Check for IP validity

Question

How do I check the validity of an IP address in a shell script, that is within the range 0.0.0.0 to 255.255.255.255?

Answer 1

If you're using bash, you can do a simple regex match for the pattern, without validating the quads:

#!/usr/bin/env bash  ip=1.2.3.4  if [[ $ip =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then   echo "success" else   echo "fail" fi 

If you're stuck with a POSIX shell, then you can use expr to do basically the same thing, using BRE instead of ERE:

#!/bin/sh  ip=1.2.3.4  if expr "$ip" : '[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' >/dev/null; then   echo "success" else   echo "fail" fi 

Note that expr assumes that your regex is anchored to the left-hand-side of the string, so the initial ^ is unnecessary.

If it's important to verify that each quad is less than 256, you'll obviously require more code:

#!/bin/sh  ip=${1:-1.2.3.4}  if expr "$ip" : '[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' >/dev/null; then   for i in 1 2 3 4; do     if [ $(echo "$ip" | cut -d. -f$i) -gt 255 ]; then       echo "fail ($ip)"       exit 1     fi   done   echo "success ($ip)"   exit 0 else   echo "fail ($ip)"   exit 1 fi 

Or perhaps even with fewer pipes:

#!/bin/sh  ip=${1:-1.2.3.4}  if expr "$ip" : '[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' >/dev/null; then   IFS=.   set $ip   for quad in 1 2 3 4; do     if eval [ \$$quad -gt 255 ]; then       echo "fail ($ip)"       exit 1     fi   done   echo "success ($ip)"   exit 0 else   echo "fail ($ip)"   exit 1 fi 

Or again, if your shell is bash, you could use a cumbersome regular expression for quad validation if you're not fond of arithmetic:

#!/usr/bin/env bash  ip=${1:-1.2.3.4}  re='^(0*(1?[0-9]{1,2}|2([0-4][0-9]|5[0-5]))\.){3}'  re+='0*(1?[0-9]{1,2}|2([‌​0-4][0-9]|5[0-5]))$'  if [[ $ip =~ $re ]]; then   echo "success" else   echo "fail" fi 

This could also be expressed in BRE, but that's more typing than I have in my fingers.

And lastly, if you like the idea of putting this functionality ... in a function:

#!/usr/bin/env bash  ip=${1:-1.2.3.4}  ipvalid() {   # Set up local variables   local ip=${1:-1.2.3.4}   local IFS=.; local -a a=($ip)   # Start with a regex format test   [[ $ip =~ ^[0-9]+(\.[0-9]+){3}$ ]] || return 1   # Test values of quads   local quad   for quad in {0..3}; do     [[ "${a[$quad]}" -gt 255 ]] && return 1   done   return 0 }  if ipvalid "$ip"; then   echo "success ($ip)"   exit 0 else   echo "fail ($ip)"   exit 1 fi 

There are many ways you could do this. I've shown you just a few.