Simple pid script

pid.sh

#!/usr/bin/env bash

_pid="$HOME/$0.pid"

while :
do
  if [ ! -s "${_pid}" ] ; then
    echo $$ >"${_pid}"
    echo "Doing real stuff"
    rm "${_pid}"
    break
  else
    echo "waiting for $(cat ${_pid}) to finish"
    sleep 10
  fi
done