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

1. Function using subnetcalc:

   IPprefix_by_netmask() {
       subnetcalc 1.1.1.1 "$1" -n  | sed -n '/^Netw/{s#.*/ #/#p;q}'
   }
   

2. In pure bash, convert IP to a long octal string and sum its bits:

   IPprefix_by_netmask () { 
      c=0 x=0$( printf '%o' ${1//./ } )
      while [ $x -gt 0 ]; do
          let c+=$((x%2)) 'x>>=1'
      done
      echo /$c ; }
   

Output of IPprefix_by_netmask 255.255.255.0 (either function):

/24