You want to get bash for your Mac, or you want to make sure you have the latest version.
According to Chet Ramey’s bash page (http://tiswww.tis.case.edu/~chet/bash/bashtop.
html), Mac OS 10.2 (Jaguar) and newer ship with bash as /bin/sh. 10.4 (Tiger) has version 2.05b.0(1)-release (powerpc-apple-darwin8.0).
There are also precompiled OS X packages of bash-2.05 available from many web sites. One such package is at HMUG.
Bash for Darwin (the base for Mac OS X) is available from Fink or DarwinPorts.
It is also possible to build a more recent version of bash from source, but this is recommended only for experienced users.