Nmap check if port is open in bash

Question

I want to make a script which will check if a port is open on a server. If not open stay in a while. If open continue. The break conditions is use are if "host is up" is present and not "closed". I assume conenction is ok. The problem is that the grep is not working as expected.

I have tried with following:

while true; do
 NMAP=$(nmap -p 1700 router.eu.thethings.network)
 if [[$NMAP | grep "Host is up" -ne ""]] && [[$NMAP | grep "closed" -eq ""]]; then
    echo "connection!!!"
    break
 fi


 echo "waiting for connectiong"
 done

I run it on a raspberry pi jessie system.

Solution 1

The problem is that [[$NMAP | grep "Host is up" -ne ""]] is very far from valid bash syntax. The error messages don't tell you exactly how to fix it, but they are a hint that something is seriously wrong.

• [[ expression ]] requires spaces inside the brackets. See Brackets in if condition: why am I getting syntax errors without whitespace?
• | is the pipe operator between commands. It isn't an operator in conditional expressions. In fact [[ foo | bar ]] is parsed as the command [[ foo piped into the command bar ]], which doesn't do anything useful.
• The -eq operator compares integers, but what you put around it aren't integers.

To test whether a string contains a substring, you can either use the == operator in a conditional expression, or a pipe through grep (which doesn't involve a conditional expression). With grep, you're not running $NMAP as a command, you want to pass this as input to grep, so you need to echo it into the pipe. Pass -q to grep since you only care about its return status, not about its output.

if echo "$NMAP" | grep "Host is up"  && echo "$NMAP" | grep "closed"; then …

With a conditional expression:

if [[ $NMAP == *"Host is up"* || $NMAP == *"closed"* ]]; then …

Do read Confused about operators [[ vs [ vs ( vs (( which explains about conditional expressions and how they aren't the only way to test whether a condition is true. Also, read Why does my shell script choke on whitespace or other special characters?

Solution 2

nmap -p 1700 router.eu.thethings.network

Will print "Host is up" if the hosts network is up and there is nothing listening on the port. Use nc instead:

if nc -w1 -z router.eu.thethings.network 1700; then
  echo "Port is listening"
else
  echo "Port is not listening"
fi

Comments

[Peter Savnik]

when i run the nc -z router.eu.thethings.network 1700 it does not return. Will it behave different in a script?

[Paranoid]

This is due to the way tcp/ip works. If it gets no response then it will resend and eventually timeout after about a minute. You could try a simple ping test before testing the port.

[111---]

Add the -w1 option to nc to try the connection for one second. This will let you get a quick failure if the host is not up.

[Peter Savnik]

Great explanation and very good reading!