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?

• Chris Down: You can use grep -q instead of sending stdout to /dev/null.
• poige: Comment you've marked as approved answer is flawed — grep and IP-addresses are pitfall, since usual notation for IP-addresses has dots inside. See my comments: unix.stackexchange.com/questions/21020/… P. S. I'd mark as answer this one: unix.stackexchange.com/a/36354/6622
• rozcietrzewiacz: I don't agree that there is such flaw (see comments). But JodieC's answer does look better, just as Gilles' one.

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
  

• poige: I advice not using grep, when there's no need in regexps fgrep fits better.
• rozcietrzewiacz: @poige But here you do have a regexp. And using fgrep (or rather grep -F, since fgrep is deprecated) in this case is useless also due to negligible potential speed benefit in a single invocation. It would take far more time to type a (longer) working filter with fgrep - and it would need to use pipe, just as in the OP's example.
• poige: My bad, I've overlooked it. Regarding fgrep: 1) GNU's man can call it deprecated (whatever), but neither does BSD's or Solaris'. So I don't care typing extra -f to grep, neither advice doing that to anyone. :)
• poige: 2) Lots of people tend blindly use grep and spending theirs time in escaping its spec. symbols, or what's a way worse, getting wrong results having forgotten to escape such symbs.. Look, $address you've used in your grep actually has that spec. symbs. — dots. That's a flaw, actually.
• poige: Not only at the beginning, there's a port as well where, say :80 would match ":80anything" — that's why I support solution using ss. And yeah, when there's terminator like :, dots can be safe. But otherwise 192.168.1.4 perfectly matches the whole 192.168.104, for e. g..

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, /:[^:]*$/);
 a["host"] = substr(s, 1, RSTART-1);
 a["port"] = substr(s, RSTART+1);
 }
 sub(/^p/,"") {pid = 0ドル}
 sub(/^n/,"") {
 split(0,ドル endpoints, "->");
 host_port(endpoints[1], from);
 host_port(endpoints[2], to);
 }
 /^TST=ESTABLISHED$/ {
 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 [email protected]:80, but lsof is only available for root user and often not installed by default, so it's additional external dependency.