HACKTUX notes from the trenches

For floating point math calculations in Bash, use /usr/bin/bc.
A simple calculator program might look something like this:

#!/bin/bash
echo "scale=2; $@" | /usr/bin/bc -l
exit $?

Alternatively, a calculator function:

calc()
{
   echo "scale=2; $@" | /usr/bin/bc -l
   return $?
}

If you are looking for more, here are other ways to do math in Bash.