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Full Discussion: Bash script, find the next closed (not in use) port from some port range.
Top Forums UNIX for Beginners Questions & Answers Bash script, find the next closed (not in use) port from some port range. Post 303036751 by drysdalk on Wednesday 10th of July 2019 10:56:17 AM
Old 07-10-2019
Hi,

In terms of your script itself as a solution to your problem, a change like this should work:

Code:
#!/bin/bash
IP=$1
first_port=$2
last_port=$3
function scanner

{
for ((port=$first_port; port<=$last_port; port++))
        do
                if ! (echo >/dev/tcp/$IP/$port)2>/dev/null
                then
                        echo $port
                        return
                fi
        done
}

scanner

This will cause the first TCP port that cannot have a blank line written to it by the echo command to be echoed out to standard output. The key is the return statement, which will cause the function to end as soon as it is executed. So once we find the first port we can't write a blank line to, we print it out and return control to the next line of the script (which doesn't exist, and so the script ends at that point).

Now, a better question: what is it you actually need to do here, and why ? This script might well work for certain ports in certain circumstances, but a far better bet here would be to use a utility that is dedicated to the purpose of port scanning (like nmap, for example) rather than trying to re-invent the wheel with a shell script.
 

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LEARN ABOUT OSF1

auscope

AUSCOPE(1)						      General Commands Manual							AUSCOPE(1)

NAME

       auscope - Network Audio System Protocol Filter

SYNOPSIS

       auscope [ option ] ...

DESCRIPTION

       auscope	is  an	audio  protocol  filter  that can be used to view the network packets being sent between an audio application and an audio
       server.

       auscope is written in Perl, so you must have Perl installed on your machine in order to run  auscope.   If  your  Perl  executable  is  not
       installed  as  /usr/local/bin/perl,  you should modify the first line of the auscope script to reflect the Perl executable's location.  Or,
       you can invoke auscope as

       perl auscope [ option ] ...

       assuming the Perl executable is in your path.

       To operate, auscope must know the port on which it should listen for audio clients, the name of the desktop  machine  on  which	the  audio
       server  is running and the port to use to connect to the audio server.  Both the output port (server) and input port (client) are automati-
       cally biased by 8000.  The output port defaults to 0 and the input port defaults to 1.

ARGUMENTS

       -i<input-port>
	       Specify the port that auscope will use to take requests from clients.

       -o<output-port>
	       Determines the port that auscope will use to connect to the audio server.

       -h<audio server name>
	       Determines the desktop machine name that auscope will use to find the audio server.

       -v<print-level>
	       Determines the level of printing which auscope will provide.  The print-level can be 0 or 1.  The larger  numbers  provide  greater
	       output detail.

EXAMPLES

       In  the	following  example,  mcxterm is the name of the desktop machine running the audio server, which is connected to the TCP/IP network
       host tcphost.  auscope uses the desktop machine with the -h command line option, will listen for client requests on port 8001  and  connect
       to the audio server on port 8000.

       Ports  (file  descriptors)  on the network host are used to read and write the audio protocol.  The audio client auplay will connect to the
       audio server via the TCP/IP network host tcphost and port 8001:

	      auscope -i1 -o0 -hmcxterm

	      auplay -audio tcp/tcphost:8001 dial.snd

       In the following example, the auscope verbosity is increased to 1, and the audio client autool will connect to the  audio  server  via  the
       network host tcphost, while displaying its graphical interface on another server labmcx:

	      auscope -i1 -o0 -hmcxterm -v1

	      autool -audio tcp/tcphost:8001 -display labmcx:0.0

SEE ALSO

       nas(1), perl(1)

COPYRIGHT

       Copyright 1994 Network Computing Devices, Inc.

AUTHOR

       Greg Renda, Network Computing Devices, Inc.

								       1.9.3								AUSCOPE(1)
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