Bash comparisons with blank variable in simple nmap script - bash

Question

I have a simple script that nmaps from a list of ip's, and returns any ips that have port 80 open. The problem is when 80 is closed, instead of no return, Im getting "=: command not found" as it is attempting to compare a blank/empty variable.
#!/bin/bash
for i in$(cat filename.txt); do
str=$(nmap $i |grep 80 | cut -d ' ' -f2)
if($str = "open"); then
echo port 80 on $i is open
fi
done
When port 80 is closed, $str is getting nothing for a value, then when I try to compare nothing to the string "open", im getting "=: command not found". How do I check if $str has a value before comparing it to a string? I tried
if (! -z $str); then
which I thought meant
if $str is not null, then,
but could not get it to work properly. I certainly believe I understand why its is behaving as it is, I just dont know how to correct it.

Answer 1

With GNU bash:
#!/bin/bash
port="80"
while read -r host; do
str=$(nmap "$host" -p "$port" | grep "^$port")
ret="${PIPESTATUS[0]}" # returncode of nmap
if [[ $ret != 0 ]]; then
echo "error"
exit 1
fi
# check if $str contains "open"
if [[ $str =~ open ]]; then
echo "port $port on $host is open"
else
echo "port $port on $host is not open"
fi
done <filename.txt
I assume filename.txt contains only one IP or hostname per line.
Output (example):
port 80 on 10.20.30.40 is not open
port 80 on localhost is open

Answer 2

Here's a version using netcat's exit status:
#!/bin/bash
while read host; do
nc -zw2 $host 80 &>/dev/null && state=open || state=closed
echo "port 80 on $host is $state"
done < filename.txt

Answer 3

Thank you all for the help, Cyrus I like yours a lot, better than what I came up with, and Cole I was very happy to see it done with netcat. Here is what I eventually arrived at
#!/bin/bash
for i in$(cat targetlist.txt); do --multiple people say this doesnt work
str=$(nmap -p 80 $i | grep 80 | cut -d ' ' -f2) --im using bash 4.2.37(1)
if [[ ! -z $str && $str = "open" ]] ; then
echo port 80 on $i is open
fi
done