CODEWITHSUNDEEP
regexbashgrep
Gaia
Gaia

Reputation: 2952

Bash Script to Check if Given IP is Valid A Record for Host

I need to use cron and bash to check if IP 111.222.333.444 is still valid for host sub.domain.com.

I have tried grep with -Pzo, but it didn't work. I don't want to install pcregrep.

#!/bin/bash

ipaddressused=$1

#Run a dig for sub.domain.com:

ipaddresscurrent='dig +short sub.domain.com'

echo "$ipaddresscurrent" | grep -Pzo "$ipaddressused" && echo "found" && exit 0 || echo "not found" && exit 1

ipaddresscurrent returns multiple IPs, one per line.

How do I make this work?

Upvotes: 1

Views: 726

Answers (1)

gniourf_gniourf
gniourf_gniourf

Reputation: 46903

Wouldn't this be enough?

#!/bin/bash

ipaddressused=$1

if grep -q -P "$ipaddressused" < <(dig +short sub.domain.com); then
    echo "found"
    exit 0
else
    echo "not found"
    exit 1
fi

What was wrong with your script?

  • The line

    ipaddresscurrent='dig +short sub.domain.com'

    assigns the string dig +short sub.domain.com to the variable ipaddresscurrent. You probably, instead, wanted to assign to the variable ipaddresscurrent the output of the command dig +short sub.domain.com. This is done either using the old and deprecated backticks:

    ipaddresscurrent=`dig +short sub.domain.com`

    (but please don't ever use backticks!) or the more modern, robust and nestable $(...) as:

    ipaddresscurrent=$(dig +short sub.domain.com)

  • grep -Pzo doesn't really do what you're expecting. Instead you want to run grep quiently (hence the -q flag) and check its output, so the following would have been valid:

    echo "$ipaddresscurrent" | grep -q -P "$ipaddressused" && echo "found" && exit 0 || echo "not found" && exit 1

Since you don't really need the variable ipaddresscurrent, I prefered to use 's process substitution to feed grep.

Also, don't use long chains of && || &&'s, it's hard to read, and can have some subtle side effects.

If you want to stick with your variable, you need a here-string as so:

#!/bin/bash

ipaddressused=$1
ipaddresscurrent=$(dig +short sub.domain.com)

if grep -q -P "$ipaddressused" <<< "$ipaddresscurrent"; then
    echo "found"
    exit 0
else
    echo "not found"
    exit 1
fi

As you note in your comment:

should be noted that if the $ipaddressused supplied is 111.222.333.4 and 111.222.333.456 is present on the list a match will also occur. this could cause problems.

I actually didn't really know if this was a requested feature or not (since the argument of the script is a regex, that's actually why I left the -P flag). If you really want to exactly match an IP, here's how you could proceed:

#!/bin/bash

if grep -q "^${1//./\.}$" < <(dig +short sub.domain.com); then
    echo "found"
    exit 0
else
    echo "not found"
    exit 1
fi

assuming that dig used this way will output only one ip per line.

Upvotes: 2

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