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}"
}

Answer 2

1. Function using subnetcalc:

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

2. In pure bash, (i.e. no external utils like sed or bc), 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

Answer 3

Based on Sasha's answer, this script works with dash (tested with Ubuntu 18.04):

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; ${octet}"| bc | sed 's/0//g') 
         bits=$(expr $bits + ${#binbits})
    done
    echo "/${bits}"
}

Answer 4

Solution based on awk

While GNU awk is not Bash, it’s installed by default in enough distributions that this may be helpful in the sense of the question:

awk -F. '{
    split($0, octets)
    for (i in octets) {
        mask += 8 - log(2**8 - octets[i])/log(2);
    }
    print "/" mask
}' <<< 255.255.255.240

This prints:

/28