existing directory, 'cd' will fail
191	instead of falling back to PHYSICAL mode.
192
193	49. When the 'cd' builtin cannot change a directory because the length
194	of the pathname constructed from '$PWD' and the directory name
195	supplied as an argument exceeds PATH_MAX when all symbolic links
196	are expanded, 'cd' will fail instead of attempting to use only the
197	supplied directory name.
198
199	50. The 'pwd' builtin verifies that the value it prints is the same as
200	the current directory, even if it is not asked to check the file
201	system with the '-P' option.
202
203	51. When listing the history, the 'fc' builtin does not include an
204	indication of whether or not a history entry has been modified.
205
206	52. The default editor used by 'fc' is 'ed'.
207
208	53. The 'type' and 'command' builtins will not report a non-executable
209	file as having been found, though the shell will attempt to execute
210	such a file if it is the only so-named file found in '$PATH'.
211
212	54. The 'vi' editing mode will invoke the 'vi' editor directly when
213	the 'v' command is run, instead of checking '$VISUAL' and
214	'$EDITOR'.
215
216	55. When the 'xpg_echo' option is enabled, Bash does not attempt to
217	interpret any arguments to 'echo' as options. Each argument is
218	displayed, after escape characters are converted.
219
220	56. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
221	and '-f' options.
222
223	57. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
224	interrupt the 'wait' builtin and cause it to return immediately.
225	The trap command is run once for each child that exits.
226
227	58. The 'read' builtin may be interrupted by a signal for which a trap
228	has been set. If Bash receives a trapped signal while executing
229	'read', the trap handler executes and 'read' returns an exit status
230	greater than 128.
231
232	59. Bash removes an exited background process's status from the list
233	of such statuses after the 'wait' builtin is used to obtain it.
234
235	There is other POSIX behavior that Bash does not implement by default
236	even when in POSIX mode. Specifically:
237
238	1. The 'fc' builtin checks '$EDITOR' as a program to edit history
239	entries if 'FCEDIT' is unset, rather than defaulting directly to
240	'ed'. 'fc' uses 'ed' if 'EDITOR' is unset.
241
242	2. As noted above, Bash requires the 'xpg_echo' option to be enabled
243	for the 'echo' builtin to be fully conformant.
244
245	Bash can be configured to be POSIX-conformant by default, by specifying
246	the '--enable-strict-posix-default' to 'configure' when building (*note
247	Optional Features::).
248