Shell Programming and Scripting

This is because /bin/sh gets you "a Bourne shell" but not any particular Bourne shell. What shell you get depends on what shell your system considers a good default. If you don't use extended shell features #!/bin/sh will work on most systems (except perhaps Solaris where /bin/sh can be really, really old.)

So, if you write bash code, you should use #!/bin/bash. If you write ksh code, you should use #!/bin/ksh. #!/bin/sh should only be used for generic Bourne code.