Bash - Convert netmask in CIDR notation?

Question

Example: I have this netmask: 255.255.255.0

Is there, in bash, a command or a simple script to convert my netmask in notation /24?

Answer 1

Example Function for RHEL6/RHEL7:

IPprefix_by_netmask() {
#function returns prefix for given netmask in arg1
 ipcalc -p 1.1.1.1 $1 | sed -n 's/^PREFIX=\(.*\)/\/\1/p'
}

The Result:

$ IPprefix_by_netmask 255.255.255.0
/24

In other Linux distributives ipcalc options may differ.

The same function without ipcalc, tested in Solaris and Linux:

IPprefix_by_netmask() {
    #function returns prefix for given netmask in arg1
    bits=0
    for octet in $(echo $1| sed 's/\./ /g'); do 
         binbits=$(echo "obase=2; ibase=10; ${octet}"| bc | sed 's/0//g') 
         let bits+=${#binbits}
    done
    echo "/${bits}"
}