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I have tried the following to no avail, the script just stops after the first input.

filename="..."

while IFS="" read -r line; do
 echo "$line" | nc <target> <port>
done < "$filename"

I have tried with a FIFO too:

filename=".."

# Create a named pipe if it doesn’t exist
PIPE=/tmp/nc_pipe
[ -p "$PIPE" ] || mkfifo "$PIPE"

# Start Netcat in the background
nc <target> <port> < "$PIPE" | while read -r response; do
    echo "Server response: $response"
done &

while IFS= read -r line; do
    echo "$line" > "$PIPE" 
    sleep 0.5            
done < "$filename"

# Cleanup
rm -f "$PIPE"

The simple command works:

echo test | nc <target> <port>

Is there an issue with netcat in Bash scripts?

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1
  • Maybe I misunderstood you and this isn't a nc problem at all. What happens if you replace echo "$line" | nc <target> <port> with echo "$line" | cat? Commented Jan 30 at 18:01

3 Answers 3

Reset to default
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Your example appears to be over-complicating the matter, and starts a fresh instance of nc for each line of input. Instead, you can feed the file straight into nc:

nc remoteHost remotePort <"$filename"

If you want two lines a second (i.e. a half-second delay between lines):

pv --quiet --line-mode --rate-limit 2 <"$filename" |
    nc remoteHost remotePort

You may find you need nc -q0 to time out and disconnect after the input has been read. (This is for nc traditional rather than openbsd.)

Ensure you have a listener on remoteHost for the port remotePort.

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2
  • Your example does not seem to work with a file containing multiple lines of input. Commented Jan 30 at 15:09
  • pv writes a line at a time. nc sends it to a remote system. What happens when you try it? Commented Jan 30 at 15:55
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nc like ssh reads from the std input. Try to redirect it to read the arguments one at a time:

nc <target> <port>  </dev/null
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1
  • This doesn't add any information past what is already in existing answers. Commented Feb 4 at 19:18
-1

You need a permanent connection between the input and nc (whether it is a pipeline or a FIFO). The echo sends the data and then breaks the connection.

while IFS="" read -r line; do
    echo "$line"
done < "$filename" | nc <target> <port>

while IFS= read -r line; do
    echo "$line"
    sleep 0.5            
done < "$filename" > "$PIPE"
Improve this answer
1
  • Your examples does not seem to work, netcat still pauses after the first connection. Commented Jan 30 at 15:13

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