Several Issues about processes

Several Issues about processes-general Linux technology-Linux programming and kernel information. The following is a detailed description. I am a newbie and want to compile a small script to implement the function:
Check if a process is not running, run it; if it is already running, close it.
The problem is:
1. How do I know whether a process is running? Some processes are on the monitor, some are not on the monitor, and some are under/var/run? What is the problem?
2. Some of pidof can be found, and some cannot be found...
3. I don't know much about the lock process...
I hope you can give me some advice...