Can't get netcat port scan to output to a file

jase4867 · 07-08-2012, 08:00 PM

Hi all,

Just starting to learn some bash scripting, and I'm having trouble understanding how the redirection operators work. I wrote a very simple bash script, where I run a netcat port scan against a text file, which contains a list of hosts (not very practical, but just messing around with it), and I can't get it to send the output to a text file. The text file itself gets created, but there is no data in it. Here's what my script looks like:

#!/bin/bash

for ip in $(cat ips.txt); do
nc -vv -z $ip 1-100 > results.txt
done

I'm sure it's a simple fix, but something isn't clicking in my head. Can anyone offer some pointers?

Thanks!

chrism01 · 07-08-2012, 08:11 PM

Do you get any output with

Code:

nc -vv -z <someip> 1-100

Lets check its working; I used my local IP addr to test and got plenty of output.
I noticed that if you re-direct to a file, only successful cxns ie open ports are written to stdout; failures go to stderr.
To collect even those

Code:

nc -vv -z <someip> 1-100>nc.out 2>&1

Try just that to test with...

jase4867 · 07-08-2012, 08:49 PM

Hi Chris,

The second example you gave worked like a charm! I'll give it a try with the ips.txt file, and see what happens there.

Thanks again!

Jason

jase4867 · 07-08-2012, 10:13 PM

So, the script is still a bit flaky acting. While it was running against the list of IP's, it would update the nc.out file, but then it would zero itself out, and start over. When the script finished, the only data that was in the nc.out file, was the last host that was scanned.

Is there a better way to write this script, than what I have?

chrism01 · 07-09-2012, 12:55 AM

Indeed, you just need to learn some bash operators

Code:

# this outputs to (overwrites) a file
prog > file

# this appends to a file
prog >> file

# this takes in put from(!) a file
prog < file

Think carefully about what your loop is doing and/or try

Code:

#!/bin/bash
set -xv

The latter shows what the parser is doing...

some useful links
http://rute.2038bug.com/index.html.gz
http://tldp.org/LDP/Bash-Beginners-G...tml/index.html
http://www.tldp.org/LDP/abs/html/

jase4867 · 07-09-2012, 09:40 PM

Making the last change on the script, fixed it, and gave me the output I was looking for. Changed the redirect from > to >>.

Thanks again for the help, and the resource links!

Jason

chrism01 · 07-09-2012, 11:31 PM

No worries mate