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A "hello world" program is a computer program that prints out "Hello, World!" on a display device. It is used in many introductory tutorials for teaching a programming language. Such a program is typically one of the simplest programs possible in a computer language. Some are surprisingly complex, especially in some graphical user interface (GUI) contexts, but most are very simple, especially those which rely heavily on a particular command line interpreter ("shell") to perform the actual output. In many embedded systems, the text may be sent to a one or two-line liquid crystal display (LCD), or some other appropriate signal, such as an LED being turned on, may substitute for the message.

A GUI Hello World program, written in Perl, using GTK+2, edited with Vim.

A "hello world" program can be a useful sanity test to make sure that a language's compiler, development environment, and run-time environment are correctly installed. Configuring a complete programming toolchain from scratch to the point where even trivial programs can be compiled and run can involve substantial amounts of work. For this reason, a simple program is used first when testing a new tool chain.

While small test programs existed since the development of programmable computers, the tradition of using the phrase "Hello world!" as a test message was influenced by an example program in the book The C Programming Language. The example program from that book prints "hello, world" (without capital letters or exclamation mark), and was inherited from a 1974 Bell Laboratories internal memorandum by Brian Kernighan, Programming in C: A Tutorial, which contains the first known version:

main() {
        printf("hello, world");
 }

The first known instance of the usage of the words "hello" and "world" together in computer literature occurred earlier, in Kernighan's 1972 Tutorial Introduction to the Language B, with the following code:

main( ) {
   extrn a, b, c;
   putchar(a); putchar(b); putchar(c); putchar('!*n');
 }
 a 'hell';
 b 'o, w';
 c 'orld';

A light-programmable biofilm displaying the Hello World message

There are many variations on the punctuation and casing of the phrase. Variations include the presence or absence of the comma and exclamation mark, and the capitalization of the 'H', both the 'H' and the 'W', or neither. Some languages are forced to implement different forms, such as "HELLO WORLD!," on systems that only support capital letters, while many "hello world" programs in esoteric languages print out a slightly modified string. For example, the first non-trivial Malbolge program printed "HEllO WORld", this having been determined to be "good enough".

There are variations in spirit, as well. Functional programming languages, like Lisp, ML, and Haskell, tend to substitute a factorial program for Hello World, as the former emphasizes recursive techniques, which are a big part of functional programming, while the latter emphasizes I/O, which violates the spirit of pure functional programming by producing side effects.

• Trabb Pardo-Knuth algorithm
• Just another Perl hacker
• List of basic computer science topics
• At Wikibooks:
  • Computer programming/Hello world
  • List of hello world programs
  • Fibonacci number program

• The Hello World Collection with 300+ programs, including "Hello World" in 50+ human languages
• Desu Desu Desu is a similar site.
• ACM "Hello World" project
• "HelloWorld online on Web, and steps beyond HelloWorld"
• http://www.99-bottles-of-beer.net 99 bottles ... over 1000 programming languages used ...
• Programming in C: A Tutorial by Brian Kernighan — internal Bell Labs memo, containing the above C program
• Humor:
  • from the GNU Humor Collection