Efficiently test if a port is open on Linux?

Question

From a bash script how can I quickly find out whether a port 445 is open/listening on a server.

I have tried a couple of options, but I want something q

Answer 1

nmap is the right tool. Simply use nmap example.com -p 80

You can use it from local or remote server. It also helps you identify if a firewall is blocking the access.

Answer 2

ss -tl4 '( sport = :22 )'

2ms is quick enough ?

Add the colon and this works on Linux

Answer 3

You can use netcat command as well

[location of netcat]/netcat -zv [ip] [port]

or

nc -zv [ip] [port]

-z – sets nc to simply scan for listening daemons, without actually sending any data to them.
-v – enables verbose mode.

Answer 4

Based on Spencer Rathbun's answer, using bash:

true &>/dev/null </dev/tcp/127.0.0.1/$PORT && echo open || echo closed

Answer 5

There's a very short with "fast answer" here : How to test if remote TCP port is opened from Shell script?

nc -z <host> <port>; echo $?

I use it with 127.0.0.1 as "remote" address.

this returns "0" if the port is open and "1" if the port is closed

e.g.

nc -z 127.0.0.1 80; echo $?

-z Specifies that nc should just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunc- tion with the -l option.

Answer 6

If you're using iptables try:

iptables -nL

or

iptables -nL | grep 445