System command

In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is either a directive to some kind of command-line interface, such as a shell, or an event in a graphical user interface triggered by the user selecting an option in a menu.

Specifically, the term command is used in imperative computer languages. These languages are called this, because statements in these languages are usually written in a manner similar to the imperative mood used in many natural languages. If one views a statement in an imperative language as being like a sentence in a natural language, then a command is generally like a verb in such a language.

Many programs allow specially formatted arguments, known as flags or options, which modify the default behaviour of the command, while further arguments describe what the command acts on. Comparing to a natural language: the flags are adverbs, whilst the other arguments are objects.

Here are some commands given to a command-line interpreter (Unix shell).

The following command changes the user's place in the directory tree from their current position to the directory /home/pete. The command is cd and the argument is /home/pete

The following command prints the text Hello World out to the standard output stream, which, in this case, will just print the text out on the screen. The command is echo and the argument is "Hello World". The quotes are used to prevent Hello and World being treated as separate arguments:

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