-Bash POSIX Mode
-===============
+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 1003.2 standard by changing the behavior to match that
-specified by POSIX in areas where the Bash default differs.
+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.
is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example,
`SIGTSTP'.
- 4. Reserved words may not be aliased.
+ 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. The POSIX 1003.2 `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.
+ 5. Reserved words appearing in a context where reserved words are
+ recognized do not undergo alias expansion.
- 6. Interactive comments are enabled by default. (Bash has them on by
- default anyway.)
+ 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 1003.2 startup files are executed (`$ENV') rather than
- the normal Bash files.
+ 7. The POSIX startup files are executed (`$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
+ 9. The `command' builtin does not prevent builtins that take
+ assignment statements as arguments from expanding them as
+ assignment statements; when not in POSIX mode, assignment builtins
+ lose their assignment statement expansion properties when preceded
+ by `command'.
+
+ 10. 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.
+ 11. The output of `kill -l' prints all the signal names on a single
+ line, separated by spaces, without the `SIG' prefix.
+
+ 12. The `kill' builtin does not accept signal names with a `SIG'
+ prefix.
- 11. Non-interactive shells exit if FILENAME in `.' FILENAME is not
+ 13. Non-interactive shells exit if FILENAME in `.' FILENAME is not
found.
- 12. Non-interactive shells exit if a syntax error in an arithmetic
+ 14. Non-interactive shells exit if a syntax error in an arithmetic
expansion results in an invalid expression.
- 13. Redirection operators do not perform filename expansion on the word
+ 15. Non-interactive shells exit if there is a syntax error in a script
+ read with the `.' or `source' builtins, or in a string processed by
+ the `eval' builtin.
+
+ 16. Redirection operators do not perform filename expansion on the word
in the redirection unless the shell is interactive.
- 14. Redirection operators do not perform word splitting on the word in
+ 17. Redirection operators do not perform word splitting on the word in
the redirection.
- 15. Function names must be valid shell `name's. That is, they may not
+ 18. 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.
- 16. POSIX 1003.2 `special' builtins are found before shell functions
- during command lookup.
+ 19. Function names may not be the same as one of the POSIX special
+ builtins.
+
+ 20. POSIX special builtins are found before shell functions during
+ command lookup.
+
+ 21. The `time' reserved word may be used by itself as a command. When
+ used in this way, it displays timing statistics for the shell and
+ its completed children. The `TIMEFORMAT' variable controls the
+ format of the timing information.
+
+ 22. When parsing and expanding a ${...} expansion that appears within
+ double quotes, single quotes are no longer special and cannot be
+ used to quote a closing brace or other special character, unless
+ the operator is one of those defined to perform pattern removal.
+ In this case, they do not have to appear as matched pairs.
- 17. If a POSIX 1003.2 special builtin returns an error status, a
+ 23. The parser does not recognize `time' as a reserved word if the next
+ token begins with a `-'.
+
+ 24. If a POSIX special builtin returns an error status, a
non-interactive shell exits. The fatal errors are those listed in
- the POSIX.2 standard, and include things like passing incorrect
+ the POSIX standard, and include things like passing incorrect
options, redirection errors, variable assignment errors for
assignments preceding the command name, and so on.
- 18. If the `cd' builtin finds a directory to change to using
- `$CDPATH', the value it assigns to the `PWD' variable does not
- contain any symbolic links, as if `cd -P' had been executed.
-
- 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
+ 25. 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
+ 26. A non-interactive shell exits with an error status if a variable
+ assignment error occurs in an assignment statement preceding a
+ special builtin, but not with any other simple command.
+
+ 27. 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.
+ 28. Process substitution is not available.
+
+ 29. While variable indirection is available, it may not be applied to
+ the `#' and `?' special parameters.
- 23. Assignment statements preceding POSIX 1003.2 special builtins
- persist in the shell environment after the builtin completes.
+ 30. 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
+ 31. 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 1003.2.
+ 32. The `export' and `readonly' builtin commands display their output
+ in the format required by POSIX.
+
+ 33. The `trap' builtin displays signal names without the leading `SIG'.
- 26. The `trap' builtin displays signal names without the leading `SIG'.
+ 34. 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.
- 27. The `.' and `source' builtins do not search the current directory
+ 35. The `.' and `source' builtins do not search the current directory
for the filename argument if it is not found by searching `PATH'.
- 28. Subshells spawned to execute command substitutions inherit the
+ 36. 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.
- 29. Alias expansion is always enabled, even in non-interactive shells.
+ 37. Alias expansion is always enabled, even in non-interactive shells.
- 30. When the `set' builtin is invoked without options, it does not
+ 38. When the `alias' builtin displays alias definitions, it does not
+ display them with a leading `alias ' unless the `-p' option is
+ supplied.
+
+ 39. When the `set' builtin is invoked without options, it does not
display shell function names and definitions.
- 31. When the `set' builtin is invoked without options, it displays
+ 40. 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.
- 32. When the `cd' builtin is invoked in LOGICAL mode, and the pathname
+ 41. 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.
-There is other POSIX 1003.2 behavior that Bash does not implement.
-Specifically:
+ 42. 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.
+
+ 43. When listing the history, the `fc' builtin does not include an
+ indication of whether or not a history entry has been modified.
+
+ 44. The default editor used by `fc' is `ed'.
+
+ 45. 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
+ `$PATH'.
+
+ 46. The `vi' editing mode will invoke the `vi' editor directly when
+ the `v' command is run, instead of checking `$VISUAL' and
+ `$EDITOR'.
+
+ 47. When the `xpg_echo' option is enabled, Bash does not attempt to
+ interpret any arguments to `echo' as options. Each argument is
+ displayed, after escape characters are converted.
+
+ 48. The `ulimit' builtin uses a block size of 512 bytes for the `-c'
+ and `-f' options.
+
+ 49. The arrival of `SIGCHLD' when a trap is set on `SIGCHLD' does not
+ interrupt the `wait' builtin and cause it to return immediately.
+ The trap command is run once for each child that exits.
+
+ 50. The `read' builtin may be interrupted by a signal for which a trap
+ has been set. If Bash receives a trapped signal while executing
+ `read', the trap handler executes and `read' returns an exit
+ status greater than 128.
+
+
+There is other POSIX behavior that Bash does not implement by default
+even when in POSIX mode. Specifically:
- 1. Assignment statements affect the execution environment of all
- builtins, not just special ones.
+ 1. The `fc' builtin checks `$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. When a subshell is created to execute a shell script with execute
- permission, but without a leading `#!', Bash sets `$0' to the full
- pathname of the script as found by searching `$PATH', rather than
- the command as typed by the user.
+ 2. As noted above, Bash requires the `xpg_echo' option to be enabled
+ for the `echo' builtin to be fully conformant.
- 3. When using `.' to source a shell script found in `$PATH', bash
- checks execute permission bits rather than read permission bits,
- just as if it were searching for a command.
+Bash can be configured to be POSIX-conformant by default, by specifying
+the `--enable-strict-posix-default' to `configure' when building (*note
+Optional Features::).
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