How do I redirect stdout from one process to be read as file descriptor 3 in another?

Question

I want to redirect (stdout,stderr) from one process to (stdin,file descriptor 3) of another. How do I do that?

For example, if I have myscript.sh:

#!/bin/sh
# myscript.sh
echo "stdout"
echo "stderr" >&2

And reader.sh:

#!/bin/sh
# reader.sh
read key
echo "stdin: $key"
read key <&3
echo "fd3: $key"

How do I pipe the output of myscript.sh to reader.sh such that the final output is:

stdin: stdout
fd3: stderr

The closest I've gotten is:

./myscript.sh 2>&3 | ./reader.sh

But that hangs waiting for input (from fd 3)

Answer 1

I would do this with a named pipe:

mkfifo err 
./myscript.sh 2>err | ./reader.sh 3<err

Doing 2>&3 in your attempt just sends stderr to wherever the parent shell has 3 pointed, and will fail if you haven't already opened 3 in that shell. It doesn't do any good for the reader shell's 3; even if inherited, it would be as a write fd, not a read one.