Failing IF in a WHILE loop in BASH script that checks for open 22 ports

Question

Im trying to create a rather basic script to run through a list of servers and check if the SSH port is open using nc. I've tried a few different ways, but can't seem to get this to work. I am definitely not great at any type of scripting.

Here is the script. I just want it to perform an action if it sees "succeeded" in the response from the nc command in OPEN.

while read SERVER
do
OPEN=$(nc -z -v -w5 $SERVER 22)
echo $SERVER
   if [[ $OPEN = *"suc"* ]];
    then
        echo "Found SSH open on $SERVER"
    else
        echo "No open ports on $SERVER!"    
   fi
done < server.txt

The list of servers is in the server.txt file that is referenced at the end on the script.

Here is the response that I get. I not

nc: connect to 10.10.51.55 port 22 (tcp) failed: No route to host
10.10.51.55
No open ports on test1!
Connection to 10.10.51.65 22 port [tcp/ssh] succeeded!
10.10.50.65
No open ports in test2!

It give me the "No open ports on $SERVER" no matter what.

I thank you for any guidance.

Answer 1

Notes, too long for a comment.

1. server.txt could better be named server-list or servers.txt if you wish.

2. Use lower case variable names like, avoid SERVER and alike, could better be named server_ip anyways, because it is unclear if you use host names or IPs.

3. Double quote all non-integer-number variables like "$server_ip".

4. Use a direct if statement instead of variable $OPEN with a redirection to the black hole (/dev/null).

---

Rewritten based on above:

#!/bin/sh

while read -r server_ip; do

    if nc -z -v -w5 "$server_ip" 22 >/dev/null 2>&1; then
        echo "Found SSH port open on $server_ip."
    else
        echo "Did not find open SSH port on $server_ip." >&2
    fi

done < server-list

Answer 2

The most simple solution seems to be:

if nc -z -v -w5 $SERVER 22 >/dev/null 2>&1;
then
    echo "Found SSH open on $SERVER"
else
    echo "No open ports on $SERVER!"    
fi

Answer 3

The issue is that nc writes to standard error when outputting diagnostic messages, while a command substitution only captures standard output.

You also should be using == in [[ ... ]]. Variable expansions should be quoted and ideally, you'd be using printf to output variable data.

This means that your code could look like

while IFS= read -r SERVER do
    OPEN=$( nc -z -v -w5 "$SERVER" 22 2>&1 )
    if [[ "$OPEN" == *succ* ]]; then
        printf 'Found SSH open on %s\n' "$SERVER"
    else
        printf 'No open ports on %s!\n' "$SERVER"    
    fi
done <server.txt

or

while IFS= read -r SERVER do
    OPEN=$( nc -z -v -w5 "$SERVER" 22 2>&1 )
    case "$OPEN" in
        *succ*) printf 'Found SSH open on %s\n' "$SERVER" ;;
        *)      printf 'No open ports on %s!\n' "$SERVER"  ;;
    esac
done <server.txt

Or, you could just use the exit status of nc:

while IFS= read -r SERVER do
    if nc -z -v -w5 "$SERVER" 22 >/dev/null 2>&1; then
        printf 'Found SSH open on %s\n' "$SERVER"
    else
        printf 'No open ports on %s!\n' "$SERVER"
    fi
done <server.txt