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Help with Shell Script

Posted on 2014-01-07
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Last Modified: 2014-01-22
Hello,

I am looking for help writing a *nix shell script, specifically to execute on OpenWRT environments.

The summary of the process is as follows:

1) The script would query/resolve a DNS (i.e: foobar.example.com) and get the IP address. If multiple IP's are returned, just the first one should be good enough.

eg:
>nslookup foobar.example.com 8.8.8.8
Server:    8.8.8.8
Address 1: 8.8.8.8 google-public-dns-a.google.com

Name:      foobar.example.com
Address 1: 3.3.3.3  n003-000-000-000.static.ge.com
Address 2: 2.2.2.2

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2) the script would then overwrite a config file with the new resolved IP. Config file: server.conf

server.conf
============

foo = bar
bar = foo
some_other_config = 1
server_ip = 5.6.7.8
more_config = true

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it should find the line which says server_ip = x.x.x.x, and replce it it server_ip = 3.3.3.3

3) Optionally - it would be nice if we could read the exiting server.conf file and make sure the IP is indeed different and needs to be updated. If yes, then go ahead with the replacement, if not, leave the server.conf alone.

I know this can be achieved by the use of sed and/or awk, but I've had my share of issues doing it, as this is a new area for me.

Any help would be much appreciated.

Thank you!
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Question by:shaf81
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Accepted Solution

by:
simon3270 earned 300 total points
ID: 39761748
To get the IP address:
SERVER=foobar.example.com
IP=$(nslookup $SERVER 8.8.8.8 | awk '/^Name:/{r=1}/^Address:/{if (r == 1){print $2;exit}}')

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To replace the existing IP address:
CONF=server.conf
sed -i 's/^server_ip.*/server_ip = '"${IP}/" $CONF

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To do this only if the address has changed:
oldip=$(awk '$1 ~ /^server_ip$/{print $3;exit}' $CONF)

if [ "$oldip" != "" ]
then
 if [ "$oldip" != "$IP" ]
 then
   echo Replacing IP
   sed -i 's/^server_ip.*/server_ip = '"${IP}/" $CONF
 fi
fi

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