setup-autossh-tunnel.sh

#!/bin/bash
# credits to: https://gist.github.com/2called-chaos/4285767
# Install:
#   curl -O https://gist.githubusercontent.com/ziogaschr/74884b8d5095c86a7cef/raw/d236c87d89a34b588f37838843279e8e02f073e9/setup-autossh-tunnel.sh
#   chmod u+x setup-autossh-tunnel.sh
#   ./setup-autossh-tunnel.sh
#
# Extra:
# it is good to make a new user on both host and remote (http://linuxaria.com/howto/permanent-ssh-tunnels-with-autossh)
# useradd -m -s /bin/false autossh

SSH_USER="autossh"
SSH_SERVER="example.com"
SSH_PORT="3000"

LOCAL_USER="autossh"
LOCAL_DAEMON_USER="root"
LOCAL_KEYFILE="/home/$LOCAL_USER/.ssh/id_rsa"

LOCAL_PORT="22"
REMOTE_PORT="4444"

# Use a different port for every tunnel to the same machine!
# Be aware that the port above the declared one is also used (e.g.: 20009: 20009 & 20010)
CONTROL_PORT="20009"

SERVICE_SCRIPT="tunnel_rpi"
SERVICE_DESC="AutoSSH-RPi" # just for display
SERVICE_PIDFILE="/var/run/$SERVICE_SCRIPT.pid"

###########


# install autossh
if [[ ! -x /usr/bin/autossh ]] ; then
  read -p "You will need autossh! Shall I invoke 'aptitude install autossh' for you (Y/n)? "
  if [ "$REPLY" != "n" ]; then
    aptitude install autossh
  fi
fi

# trigger ssh connect to accept key
set -e
echo "Please accept the key once"
su -s /bin/sh $LOCAL_USER -c "ssh -i $LOCAL_KEYFILE -p $SSH_PORT $SSH_USER@$SSH_SERVER & echo SSH works"
set +e

echo "Start writing the file"

# write init.d script
cat > /etc/init.d/$SERVICE_SCRIPT <<EOF
#!/bin/bash
#
### BEGIN INIT INFO
# Provides:          $SERVICE_SCRIPT
# Required-Start:
# Required-Stop:
# Should-Start:
# Should-Stop:
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start and stop a autossh $SERVICE_DESC
# Description:       Creates a SSH Tunnel for $SERVICE_DESC
### END INIT INFO
#

export PATH="\${PATH:+\$PATH:}/usr/sbin:/sbin"

# config
USER="$LOCAL_DAEMON_USER"
PIDFILE="$SERVICE_PIDFILE"
# TUNNEL="-N -R $REMOTE_PORT:127.0.0.1:$LOCAL_PORT -i $LOCAL_KEYFILE -p $SSH_PORT $SSH_USER@$SSH_SERVER";
TUNNEL_ARGS=(  -N
               -o ServerAliveInterval=60
               -o ServerAliveCountMax=3
               -o BatchMode=yes
               -o StrictHostKeyChecking=no
               -R $REMOTE_PORT:127.0.0.1:$LOCAL_PORT
               -i $LOCAL_KEYFILE
               -p $SSH_PORT
               $SSH_USER@$SSH_SERVER
            )
DAEMON="/usr/bin/autossh"
DAEMON_OPTS="-M $CONTROL_PORT ${TUNNEL_ARGS[@]}"

# autossh config
export AUTOSSH_GATETIME=20
export AUTOSSH_POLL=60
export AUTOSSH_FIRST_POLL=30
export AUTOSSH_DEBUG=1
export AUTOSSH_LOGFILE=/var/log/$SERVICE_SCRIPT

# exit if autossh is missing
if [[ ! -x /usr/bin/autossh ]]; then
  echo "Please install autossh: aptitude install autossh"
  exit 1
fi

do_start() {
  echo "Starting: $SERVICE_DESC (daemon)"
  start-stop-daemon -u \$USER --name "autossh" --make-pidfile --pidfile \$PIDFILE --start --background --exec \$DAEMON -- \$DAEMON_OPTS
}

do_stop() {
  echo "Stopping: $SERVICE_DESC (daemon)"
  start-stop-daemon -u \$USER --name "autossh" --pidfile \$PIDFILE --stop --retry 30
}

do_restart() {
  echo "Restarting: $SERVICE_DESC (daemon)"
  do_stop
  sleep 5
  do_start
}

do_status() {
  echo "Check Status: $SERVICE_DESC (daemon)"
  if start-stop-daemon -u \$USER --pidfile \$PIDFILE --status; then
    echo "Running"
  else
    echo "Stopped"
    exit 1
  fi
}

uninstall_script() {
  read -p "This will remove the init.d script! Continue (Y/n)? "
  if [ "\$REPLY" == "n" ]; then
    echo "Skipped delete!"
    exit 1
  else
    do_stop
    update-rc.d -f $SERVICE_SCRIPT remove
    rm /etc/init.d/$SERVICE_SCRIPT

    read -p "Remove autossh (y/N)? "
    if [ "\$REPLY" == "y" ]; then
      aptitude remove autossh
    fi
  fi
}

case "\$1" in
    start)
        do_start
        ;;
    stop)
        do_stop
        ;;
    restart)
        do_restart
        ;;
    status)
        do_status
        ;;
    uninstall)
        uninstall_script
        ;;
    *)
        echo "Usage: "\$1" {start|stop|restart|status|uninstall}"
        exit 1
        ;;
esac

exit 0
EOF

# add init script
chmod 755 /etc/init.d/$SERVICE_SCRIPT
update-rc.d $SERVICE_SCRIPT defaults
echo "done"
exit 0

I suspect you might be missing an ampersand?
su -s /bin/sh $LOCAL_USER -c "ssh -i $LOCAL_KEYFILE -p $SSH_PORT $SSH_USER@$SSH_SERVER & echo SSH works"

perhaps should be:
su -s /bin/sh $LOCAL_USER -c "ssh -i $LOCAL_KEYFILE -p $SSH_PORT $SSH_USER@$SSH_SERVER && echo SSH works"