Bash supports arithmetic but it does not support floating point arithmetic.
For example
bash$ echo $((1 + 1))
2
bash $ echo $((1 + 1.1))
-bash: 1 + 1.1: syntax error in expression (error token is ".1")
Bash will treat the floating point number as a string.
However, the utility bc does handle floating point arithmetic and can
be called from within bash scripts.
Here we call it from the command line:
bash$ echo "1+1" | bc
2
bash$ echo "1+1.1" | bc
2.1