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1 | 6.11 Bash POSIX Mode |
2 | ==================== | |
726f6388 | 3 | |
a0c0a00f | 4 | Starting Bash with the '--posix' command-line option or executing 'set |
ccc6cda3 | 5 | -o posix' while Bash is running will cause Bash to conform more closely |
0628567a JA |
6 | to the POSIX standard by changing the behavior to match that specified |
7 | by POSIX in areas where the Bash default differs. | |
726f6388 | 8 | |
a0c0a00f | 9 | When invoked as 'sh', Bash enters POSIX mode after reading the startup |
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10 | files. |
11 | ||
a0c0a00f | 12 | The following list is what's changed when 'POSIX mode' is in effect: |
726f6388 | 13 | |
d233b485 CR |
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 | |
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17 | re-search '$PATH' to find the new location. This is also available |
18 | with 'shopt -s checkhash'. | |
726f6388 | 19 | |
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20 | 3. Bash will not insert a command without the execute bit set into the |
21 | command hash table, even if it returns it as a (last-ditch) result | |
22 | from a '$PATH' search. | |
726f6388 | 23 | |
d233b485 | 24 | 4. The message printed by the job control code and builtins when a job |
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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 | |
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28 | is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example, |
29 | 'SIGTSTP'. | |
726f6388 | 30 | |
712f80b0 | 31 | 6. Alias expansion is always enabled, even in non-interactive shells. |
726f6388 | 32 | |
712f80b0 | 33 | 7. Reserved words appearing in a context where reserved words are |
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34 | recognized do not undergo alias expansion. |
35 | ||
712f80b0 | 36 | 8. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number |
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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. | |
726f6388 | 40 | |
712f80b0 | 41 | 9. The POSIX startup files are executed ('$ENV') rather than the |
0628567a | 42 | normal Bash files. |
726f6388 | 43 | |
712f80b0 | 44 | 10. Tilde expansion is only performed on assignments preceding a |
ccc6cda3 | 45 | command name, rather than on all assignment statements on the line. |
726f6388 | 46 | |
712f80b0 | 47 | 11. The default history file is '~/.sh_history' (this is the default |
a0c0a00f | 48 | value of '$HISTFILE'). |
ac50fbac | 49 | |
712f80b0 | 50 | 12. Redirection operators do not perform filename expansion on the |
a0c0a00f | 51 | word in the redirection unless the shell is interactive. |
726f6388 | 52 | |
712f80b0 | 53 | 13. Redirection operators do not perform word splitting on the word in |
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54 | the redirection. |
55 | ||
712f80b0 | 56 | 14. Function names must be valid shell 'name's. That is, they may not |
ccc6cda3 | 57 | contain characters other than letters, digits, and underscores, and |
cce855bc | 58 | may not start with a digit. Declaring a function with an invalid |
ccc6cda3 | 59 | name causes a fatal syntax error in non-interactive shells. |
726f6388 | 60 | |
712f80b0 | 61 | 15. Function names may not be the same as one of the POSIX special |
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62 | builtins. |
63 | ||
712f80b0 | 64 | 16. POSIX special builtins are found before shell functions during |
0628567a | 65 | command lookup. |
726f6388 | 66 | |
712f80b0 | 67 | 17. When printing shell function definitions (e.g., by 'type'), Bash |
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68 | does not print the 'function' keyword. |
69 | ||
712f80b0 | 70 | 18. Literal tildes that appear as the first character in elements of |
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71 | the 'PATH' variable are not expanded as described above under *note |
72 | Tilde Expansion::. | |
73 | ||
712f80b0 | 74 | 19. The 'time' reserved word may be used by itself as a command. When |
495aee44 | 75 | used in this way, it displays timing statistics for the shell and |
a0c0a00f | 76 | its completed children. The 'TIMEFORMAT' variable controls the |
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77 | format of the timing information. |
78 | ||
712f80b0 | 79 | 20. When parsing and expanding a ${...} expansion that appears within |
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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 | ||
712f80b0 | 85 | 21. The parser does not recognize 'time' as a reserved word if the |
a0c0a00f | 86 | next token begins with a '-'. |
495aee44 | 87 | |
712f80b0 | 88 | 22. The '!' character does not introduce history expansion within a |
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89 | double-quoted string, even if the 'histexpand' option is enabled. |
90 | ||
712f80b0 | 91 | 23. If a POSIX special builtin returns an error status, a |
ccc6cda3 | 92 | non-interactive shell exits. The fatal errors are those listed in |
0628567a | 93 | the POSIX standard, and include things like passing incorrect |
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94 | options, redirection errors, variable assignment errors for |
95 | assignments preceding the command name, and so on. | |
726f6388 | 96 | |
712f80b0 | 97 | 24. A non-interactive shell exits with an error status if a variable |
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98 | assignment error occurs when no command name follows the assignment |
99 | statements. A variable assignment error occurs, for example, when | |
cce855bc | 100 | trying to assign a value to a readonly variable. |
ccc6cda3 | 101 | |
712f80b0 | 102 | 25. A non-interactive shell exits with an error status if a variable |
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103 | assignment error occurs in an assignment statement preceding a |
104 | special builtin, but not with any other simple command. | |
105 | ||
712f80b0 | 106 | 26. A non-interactive shell exits with an error status if the |
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107 | iteration variable in a 'for' statement or the selection variable |
108 | in a 'select' statement is a readonly variable. | |
109 | ||
712f80b0 | 110 | 27. Non-interactive shells exit if FILENAME in '.' FILENAME is not |
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111 | found. |
112 | ||
712f80b0 | 113 | 28. Non-interactive shells exit if a syntax error in an arithmetic |
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114 | expansion results in an invalid expression. |
115 | ||
712f80b0 | 116 | 29. Non-interactive shells exit if a parameter expansion error occurs. |
a0c0a00f | 117 | |
712f80b0 | 118 | 30. Non-interactive shells exit if there is a syntax error in a script |
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119 | read with the '.' or 'source' builtins, or in a string processed by |
120 | the 'eval' builtin. | |
ccc6cda3 | 121 | |
d233b485 | 122 | 31. While variable indirection is available, it may not be applied to |
a0c0a00f | 123 | the '#' and '?' special parameters. |
ac50fbac | 124 | |
d233b485 CR |
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 | |
0628567a | 130 | the shell environment after the builtin completes. |
ccc6cda3 | 131 | |
712f80b0 | 132 | 34. The 'command' builtin does not prevent builtins that take |
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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 | ||
712f80b0 | 138 | 35. The 'bg' builtin uses the required format to describe each job |
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139 | placed in the background, which does not include an indication of |
140 | whether the job is the current or previous job. | |
141 | ||
712f80b0 | 142 | 36. The output of 'kill -l' prints all the signal names on a single |
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143 | line, separated by spaces, without the 'SIG' prefix. |
144 | ||
712f80b0 | 145 | 37. The 'kill' builtin does not accept signal names with a 'SIG' |
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146 | prefix. |
147 | ||
712f80b0 | 148 | 38. The 'export' and 'readonly' builtin commands display their output |
0628567a | 149 | in the format required by POSIX. |
d166f048 | 150 | |
712f80b0 | 151 | 39. The 'trap' builtin displays signal names without the leading |
a0c0a00f | 152 | 'SIG'. |
28ef6c31 | 153 | |
712f80b0 | 154 | 40. The 'trap' builtin doesn't check the first argument for a possible |
b80f6443 | 155 | signal specification and revert the signal handling to the original |
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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 | |
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158 | handler for a given signal to the original disposition, they should |
159 | use '-' as the first argument. | |
b80f6443 | 160 | |
712f80b0 CR |
161 | 41. 'trap -p' displays signals whose dispositions are set to SIG_DFL |
162 | and those that were ignored when the shell started. | |
163 | ||
d233b485 | 164 | 42. The '.' and 'source' builtins do not search the current directory |
a0c0a00f | 165 | for the filename argument if it is not found by searching 'PATH'. |
28ef6c31 | 166 | |
d233b485 | 167 | 43. Enabling POSIX mode has the effect of setting the |
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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. | |
28ef6c31 | 172 | |
d233b485 CR |
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 | |
a0c0a00f | 178 | display them with a leading 'alias ' unless the '-p' option is |
b80f6443 | 179 | supplied. |
28ef6c31 | 180 | |
d233b485 | 181 | 46. When the 'set' builtin is invoked without options, it does not |
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182 | display shell function names and definitions. |
183 | ||
d233b485 | 184 | 47. When the 'set' builtin is invoked without options, it displays |
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185 | variable values without quotes, unless they contain shell |
186 | metacharacters, even if the result contains nonprinting characters. | |
187 | ||
d233b485 | 188 | 48. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname |
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189 | constructed from '$PWD' and the directory name supplied as an |
190 | argument does not refer to an existing directory, 'cd' will fail | |
7117c2d2 | 191 | instead of falling back to PHYSICAL mode. |
ccc6cda3 | 192 | |
d233b485 CR |
193 | 49. When the 'cd' builtin cannot change a directory because the length |
194 | of the pathname constructed from '$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 | |
95732b49 | 200 | the current directory, even if it is not asked to check the file |
a0c0a00f | 201 | system with the '-P' option. |
95732b49 | 202 | |
d233b485 | 203 | 51. When listing the history, the 'fc' builtin does not include an |
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204 | indication of whether or not a history entry has been modified. |
205 | ||
d233b485 | 206 | 52. The default editor used by 'fc' is 'ed'. |
95732b49 | 207 | |
d233b485 | 208 | 53. The 'type' and 'command' builtins will not report a non-executable |
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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 '$PATH'. | |
95732b49 | 211 | |
d233b485 | 212 | 54. The 'vi' editing mode will invoke the 'vi' editor directly when |
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213 | the 'v' command is run, instead of checking '$VISUAL' and |
214 | '$EDITOR'. | |
95732b49 | 215 | |
d233b485 | 216 | 55. When the 'xpg_echo' option is enabled, Bash does not attempt to |
a0c0a00f | 217 | interpret any arguments to 'echo' as options. Each argument is |
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218 | displayed, after escape characters are converted. |
219 | ||
d233b485 | 220 | 56. The 'ulimit' builtin uses a block size of 512 bytes for the '-c' |
a0c0a00f | 221 | and '-f' options. |
3185942a | 222 | |
d233b485 | 223 | 57. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not |
a0c0a00f | 224 | interrupt the 'wait' builtin and cause it to return immediately. |
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225 | The trap command is run once for each child that exits. |
226 | ||
d233b485 | 227 | 58. The 'read' builtin may be interrupted by a signal for which a trap |
ac50fbac | 228 | has been set. If Bash receives a trapped signal while executing |
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229 | 'read', the trap handler executes and 'read' returns an exit status |
230 | greater than 128. | |
ac50fbac | 231 | |
d233b485 | 232 | 59. Bash removes an exited background process's status from the list |
a0c0a00f | 233 | of such statuses after the 'wait' builtin is used to obtain it. |
95732b49 | 234 | |
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235 | There is other POSIX behavior that Bash does not implement by default |
236 | even when in POSIX mode. Specifically: | |
ccc6cda3 | 237 | |
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238 | 1. The 'fc' builtin checks '$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. | |
28ef6c31 | 241 | |
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242 | 2. As noted above, Bash requires the 'xpg_echo' option to be enabled |
243 | for the 'echo' builtin to be fully conformant. | |
28ef6c31 | 244 | |
95732b49 | 245 | Bash can be configured to be POSIX-conformant by default, by specifying |
a0c0a00f | 246 | the '--enable-strict-posix-default' to 'configure' when building (*note |
95732b49 | 247 | Optional Features::). |
28ef6c31 | 248 |