source: Daodan/MSYS2/usr/share/doc/bash/POSIX@ 1173

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[1166]16.11 Bash POSIX Mode
2====================
3
4Starting Bash with the '--posix' command-line option or executing 'set
5-o posix' while Bash is running will cause Bash to conform more closely
6to the POSIX standard by changing the behavior to match that specified
7by POSIX in areas where the Bash default differs.
8
9When invoked as 'sh', Bash enters POSIX mode after reading the startup
10files.
11
12The 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 '$EDITOR'.
215
216 55. When the 'xpg_echo' option is enabled, Bash does not attempt to
217 interpret any arguments to 'echo' as options. Each argument is
218 displayed, after escape characters are converted.
219
220 56. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
221 and '-f' options.
222
223 57. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
224 interrupt the 'wait' builtin and cause it to return immediately.
225 The trap command is run once for each child that exits.
226
227 58. The 'read' builtin may be interrupted by a signal for which a trap
228 has been set. If Bash receives a trapped signal while executing
229 'read', the trap handler executes and 'read' returns an exit status
230 greater than 128.
231
232 59. Bash removes an exited background process's status from the list
233 of such statuses after the 'wait' builtin is used to obtain it.
234
235There is other POSIX behavior that Bash does not implement by default
236even when in POSIX mode. Specifically:
237
238 1. The 'fc' builtin checks '$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
245Bash can be configured to be POSIX-conformant by default, by specifying
246the '--enable-strict-posix-default' to 'configure' when building (*note
247Optional Features::).
248
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