1

I am facing a problem using netcat in a bash script.

I would like to match a specific output after sending a command and continue the script execution as soon as possible (not waiting for a netcat timeout)

$> echo 'my_command' | nc -q 10 <IP> <PORT> | grep -m 1 EXPECTED_OUTPUT
# ISSUE: Closes the connection quite instantly
$> echo $?
$> 1 # grep did not get (yet) the output of nc

Another try:

$> echo 'my_command' | nc -w 1 <IP> <PORT> | grep -m 1 EXPECTED_OUTPUT
Binary file (standard input) matches
# ISSUE: Wait until the timeout expires
$> echo $?
$> 0

For information:

without command, netcat prints a banner message:

$>nc <IP> <PORT>                                                                                        
welcome message

I am not against other tools (telnet, ...)

I would like a bash-compliant solution.

As the expected message should come within a second I use the timeout -w 1 of nc

2

You want to set it up so that nc is killed as soon as grep finishes. Here is one way:

( subshell_pid=$BASHPID ; echo 'my_command' | nc $IP $PORT > >(grep -m 1 EXPECTED_OUTPUT ; kill -13 -- -$subshell_pid ; ) )

This all runs in a subshell, and then kills all processes started by the subshell when grep finishes.

The >() is process substitution, which lets you pipe from one command to multiple commands.

  • Thank you very much for your answer and the SIGPIPE reminder; Your proposal is pretty much what I'm using. I upvoted and will accept your answer in a few days unless someone proposes something more 'straight forward' – Zermingore Feb 5 at 6:51

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