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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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ports(7)						 Miscellaneous Information Manual						  ports(7)

NAME
ports, port_names - Device (tty and lp) names for serial and parallel ports SYNOPSIS
Default Serial Ports: /dev/tty00 /dev/tty01 (not present on a single-port system) Parallel Port: /dev/lp0 DESCRIPTION
AlphaStation and AlphaServer systems provide one or two 9-pin serial communication ports. These ports are usually labelled 1 (COMM1) and 2 (COMM2), but they may be identified by different icons. Using the appropriate serial cable and terminator, you can connect a serial printer, external modem, or character-cell terminal to a serial port. Most AlphaStation and AlphaServer systems also provide one parallel port, for use with a parallel printer. When you add a device to your system, the installation documentation may instruct you to map the device pathname to the port. These devices are located in the /dev directory. For serial-line ports, the two default device pathnames are: This pathname always maps to 1, COMM1, the lowest port number, an icon for a terminal console, or the only serial port (on a single-port system). This pathname always maps to 2, COMM2, the next numbered port, or (if one serial port is labeled with an icon for a terminal console) the remaining serial port. If your system hardware has been extended to include additional serial ports, the pathnames /dev/tty02, /dev/tty03, and so forth, may also be available to you. However, most systems have only /dev/tty00 and /dev/tty01 as the device pathnames for serial ports. The one parallel port on an AlphaStation or AlphaServer may be labeled with the word printer or a printer icon. On some systems, the paral- lel port may not be labeled. The device pathname for the parallel port is /dev/lp0. Currently, Tru64 UNIX does not fully support parallel printers, so fewer devices are connected to this port as compared to serial ports. If you are connecting a terminal console to your system, it must be connected to the serial port mapped to /dev/tty00. For other serial devices, it does not matter which of the serial ports you choose for the connection. For example, suppose you are setting up a system that has two serial ports, labeled 1 and 2. You intend to use a serial-line terminal rather than a workstation monitor as the system console and also want to connect a serial-line printer to the system. In this case, you must connect the terminal to the port labeled 1 (with the device pathname /dev/tty00). Therefore, you must connect the printer to the remaining port labeled 2 (with the device pathname /dev/tty01). If, for the same type of system, you intend to use a workstation monitor as the system console, it does not matter which serial port you use for a serial-line printer or modem. In other words, you can connect the printer to either port 1 (with pathname /dev/tty00) or port 2 (with pathname /dev/tty01). When prompted to enter a /dev/tty** pathname by the lprsetup script or the Print configuration tool in the CDE Application Manager, you would specify /dev/tty00 if you connected the printer to port 1 or /dev/tty01 if you connected the printer to port 2. See the System Administration manual for more information on setting up consoles (including remote consoles) and printers. See the modem(7) reference page for more information on setting up modems. SEE ALSO
Commands: lprsetup(8) Devices: ace(7), modem(7) System Administration delim off ports(7)
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