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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 |
7117c2d2 JA |
10 | files. |
11 | ||
a0c0a00f | 12 | The following list is what's changed when 'POSIX mode' is in effect: |
726f6388 | 13 | |
ccc6cda3 | 14 | 1. When a command in the hash table no longer exists, Bash will |
a0c0a00f CR |
15 | re-search '$PATH' to find the new location. This is also available |
16 | with 'shopt -s checkhash'. | |
726f6388 | 17 | |
28ef6c31 | 18 | 2. The message printed by the job control code and builtins when a job |
a0c0a00f | 19 | exits with a non-zero status is 'Done(status)'. |
726f6388 | 20 | |
ccc6cda3 | 21 | 3. The message printed by the job control code and builtins when a job |
a0c0a00f CR |
22 | is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example, |
23 | 'SIGTSTP'. | |
726f6388 | 24 | |
a0c0a00f | 25 | 4. Alias expansion is always enabled, even in non-interactive shells. |
726f6388 | 26 | |
95732b49 JA |
27 | 5. Reserved words appearing in a context where reserved words are |
28 | recognized do not undergo alias expansion. | |
29 | ||
a0c0a00f CR |
30 | 6. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number |
31 | and '!!' to '!' are enabled, and parameter expansion is performed | |
32 | on the values of 'PS1' and 'PS2' regardless of the setting of the | |
33 | 'promptvars' option. | |
726f6388 | 34 | |
a0c0a00f | 35 | 7. The POSIX startup files are executed ('$ENV') rather than the |
0628567a | 36 | normal Bash files. |
726f6388 | 37 | |
95732b49 | 38 | 8. Tilde expansion is only performed on assignments preceding a |
ccc6cda3 | 39 | command name, rather than on all assignment statements on the line. |
726f6388 | 40 | |
a0c0a00f CR |
41 | 9. The default history file is '~/.sh_history' (this is the default |
42 | value of '$HISTFILE'). | |
ac50fbac | 43 | |
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44 | 10. Redirection operators do not perform filename expansion on the |
45 | word in the redirection unless the shell is interactive. | |
726f6388 | 46 | |
a0c0a00f | 47 | 11. Redirection operators do not perform word splitting on the word in |
bb70624e JA |
48 | the redirection. |
49 | ||
a0c0a00f | 50 | 12. Function names must be valid shell 'name's. That is, they may not |
ccc6cda3 | 51 | contain characters other than letters, digits, and underscores, and |
cce855bc | 52 | may not start with a digit. Declaring a function with an invalid |
ccc6cda3 | 53 | name causes a fatal syntax error in non-interactive shells. |
726f6388 | 54 | |
a0c0a00f | 55 | 13. Function names may not be the same as one of the POSIX special |
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56 | builtins. |
57 | ||
a0c0a00f | 58 | 14. POSIX special builtins are found before shell functions during |
0628567a | 59 | command lookup. |
726f6388 | 60 | |
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61 | 15. Literal tildes that appear as the first character in elements of |
62 | the 'PATH' variable are not expanded as described above under *note | |
63 | Tilde Expansion::. | |
64 | ||
65 | 16. The 'time' reserved word may be used by itself as a command. When | |
495aee44 | 66 | used in this way, it displays timing statistics for the shell and |
a0c0a00f | 67 | its completed children. The 'TIMEFORMAT' variable controls the |
495aee44 CR |
68 | format of the timing information. |
69 | ||
a0c0a00f | 70 | 17. When parsing and expanding a ${...} expansion that appears within |
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71 | double quotes, single quotes are no longer special and cannot be |
72 | used to quote a closing brace or other special character, unless | |
73 | the operator is one of those defined to perform pattern removal. | |
74 | In this case, they do not have to appear as matched pairs. | |
75 | ||
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76 | 18. The parser does not recognize 'time' as a reserved word if the |
77 | next token begins with a '-'. | |
495aee44 | 78 | |
a0c0a00f CR |
79 | 19. The '!' character does not introduce history expansion within a |
80 | double-quoted string, even if the 'histexpand' option is enabled. | |
81 | ||
82 | 20. If a POSIX special builtin returns an error status, a | |
ccc6cda3 | 83 | non-interactive shell exits. The fatal errors are those listed in |
0628567a | 84 | the POSIX standard, and include things like passing incorrect |
ccc6cda3 JA |
85 | options, redirection errors, variable assignment errors for |
86 | assignments preceding the command name, and so on. | |
726f6388 | 87 | |
a0c0a00f | 88 | 21. A non-interactive shell exits with an error status if a variable |
ccc6cda3 JA |
89 | assignment error occurs when no command name follows the assignment |
90 | statements. A variable assignment error occurs, for example, when | |
cce855bc | 91 | trying to assign a value to a readonly variable. |
ccc6cda3 | 92 | |
a0c0a00f | 93 | 22. A non-interactive shell exits with an error status if a variable |
495aee44 CR |
94 | assignment error occurs in an assignment statement preceding a |
95 | special builtin, but not with any other simple command. | |
96 | ||
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97 | 23. A non-interactive shell exits with an error status if the |
98 | iteration variable in a 'for' statement or the selection variable | |
99 | in a 'select' statement is a readonly variable. | |
100 | ||
101 | 24. Non-interactive shells exit if FILENAME in '.' FILENAME is not | |
102 | found. | |
103 | ||
104 | 25. Non-interactive shells exit if a syntax error in an arithmetic | |
105 | expansion results in an invalid expression. | |
106 | ||
107 | 26. Non-interactive shells exit on word expansion errors. | |
108 | ||
109 | 27. Non-interactive shells exit if there is a syntax error in a script | |
110 | read with the '.' or 'source' builtins, or in a string processed by | |
111 | the 'eval' builtin. | |
ccc6cda3 | 112 | |
a0c0a00f | 113 | 28. Process substitution is not available. |
ccc6cda3 | 114 | |
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115 | 29. While variable indirection is available, it may not be applied to |
116 | the '#' and '?' special parameters. | |
ac50fbac | 117 | |
a0c0a00f | 118 | 30. Assignment statements preceding POSIX special builtins persist in |
0628567a | 119 | the shell environment after the builtin completes. |
ccc6cda3 | 120 | |
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121 | 31. Assignment statements preceding shell function calls persist in |
122 | the shell environment after the function returns, as if a POSIX | |
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123 | special builtin command had been executed. |
124 | ||
a0c0a00f CR |
125 | 32. The 'command' builtin does not prevent builtins that take |
126 | assignment statements as arguments from expanding them as | |
127 | assignment statements; when not in POSIX mode, assignment builtins | |
128 | lose their assignment statement expansion properties when preceded | |
129 | by 'command'. | |
130 | ||
131 | 33. The 'bg' builtin uses the required format to describe each job | |
132 | placed in the background, which does not include an indication of | |
133 | whether the job is the current or previous job. | |
134 | ||
135 | 34. The output of 'kill -l' prints all the signal names on a single | |
136 | line, separated by spaces, without the 'SIG' prefix. | |
137 | ||
138 | 35. The 'kill' builtin does not accept signal names with a 'SIG' | |
139 | prefix. | |
140 | ||
141 | 36. The 'export' and 'readonly' builtin commands display their output | |
0628567a | 142 | in the format required by POSIX. |
d166f048 | 143 | |
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144 | 37. The 'trap' builtin displays signal names without the leading |
145 | 'SIG'. | |
28ef6c31 | 146 | |
a0c0a00f | 147 | 38. The 'trap' builtin doesn't check the first argument for a possible |
b80f6443 | 148 | signal specification and revert the signal handling to the original |
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149 | disposition if it is, unless that argument consists solely of |
150 | digits and is a valid signal number. If users want to reset the | |
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151 | handler for a given signal to the original disposition, they should |
152 | use '-' as the first argument. | |
b80f6443 | 153 | |
a0c0a00f CR |
154 | 39. The '.' and 'source' builtins do not search the current directory |
155 | for the filename argument if it is not found by searching 'PATH'. | |
28ef6c31 | 156 | |
a0c0a00f CR |
157 | 40. Enabling POSIX mode has the effect of setting the |
158 | 'inherit_errexit' option, so subshells spawned to execute command | |
159 | substitutions inherit the value of the '-e' option from the parent | |
160 | shell. When the 'inherit_errexit' option is not enabled, Bash | |
161 | clears the '-e' option in such subshells. | |
28ef6c31 | 162 | |
a0c0a00f CR |
163 | 41. When the 'alias' builtin displays alias definitions, it does not |
164 | display them with a leading 'alias ' unless the '-p' option is | |
b80f6443 | 165 | supplied. |
28ef6c31 | 166 | |
a0c0a00f | 167 | 42. When the 'set' builtin is invoked without options, it does not |
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168 | display shell function names and definitions. |
169 | ||
a0c0a00f | 170 | 43. When the 'set' builtin is invoked without options, it displays |
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171 | variable values without quotes, unless they contain shell |
172 | metacharacters, even if the result contains nonprinting characters. | |
173 | ||
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174 | 44. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname |
175 | constructed from '$PWD' and the directory name supplied as an | |
176 | argument does not refer to an existing directory, 'cd' will fail | |
7117c2d2 | 177 | instead of falling back to PHYSICAL mode. |
ccc6cda3 | 178 | |
a0c0a00f | 179 | 45. The 'pwd' builtin verifies that the value it prints is the same as |
95732b49 | 180 | the current directory, even if it is not asked to check the file |
a0c0a00f | 181 | system with the '-P' option. |
95732b49 | 182 | |
a0c0a00f | 183 | 46. When listing the history, the 'fc' builtin does not include an |
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184 | indication of whether or not a history entry has been modified. |
185 | ||
a0c0a00f | 186 | 47. The default editor used by 'fc' is 'ed'. |
95732b49 | 187 | |
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188 | 48. The 'type' and 'command' builtins will not report a non-executable |
189 | file as having been found, though the shell will attempt to execute | |
190 | such a file if it is the only so-named file found in '$PATH'. | |
95732b49 | 191 | |
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192 | 49. The 'vi' editing mode will invoke the 'vi' editor directly when |
193 | the 'v' command is run, instead of checking '$VISUAL' and | |
194 | '$EDITOR'. | |
95732b49 | 195 | |
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196 | 50. When the 'xpg_echo' option is enabled, Bash does not attempt to |
197 | interpret any arguments to 'echo' as options. Each argument is | |
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198 | displayed, after escape characters are converted. |
199 | ||
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200 | 51. The 'ulimit' builtin uses a block size of 512 bytes for the '-c' |
201 | and '-f' options. | |
3185942a | 202 | |
a0c0a00f CR |
203 | 52. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not |
204 | interrupt the 'wait' builtin and cause it to return immediately. | |
0001803f CR |
205 | The trap command is run once for each child that exits. |
206 | ||
a0c0a00f | 207 | 53. The 'read' builtin may be interrupted by a signal for which a trap |
ac50fbac | 208 | has been set. If Bash receives a trapped signal while executing |
a0c0a00f CR |
209 | 'read', the trap handler executes and 'read' returns an exit status |
210 | greater than 128. | |
ac50fbac | 211 | |
a0c0a00f CR |
212 | 54. Bash removes an exited background process's status from the list |
213 | of such statuses after the 'wait' builtin is used to obtain it. | |
95732b49 | 214 | |
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215 | There is other POSIX behavior that Bash does not implement by default |
216 | even when in POSIX mode. Specifically: | |
ccc6cda3 | 217 | |
a0c0a00f CR |
218 | 1. The 'fc' builtin checks '$EDITOR' as a program to edit history |
219 | entries if 'FCEDIT' is unset, rather than defaulting directly to | |
220 | 'ed'. 'fc' uses 'ed' if 'EDITOR' is unset. | |
28ef6c31 | 221 | |
a0c0a00f CR |
222 | 2. As noted above, Bash requires the 'xpg_echo' option to be enabled |
223 | for the 'echo' builtin to be fully conformant. | |
28ef6c31 | 224 | |
95732b49 | 225 | Bash can be configured to be POSIX-conformant by default, by specifying |
a0c0a00f | 226 | the '--enable-strict-posix-default' to 'configure' when building (*note |
95732b49 | 227 | Optional Features::). |
28ef6c31 | 228 |