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

#!/bin/bash
echo $((2 - 1))

Which should return the following.

1

Or like this.

#!/bin/bash
sum=$((2 - 1))
echo $sum

Or like this.

#!/bin/bash
x=2
y=1
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