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?
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.
nc.openbsd(packagenetcat-openbsd) if I remove the-koption-k, thencwill exit if the wrong command comes in.-kit just stays alive even when thegrepexits... Now I'm wondering if there's differences between the variants and versions of netcat. I have two on my Debian,nc.openbsdandnc.traditional, I'm not sure if there's a third one too. But.traditionaldoesn't seem to have-k, so yours can't be that.