How can I test whether connection to the given host/port is established in Bash?

Question

Currently I'm using netstat for this:

if netstat -an | grep ESTABLISHED | grep $address:$port > /dev/null; then
    # command
fi  

Is there a more elegant solution?

Answer 1

As far as elegance is concerned, I'd modify two things in your command:

• As mentioned in comment by Chris, you can use -q instead of output redirection.

• Use one grep instead of two:

  if netstat -an | grep -q " $address:$port .* ESTABLISHED"; then
  

Answer 2

lsof should do the job. Ask it to give you machine-parseable output with the -F option.

lsof -n -i @${hostname}:${port} -F nT | grep '^TST=ESTABLISHED$'

If you need more information:

lsof -n -i -F nT | awk '
    function host_port(s, a) {
        match(s, /:[^:]*$/);
        a["host"] = substr(s, 1, RSTART-1);
        a["port"] = substr(s, RSTART+1);
    }
    sub(/^p/,"") {pid = $0}
    sub(/^n/,"") {
        split($0, endpoints, "->");
        host_port(endpoints[1], from);
        host_port(endpoints[2], to);
    }
    /^TST=ESTABLISHED$/ {
        print "Established from", from["host"] ":" from["port"],
              "to", to["host"] ":" to["port"]
    }
'

Answer 3

with ss:

if ss -n -o state established '( dport = $hostname:$portnumber )'|awk 'NR==2{exit 0}END{exit 1}';then 

Answer 4

That works well enough for now, for Linux.

On other UNIX systems (the ones I have my hands on are Mac OS X and Solaris) the port is separated by . instead of :.

And this will most likely fail for any IPv6 connection. netstat truncates IPv6 addresses, so make sure to use --wide. But again, that's not a portable option.

Answer 5

You can use lsof instead of netstat: something like this: sudo /usr/sbin/lsof -i tcp@1.2.3.4:80, but lsof is only available for root user and often not installed by default, so it's additional external dependency.