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. When a command in the hash table no longer exists, Bash will 15 re-search `$PATH' to find the new location. This is also 16 available with `shopt -s checkhash'. 17 18 2. The message printed by the job control code and builtins when a job 19 exits with a non-zero status is `Done(status)'. 20 21 3. The message printed by the job control code and builtins when a job 22 is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example, 23 `SIGTSTP'. 24 25 4. The `bg' builtin uses the required format to describe each job 26 placed in the background, which does not include an indication of 27 whether the job is the current or previous job. 28 29 5. Reserved words appearing in a context where reserved words are 30 recognized do not undergo alias expansion. 31 32 6. The POSIX `PS1' and `PS2' expansions of `!' to the history number 33 and `!!' to `!' are enabled, and parameter expansion is performed 34 on the values of `PS1' and `PS2' regardless of the setting of the 35 `promptvars' option. 36 37 7. The POSIX startup files are executed (`$ENV') rather than the 38 normal Bash files. 39 40 8. Tilde expansion is only performed on assignments preceding a 41 command name, rather than on all assignment statements on the line. 42 43 9. The default history file is `~/.sh_history' (this is the default 44 value of `$HISTFILE'). 45 46 10. The output of `kill -l' prints all the signal names on a single 47 line, separated by spaces, without the `SIG' prefix. 48 49 11. The `kill' builtin does not accept signal names with a `SIG' 50 prefix. 51 52 12. Non-interactive shells exit if FILENAME in `.' FILENAME is not 53 found. 54 55 13. Non-interactive shells exit if a syntax error in an arithmetic 56 expansion results in an invalid expression. 57 58 14. Redirection operators do not perform filename expansion on the word 59 in the redirection unless the shell is interactive. 60 61 15. Redirection operators do not perform word splitting on the word in 62 the redirection. 63 64 16. Function names must be valid shell `name's. That is, they may not 65 contain characters other than letters, digits, and underscores, and 66 may not start with a digit. Declaring a function with an invalid 67 name causes a fatal syntax error in non-interactive shells. 68 69 17. POSIX special builtins are found before shell functions during 70 command lookup. 71 72 18. If a POSIX special builtin returns an error status, a 73 non-interactive shell exits. The fatal errors are those listed in 74 the POSIX standard, and include things like passing incorrect 75 options, redirection errors, variable assignment errors for 76 assignments preceding the command name, and so on. 77 78 19. If `CDPATH' is set, the `cd' builtin will not implicitly append 79 the current directory to it. This means that `cd' will fail if no 80 valid directory name can be constructed from any of the entries in 81 `$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. 83 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. 88 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. 92 93 22. Process substitution is not available. 94 95 23. Assignment statements preceding POSIX special builtins persist in 96 the shell environment after the builtin completes. 97 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. 101 102 25. The `export' and `readonly' builtin commands display their output 103 in the format required by POSIX. 104 105 26. The `trap' builtin displays signal names without the leading `SIG'. 106 107 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. 113 114 28. The `.' and `source' builtins do not search the current directory 115 for the filename argument if it is not found by searching `PATH'. 116 117 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. 120 121 30. Alias expansion is always enabled, even in non-interactive shells. 122 123 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. 126 127 32. When the `set' builtin is invoked without options, it does not 128 display shell function names and definitions. 129 130 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. 133 134 34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname 135 constructed from `$PWD' and the directory name supplied as an 136 argument does not refer to an existing directory, `cd' will fail 137 instead of falling back to PHYSICAL mode. 138 139 35. When the `pwd' builtin is supplied the `-P' option, it resets 140 `$PWD' to a pathname containing no symlinks. 141 142 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. 145 146 37. When listing the history, the `fc' builtin does not include an 147 indication of whether or not a history entry has been modified. 148 149 38. The default editor used by `fc' is `ed'. 150 151 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 `$PATH'. 155 156 40. The `vi' editing mode will invoke the `vi' editor directly when 157 the `v' command is run, instead of checking `$FCEDIT' and 158 `$EDITOR'. 159 160 41. When the `xpg_echo' option is enabled, Bash does not attempt to 161 interpret any arguments to `echo' as options. Each argument is 162 displayed, after escape characters are converted. 163 164 165There is other POSIX behavior that Bash does not implement by default 166even when in POSIX mode. Specifically: 167 168 1. The `fc' builtin checks `$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. 171 172 2. As noted above, Bash requires the `xpg_echo' option to be enabled 173 for the `echo' builtin to be fully conformant. 174 175 176Bash can be configured to be POSIX-conformant by default, by specifying 177the `--enable-strict-posix-default' to `configure' when building (*note 178Optional Features::). 179 180