| 6.11 Bash POSIX Mode |
| ==================== |
| |
| Starting Bash with the `--posix' command-line option or executing `set |
| -o posix' while Bash is running will cause Bash to conform more closely |
| to the POSIX standard by changing the behavior to match that specified |
| by POSIX in areas where the Bash default differs. |
| |
| When invoked as `sh', Bash enters POSIX mode after reading the startup |
| files. |
| |
| The following list is what's changed when `POSIX mode' is in effect: |
| |
| 1. When a command in the hash table no longer exists, Bash will |
| re-search `$PATH' to find the new location. This is also |
| available with `shopt -s checkhash'. |
| |
| 2. The message printed by the job control code and builtins when a job |
| exits with a non-zero status is `Done(status)'. |
| |
| 3. The message printed by the job control code and builtins when a job |
| is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example, |
| `SIGTSTP'. |
| |
| 4. The `bg' builtin uses the required format to describe each job |
| placed in the background, which does not include an indication of |
| whether the job is the current or previous job. |
| |
| 5. Reserved words appearing in a context where reserved words are |
| recognized do not undergo alias expansion. |
| |
| 6. The POSIX `PS1' and `PS2' expansions of `!' to the history number |
| and `!!' to `!' are enabled, and parameter expansion is performed |
| on the values of `PS1' and `PS2' regardless of the setting of the |
| `promptvars' option. |
| |
| 7. The POSIX startup files are executed (`$ENV') rather than the |
| normal Bash files. |
| |
| 8. Tilde expansion is only performed on assignments preceding a |
| command name, rather than on all assignment statements on the line. |
| |
| 9. The default history file is `~/.sh_history' (this is the default |
| value of `$HISTFILE'). |
| |
| 10. The output of `kill -l' prints all the signal names on a single |
| line, separated by spaces, without the `SIG' prefix. |
| |
| 11. The `kill' builtin does not accept signal names with a `SIG' |
| prefix. |
| |
| 12. Non-interactive shells exit if FILENAME in `.' FILENAME is not |
| found. |
| |
| 13. Non-interactive shells exit if a syntax error in an arithmetic |
| expansion results in an invalid expression. |
| |
| 14. Redirection operators do not perform filename expansion on the word |
| in the redirection unless the shell is interactive. |
| |
| 15. Redirection operators do not perform word splitting on the word in |
| the redirection. |
| |
| 16. Function names must be valid shell `name's. That is, they may not |
| contain characters other than letters, digits, and underscores, and |
| may not start with a digit. Declaring a function with an invalid |
| name causes a fatal syntax error in non-interactive shells. |
| |
| 17. POSIX special builtins are found before shell functions during |
| command lookup. |
| |
| 18. If a POSIX special builtin returns an error status, a |
| non-interactive shell exits. The fatal errors are those listed in |
| the POSIX standard, and include things like passing incorrect |
| options, redirection errors, variable assignment errors for |
| assignments preceding the command name, and so on. |
| |
| 19. If `CDPATH' is set, the `cd' builtin will not implicitly append |
| the current directory to it. This means that `cd' will fail if no |
| valid directory name can be constructed from any of the entries in |
| `$CDPATH', even if the a directory with the same name as the name |
| given as an argument to `cd' exists in the current directory. |
| |
| 20. A non-interactive shell exits with an error status if a variable |
| assignment error occurs when no command name follows the assignment |
| statements. A variable assignment error occurs, for example, when |
| trying to assign a value to a readonly variable. |
| |
| 21. A non-interactive shell exits with an error status if the iteration |
| variable in a `for' statement or the selection variable in a |
| `select' statement is a readonly variable. |
| |
| 22. Process substitution is not available. |
| |
| 23. Assignment statements preceding POSIX special builtins persist in |
| the shell environment after the builtin completes. |
| |
| 24. Assignment statements preceding shell function calls persist in the |
| shell environment after the function returns, as if a POSIX |
| special builtin command had been executed. |
| |
| 25. The `export' and `readonly' builtin commands display their output |
| in the format required by POSIX. |
| |
| 26. The `trap' builtin displays signal names without the leading `SIG'. |
| |
| 27. The `trap' builtin doesn't check the first argument for a possible |
| signal specification and revert the signal handling to the original |
| disposition if it is, unless that argument consists solely of |
| digits and is a valid signal number. If users want to reset the |
| handler for a given signal to the original disposition, they |
| should use `-' as the first argument. |
| |
| 28. The `.' and `source' builtins do not search the current directory |
| for the filename argument if it is not found by searching `PATH'. |
| |
| 29. Subshells spawned to execute command substitutions inherit the |
| value of the `-e' option from the parent shell. When not in POSIX |
| mode, Bash clears the `-e' option in such subshells. |
| |
| 30. Alias expansion is always enabled, even in non-interactive shells. |
| |
| 31. When the `alias' builtin displays alias definitions, it does not |
| display them with a leading `alias ' unless the `-p' option is |
| supplied. |
| |
| 32. When the `set' builtin is invoked without options, it does not |
| display shell function names and definitions. |
| |
| 33. When the `set' builtin is invoked without options, it displays |
| variable values without quotes, unless they contain shell |
| metacharacters, even if the result contains nonprinting characters. |
| |
| 34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname |
| constructed from `$PWD' and the directory name supplied as an |
| argument does not refer to an existing directory, `cd' will fail |
| instead of falling back to PHYSICAL mode. |
| |
| 35. When the `pwd' builtin is supplied the `-P' option, it resets |
| `$PWD' to a pathname containing no symlinks. |
| |
| 36. The `pwd' builtin verifies that the value it prints is the same as |
| the current directory, even if it is not asked to check the file |
| system with the `-P' option. |
| |
| 37. When listing the history, the `fc' builtin does not include an |
| indication of whether or not a history entry has been modified. |
| |
| 38. The default editor used by `fc' is `ed'. |
| |
| 39. The `type' and `command' builtins will not report a non-executable |
| file as having been found, though the shell will attempt to |
| execute such a file if it is the only so-named file found in |
| `$PATH'. |
| |
| 40. The `vi' editing mode will invoke the `vi' editor directly when |
| the `v' command is run, instead of checking `$VISUAL' and |
| `$EDITOR'. |
| |
| 41. When the `xpg_echo' option is enabled, Bash does not attempt to |
| interpret any arguments to `echo' as options. Each argument is |
| displayed, after escape characters are converted. |
| |
| 42. The `ulimit' builtin uses a block size of 512 bytes for the `-c' |
| and `-f' options. |
| |
| |
| There is other POSIX behavior that Bash does not implement by default |
| even when in POSIX mode. Specifically: |
| |
| 1. The `fc' builtin checks `$EDITOR' as a program to edit history |
| entries if `FCEDIT' is unset, rather than defaulting directly to |
| `ed'. `fc' uses `ed' if `EDITOR' is unset. |
| |
| 2. As noted above, Bash requires the `xpg_echo' option to be enabled |
| for the `echo' builtin to be fully conformant. |
| |
| |
| Bash can be configured to be POSIX-conformant by default, by specifying |
| the `--enable-strict-posix-default' to `configure' when building (*note |
| Optional Features::). |
| |