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I want to open a port and listen for a command, and when that command comes in, immediately run a script.

The best I've come up with is (the echo is a substitute for my script):

nc -lk 9999 | grep -q runcommand && echo "running command"

, then I run this to test:

echo runcommand | nc localhost 9999

Unfortunately it only seems to echo the text on the second time I send "runcommand"??

How can I make grep exit immediately, so my script will run immediately?

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  • that works for me with Debian's nc.openbsd (package netcat-openbsd) if I remove the -k option
    – ilkkachu
    Jul 31, 2021 at 8:45
  • @ilkkachu I wanted it to ignore the wrong command coming in, and stay listening. But I think without -k, the nc will exit if the wrong command comes in.
    – localhost
    Jul 31, 2021 at 9:26
  • ah, right. Though for what I got when testing was that with -k it just stays alive even when the grep exits... Now I'm wondering if there's differences between the variants and versions of netcat. I have two on my Debian, nc.openbsd and nc.traditional, I'm not sure if there's a third one too. But .traditional doesn't seem to have -k, so yours can't be that.
    – ilkkachu
    Jul 31, 2021 at 9:52
  • I threw some ideas in an answer, not sure if it works for you. Also, now that I check, I get your behavior on Mac.
    – ilkkachu
    Jul 31, 2021 at 13:24

1 Answer 1

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One problem a construct like something | grep -q can have is that after grep exits, the left-hand command doesn't see the pipe close until it tries to write to it. But with my netcat (nc.openbsd from Debian), it doesn't seem to ever exit if -k is given, even if grep does. Hence the && never triggers. I did get the behavior you described with the netcat on macOS, though.

One possibility is to wrap netcat in a loop, this seems to work for me:

while true; do
    if nc -l 9999 | grep -q runcommand; then
        echo got correct string, exiting...
        break
    else
        echo got a connection, but not the correct string...
    fi
done

And of course if there's still a problem that just sending one line doesn't trigger netcat to exit, you could always send a dummy line after it to poke it:

printf "runcommand\n.\n" | nc localhost 9999

Not sure if this helps, and there might still be differences between the various netcats.

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