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1 6.11 Bash POSIX Mode
2 ====================
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.
9 When invoked as `sh', Bash enters POSIX mode after reading the startup
10 files.
12 The following list is what's changed when `POSIX mode' is in effect:
14 1. When a command in the hash table no longer exists, Bash will
15 re-search `$PATH' to find the new location. This is also
16 available with `shopt -s checkhash'.
18 2. The message printed by the job control code and builtins when a job
19 exits with a non-zero status is `Done(status)'.
21 3. The message printed by the job control code and builtins when a job
22 is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example,
23 `SIGTSTP'.
25 4. The `bg' builtin uses the required format to describe each job
26 placed in the background, which does not include an indication of
27 whether the job is the current or previous job.
29 5. Reserved words appearing in a context where reserved words are
30 recognized do not undergo alias expansion.
32 6. The POSIX `PS1' and `PS2' expansions of `!' to the history number
33 and `!!' to `!' are enabled, and parameter expansion is performed
34 on the values of `PS1' and `PS2' regardless of the setting of the
35 `promptvars' option.
37 7. The POSIX startup files are executed (`$ENV') rather than the
38 normal Bash files.
40 8. Tilde expansion is only performed on assignments preceding a
41 command name, rather than on all assignment statements on the line.
43 9. The default history file is `~/.sh_history' (this is the default
44 value of `$HISTFILE').
46 10. The output of `kill -l' prints all the signal names on a single
47 line, separated by spaces, without the `SIG' prefix.
49 11. The `kill' builtin does not accept signal names with a `SIG'
50 prefix.
52 12. Non-interactive shells exit if FILENAME in `.' FILENAME is not
53 found.
55 13. Non-interactive shells exit if a syntax error in an arithmetic
56 expansion results in an invalid expression.
58 14. Redirection operators do not perform filename expansion on the word
59 in the redirection unless the shell is interactive.
61 15. Redirection operators do not perform word splitting on the word in
62 the redirection.
64 16. Function names must be valid shell `name's. That is, they may not
65 contain characters other than letters, digits, and underscores, and
66 may not start with a digit. Declaring a function with an invalid
67 name causes a fatal syntax error in non-interactive shells.
69 17. POSIX special builtins are found before shell functions during
70 command lookup.
72 18. If a POSIX special builtin returns an error status, a
73 non-interactive shell exits. The fatal errors are those listed in
74 the POSIX standard, and include things like passing incorrect
75 options, redirection errors, variable assignment errors for
76 assignments preceding the command name, and so on.
78 19. If `CDPATH' is set, the `cd' builtin will not implicitly append
79 the current directory to it. This means that `cd' will fail if no
80 valid directory name can be constructed from any of the entries in
81 `$CDPATH', even if the a directory with the same name as the name
82 given as an argument to `cd' exists in the current directory.
84 20. A non-interactive shell exits with an error status if a variable
85 assignment error occurs when no command name follows the assignment
86 statements. A variable assignment error occurs, for example, when
87 trying to assign a value to a readonly variable.
89 21. A non-interactive shell exits with an error status if the iteration
90 variable in a `for' statement or the selection variable in a
91 `select' statement is a readonly variable.
93 22. Process substitution is not available.
95 23. Assignment statements preceding POSIX special builtins persist in
96 the shell environment after the builtin completes.
98 24. Assignment statements preceding shell function calls persist in the
99 shell environment after the function returns, as if a POSIX
100 special builtin command had been executed.
102 25. The `export' and `readonly' builtin commands display their output
103 in the format required by POSIX.
105 26. The `trap' builtin displays signal names without the leading `SIG'.
107 27. The `trap' builtin doesn't check the first argument for a possible
108 signal specification and revert the signal handling to the original
109 disposition if it is, unless that argument consists solely of
110 digits and is a valid signal number. If users want to reset the
111 handler for a given signal to the original disposition, they
112 should use `-' as the first argument.
114 28. The `.' and `source' builtins do not search the current directory
115 for the filename argument if it is not found by searching `PATH'.
117 29. Subshells spawned to execute command substitutions inherit the
118 value of the `-e' option from the parent shell. When not in POSIX
119 mode, Bash clears the `-e' option in such subshells.
121 30. Alias expansion is always enabled, even in non-interactive shells.
123 31. When the `alias' builtin displays alias definitions, it does not
124 display them with a leading `alias ' unless the `-p' option is
125 supplied.
127 32. When the `set' builtin is invoked without options, it does not
128 display shell function names and definitions.
130 33. When the `set' builtin is invoked without options, it displays
131 variable values without quotes, unless they contain shell
132 metacharacters, even if the result contains nonprinting characters.
134 34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname
135 constructed from `$PWD' and the directory name supplied as an
136 argument does not refer to an existing directory, `cd' will fail
137 instead of falling back to PHYSICAL mode.
139 35. When the `pwd' builtin is supplied the `-P' option, it resets
140 `$PWD' to a pathname containing no symlinks.
142 36. The `pwd' builtin verifies that the value it prints is the same as
143 the current directory, even if it is not asked to check the file
144 system with the `-P' option.
146 37. When listing the history, the `fc' builtin does not include an
147 indication of whether or not a history entry has been modified.
149 38. The default editor used by `fc' is `ed'.
151 39. The `type' and `command' builtins will not report a non-executable
152 file as having been found, though the shell will attempt to
153 execute such a file if it is the only so-named file found in
154 `$PATH'.
156 40. The `vi' editing mode will invoke the `vi' editor directly when
157 the `v' command is run, instead of checking `$FCEDIT' and
158 `$EDITOR'.
160 41. When the `xpg_echo' option is enabled, Bash does not attempt to
161 interpret any arguments to `echo' as options. Each argument is
162 displayed, after escape characters are converted.
165 There is other POSIX behavior that Bash does not implement by default
166 even when in POSIX mode. Specifically:
168 1. The `fc' builtin checks `$EDITOR' as a program to edit history
169 entries if `FCEDIT' is unset, rather than defaulting directly to
170 `ed'. `fc' uses `ed' if `EDITOR' is unset.
172 2. As noted above, Bash requires the `xpg_echo' option to be enabled
173 for the `echo' builtin to be fully conformant.
176 Bash can be configured to be POSIX-conformant by default, by specifying
177 the `--enable-strict-posix-default' to `configure' when building (*note
178 Optional Features::).