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awk search and replace

Posted on 2006-06-27
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Last Modified: 2010-05-18
Hi,

Can I use awk to change a file like the following?  I want to add the second line but am not getting correct results.  See below for my current script.

BEFORE:
domain foobar.com
nameserver 1.2.3.4
nameserver 2.3.4.5
nameserver 3.4.5.6

AFTER:
domain foobar.com
nameserver 5.5.5.5
nameserver 1.2.3.4
nameserver 2.3.4.5
nameserver 3.4.5.6

My current script _replaces_ line 2 instead of _inserting_ it.  Can anyone show me how to do it correctly?

awk 'NR==2{$1 = "nameserver"} NR==2{$2 = "5.5.5.5"}{print}' /etc/resolv.conf,backup
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Question by:bryanlloydharris
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6 Comments
 
LVL 51

Assisted Solution

by:ahoffmann
ahoffmann earned 500 total points
ID: 16996245
awk '/^domain/{print; print "nameserver 5.5.5.5"}{print}' /etc/resolv.conf,backup
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Accepted Solution

by:
bpmurray earned 500 total points
ID: 16996538
You're simply replacing the current values of line 2 in your code. What you really want to do is add an extra line:

awk 'NR==2{print "nameserver 5.5.5.5"}{print}' /etc/resolv.conf,backup
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LVL 3

Author Comment

by:bryanlloydharris
ID: 16996755
Thanks to both for the tips.  It seems ahoffman that yours has a slightly strange added effect of adding domain.com twice.  But that's okay I can work with it.
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LVL 3

Author Comment

by:bryanlloydharris
ID: 16996783
Sorry ahoffman I meant it adds foobar.com twice.
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LVL 15

Expert Comment

by:bpmurray
ID: 16997226
The fix for ahoffmann's script is to remove the first "print;" because the first line fits both the pattern and the section with no pattern. Actually, the section without a pattern should be first, or the extra nameserver line will be printed before the domain line.
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LVL 51

Expert Comment

by:ahoffmann
ID: 16998938
> It seems ahoffman that yours has a slightly strange added effect of adding domain.com twice.
sorry, printed a lazy solution just out of my finger tips, there should be a
 next;
at end of the first block

> The fix for ahoffmann's script is to remove the first "print;"
yes, no, perhaps
that's a dirty solution 'cause it breaks the order of the keywords
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