Why let command doesn't work to add real numbers?

Question

I need to add some real numbers in a script. I tried:

let var=2.5+2.5 

which gives me an error - invalid arithmetic operator and then I tried:

let var=2,5+2,5 

which does not give me an error, but it gives me a wrong result -2 in this case.

Why? How to add real numbers using let or other command?

Answer 1

The first variant (let var=2.5+2.5) doesn't work because bash doesn't support floating point.

The second variant (let var=2,5+2,5) works, but it may not be what you wish because comma has another meaning in this case: it's a separator, command separator. So, your command is equivalent in this case with the following three commands:

let var=2
let 5+2
let 5

which are all valid and because of this you get var=2.

But, the good news is that you can use bc to accomplish what you wish. For example:

var=$(bc <<< "2.5+2.5")

Or awk:

var=$(awk "BEGIN {print 2.5+2.5; exit}")

Or perl:

var=$(perl -e "print 2.5+2.5")

Or python:

var=$(python -c "print 2.5+2.5")