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I need a command that will wait for a process to start accepting requests on a specific port.

Is there something in linux that does that?

while (checkAlive -host localhost -port 13000 == false)
  do some waiting

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up vote 24 down vote accepted

The best test to see if a server is accepting connections is to actually try connecting. Use a regular client for whatever protocol your server speaks and try a no-op command.

If you want a lightweight TCP or UDP client you can drive simply from the shell, use netcat. How to program a conversation depends on the protocol; many protocols have the server close the connection on a certain input, and netcat will then exit.

while ! echo exit | nc localhost 13000; do sleep 10; done

You can also tell netcat to exit after establishing the connection. It returns 1 if there's no connection and 0 if there is so we negate its output.

while ! nc -q 1 localhost 13000 </dev/null; do sleep 10; done

An alternative approach is to wait for the server process to open a listening socket.

while netstat -lnt | awk '$4 ~ /:13000$/ {exit 1}'; do sleep 10; done

Or you might want to target a specific process ID:

while ! lsof -n -Fn -p $pid | grep -q '^n.*:13000$'; do sleep 10; done

I can't think of any way to react to the process starting to listen to the socket (which would avoid a polling approach) short of using ptrace.

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I think netcat is the answer, so thank you. To clarify, what I'm trying to do is write a script as part of a load balancing procedure. I need to start a process, wait for it to accept requests on the port and then shutdown the original. If there are better ways of doing this, rather than writing my own script, I'm all ears. – Will Dec 31 '10 at 16:36
@Will: That's a very different question! I've written a different answer. – Gilles Dec 31 '10 at 17:37
I like netcat solution too. I have a script using nc -w 2 </dev/null >/dev/null — if the connection takes more than 2 seconds, it times out and fails — which is handy for my usage. – ephemient Jan 9 '11 at 23:10
FYI, I can't get the 'while nc -q 1 localhost 13000 </dev/null; do sleep 10; done' one to work. It just returns immediately. The first one works fine though. Thanks! – Flyte Mar 19 '15 at 16:04
@Flyte nc -q 1 localhost 13000 </dev/null returns immediately if no server is listening, but it returns with an error code, so the loop makes it sleep and try again a few seconds later. – Gilles Mar 19 '15 at 17:16

If you have bash and coreutils (e.g. timeout, sleep), but not nc/lsof/netstat, you can use this solution which uses bash magic tcp sockets:

while ! timeout 1 bash -c "echo > /dev/tcp/localhost/13000"; do sleep 10; done
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To elaborate, Bash has the optional feature of connecting to TCP sockets using "network redirections" -- gnu.org/software/bash/manual/html_node/… – johntellsall Dec 23 '14 at 17:25

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