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1 | 6.11 Bash POSIX Mode |
2 | ==================== | |
726f6388 | 3 | |
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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 | |
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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 | |
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9 | When invoked as `sh', Bash enters POSIX mode after reading the startup |
10 | files. | |
11 | ||
ccc6cda3 | 12 | The following list is what's changed when `POSIX mode' is in effect: |
726f6388 | 13 | |
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14 | 1. When a command in the hash table no longer exists, Bash will |
15 | re-search `$PATH' to find the new location. This is also | |
16 | available with `shopt -s checkhash'. | |
726f6388 | 17 | |
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18 | 2. The message printed by the job control code and builtins when a job |
19 | exits with a non-zero status is `Done(status)'. | |
726f6388 | 20 | |
ccc6cda3 | 21 | 3. The message printed by the job control code and builtins when a job |
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22 | is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example, |
23 | `SIGTSTP'. | |
726f6388 | 24 | |
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25 | 4. The `bg' builtin uses the required format to describe each job |
26 | placed in the background, which does not include an indication of | |
27 | whether the job is the current or previous job. | |
726f6388 | 28 | |
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29 | 5. Reserved words appearing in a context where reserved words are |
30 | recognized do not undergo alias expansion. | |
31 | ||
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32 | 6. The POSIX `PS1' and `PS2' expansions of `!' to the history number |
33 | and `!!' to `!' are enabled, and parameter expansion is performed | |
34 | on the values of `PS1' and `PS2' regardless of the setting of the | |
35 | `promptvars' option. | |
726f6388 | 36 | |
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37 | 7. The POSIX startup files are executed (`$ENV') rather than the |
38 | normal Bash files. | |
726f6388 | 39 | |
95732b49 | 40 | 8. Tilde expansion is only performed on assignments preceding a |
ccc6cda3 | 41 | command name, rather than on all assignment statements on the line. |
726f6388 | 42 | |
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43 | 9. The `command' builtin does not prevent builtins that take |
44 | assignment statements as arguments from expanding them as | |
45 | assignment statements; when not in POSIX mode, assignment builtins | |
46 | lose their assignment statement expansion properties when preceded | |
47 | by `command'. | |
48 | ||
49 | 10. The default history file is `~/.sh_history' (this is the default | |
ccc6cda3 | 50 | value of `$HISTFILE'). |
726f6388 | 51 | |
ac50fbac | 52 | 11. The output of `kill -l' prints all the signal names on a single |
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53 | line, separated by spaces, without the `SIG' prefix. |
54 | ||
ac50fbac | 55 | 12. The `kill' builtin does not accept signal names with a `SIG' |
b80f6443 | 56 | prefix. |
726f6388 | 57 | |
ac50fbac | 58 | 13. Non-interactive shells exit if FILENAME in `.' FILENAME is not |
ccc6cda3 | 59 | found. |
726f6388 | 60 | |
ac50fbac | 61 | 14. Non-interactive shells exit if a syntax error in an arithmetic |
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62 | expansion results in an invalid expression. |
63 | ||
ac50fbac | 64 | 15. Non-interactive shells exit if there is a syntax error in a script |
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65 | read with the `.' or `source' builtins, or in a string processed by |
66 | the `eval' builtin. | |
67 | ||
ac50fbac | 68 | 16. Redirection operators do not perform filename expansion on the word |
ccc6cda3 | 69 | in the redirection unless the shell is interactive. |
726f6388 | 70 | |
ac50fbac | 71 | 17. Redirection operators do not perform word splitting on the word in |
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72 | the redirection. |
73 | ||
ac50fbac | 74 | 18. Function names must be valid shell `name's. That is, they may not |
ccc6cda3 | 75 | contain characters other than letters, digits, and underscores, and |
cce855bc | 76 | may not start with a digit. Declaring a function with an invalid |
ccc6cda3 | 77 | name causes a fatal syntax error in non-interactive shells. |
726f6388 | 78 | |
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79 | 19. Function names may not be the same as one of the POSIX special |
80 | builtins. | |
81 | ||
82 | 20. POSIX special builtins are found before shell functions during | |
0628567a | 83 | command lookup. |
726f6388 | 84 | |
ac50fbac | 85 | 21. The `time' reserved word may be used by itself as a command. When |
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86 | used in this way, it displays timing statistics for the shell and |
87 | its completed children. The `TIMEFORMAT' variable controls the | |
88 | format of the timing information. | |
89 | ||
ac50fbac | 90 | 22. When parsing and expanding a ${...} expansion that appears within |
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91 | double quotes, single quotes are no longer special and cannot be |
92 | used to quote a closing brace or other special character, unless | |
93 | the operator is one of those defined to perform pattern removal. | |
94 | In this case, they do not have to appear as matched pairs. | |
95 | ||
ac50fbac | 96 | 23. The parser does not recognize `time' as a reserved word if the next |
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97 | token begins with a `-'. |
98 | ||
ac50fbac | 99 | 24. If a POSIX special builtin returns an error status, a |
ccc6cda3 | 100 | non-interactive shell exits. The fatal errors are those listed in |
0628567a | 101 | the POSIX standard, and include things like passing incorrect |
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102 | options, redirection errors, variable assignment errors for |
103 | assignments preceding the command name, and so on. | |
726f6388 | 104 | |
ac50fbac | 105 | 25. A non-interactive shell exits with an error status if a variable |
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106 | assignment error occurs when no command name follows the assignment |
107 | statements. A variable assignment error occurs, for example, when | |
cce855bc | 108 | trying to assign a value to a readonly variable. |
ccc6cda3 | 109 | |
ac50fbac | 110 | 26. A non-interactive shell exits with an error status if a variable |
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111 | assignment error occurs in an assignment statement preceding a |
112 | special builtin, but not with any other simple command. | |
113 | ||
ac50fbac | 114 | 27. A non-interactive shell exits with an error status if the iteration |
ccc6cda3 | 115 | variable in a `for' statement or the selection variable in a |
cce855bc | 116 | `select' statement is a readonly variable. |
ccc6cda3 | 117 | |
ac50fbac | 118 | 28. Process substitution is not available. |
ccc6cda3 | 119 | |
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120 | 29. While variable indirection is available, it may not be applied to |
121 | the `#' and `?' special parameters. | |
122 | ||
123 | 30. Assignment statements preceding POSIX special builtins persist in | |
0628567a | 124 | the shell environment after the builtin completes. |
ccc6cda3 | 125 | |
ac50fbac | 126 | 31. Assignment statements preceding shell function calls persist in the |
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127 | shell environment after the function returns, as if a POSIX |
128 | special builtin command had been executed. | |
129 | ||
ac50fbac | 130 | 32. The `export' and `readonly' builtin commands display their output |
0628567a | 131 | in the format required by POSIX. |
d166f048 | 132 | |
ac50fbac | 133 | 33. The `trap' builtin displays signal names without the leading `SIG'. |
28ef6c31 | 134 | |
ac50fbac | 135 | 34. The `trap' builtin doesn't check the first argument for a possible |
b80f6443 | 136 | signal specification and revert the signal handling to the original |
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137 | disposition if it is, unless that argument consists solely of |
138 | digits and is a valid signal number. If users want to reset the | |
139 | handler for a given signal to the original disposition, they | |
140 | should use `-' as the first argument. | |
b80f6443 | 141 | |
ac50fbac | 142 | 35. The `.' and `source' builtins do not search the current directory |
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143 | for the filename argument if it is not found by searching `PATH'. |
144 | ||
ac50fbac | 145 | 36. Subshells spawned to execute command substitutions inherit the |
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146 | value of the `-e' option from the parent shell. When not in POSIX |
147 | mode, Bash clears the `-e' option in such subshells. | |
148 | ||
ac50fbac | 149 | 37. Alias expansion is always enabled, even in non-interactive shells. |
b80f6443 | 150 | |
ac50fbac | 151 | 38. When the `alias' builtin displays alias definitions, it does not |
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152 | display them with a leading `alias ' unless the `-p' option is |
153 | supplied. | |
28ef6c31 | 154 | |
ac50fbac | 155 | 39. When the `set' builtin is invoked without options, it does not |
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156 | display shell function names and definitions. |
157 | ||
ac50fbac | 158 | 40. When the `set' builtin is invoked without options, it displays |
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159 | variable values without quotes, unless they contain shell |
160 | metacharacters, even if the result contains nonprinting characters. | |
161 | ||
ac50fbac | 162 | 41. When the `cd' builtin is invoked in LOGICAL mode, and the pathname |
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163 | constructed from `$PWD' and the directory name supplied as an |
164 | argument does not refer to an existing directory, `cd' will fail | |
165 | instead of falling back to PHYSICAL mode. | |
ccc6cda3 | 166 | |
ac50fbac | 167 | 42. The `pwd' builtin verifies that the value it prints is the same as |
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168 | the current directory, even if it is not asked to check the file |
169 | system with the `-P' option. | |
170 | ||
ac50fbac | 171 | 43. When listing the history, the `fc' builtin does not include an |
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172 | indication of whether or not a history entry has been modified. |
173 | ||
ac50fbac | 174 | 44. The default editor used by `fc' is `ed'. |
95732b49 | 175 | |
ac50fbac | 176 | 45. The `type' and `command' builtins will not report a non-executable |
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177 | file as having been found, though the shell will attempt to |
178 | execute such a file if it is the only so-named file found in | |
179 | `$PATH'. | |
180 | ||
ac50fbac | 181 | 46. The `vi' editing mode will invoke the `vi' editor directly when |
3185942a | 182 | the `v' command is run, instead of checking `$VISUAL' and |
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183 | `$EDITOR'. |
184 | ||
ac50fbac | 185 | 47. When the `xpg_echo' option is enabled, Bash does not attempt to |
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186 | interpret any arguments to `echo' as options. Each argument is |
187 | displayed, after escape characters are converted. | |
188 | ||
ac50fbac | 189 | 48. The `ulimit' builtin uses a block size of 512 bytes for the `-c' |
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190 | and `-f' options. |
191 | ||
ac50fbac | 192 | 49. The arrival of `SIGCHLD' when a trap is set on `SIGCHLD' does not |
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193 | interrupt the `wait' builtin and cause it to return immediately. |
194 | The trap command is run once for each child that exits. | |
195 | ||
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196 | 50. The `read' builtin may be interrupted by a signal for which a trap |
197 | has been set. If Bash receives a trapped signal while executing | |
198 | `read', the trap handler executes and `read' returns an exit | |
199 | status greater than 128. | |
200 | ||
95732b49 | 201 | |
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202 | There is other POSIX behavior that Bash does not implement by default |
203 | even when in POSIX mode. Specifically: | |
ccc6cda3 | 204 | |
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205 | 1. The `fc' builtin checks `$EDITOR' as a program to edit history |
206 | entries if `FCEDIT' is unset, rather than defaulting directly to | |
207 | `ed'. `fc' uses `ed' if `EDITOR' is unset. | |
726f6388 | 208 | |
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209 | 2. As noted above, Bash requires the `xpg_echo' option to be enabled |
210 | for the `echo' builtin to be fully conformant. | |
28ef6c31 | 211 | |
28ef6c31 | 212 | |
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213 | Bash can be configured to be POSIX-conformant by default, by specifying |
214 | the `--enable-strict-posix-default' to `configure' when building (*note | |
215 | Optional Features::). | |
28ef6c31 | 216 |