Validate IP Addresses in Bash: Two Reliable Shell Scripts

From an operations perspective, checking whether a user‑provided value such as an IP address is syntactically correct is a common task in shell scripts. An IP consists of four decimal numbers separated by dots, each ranging from 0 to 255.

Method 1: awk + grep

#!/bin/bash
# blog: http://lizhenliang.blog.51cto.com
function check_ip() {
    IP=$1
    VALID_CHECK=$(echo $IP | awk -F. '$1<=255&&$2<=255&&$3<=255&&$4<=255{print "yes"}')
    if echo $IP | grep -E "^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$" >/dev/null; then
        if [ ${VALID_CHECK:-no} == "yes" ]; then
            echo "IP $IP available."
        else
            echo "IP $IP not available!"
        fi
    else
        echo "IP format error!"
    fi
}
# Example usage
check_ip 192.168.1.1
check_ip 256.1.1.1

This script first ensures the string matches the dotted‑decimal pattern with grep -E, then uses awk to verify each octet does not exceed 255.

Method 2: Bash regex + cut

#!/bin/bash
# blog: http://lizhenliang.blog.51cto.com
function check_ip() {
    IP=$1
    if [[ $IP =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
        FIELD1=$(echo $IP | cut -d. -f1)
        FIELD2=$(echo $IP | cut -d. -f2)
        FIELD3=$(echo $IP | cut -d. -f3)
        FIELD4=$(echo $IP | cut -d. -f4)
        if [ $FIELD1 -le 255 -a $FIELD2 -le 255 -a $FIELD3 -le 255 -a $FIELD4 -le 255 ]; then
            echo "IP $IP available."
        else
            echo "IP $IP not available!"
        fi
    else
        echo "IP format error!"
    fi
}
# Example usage
check_ip 192.168.1.1
check_ip 256.1.1.1

This version relies on Bash’s built‑in regular‑expression operator to filter the format, then extracts each octet with cut and checks the numeric range.

Looping Until a Valid IP Is Entered

#!/bin/bash
# blog: http://lizhenliang.blog.51cto.com
function check_ip() {
    local IP=$1
    VALID_CHECK=$(echo $IP | awk -F. '$1<=255&&$2<=255&&$3<=255&&$4<=255{print "yes"}')
    if echo $IP | grep -E "^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$" >/dev/null; then
        if [ "$VALID_CHECK" == "yes" ]; then
            echo "IP $IP available!"
            return 0
        else
            echo "IP $IP not available!"
            return 1
        fi
    else
        echo "IP format error!"
        return 1
    fi
}
while true; do
    read -p "Please enter IP: " IP
    check_ip $IP
    [ $? -eq 0 ] && break
    # loop repeats until a correct IP is provided
    done

The loop repeatedly prompts the user, calls check_ip, and exits only when the function returns success, ensuring the script proceeds with a valid address.