How to use expr on float?

bc will do this for you, but the order is important.

> echo "scale = 2; 20 * 100 / 30" | bc
66.66
> echo "scale = 2; 20 / 30 * 100" | bc
66.00

or, for your specific case:

> export ach_gs=2
> export ach_gs_max=3
> x=$(echo "scale = 2; $ach_gs * 100 / $ach_gs_max" | bc)
> echo $x
66.66

Whatever method you choose, this is ripe for inclusion as a function to make your life easier:

#!/bin/bash
function pct () {
    echo "scale = $3; $1 * 100 / $2" | bc
}

x=$(pct 2 3 2) ; echo $x # gives 66.66
x=$(pct 1 6 0) ; echo $x # gives 16

just do it in awk

# awk 'BEGIN{print 20 / 30 * 100}'
66.6667

save it to variable

# result=$(awk 'BEGIN{print 20 / 30 * 100}')
# echo $result
66.6667

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