Bash function for creating a SSH tunnel

ssh_tunnel.sh

function ssh_tunnel() {
    target_host="$1"
    target_port="$2"
    tunnel_host="$3"
    source_port="$4"
    [[ `ps axu | grep $target_port | grep ssh` ]] && return
    ssh -Nf -L "$source_port:$target_host:$target_port" "$tunnel_host"
}