Best practice to set up command that will run only at first session

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1

What is the good way to make command run only one time and never run it again if once it succeeds?
I think it might be done by triggering a handler which creates a flag-file after command succeds and also by defining a condition which prevents command run if that flag-file exists.
But how to accomplish that? The problems that I can't solve here is:
1) How to trigger a handler only if command succeeds?
2) How to run commands only if certain file exists?

2

The command/shell modules already do this with the creates/removes options. See the documentation here:

http://docs.ansible.com/command_module.html