POSIX

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 `$FCEDIT' and
     `$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.


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::).