How do I tell a script to wait for a process to start accepting requests on a port?

Question

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

...

Answer 1

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.

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.