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I/O Redirection: how can I redirect error msgs to a temp file

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# 1  
Old 01-05-2011
I/O Redirection: how can I redirect error msgs to a temp file
Hi there,

I just want to know how can I redirect error msgs returned from running a command (e.g. nslookup) and then only print out the correct output using an if statement?.

I've tried the following:

Quote:
nslookup "$a" 2>bad_command
if [ ! -s bad_command ]
then
nslookup "$a" | grep -i "name =" | awk '{print $4}' >> hostnames
fi
where $a is a list of IPs.

but I got all the error msgs printed out to screen and the corret values hasn't enhanced Smilie

any help is apprieciated
# 2  
Old 01-05-2011
You might find it easier to do one at a time, since nslookup dos not do lists as I recall:
Code:
for i in $a
do
 ... $i ....
done

You might use "ping" or "ping -s $i 100 1" if they are supposed to be live and accessible, and get the name from that.
This User Gave Thanks to DGPickett For This Post:
Abdulelah
# 3  
Old 01-06-2011
You can try to redirect the output to:

2>/dev/null
These 2 Users Gave Thanks to LiorAmitai For This Post:
Abdulelah numele
# 4  
Old 01-06-2011
nslookup does not produce an error message on stderr or produce a return status when a host is not found. It just reports on stdout that a host could not be found.
You might want to try host instead if available, but host only works with a single name, not with a list of names...
This User Gave Thanks to Scrutinizer For This Post:
Abdulelah
# 5  
Old 01-06-2011
thanks for your help and input guys,

what I need is to give the nslookup an IP address each time using a while loop (done)

and than, check if a host name was returned.. if it does then great! and it will be added to a temporary file. if it is not, than I want a customized error msg to appear.

what I am doing now is taking only the host name part of the nslookup output.. which can has something like "ns9.google.com" or sometimes.. nothing!

I want to replace the "nothing" with an error msg that I wrote.

---------- Post updated at 06:21 PM ---------- Previous update was at 05:25 PM ----------

here is the part I am trying to do that through:

Quote:
awk ' {if ( ! -s (print $2)) then "you've got an error pal!" else '{print $2}' fi}'
what I'm trying to achieve here is that, when ever that value of print $2 is null, than print an error mssage, otherwise print the value in print $2

this should make things clearer to you I wish

---------- Post updated at 07:41 PM ---------- Previous update was at 06:21 PM ----------

this code seems to be partially working:

Quote:
awk '{if ( ! -n $4) { print $4 } else { print "Error"}}
but I am getting no "Error" messages. any suggestions please.
Last edited by Abdulelah; 01-06-2011 at 02:44 PM..
# 6  
Old 01-07-2011
Some messages are on stdout (1), some on stderr (2) (in bits!), none on /dev/tty, for my HPUX system:
Code:
$ truss -o /tmp/nsl.tr nslookup 12.23.34.45 ; grep write /tmp/nsl.tr          
Using /etc/hosts on:  hideme
looking up FILES
Trying DNS
Trying NIS
*** No hostname information is available for "12.23.34.45"
write(1, "U s i n g   / e t c / h o s t s ".., 32) ....... = 32
write(1, "l o o k i n g   u p   F I L E S ".., 17) ....... = 17
write(1, "T r y i n g   D N S \n", 11) ................... = 11
write(1, "T r y i n g   N I S \n", 11) ................... = 11
write(2, "* * *   N o   h ", 8) .......................... = 8
write(2, "o s t n a m e ", 7) ............................ = 7
write(2, "  i n f o r m a ", 8) .......................... = 8
write(2, "t ", 1) ........................................ = 1
write(2, "i o n   i s   a ", 8) .......................... = 8
write(2, "v ", 1) ........................................ = 1
write(2, "a i l a b l e   ", 8) .......................... = 8
write(2, "f ", 1) ........................................ = 1
write(2, "o r   " 1 2 . 2 ", 8) .......................... = 8
write(2, "3 . 3 4 . 4 5 ", 7) ............................ = 7
write(2, "" \n", 2) ...................................... = 2
$ 
 
 
 
Good IP:
 
$ truss -o /tmp/nsl.tr nslookup 89.67.45.123 ; grep write /tmp/nsl.tr
Using /etc/hosts on:  hideme
looking up FILES
Name:    hideme
Address:  89.67.45.123
Aliases:  hideme.somedomain.com
write(1, "U s i n g   / e t c / h o s t s ".., 32) ....... = 32
write(1, "l o o k i n g   u p   F I L E S ".., 17) ....... = 17
write(1, "N a m e :         h i d e m e \n".., 26) ....... = 26
write(1, "    8 9 . 6 7 . 4 5 . 1 2 3 ".., 26) ....... = 26
write(1, "    h i d e m e . s o m e d o ".., 34) ....... = 34
$
 
 
Good IP, PTR specific query for pointer-reverse = ip to name:
 
$ truss -o /tmp/nsl.tr nslookup -q=PTR 89.67.45.123 ; grep write /tmp/nsl.tr
Using /etc/hosts on:  hideme
looking up FILES
Trying DNS
123.45.67.89.in-addr.arpa    name = hideme.somedomain.com
write(1, "U s i n g   / e t c / h o s t s ".., 32) ....... = 32
write(1, "l o o k i n g   u p   F I L E S ".., 17) ....... = 17
write(1, "T r y i n g   D N S \n", 11) ................... = 11
write(1, "1 2 3 . 4 5 . 6 7 . 8 9 . i n - ".., 28) ....... = 28
write(1, "\tn a m e   =   h i d e m e . s ".., 34) ....... = 34
write(1, "\n", 1) ........................................ = 1
$
 
 
Good IP, specific DNS server:
 
$ truss -o /tmp/nsl.tr nslookup 89.67.45.123 89.123.45.67 ; grep write /tmp/nsl.tr 
Name Server:  nameserver1.somedomain.com
Address:  89.123.45.67
Trying DNS
Name:    hideme.somedomain.com
Address:  89.67.45.123 
write(1, "N a m e   S e r v e r :     n a ".., 50) ....... = 50
write(1, "    8 9 . 1 2 3 . 4 5 . 6 7 \n".., 17) ....... = 17
write(1, "T r y i n g   D N S \n", 11) ................... = 11
write(1, "N a m e :         h i d e m e ".., 44) ....... = 44
write(1, "    8 9 . 6 7 . 4 5 . 1 2 3 ".., 19) ....... = 19
$

Tools like this, called truss, tusc or strace, can tell you what you are seeing in more detail.
Last edited by DGPickett; 01-07-2011 at 01:44 PM..
# 7  
Old 01-07-2011
This thread seems to be a repeat of a previous thread:
https://www.unix.com/shell-programmin...ing-right.html
In the previous thread the O/P posted sample output from a version of "nslookup" which I do not recognise.

Anybody worked out what Operating System and TCP/IP package this is?

Just for interest, unix versions of "nslookup" do use the output channels in a strange manner. I use an unusual (but portable) method to capture the whole output of one enquiry to a file having failed to get the combined output to a pipe by normal means.
Code:
>mylogfile
nslookup server_name 2>>mylogfile 1>>mylogfile

It is then possible to search "mylogfile" for error messages such as "No hostname information is available".
If anybody recognises the O/P version of "nslookup" can they try various techniques to capture the whole output from one enquiry?
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