I mean I execute ./pants binary my/fs/path so how I can find inside execute method that path my/fs/path? Somewhere in context?

Yes, in the Context. For the targets explicitly listed on the command line (my/fs/path) in your example: https://github.com/pantsbuild/pants/blob/bfa45c8f46f285879f0b60207ed892cf0d89eeb5/src/python/pants/goal/context.py#L119-L129