A "Hello world" program is a computer program that outputs "Hello World" (or some variant) on a display device. The first known version of this program comes from Brian Kernighan's paper A Tutorial Introduction to the Language B from 1972 (chapter 7). In it's first form it was used to show how to use external variables in B but since then it has become pretty much the standard example to show the very basic syntax and most simple program in any given programming language.
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