Programming Netcat TCP with Bash

Question

I am trying to do some basic communication with a TCP client using strictly bash scripting. I have netcat at my disposal, and so I wrote this loop so far:

nc 10.0.0.104 4646 | while read line
do
   if [ "$line" == '{"cmd": 1}' ]
   then
      # Send text back to the TCP server
      echo '{"error": 0}'
   fi
done

The script can successfully connect to the server application you are using, but it's hard for me to figure out how to send text back to the netcat process.

Answer 1

With Bash ≥4 you can use coproc:

#!/bin/bash

coproc nc { nc 10.0.0.104 4646; }

while [[ $nc_PID ]] && IFS= read -r -u${nc[0]} line; do
    case $line in
        ('{"cmd": 1}')
            printf >&${nc[1]} '%s\n' '{"error": 0}'
            ;;
        (*)
            printf >&2 '%s\n' "Received line:" "$line"
            ;;
    esac
done

This avoids the use of temporary fixes. Without coproc, I think, the only option is to use fifos explicitly. Here is an example:

#!/bin/bash

mkfifo fifo_in

while IFS= read -r line; do
    case $line in
        ('{"cmd": 1}')
            printf '%s\n' '{"error": 0}'
            ;;
        (*)
            printf >&2 '%s\n' "Received line:" "$line"
            ;;
    esac
done < <(nc 10.0.0.104 4646 < fifo_in) > fifo_in

fifo: mktemp, fifo, trap script, .

---

/dev/tcp

Bash , nc, :

#!/bin/bash

# open TCP connection, available on file descriptor 3
exec 3<> /dev/tcp/10.0.0.104/4646 || exit

while IFS= read -r -u3 line; do
    case $line in
        ('{"cmd": 1}')
            printf >&3 '%s\n' '{"error": 0}'
            ;;
        (*)
            printf >&2 '%s\n' "Received line:" "$line"
            ;;
    esac
done

!