Automatically start ssh-agent

The following in my .bashrc starts ssh-agent and adds keys automatically whenever I open a shell, if it's not already running:

# Check PS1 to determine if it's a login shell
if [ "$PS1" ]; then
  SSH_ENV=$HOME/.ssh/env-$HOSTNAME
  if [ -f "$SSH_ENV" ]; then
    source $SSH_ENV > /dev/null
  fi

  ps $SSH_AGENT_PID 2> /dev/null 1>&2
  if [ $? != 0 ]; then
    echo "$(date) : Starting SSH agent on $HOSTNAME" | tee -a $HOME/.ssh/agent.log
    ssh-agent > $SSH_ENV
    chmod 600 $SSH_ENV
    source $SSH_ENV
    ssh-add
  fi
fi