Arithmetic operators are simple calculations, like addition, subtraction, multiplication, and etc. This is simple and straightforward topic, I believe no need to explain here. Because we all did calculations in our school.
Your expression goes inside the 2 round brackets with the dollar prefix.
$( command )means shell execution.
$(( expression ))means do some calculation.
Here are few examples for you to understand more.
1 2 3 4 5 6 7 8 9 10 11 12 echo $(( expression )) # dummy expression echo $(( 3 + 2 )) # simple expression, without variable # calculation with variable number=3 echo $((number += 2)) # addition echo $((number -= 2)) # subtract echo $((number *= 2)) # multiplication echo $((number /= 2)) # division echo $((number %= 2)) # modulus or remainder echo $((number++)) # increament echo $((number--)) # descrement