How to make sure only one instance of a Bash script is running at a time?

Question

I want to make a sh script that will only run at most once at any point.

Say, if I exec the script then I go to exec the script again, how do I make it so that if the f

Answer 1

You want a pid file, maybe something like this:

pidfile=/path/to/pidfile
if [ -f "$pidfile" ] && kill -0 `cat $pidfile` 2>/dev/null; then
    echo still running
    exit 1
fi  
echo $$ > $pidfile