Not only is it guaranteed by POSIX, it's also a built-in in bash. (And the author, I believe, is writing bash, not generic shell: the shebang in part 1 is to ba

Not only is it guaranteed by POSIX, it's also a built-in in bash. (And the author, I believe, is writing bash, not generic shell: the shebang in part 1 is to bash, and "local", which appears in this article, is not defined by POSIX.) I suppose there could be an (early?) version of bash that it wasn't built it (and then the POSIX argument applies); but OS X ships a version of bash over a decade old (2007), and it's a builtin there, so it's been there for at least that long…