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

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))

In case you do not need floating point precision, you may simply strip off the decimal part.

echo $var | cut -d "." -f 1 | cut -d "," -f 1

cuts the integer part of the value. The reason to use cut twice is to parse integer part in case a regional setting may use dots to separate decimals and some others may use commas.

Edit:

Or, to automate the regional settings one may use locale.

echo $var | cut -d $(locale decimal_point) -f 1

Another way to calculate floating numbers, is by using AWK rounding capability, for example:

a=502.709672592
b=501.627497268
echo "$a $b" | awk '{print $1 - $2}'

1.08218