On a sunny day (Thu, 5 Jul 2018 14:22:06 +0100) it happened Peter Percival
 wrote in :

>I wrote a "Hello world" program in C.
>
>gcc hello.c
>
>creates a.out without warnings or error messages.  ls shows that a.out
>exists.
>
>a.out
>
>gets the response
>
>-bash: a.out: command not found

Try
./aout

For compile, better use
gcc -Wall -o hello hello.c

./hello

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