Bourne Again SHell
In functionality bash goes well beyond the original Bourne shells and closely matches the extensions of David Korn's "Korn Shell" (ksh), though bash and ksh use different syntax to create a coprocess or an associative array. bash also adds a number of interactive features drawn from Bill Joy's C Shell (including the many "bang expansion operators" for things like !! (expand to entire previous command), !-2 (expand to penultimate command), !$ (expand to last argument of previous command) etc).