Shell script function to validate an IP address (v4).

#
# Check for IP validity
# http://stackoverflow.com/a/13778973
is_ip() {
    local ip=$1

    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
                return 1
            fi
        done
        return 0
    else
        return 1
    fi
}

# 
# Usage:
is_ip '0.0.0.1'
if [[ $? -eq 0 ]]; then
    echo "Valid IP4 address."
else
    echo "Invalid IP4 address."
fi