grep reverses # output, but not text?

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04-21-2010

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grep reverses # output, but not text?

I have a working script to find out the IP of a hostname like this:

Code:

#!/bin/bash
h="www.google.com"
echo $h
j=`host $h | grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}'`
echo "ip: $j"

output

Code:

ip: 74.125.19.99

the strange thing is if I use an IP instead of a hostname for $h, it inverts the order of the numbers on the output:

Code:

h="74.125.19.99"
echo $h
j=`host $h | grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}'`
echo "ip: $j"

outputs:

Code:

ip: 99.19.125.74

why? Also, is there a better way to do this?

unclecameron

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04-21-2010

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Quote:

Originally Posted by unclecameron

I have a working script to find out the IP of a hostname like this:

Code:

#!/bin/bash
h="www.google.com"
echo $h
j=`host $h | grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}'`
echo "ip: $j"

output

Code:

ip: 74.125.19.99

the strange thing is if I use an IP instead of a hostname for $h, it inverts the order of the numbers on the output:

Code:

h="74.125.19.99"
echo $h
j=`host $h | grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}'`
echo "ip: $j"

outputs:

Code:

ip: 99.19.125.74

why? ...

Seems like the IP is inverted in the output of the host command itself. grep is simply extracting the general pattern it matches.

If you run the following two commands:

Code:

host "www.google.com"
host "<one_of_the_ip_addresses_that_is_returned_by_the_command_above>"

then you'll see reversed IP addresses. I have no idea why that's the case.

tyler_durden

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04-21-2010

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Read the host man page. It should tell you that when you give it an ip address it does a reverse lookup.

http://en.wikipedia.org/wiki/Reverse_DNS_lookup

Last edited by alister; 04-21-2010 at 11:36 PM..

alister

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