Use SSH agent from other client

ssh-auth-caret.sh

#!/bin/bash

# See blog for more details:
# https://qistoph.blogspot.nl/2017/07/use-ssh-agent-socket-from-remote-host.html

# Exit if undeclared variables are used
set -o nounset

# Exit if any command exits with error
set -o errexit

# Print each command to stdout before executing it
# set -o verbose

function enternum() {
        if [ $# -lt 4 ]; then
                echo "Invalid call to enternum (enternum $*)" >&2
                return 1
        fi

        local __prompt=$1
        local __min=$2
        local __max=$3
        local __resultvar=$4

        if [ $__min -eq $__max ]; then
                __prompt="$__prompt [$__min]"
        else
                __prompt="$__prompt [$__min-$__max]"
        fi

        while [ 1 ]; do
                echo -n "$__prompt: "
                read ans

                [ -z "$ans" ] && [ $__min -eq $__max ] && ans="$__min"

                if [[ $ans =~ ^[0-9]+$ ]]; then
                        if [[ $ans -ge $__min && $ans -le $__max ]]; then
                                eval $__resultvar="'$ans'"
                                return 0
                        fi
                fi
        done
}

# Setup SSH master channel
SSH_CONTROL_PATH="~/.ssh/control:%h:%p:%r"
LOCAL_SOCK="/tmp/ssh-auth-caret.$USER"

SSH_SERVER="$1"

ssh -N -f -M -o StreamLocalBindUnlink=yes -S "$SSH_CONTROL_PATH" "$SSH_SERVER"

ssh -S "$SSH_CONTROL_PATH" -O check "$SSH_SERVER"

exec 3< <(ssh -S "$SSH_CONTROL_PATH" "$SSH_SERVER" 'ls -1 /tmp/ssh-*/agent*')

SOCKS=()
while read -u 3 SOCK; do
        echo "Sock :$SOCK"
        SOCKS+=("$SOCK")
        echo "${#SOCKS[@]}) $SOCK"
        ssh -S "$SSH_CONTROL_PATH" "$SSH_SERVER" 'SSH_AUTH_SOCK="'"$SOCK"'" ssh-add -l'
        echo
done

if [ ${#SOCKS[@]} -le 0 ]; then
        echo "No sockets found on $SSH_SERVER" >&2
        eval "$SAVED_OPTIONS"
        exit 1
elif [ ${#SOCKS[@]} -eq 1 ]; then
        socket=1
else
        enternum "Socket" 1 ${#SOCKS[@]} socket
fi

ssh -N -f -S "$SSH_CONTROL_PATH" -L"$LOCAL_SOCK":"${SOCKS[$socket - 1]}" "$SSH_SERVER"

echo export SSH_AUTH_SOCK="$LOCAL_SOCK"

#ssh -S "$SSH_CONTROL_PATH" -O exit "$SSH_SERVER"