Shell script to start up an SSH Port Forward to a remote

ssh-forward-start.sh

#!/bin/bash

# *********************************************************************
# script: ssh-forward-start.sh
# summary: Starts an SSH Port Forward process, forwarding a localhost
#   port to a remote host. The PID of the SSH process will be saved to
#   the file defined in the pid_filename variable.
#   See the companion script ssh-forward-stop.sh, which stops the tunnel
#   using the process information in the PID file.
#
# Tested on macOS Sierra.
#    
# dependencies: 
#   - The local port must not be in use by another process
#   - The remote must be set listening on the port
# 
# *********************************************************************

pid_filename="ssh-forward.pid"
ssh_username="" # Will be prompted
# set your host name here
ssh_host="some.remote.host.net"
# set the local port to map
local_port=1234
# set the remote port to map
remote_port=1234
# If your private key is not properly registered with SSH or if you
# don't use ssh's CONFIG, you might need to explicitly include the 
# path to your key!
#ssh_more_options="-i /path/to/private/key"

echo ""
echo "ssh-forward-start.sh"
echo "Starts an SSH Port Forward Session."
echo "Assumes you have the proper private key for SSH."
echo ""

if [[ ! -f ${pid_filename} ]]; then

  if [[ -z "${ssh_username}"  ]]; then
    read -p 'Please enter a username on the remote host: ' ssh_username
  fi

  if [[ -z "${ssh_username}"  ]]; then
    echo "error: username is required!"
    echo ""
    exit 1
  fi

  echo "Will forward local port ${local_port} to port ${remote_port} on ${ssh_username}@${ssh_host}"
  read -n1 -rsp $'Press any key to continue or Ctrl+C to exit...\n'

  # perform the ssh tunnel / port redirect (and background it)
  ssh -nNT ${ssh_more_options} -L ${local_port}:localhost:${remote_port} ${ssh_username}@${ssh_host} &
  ssh_pid=$!
  echo ${ssh_pid} > ${pid_filename}
  echo "done: ssh fw started!"
else
  echo "error: ssh fw process already started! Use ssh-forward-stop.sh to stop it first."
fi

echo ""