shell - "Invalid Arithmetic Operator" when doing floating-point math in bash

Question

Here is my script:

d1=0.003
d2=0.0008
d1d2=$((d1 + d2))

mean1=7
mean2=5
meandiff=$((mean1 - mean2))

echo $meandiff
echo $d1d2

But instead of getting my intended output of:

0.0038
2

I am getting the error Invalid Arithmetic Operator, (error token is ".003")?

Answer 1

bash does not support floating-point arithmetic. You need to use an external utility like bc.

# Like everything else in shell, these are strings, not
# floating-point values
d1=0.003
d2=0.0008

# bc parses its input to perform math
d1d2=$(echo "$d1 + $d2" | bc)

# These, too, are strings (not integers)
mean1=7
mean2=5

# $((...)) is a built-in construct that can parse
# its contents as integers; valid identifiers
# are recursively resolved as variables.
meandiff=$((mean1 - mean2))