myhome/.local/bin/mask2cidr

#!/bin/bash
mask2cidr() {
    local mask=$1

    # In RFC 4632 netmasks there's no "255." after a non-255 byte in the mask
    local left_stripped_mask=${mask##*255.}
    local len_mask=${#mask}
    local len_left_stripped_mask=${#left_stripped_mask}

    local conversion_table=0^^^128^192^224^240^248^252^254^
    local number_of_bits_stripped=$(((len_mask - len_left_stripped_mask) * 2))
    local signifacant_octet=${left_stripped_mask%%.*}

    local right_stripped_conversion_table=${conversion_table%%$signifacant_octet*}
    local len_right_stripped_conversion_table=${#right_stripped_conversion_table}
    local number_of_bits_from_conversion_table=$((len_right_stripped_conversion_table / 4))
    echo $((number_of_bits_stripped + number_of_bits_from_conversion_table))
}

mask2cidr "$@"