不以bash打印十进制值？

Question

I want to evaluate the expression and display the output correct to 3 decimal places i tried with below code but it is not working help me how to do that in bash.

echo -e "Enter expression to calculate : \c"
read num
let a=num
printf '%f\n' "$a"

Input :  5+50*3/20 + (19*2)/7

Output :  17.000000

Desired Output : 17.929

Answer 1

With bc :

echo 'scale=3; 5+50*3/20 + (19*2)/7' | bc -l

Output:

17.928

Answer 2

Instead of let a=num , you could for example:

echo $num | bc 
17.92857142857142857142

Answer 3

This is one of the very rare cases where it's acceptable to let a shell variable expand to become part of the body of an awk script:

$ num='5+50*3/20 + (19*2)/7'

$ awk 'BEGIN{print '"$num"'}'
17.9286

$ awk 'BEGIN{printf "%0.3f\n", '"$num"'}'
17.929