Here is a basic example of how to multiply whole numbers in a Bash Shell script.

#!/bin/bash
echo $((4 * 2))

Which should return the following.

8

Or like this.

#!/bin/bash
sum=$((4 * 2))
echo $sum

Or like this.

#!/bin/bash
x=4
y=2
sum=$((x - y))
echo $sum

decimal

The following can be used when dealing with non-whole numbers (decimals), piping to the bc command.

#!/bin/bash
x="2.47"
y="1.178"
sum=$(echo "$x * $y" | bc)
echo $sum

Or using the awk command.

#!/bin/bash
x="2.47"
y="1.178"
sum=$(awk "BEGIN{print $x * $y}")
echo $sum