[Solved] About commands for linux executable files

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1

I have seen commands like this: (hello is the executable file or maybe script file)
./hello
hello
.hello
So, anyone could tell me the differences among these commands?

2 
./hello  # run the hello executable in this directory
hello # run the first instance of the hello executable you find on $PATH
.hello # Run the hidden file .hello from $PATH
3

That's it. Thx.