SSH Tunnel to MySQL via Bastion Host

bastion-host.sh

#!/bin/bash

# The following environment variables are required for connecting to the DB via an SSH tunnel:
# DBHOST -> the MySQL DB endpoint
# BASTIONUSER -> username for connecting to the bastion host, associated with your SSH key
# BASTIONHOST -> the IP or hostname of the bastion host
#
# Overriding the SSH key path is optional:
# SSHKEYPATH -> if using a non-default SSH key, set this to override ~/.ssh/id_rsa

if [ -z "$DBHOST" ]; then
    echo "Missing DBHOST environment variable."
    exit -1
fi

if [ -z "$BASTIONUSER" ]; then
    echo "Missing BASTIONUSER environment variable."
    exit -1
fi

if [ -z "$BASTIONHOST" ]; then
    echo "Missing BASTIONHOST environment variable."
    exit -1
fi

if [ -n "$SSHKEYPATH" ]; then
    PUBLICKEYPATH="$SSHKEYPATH"
else
    PUBLICKEYPATH="~/.ssh/id_rsa"
fi

SERVER="$BASTIONUSER@$BASTIONHOST"
SOCKET=/tmp/dbssh

if [ -S "$SOCKET" ]; then
    echo "SSH tunnel already exists so sending exit command before creating a new one."
    ssh -S $SOCKET -O exit $SERVER
fi

echo "Creating new ssh tunnel."
ssh -i $PUBLICKEYPATH -f -N -M -S $SOCKET -L 3306:$DBHOST:3306 $SERVER

Thanks to the following Stack Overflow questions for help:
https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash
https://stackoverflow.com/questions/37618443/why-cant-bash-recognize-the-existence-of-a-socket-file
https://unix.stackexchange.com/a/164656
https://unix.stackexchange.com/a/328824

Once environment variables are set, this could be integrated with something like a Node.js project via a script like
"start-with-db": "./bastion-host.sh && npm run start"

Then running npm run start-with-db would spin up the SSH tunnel and then start the application. With this setup, the application would connect to the database as if it's local, using the hostname "localhost".