-Bash POSIX Mode
-===============
+6.11 Bash POSIX Mode
+====================
-Starting Bash with the `--posix' command-line option or executing `set
+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
+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:
+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 `$PATH' to find the new location. This is also
- available with `shopt -s checkhash'.
+ re-search '$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)'.
+ 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'.
+ is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
+ 'SIGTSTP'.
- 4. Reserved words may not be aliased.
+ 4. Alias expansion is always enabled, even in non-interactive shells.
- 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. The POSIX 1003.2 startup files are executed (`$ENV') rather than
- the normal Bash files.
+ 6. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number
+ and '!!' to '!' are enabled, and parameter expansion is performed
+ on the values of 'PS1' and 'PS2' regardless of the setting of the
+ 'promptvars' option.
- 7. Tilde expansion is only performed on assignments preceding a
- command name, rather than on all assignment statements on the line.
-
- 8. The default history file is `~/.sh_history' (this is the default
- value of `$HISTFILE').
-
- 9. The output of `kill -l' prints all the signal names on a single
- line, separated by spaces, without the `SIG' prefix.
-
- 10. The `kill' builtin does not accept signal names with a `SIG'
- prefix.
+ 7. The POSIX startup files are executed ('$ENV') rather than the
+ normal Bash files.
- 11. Non-interactive shells exit if FILENAME in `.' FILENAME is not
- found.
+ 8. Tilde expansion is only performed on assignments preceding a
+ command name, rather than on all assignment statements on the line.
- 12. Non-interactive shells exit if a syntax error in an arithmetic
- expansion results in an invalid expression.
+ 9. The default history file is '~/.sh_history' (this is the default
+ value of '$HISTFILE').
- 13. Redirection operators do not perform filename expansion on the word
- in the redirection unless the shell is interactive.
+ 10. 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
+ 11. 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
+ 12. 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.
+ 13. Function names may not be the same as one of the POSIX special
+ builtins.
+
+ 14. POSIX special builtins are found before shell functions during
+ command lookup.
- 17. If a POSIX 1003.2 special builtin returns an error status, a
+ 15. Literal tildes that appear as the first character in elements of
+ the 'PATH' variable are not expanded as described above under *note
+ Tilde Expansion::.
+
+ 16. 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.
+
+ 17. When parsing and expanding a ${...} expansion that appears within
+ double quotes, single quotes are no longer special and cannot be
+ used to quote a closing brace or other special character, unless
+ the operator is one of those defined to perform pattern removal.
+ In this case, they do not have to appear as matched pairs.
+
+ 18. The parser does not recognize 'time' as a reserved word if the
+ next token begins with a '-'.
+
+ 19. The '!' character does not introduce history expansion within a
+ double-quoted string, even if the 'histexpand' option is enabled.
+
+ 20. If a POSIX special builtin returns an error status, a
non-interactive shell exits. The fatal errors are those listed in
- the POSIX.2 standard, and include things like passing incorrect
+ the POSIX standard, and include things like passing incorrect
options, redirection errors, variable assignment errors for
assignments preceding the command name, and so on.
- 18. If the `cd' builtin finds a directory to change to using
- `$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
+ 21. 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. 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.
+
+ 23. 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.
+
+ 24. Non-interactive shells exit if FILENAME in '.' FILENAME is not
+ found.
+
+ 25. Non-interactive shells exit if a syntax error in an arithmetic
+ expansion results in an invalid expression.
+
+ 26. Non-interactive shells exit on word expansion errors.
+
+ 27. 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.
- 22. Process substitution is not available.
+ 28. Process substitution is not available.
- 23. Assignment statements preceding POSIX 1003.2 special builtins
- persist in the shell environment after the builtin completes.
+ 29. While variable indirection is available, it may not be applied to
+ the '#' and '?' special parameters.
- 24. Assignment statements preceding shell function calls persist in the
- shell environment after the function returns, as if a POSIX
+ 30. Assignment statements preceding POSIX special builtins persist in
+ the shell environment after the builtin completes.
+
+ 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 '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'.
- 26. The `trap' builtin displays signal names without the leading `SIG'.
+ 33. 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.
- 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. If users want to reset the handler for a
- given signal to the original disposition, they should use `-' as
- the first argument.
+ 34. The output of 'kill -l' prints all the signal names on a single
+ line, separated by spaces, without the 'SIG' prefix.
- 28. The `.' and `source' builtins do not search the current directory
- for the filename argument if it is not found by searching `PATH'.
+ 35. The 'kill' builtin does not accept signal names with a 'SIG'
+ prefix.
- 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.
+ 36. The 'export' and 'readonly' builtin commands display their output
+ in the format required by POSIX.
- 30. Alias expansion is always enabled, even in non-interactive shells.
+ 37. The 'trap' builtin displays signal names without the leading
+ 'SIG'.
- 31. When the `alias' builtin displays alias definitions, it does not
- display them with a leading `alias ' unless the `-p' option is
+ 38. 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.
+
+ 39. The '.' and 'source' builtins do not search the current directory
+ for the filename argument if it is not found by searching 'PATH'.
+
+ 40. Enabling POSIX mode has the effect of setting the
+ 'inherit_errexit' option, so subshells spawned to execute command
+ substitutions inherit the value of the '-e' option from the parent
+ shell. When the 'inherit_errexit' option is not enabled, Bash
+ clears the '-e' option in such subshells.
+
+ 41. 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
+ 42. 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
+ 43. 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
+ 44. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
+ constructed from '$PWD' and the directory name supplied as an
+ argument does not refer to an existing directory, 'cd' will fail
instead of falling back to PHYSICAL mode.
-There is other POSIX 1003.2 behavior that Bash does not implement.
-Specifically:
+ 45. The 'pwd' builtin verifies that the value it prints is the same as
+ the current directory, even if it is not asked to check the file
+ system with the '-P' option.
+
+ 46. When listing the history, the 'fc' builtin does not include an
+ indication of whether or not a history entry has been modified.
+
+ 47. The default editor used by 'fc' is 'ed'.
+
+ 48. The 'type' and 'command' builtins will not report a non-executable
+ file as having been found, though the shell will attempt to execute
+ such a file if it is the only so-named file found in '$PATH'.
+
+ 49. The 'vi' editing mode will invoke the 'vi' editor directly when
+ the 'v' command is run, instead of checking '$VISUAL' and
+ '$EDITOR'.
+
+ 50. 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.
+
+ 51. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
+ and '-f' options.
+
+ 52. 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.
+
+ 53. 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.
+
+ 54. Bash removes an exited background process's status from the list
+ of such statuses after the 'wait' builtin is used to obtain it.
- 1. Assignment statements affect the execution environment of all
- builtins, not just special ones.
+There is other POSIX behavior that Bash does not implement by default
+even when in POSIX mode. Specifically:
- 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.
+ 1. The 'fc' builtin checks '$EDITOR' as a program to edit history
+ entries if 'FCEDIT' is unset, rather than defaulting directly to
+ 'ed'. 'fc' uses 'ed' if 'EDITOR' is unset.
- 3. When using `.' to source a shell script found in `$PATH', bash
- checks execute permission bits rather than read permission bits,
- just as if it were searching for a command.
+ 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::).
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