Validating an IP Address in a Bash Script

I've recently written about using bash arrays and bash regular expressions, so here's a more useful example of using them to test IP addresses for validity.

To belabor the obvious: IP addresses are 32 bit values written as four numbers (the individual bytes of the IP address) separated by dots (periods). Each of the four numbers has a valid range of 0 to 255.

The following bash script contains a bash function which returns true if it is passed a valid IP address and false otherwise. In bash speak true means it exits with a zero status, anything else is false. The status of a command/function is stored in the bash variable "$?".

#!/bin/bash

# Test an IP address for validity:
# Usage:
#      valid_ip IP_ADDRESS
#      if [[ $? -eq 0 ]]; then echo good; else echo bad; fi
#   OR
#      if valid_ip IP_ADDRESS; then echo good; else echo bad; fi
#
function valid_ip()
{
    local  ip=$1
    local  stat=1

    if [[ $ip =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
        OIFS=$IFS
        IFS='.'
        ip=($ip)
        IFS=$OIFS
        [[ ${ip[0]} -le 255 && ${ip[1]} -le 255 \
            && ${ip[2]} -le 255 && ${ip[3]} -le 255 ]]
        stat=$?
    fi
    return $stat
}

# If run directly, execute some tests.
if [[ "$(basename $0 .sh)" == 'valid_ip' ]]; then
    ips='
        4.2.2.2
        a.b.c.d
        192.168.1.1
        0.0.0.0
        255.255.255.255
        255.255.255.256
        192.168.0.1
        192.168.0
        1234.123.123.123
        '
    for ip in $ips
    do
        if valid_ip $ip; then stat='good'; else stat='bad'; fi
        printf "%-20s: %s\n" "$ip" "$stat"
    done
fi

If you save this script as "valid_ip.sh" and then run it directly it will run some tests and prints the results:

  # sh valid_ip.sh
  4.2.2.2             : good
  a.b.c.d             : bad
  192.168.1.1         : good
  0.0.0.0             : good
  255.255.255.255     : good
  255.255.255.256     : bad
  192.168.0.1         : good
  192.168.0           : bad
  1234.123.123.123    : bad

In the function valid_ip, the if statement uses a regular expression to make sure the subject IP address consists of four dot separated numbers:

  if [[ $ip =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
If that test passes then the code inside the if statement separates the subject IP address into four parts at the dots and places the parts in an array:
  OIFS=$IFS
  IFS='.'
  ip=($ip)
  IFS=$OIFS
It does this by momentarily changing bash's Internal Field Separator variable so that rather than parsing words as whitespace separated items, bash parses them as dot separated. Putting the value of the subject IP address inside parenthesis and assigning it to itself thereby turns it into an array where each dot separated number is assigned to an array slot. Now the individual pieces are tested to make sure they're all less than or equal to 255 and the status of the test is saved so that it can be returned to the caller:
  [[ ${ip[0]} -le 255 && ${ip[1]} -le 255 \
          && ${ip[2]} -le 255 && ${ip[3]} -le 255 ]]
  stat=$?
Note that there's no need to test that the numbers are greater than or equal to zero because the regular expression test has already eliminated any thing that doesn't consist of only dots and digits.

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