Netcat TCP Programming with Bash

Question

I'm attempting to do some basic TCP client communication by using strictly bash scripting. I have netcat at my disposal and so I've written 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 I'm using but I'm having difficulties figuring 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 using temporary fifos. Without coproc, I guess the only option left is to use fifos explicitly. Here's 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

For this, you'll have to manage the creation and deletion of the fifo: you'll need to create a temporary directory with mktemp, in there create the fifo, then trap your script so that on exit everything is cleaned.

---

/dev/tcp

If your Bash has been compiled with net redirections support, you can get rid of nc and of the fifos and coprocesses altogether:

#!/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

This is very likely the sweetest solution!