Sure, but what's wrong with his approach? POSIX sh supports all the things you need to do 'if GNU grep run this, else if BSD grep run that' - functions, case s

Sure, but what's wrong with his approach? POSIX sh supports all the things you need to do 'if GNU grep run this, else if BSD grep run that' - functions, case statements, conditionals. He's not talking about building up major libraries in shell, just wrapping the invocations in functions based on the environment you're in.