How to use 'coproc' to interact with another command driven program

Ok, obviously I am NOT a bash guru and am in need of one!

I have never used 'coproc' before, but it seems to be just what I need. But, I have to admit that I can not extrapolate from the various 'ping' examples out there! [I did try for a couple of hours...]

All I want to do is to start a 'coproc' shell script that can take input from standard in and writes it's results to standard out. I want the main script to do the sending and processing of those commands and results respectively.

Here is one of the simplest outlines of what I am trying to do: EDITED WITH BETTER DETAIL

#! /bin/bash

coproc bkgndProc {
    /some/path/to/usefulScript.sh  maybeSomeArgsHere
}

// send command #1 to bkgndProc here
result=$(echo 'command' <&${bkgndProc[0]})    ### Doesn't work for me
echo "Did it work? $result"   ### this just prints back the 'command' I used

// here execute conditional logic based on result:
// if result1; then
//     send command #2 here, getting results
// else
//     send command #3 here, again getting results
// fi

Sorry about using pseudo-code above, but I'm not sure what those sending commands should be! If anyone could supply the details that would be greatly appreciated!

Solution

result = $(echo 'command' <&${bkgndProc[0]})    ### Doesn't work for me

wouldn't work at least basically since you have spaces on it

result=$(echo 'command' <&${bkgndProc[0]})

---- Update ----

A simple concept could be shown in a script like this:

#!/bin/bash

# create the co-process
coproc myproc {
    bash
}

# send a command to it (echo a)
echo 'echo a' >&"${myproc[1]}"

# read a line from its output
read line <&"${myproc[0]}"

# show the line
echo "$line"

Outputs:

Another which reads multiple lines using a timeout:

#!/bin/bash

coproc myproc {
    bash
}

# send a command to message 4 random numbers in 4 lines
echo 'echo "$RANDOM"; echo "$RANDOM"; echo "$RANDOM"; echo "$RANDOM"' >&"${myproc[1]}"

# keep reading the line until it times out
while read -t 1 -u "${myproc[0]}" line; do
    echo "$line"
done

Output:

If we use cat, it would no longer quit since the other end is still alive and connected, and EOF is not yet reached. It's the reason why we used timeouts.

cat <&"${myproc[0]}"