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Diffstat (limited to 'POSIX')
| -rw-r--r-- | POSIX | 186 |
1 files changed, 186 insertions, 0 deletions
@@ -0,0 +1,186 @@ +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 + `$EDITOR'. + + 41. When the `xpg_echo' option is enabled, Bash does not attempt to + interpret any arguments to `echo' as options. Each argument is + displayed, after escape characters are converted. + + 42. The `ulimit' builtin uses a block size of 512 bytes for the `-c' + and `-f' options. + + 43. The arrival of `SIGCHLD' when a trap is set on `SIGCHLD' does not + interrupt the `wait' builtin and cause it to return immediately. + The trap command is run once for each child that exits. + + +There is other POSIX behavior that Bash does not implement by default +even when in POSIX mode. Specifically: + + 1. The `fc' builtin checks `$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::). + |