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.

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 $PATH to find the new location. This is also available with 'shopt -s checkhash'.
2. The message printed by the job control code and builtins when a job exits with a non-zero status is `Done(status)'.
3. The message printed by the job control code and builtins when a job is stopped is `Stopped(signame)', where signame is, for example, SIGTSTP.
4. Reserved words may not be aliased.
5. The POSIX 1003.2 PS1 and PS2 expansions of '!' to the history number and '!!' to '!' are enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option.
6. Interactive comments are enabled by default. (Bash has them on by default anyway.)
7. The POSIX 1003.2 startup files are executed ($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.
11. Non-interactive shells exit if filename in . filename is not found.
12. 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 in the redirection unless the shell is interactive.
14. Redirection operators do not perform word splitting on the word in the redirection.
15. Function names must be valid shell names. 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.
17. If a POSIX 1003.2 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 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 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 1003.2 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 1003.2.
26. The trap builtin displays signal names without the leading SIG.
27. 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 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.
30. 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 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 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:

1. Assignment statements affect the execution environment of all builtins, not just special ones.
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.
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.