Extracting netmask from ifconfig output and printing it in CIDR format

Question

I need to extract the netmask for two sets of IPs and print it in CIDR format, out on to the screen. Thinking of using either shell or tcl to do this. Guess the major sticky points here is converting the hexadecimal netmask into CIDR format.

Tried a few different things, but does not seem to be working as expected.

Answer 1

We can just write a function that iterates through the digits of the mask, adding up the number of bits set in each digit. There are only five legal digits in a CIDR-compliant netmask.

Bash has the ${variable:offset} and ${variable:offset:length} substitutions for getting substrings of a variable, so it's pretty easy to pick off the digits one at a time.

function bitCountForMask {
    local -i count=0
    local mask="${1##0x}"
    local digit

    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"
                return 1
                ;;
        esac
    done

    echo $count
}

Answer 2

Just found out that there's a tcl package utility which allows us to do this.

package require ip
::ip::maskToLength <hex_netmask> 

This will output the CIDR format of the netmask.