POSIX Is Not a Shell

POSIX is a specification, not an actual shell program, and different shell implementations (bash, dash, ksh, etc.) interpret it inconsistently—for example, their `echo` commands handle backslash escapes differently despite both claiming POSIX compliance. True portability requires testing scripts across the specific shells they'll actually run on rather than assuming "POSIX compliance" guarantees identical behavior, as the author demonstrates through a cross-shell validation tool that reveals divergences in common features.