Bash - Convert netmask in CIDR notation?

Bash - Convert netmask in CIDR notation?

Converting a netmask to CIDR notation in Bash involves counting the number of '1' bits in the netmask, which corresponds to the prefix length of the CIDR notation. Here's a Bash function that can perform this conversion:

#!/bin/bash # Function to convert netmask to CIDR notation netmask_to_cidr() { # Check if exactly one argument is provided if [ "$#" -ne 1 ]; then echo "Usage: netmask_to_cidr <netmask>" return 1 fi # Validate the netmask format (xxx.xxx.xxx.xxx) if ! [[ "$1" =~ ^([0-9]{1,3}\.){3}[0-9]{1,3}$ ]]; then echo "Invalid netmask format. Expected format: xxx.xxx.xxx.xxx" return 1 fi # Count '1' bits in the netmask cidr=0 IFS='.' read -r -a octets <<< "$1" for octet in "${octets[@]}"; do case "$octet" in 255) cidr=$((cidr + 8)) ;; 254) cidr=$((cidr + 7)) ;; 252) cidr=$((cidr + 6)) ;; 248) cidr=$((cidr + 5)) ;; 240) cidr=$((cidr + 4)) ;; 224) cidr=$((cidr + 3)) ;; 192) cidr=$((cidr + 2)) ;; 128) cidr=$((cidr + 1)) ;; 0) continue ;; *) echo "Invalid netmask."; return 1 ;; esac done echo "$cidr" } # Example usage: netmask="255.255.255.0" cidr=$(netmask_to_cidr "$netmask") echo "Netmask $netmask is equivalent to CIDR notation /$cidr"

Explanation:

  • Function netmask_to_cidr():

    • Checks if exactly one argument (netmask) is provided.
    • Validates the netmask format using a regular expression.
    • Splits the netmask into octets (IFS='.' read -r -a octets <<< "$1").
    • Uses a case statement to count the number of '1' bits in each octet, adding the corresponding value to cidr.
    • Prints the resulting CIDR notation prefix length ($cidr).

  • Example Usage:

    • Defines a netmask (255.255.255.0).
    • Calls netmask_to_cidr() with the netmask and assigns the result to cidr.
    • Prints the equivalent CIDR notation.

Notes:

  • This script assumes IPv4 netmasks and does not handle IPv6.
  • Ensure the netmask format (xxx.xxx.xxx.xxx) is strictly followed for accurate results.
  • Adjust the function as needed for specific requirements or additional validation.
  • Error handling includes checking the number of arguments and validating the netmask format.

This function provides a practical way to convert a netmask to CIDR notation within a Bash script, facilitating network configuration and administration tasks.

Examples

  1. "Bash convert netmask to CIDR notation script"

    • Description: This script converts a netmask in dotted-decimal format to CIDR notation.
    • Code:
      #!/bin/bash # Converts a netmask to CIDR notation netmask_to_cidr() { local netmask=$1 local octets=($(echo $netmask | tr '.' ' ')) local cidr=0 for octet in "${octets[@]}"; do while [ $octet -gt 0 ]; do if [ $((octet & 1)) -eq 1 ]; then cidr=$((cidr + 1)) fi octet=$((octet >> 1)) done done echo $cidr } # Example usage netmask="255.255.255.0" cidr=$(netmask_to_cidr $netmask) echo "CIDR notation for $netmask is /$cidr"

  2. "Bash script to convert subnet mask to CIDR"

    • Description: This script converts a subnet mask to CIDR notation by calculating the number of bits set to 1.
    • Code:
      #!/bin/bash # Converts a subnet mask to CIDR notation subnet_mask_to_cidr() { local mask=$1 local binary_mask=$(echo $mask | awk -F. '{print sprintf("%08b%08b%08b%08b", $1, $2, $3, $4)}') local cidr=$(echo -n $binary_mask | grep -o "1" | wc -l) echo $cidr } # Example usage subnet_mask="255.255.255.192" cidr=$(subnet_mask_to_cidr $subnet_mask) echo "CIDR notation for $subnet_mask is /$cidr"

  3. "Convert dotted-decimal netmask to CIDR using Bash"

    • Description: This script converts a netmask from dotted-decimal notation to CIDR notation by analyzing each octet.
    • Code:
      #!/bin/bash # Converts dotted-decimal netmask to CIDR notation convert_to_cidr() { local netmask=$1 local binary="" IFS='.' read -r -a octets <<< "$netmask" for octet in "${octets[@]}"; do binary=$(printf "%08d" $(echo "obase=2; $octet" | bc)) binary=$binary done local cidr=$(echo -n $binary | grep -o '1' | wc -l) echo $cidr } # Example usage netmask="255.255.255.128" cidr=$(convert_to_cidr $netmask) echo "CIDR notation for $netmask is /$cidr"

  4. "Bash convert netmask to prefix length"

    • Description: This script converts a netmask to prefix length (CIDR notation) by counting the number of 1s in the binary representation.
    • Code:
      #!/bin/bash # Converts netmask to prefix length (CIDR notation) netmask_to_prefix() { local netmask=$1 local prefix_length=0 IFS=. read -r i1 i2 i3 i4 <<< "$netmask" for i in $i1 $i2 $i3 $i4; do while [ $i -gt 0 ]; do ((i &= (i - 1))) ((prefix_length++)) done done echo $prefix_length } # Example usage netmask="255.255.254.0" prefix_length=$(netmask_to_prefix $netmask) echo "Prefix length for $netmask is /$prefix_length"

  5. "Convert subnet mask to CIDR notation with Bash function"

    • Description: This function converts a subnet mask to CIDR notation, returning the prefix length.
    • Code:
      #!/bin/bash # Function to convert subnet mask to CIDR notation subnet_mask_to_cidr() { local mask=$1 local bin_mask=$(printf '%08b%08b%08b%08b\n' $(echo $mask | tr '.' ' ')) local cidr=$(echo -n $bin_mask | tr -d '0' | wc -c) echo $cidr } # Example usage mask="255.255.240.0" cidr=$(subnet_mask_to_cidr $mask) echo "CIDR notation for $mask is /$cidr"

  6. "Convert netmask to CIDR using bit manipulation in Bash"

    • Description: This script uses bit manipulation to convert a netmask to CIDR notation.
    • Code:
      #!/bin/bash # Converts netmask to CIDR using bit manipulation netmask_to_cidr() { local netmask=$1 local cidr=0 IFS=. read -r i1 i2 i3 i4 <<< "$netmask" for i in $i1 $i2 $i3 $i4; do while [ $i -ne 0 ]; do ((cidr++)) i=$((i & (i - 1))) done done echo $cidr } # Example usage netmask="255.255.248.0" cidr=$(netmask_to_cidr $netmask) echo "CIDR notation for $netmask is /$cidr"

  7. "Bash netmask to CIDR converter with error handling"

    • Description: This script includes error handling for invalid netmask formats when converting to CIDR notation.
    • Code:
      #!/bin/bash # Converts netmask to CIDR with error handling netmask_to_cidr() { local netmask=$1 local binary="" if [[ ! $netmask =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then echo "Invalid netmask format" return 1 fi IFS='.' read -r -a octets <<< "$netmask" for octet in "${octets[@]}"; do binary=$(printf "%08b" $octet) binary+="$binary" done local cidr=$(echo -n $binary | grep -o '1' | wc -l) echo $cidr } # Example usage netmask="255.255.0.0" cidr=$(netmask_to_cidr $netmask) echo "CIDR notation for $netmask is /$cidr"

  8. "Convert subnet mask to CIDR in Bash with awk"

    • Description: This script uses awk to convert a subnet mask to CIDR notation by processing each octet.
    • Code:
      #!/bin/bash # Converts subnet mask to CIDR using awk subnet_mask_to_cidr() { local mask=$1 local binary_mask=$(echo $mask | awk -F. '{printf("%08b%08b%08b%08b", $1, $2, $3, $4)}') local cidr=$(echo $binary_mask | awk '{print length($0) - length(gensub(/0/, "", "g", $0))}') echo $cidr } # Example usage mask="255.255.128.0" cidr=$(subnet_mask_to_cidr $mask) echo "CIDR notation for $mask is /$cidr"

  9. "Calculate CIDR prefix length from netmask in Bash"

    • Description: This script calculates the CIDR prefix length from a given netmask by counting the number of 1s.
    • Code:
      #!/bin/bash # Calculates CIDR prefix length from netmask calculate_cidr() { local netmask=$1 local binary_mask=$(echo $netmask | awk -F. '{printf("%08b%08b%08b%08b", $1, $2, $3, $4)}') local prefix_length=$(echo $binary_mask | grep -o "1" | wc -l) echo $prefix_length } # Example usage netmask="255.255.252.0" prefix_length=$(calculate_cidr $netmask) echo "CIDR prefix length for $netmask is /$prefix_length"

  10. "Bash function to convert network mask to CIDR"

    • Description: This function converts a network mask to CIDR notation and is useful for network-related scripting tasks.
    • Code:
      #!/bin/bash # Function to convert network mask to CIDR network_mask_to_cidr() { local mask=$1 local binary_mask=$(echo $mask | awk -F. '{printf("%08b%08b%08b%08b", $1, $2, $3, $4)}') local cidr=$(echo -n $binary_mask | grep -o '1' | wc -l) echo $cidr } # Example usage network_mask="255.255.224.0" cidr=$(network_mask_to_cidr $network_mask) echo "CIDR notation for $network_mask is /$cidr"

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