When you run "bash hello" you are calling exec() on bash, passing "hello" as an argument, which bash then reads; when you run "./hello" you are calling exec() o

When you run "bash hello" you are calling exec() on bash, passing "hello" as an argument, which bash then reads; when you run "./hello" you are calling exec() on hello: the kernel then treats "hello" as an executable, but notes that "hello" starts with "#!" and then will run the specified interpreter for you, passing "./hello" as an argument. The kernel doesn't think of "hello" as a program when you run "bash hello".