Shell script ip address format check.
 

I have a script that receives an ip address from the user and then sets up a ifcfg-bond0 file. I want to check to user input, to make sure it is in ip address format.

I'm guessing I need to use a regular expression. But I have't implemented one before. It basically needs to check for 4 sets of numbers with three dots in between, it can check that each number is between 0 and 255, but that isn't as important.

Thanks

Chris

This regexp will do what you want: 4 sets of numbers in the range 0 - 255, seperated by a dot.

\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b

Using egrep you can determine if the given ip is correct. Short example:

$ echo "1.2.3.4" | egrep '<regexp>'
1.2.3.4

$ echo "256.2.3.4" | egrep '<regexp>'
<empty = incorrect>

How the regexp works:

\b
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
\.
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
\.
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
\.
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
\b

The \b at the beginning and end are wordboundries and needed to limit the range (greediness).
The (25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?) part (4 sets of them) make sure that the range for each set is between 0 and 255.
The \., three times, are for the dot seperating the 4 sets of numbers.

Hope this gets you going again.

Works a treat. I've set it out like this:

echo -n "Please enter the IP Address for this machine: "
read ipaddr

regex="\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b"

cd /tmp

touch check_file

echo $ipaddr | egrep $regex | tee check_file

until -s check_file
do
echo -n "Incorrect IP address, please try again: "
input ipaddr
echo $ipaddr | egrep $regex | tee check_file
done


If there is a better method the please let me know.

Thanks

Chris

Just spotted the mistake...Bloody infinate loops!

You don't need (want) the check_file, this I/O is not necessary.

This can be done within the script:

regex="\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(2
5[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b"

echo -n "Please enter the IP Address for this machine: "

read ipaddr

CHECK=$(echo $ipaddr | egrep $regex)
if [[ "$?" -eq 0 ]]
then
echo -n "Correct IP address"
else
echo -n "Incorrect IP address, please try again: "
fi


This way you check the exit code that echo $ipaddr | egrep $regex produces (0 = ok, 1 = not ok).

Script needs some work, but the priciple is there.

That also works a treat. But I don't undertand the "$?" in the if statement

This ($?) holds the exit status of the last command executed.

You are probably familiar with: $1, $2 etc. This is just one more of the same 'family'

Take a look at the 'Special Parameters' section in man bash to see some more of these special parameters.