Hello world program

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 a LED being turned on, may substitute for the message.

A GUI Hello World program, written in Perl.

[edit] Purpose

A "hello world" program has become the traditional first program that many people learn. In general, it is simple enough such that people who have no previous experience with computer programming can easily understand it, especially with the guidance of a text or teacher. Using this simple program as a basis, computer science principles or elements of a specific programming language can be explained to novice programmers. Experienced programmers learning new languages can also gain a lot of information about a given language's syntax and structure from a hello world program.

In addition, hello world 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.

"Hello world" is also used by computer hackers as a proof of concept that arbitary code can be executed through an exploit where code should not be allowed to be executed, for example, on Sony's Playstation Portable. This is the first step into allowing home-made content ("homebrew") usable on such a device.

[edit] History

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 seminal 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^[1], with the following code:

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

An exposition: the only fundamental data type in B is the machine word. On the Honeywell 6070, the machine that the particular version of B that was described ran on, this is 4 bytes long. The character constant correctly expresses this fact. There is no = in initializers in B.

[edit] Variations

Hackers execute unsigned HelloWorld code on Sony's PlayStation Portable homebrew

A light-programmable biofilm displaying the Hello World message

CNC machining test in Perspex

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.

The Debian and Ubuntu operating systems provide the "hello world" program through the apt packaging system; this allows users to simply type "apt-get install hello" for the program to be installed, along with any software dependencies.