How to connect input/output to SSH session

Question

What is a good way to be able to directly send to STDIN and receive from STDOUT of a process? I'm specifically interested in SSH, as I want to do the following:

[ssh into a remote server]
[run remote commands]
[run local commands]
[run remote commands]
 etc...

For example, let's say I have a local script "localScript" that will output the next command I want to run remotely, depending on the output of "remoteScript". I could do something like:

output=$(ssh myServer "./remoteScript")
nextCommand=$(./localScript $output)
ssh myServer "$nextCommand"

But it would be nice to do this without closing/reopening the SSH connection at every step.

Answer 1

You can redirect SSH input and output to FIFO-s and then use these for two-way communication.

For example local.sh:

#!/bin/sh

SSH_SERVER="myServer"

# Redirect SSH input and output to temporary named pipes (FIFOs)
SSH_IN=$(mktemp -u)
SSH_OUT=$(mktemp -u)
mkfifo "$SSH_IN" "$SSH_OUT"
ssh "$SSH_SERVER" "./remote.sh" < "$SSH_IN" > "$SSH_OUT" &

# Open the FIFO-s and clean up the files
exec 3>"$SSH_IN"
exec 4<"$SSH_OUT"
rm -f "$SSH_IN" "$SSH_OUT"

# Read and write
counter=0
echo "PING${counter}" >&3
cat <&4 | while read line; do
    echo "Remote responded: $line"
    sleep 1
    counter=$((counter+1))
    echo "PING${counter}" >&3
done

And simple remote.sh:

#!/bin/sh

while read line; do
    echo "$line PONG"
done