Re: [Sheflug] quick bash question

[(Ted Harding) <ted.harding@xxxxxxxxxxxx>]

On 29-Jan-11 20:10:19, Christopher Brown wrote:
> Hi folks,
> I'm trying to run various rDNS (PTR) lookups from command line.
> My shell-foo is very poor. Can someone help?
> 
> I know the hostname I want to lookup - e.g. mail.lug.org.uk and
> want to get the PTR record in one line and the shortest number
> of key-strokes as possible.
> 
> I can get the IP in one command:
> 
> host mail.lug.org.uk
> 
> and get the PTR record in another:
> 
> dig -x 217.147.93.68
> 
> but have no idea how to combine the two or pipe output from the first
> to the second. I can probably get it in a single command but would be
> interested in the solution anyway.
> 
> Thanks
> Chris

I'm not sure exactly what you want to do, but the following
does what I think you mean!

host mail.lug.org.uk | dig -x `awk '{print $4}'` | grep PTR | awk
'{if($5){print $5}}' | sed 's/\.$//'

Result:
-------
web-01.lug.org.uk


Explanation:
[1] host mail.lug.org.uk
produces
mail.lug.org.uk has address 217.147.93.68

of which the 4th field it the IP address you want.

[2] host mail.lug.org.uk | awk '{print $4}'
produces that 4th field:
217.147.93.68

[3] When you enclose the "awk '{print $4}'" in
backquotes in a command, as in
  dig -x `awk '{print $4}'`
this has the effect of substituting the output of
"awk '{print $4}'" for the back-quoted bit, so the
effect is the same as
  dig -x 217.147.93.68

[4] Now that last command produces several lines of output,
of which two contain "PTR":
;68.93.147.217.in-addr.arpa.    IN      PTR
68.93.147.217.in-addr.arpa. 9924 IN     PTR     web-01.lug.org.uk.

You want the 5th field "web-01.lug.org.uk." from the one which
has 5 fields. So a further call to 'awk' picks out the one
in which $5 is non-null, and outputs $5:
  awk '{if($5){print $5}}'

[5] The result of that is "web-01.lug.org.uk.", but I guess
you do not want the final ".", so get rid of it:

  sed 's/\.$//'

I.e. if there is a "." in final position, replace it with null
(the "." needs to be escaped as "\." since on its own "." is
a wildcard).

Hoping that helps!
Ted.

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E-Mail: (Ted Harding) <ted.harding@xxxxxxxxxxxx>
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Date: 30-Jan-11                                       Time: 12:04:45
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