> The program [ in Debian supports -o. Do you mean a POSIX shell can > include a built-in '[' command that does not support this option ? > Can it include a built-in 'test' command ? Yes, [ and test are almost always builtins. The binary operators -a and -o are X/Open extensions to test/[. test or [, whether built-in or a strict POSIX version in /usr/local/bin, are allowed to fail on such input.