This may come as a surprise, but Bash only supports integer arithmetic natively. If you need to perform calculations on floating point numbers, you will need to call out to a utility program like bc or Python.
As a quick proof to yourself, try multiplying integers, then do the same with floating point numbers.
$ echo $((2*3)) 6 $ echo $((2.5*3)) bash: 2.5*3: syntax error: invalid arithmetic operator (error token is ".5*3")
To perform floating point operations, one way is to use the simple calculator utility, bc.
$ echo "2.5*3" | bc
7.5
# formatting using awk
$ echo "2.5*3" | bc | awk '{ printf("%.4f\n",$1) '}
7.500
Another way is to use Python.
python -c "print('{0:0.4f}'.format(2.5*3))"
And yet another way is to use awk.
awk 'BEGIN {printf "%.4f\n", 2.5*3}'
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