Hello world program

Hello world" program is a computer program that outputs "Hello, world" on a display device. Because it is typically one of the simplest programs possible in most programming languages, it is by tradition often used to illustrate to beginners the most basic syntax of a programming language, or to verify that a language or system is operating correctly.
In a device that does not display text, a simple program to produce a signal, such as turning on an LED, is often substituted for "Hello world" as the introductory program.


Variations


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 large 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 Linux distributions 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. While of itself useless, it serves as a sanity check and a simple example to newcomers of how to install a package. It is significantly more useful for developers, however, as it provides an example of how to create a .deb package, either traditionally or using debhelper, and the version of hello used, GNU hello, serves as an example of how to write a GNU program.