Let’s say that we have a Bash script that accepts a parameter that is supposed to be an IP. How do we verify that input, before proceeding with any further actions? Here’s what I do so far.

Breakdown of the conditions

First, check that exactly three dots exist in the string:

if [ `echo $IsIP | grep -o '\.' | wc -l` -ne 3 ]; then
    echo "Parameter '$1' does not look like an IP Address.";
    exit 1;
fi

Then, check that exactly four parts exist if the string is broken down by dots:

if [ `echo $IsIP | tr '.' ' ' | wc -w` -ne 4 ]; then
    echo "Parameter '$1' does not look like an IP Address.";
    exit 1;
fi

Then, check that all four parts are numeric:

for OCTET in `echo $IsIP | tr '.' ' '; do
    if ! [[ $OCTET =~ ^[0-9]+$ ]]; then
        echo "Parameter '$1' does not look like an IP Address.";
        exit 1;
    fi
done

Finally check that all four parts are in the range 0 to 255:

for OCTET in `echo $IsIP | tr '.' ' '; do
    if [[ $OCTET -lt 0 || $OCTET -gt 255 ]]; then
        echo "Parameter '$IsIP' does not look like in IP Address (octet '$OCTET' in not in range 0-255).";
    fi
done

So the parameter passed to the script must fulfill the following four criteria:

1. Contain exactly three dots.
2. Contain exactly four parts, if broken down by dots.
3. All four parts are numeric.
4. All four parts are between 0 and 255.

A Bash function

Here’s the entire thing in the form of a Bash function, with an additional comment on what went wrong:

# Verify that the parameter passed is an IP Address:
function is_IP() {
if [ `echo $1 | grep -o '\.' | wc -l` -ne 3 ]; then
        echo "Parameter '$1' does not look like an IP Address (does not contain 3 dots).";
        exit 1;
elif [ `echo $1 | tr '.' ' ' | wc -w` -ne 4 ]; then
        echo "Parameter '$1' does not look like an IP Address (does not contain 4 octets).";
        exit 1;
else
        for OCTET in `echo $1 | tr '.' ' '`; do
                if ! [[ $OCTET =~ ^[0-9]+$ ]]; then
                        echo "Parameter '$1' does not look like in IP Address (octet '$OCTET' is not numeric).";
                        exit 1;
                elif [[ $OCTET -lt 0 || $OCTET -gt 255 ]]; then
                        echo "Parameter '$1' does not look like in IP Address (octet '$OCTET' in not in range 0-255).";
                        exit 1;
                fi
        done
fi

return 0;
}

Example output of the function when saved as a standalone script:

marios@yovan ~ $ ./is_IP 1234
Parameter '1234' does not look like an IP Address (does not contain 3 dots).
marios@yovan ~ $ ./is_IP 1234...
Parameter '1234...' does not look like an IP Address (does not contain 4 octets).
marios@yovan ~ $ ./is_IP 1.2.3.A
Parameter '1.2.3.A' does not look like in IP Address (octet 'A' is not numeric).
marios@yovan ~ $ ./is_IP 1.2.3.300
Parameter '1.2.3.300' does not look like in IP Address (octet '300' in not in range 0-255).
marios@yovan ~ $ ./is_IP 1.2.3.4
marios@yovan ~ $

If someone has any better ways to check this, or improvements on the above, please step forward :)

Fork these scripts on GitHub.

Δύο μικρά tips:

1. Για να δημιουργήσετε μια αντίστροφη σειρά αριθμών με το seq, από έναν μεγαλύτερο σε έναν μικρότερο ακέραιο, χρησιμοποιήστε το ως seq ΜΕΓΑΛΟΣ ΒΗΜΑ ΜΙΚΡΟΣ, για παράδειγμα:

   marios@yovan ~ $ seq 10 -1 1
   10
   9
   8
   7
   6
   5
   4
   3
   2
   1

2. Για να μορφοποιήσετε ακέραιους με με διαφορετικό πλήθος ψηφίων με μηδενικά ώστε να εμφανίζονται με το ίδιο πλάτος, εκτυπώστε τους με την printf, συντάσσοντάς την ως printf "%0ΠΛΑΤΟΣd" ΑΚΕΡΑΙΟΣ. Για παράδειγμα:

   printf "%04d" 1
   0001

Συνδυασμός των δύο:

marios@yovan ~ $ for i in `seq 10 -1 1`; do echo `printf "%04d" $i`; done
0010
0009
0008
0007
0006
0005
0004
0003
0002
0001

Το echo χρησιμοποιείται για να τυπώνει τον επόμενο αριθμό σε καινούρια γραμμή, αλλιώς εμφανίζονται συνεχόμενα.