example: 0xffffff00 => 24

netmask-bit-count

#!/usr/bin/env bash

# read from argument or stdin
mask="$1"; [[ -z "$mask" ]] && read -r mask < /dev/stdin
# remove 0x
mask="${mask##0x}"

count=0

while [ "$mask" != "" ]; do
    digit="${mask:0:1}"
    mask="${mask:1}"
    case "$digit" in
        [fF]) count=$((count+4));;
        [eE]) count=$((count+3));;
        [cC]) count=$((count+2));;
        8) count=$((count+1));;
        0) ;;
        *) echo 1>&2 "error: illegal digit $digit in netmask"; exit 1;;
    esac
done

echo $count